Tulum, Quintana Roo, in the Yucatán Peninsula of Mexico — It was in May of 2007 when the divers approached it—a cenote deep inside the jungle. Cenotes, or water-filled sinkholes, are not uncommon in these parts. In fact, in the Yucatán, they exist by the thousands. Divers and archaeologists alike love them for the secrets they hide — secrets that could reveal much about a past human presence in a subtropical world that seemed to serve as an unlikely setting for one of the greatest of ancient civilizations, the Maya. Alberto Nava, who took the lead on this dive, is something of a veteran when it comes to such ventures. He is a professional diver with the Bay Area Underwater Explorers, a California-based nonprofit organization dedicated to exploration and conservation of the world’s underwater regions. With his two other diving companions, Alex Alvarez and Franco Attolini, he plunged into the cenote, an entry into an elaborate underwater cave system known as the Sac Actun. It was not unlike most any other dive — until they encountered an unexpected drop.

“We traveled through a flooded tunnel for about 1 mile before we reached the edge of this pit,” recounts Nava. “The floor disappeared under us, and we could not see across to the other side. We pointed our lights down and to the sides. All we could see was darkness. We felt as if our powerful underwater lights were being absorbed by this void, so we called it Black Hole, which in Spanish is Hoyo Negro.”

Nava and his colleagues could see that there was much more to this than they could absorb on a first dive. So they waited for another day.

“A couple of months later, we ventured deeper into this darkness and reached the floor of the pit at about 170 feet. It was a bell-shaped structure 200 feet in diameter. The center was littered with large boulders stacked on top of each other. As our eyes got accustomed to the environment, we started to notice large animal bones. The first one we found was a 3- foot-long femur resting against one of the boulders. My teammates started signaling in all directions as they pointed to animal remains that were resting at the bottom and on the walls of the pit. At that time, we were not sure what kind of bones we were looking at, but we knew they were old and big. All of the sudden, Alex pointed to a human skull resting on the top of a small ledge. It was a small cranium laying upside down with a perfect set of teeth and dark eye sockets looking back at us. The skull was resting on its humerus, and we could see the rest of the upper torso spread to the left and down on the ledge.”

As they usually did when encountering new finds, they began documenting the site. But they could see that this was a discovery that required more specialized attention.

“In 2009, we reported the site to archaeologist Pilar Luna from Mexico’s National Institute of Anthropology and History (INAH), and together we created the Hoyo Negro Consortium (www.hoyonegro.org ), a group of cave divers and researchers working to unravel the mysteries of Hoyo Negro.”

With this, the real scientific journey began at full throttle.

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A broad view of Hoyo Negro, shot from the floor near the south edge, showing the immensity of the chamber and the complexity of the boulder-strewn bottom. One access tunnel can be seen near the ceiling at top left. This photo was taken by the “painting with light” method on a 30 second exposure. Text and photo credit: Roberto Chavez Arce

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The skull as it was discovered in 2007, resting against the left humerus (upper arm bone).
Photo and text credit: Daniel Riordan Araujo

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The skull as it appeared in December 2011, after rolling into a near-upright position.
Photo credit: Roberto Chavez Arce

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The Cave and The Water Nymph

Under the auspices of the new consortium, the task now fell to an international interdisciplinary team of scientist-specialists to undertake the meticulous job of additional recording, recovery, examination, and analysis of the various finds — finds that would eventually lead to some headline-making conclusions.

For the anthropologists and archaeologists of the group, the greatest excitement naturally focused on the human remains, what was turning out to be a nearly complete skeleton. But before anyone could understand the age and significance of the skeleton, they needed to understand the context. This required the efforts of specialists with knowledge of things like geology and cave formation, paleontology, dating techniques, and genetics. 

Patricia Beddows of Northwestern University is one of those specialists. Her work has focused on cave systems formed by the dissolution of soluble carbonate rocks like limestone and dolomite, especially the flooded caves of the Yucatán Peninsula, including Hoyo Negro. She has logged countless hours diving into the cave systems. “Research in flooded caves is much like space exploration, with divers similar to astronauts reporting back to ‘mission control’ — a much larger scientific team at the surface,” said Beddows. “It all has to be done on SCUBA, which is our life support system.” Beddows’ studies have shown how the extensive Yucatán cave system has drained groundwater to the coast and how the water level in the caves has closely matched the sea level changes over time. “Using this knowledge, we understand how Hoyo Negro has changed over thousands of years,” she says. This has helped to date Hoyo Negro, once a drier cave chamber with lower water levels, to more than 12,000 years ago, when many of the bones or animals, including the human, had apparently fallen into or entered the cave. Her work with the re-crystalized rock sediments in the cave system also helped. The rocks and the bones in Hoyo Negro are coated with rock crystals, including a new form of crystal that Beddows calls “florets”. Using Uranium/Thorium dating analysis, scientists have been able to accurately and reliably date the bone. “An impressive aspect of this research is that we have dated the skeleton directly, but we also have supported these dates with additional dates on the florets,” Beddows said.

But narrowing down the dating has been very much a team effort, with a multi-pronged approach. To begin with, the cave-diving scientists knew that the human skeleton was found within the context of the skeletal remains of 26 large mammals, many of which were identified as the bones of long-extinct fauna, such as saber-toothed cats and gomphotheres (extinct relatives of mastodons). These are mammals that lived more than 11,000 years ago. The human skeleton was found near the bones of a gomphothere. Douglas Kennett, professor of environmental archaeology at Penn State, and Brendan J. Culleton, postdoctoral fellow in anthropology at Penn State, worked with colleagues to construct a geochronological framework for the skeleton by using a combination of methodologies that successfully constrained the age to the end of the Ice Age, the Late Pleistocene. Working with Yemane Asmerom and Victor Polyak from the University of New Mexico, they applied global sea level rise data to determine when the cave system filled with water. As the bones now lie about 130 feet below sea level, they estimated that sea level rise would have raised the groundwater level in the cave system and submerged the bones between 9,700 and 10,200 years ago. Thus, the latest animals and humans could have entered the cave system in a dry state was about 9,700 years ago.

In addition, enamel extracted from a tooth of the skeleton was radiocarbon dated to 12,900 years ago by Kennett’s lab.

“Unfortunately, we can’t rule out that the tooth enamel is contaminated with secondary carbonates from the cave system, but we removed potential contaminates using standard techniques and Tom Stafford, Stafford Research Laboratories, produced a comparable age,” said Kennett. “We consider this a maximum age and when combined with the uranium thorium dates from the adhering speleothems, we argue that the skeleton dates between 12,000 and 13,000 years ago. Well placed as a Paleoamerican.”

A Paleoamerican. This was something quite different than an ancient Mayan. The Maya, best known for the iconic monumental Mesoamerican cities like Chichen Itza, Tikal, and Copan, lived and dominated Central America from about 2000 B.C to about 1500 A.D. The Paleoamericans lived thousands of years earlier, inhabiting the American continents during the final glacial episodes of the late Pleistocene period, as far back as 15,000 years ago. The bones of this skeleton were dated to between 12,000 and 13,000 years ago. This represented one of the six oldest humans found in America. And it was certainly the most complete find for that time period. 

But there was more to the skeleton than age. After examination, the team determined that the skeleton belonged to a slightly-built, 4-foot-10-inch girl of between 15 and 16 years of age. A teenager. She may have fallen into the pit while searching for water more than 12,000 years ago, the team surmised. A plausible scenario, as there was no evidence that she had been attacked, killed and dragged into the cave chamber by another preying animal. Scientific teams like to give nick-names to their biggest finds, especially if they are human skeletons. So the Hoyo Negro team called this one Naia, a greek reference to a water nymph. The name stuck and, along with names like “Lucy”, “Turkana Boy” and “Kennewick Man”, “Naia” was soon becoming something of a household word among scientists and the public, alike.

More important to the scientists, however, was that her features were consistent with those often attributed to Paleoamericans based on previous excavations and studies. “The earliest human skeletons found in the Americas differ markedly from those of modern Native Americans in the shapes of their skulls, faces and teeth,” said Dr. James C. Chatters of Applied Paleoscience, lead author of a recently published study related to the Hoyo Negro finds. The 9,000+-year-old Kennewick Man, found near Kennewick in Washington State in 1996, and also examined by Chatters, was a prime example.

Deciphering the ancestry of the first people to populate the Americas has been a challenge. On the basis of some genetic studies, modern Native Americans are thought to descend from Siberians who moved into eastern Beringia (the landmass that anciently connected Asia and North America) between 26,000 and 18,000 years ago. These people, considered the earliest Americans, are suggested to have then spread southward and populated the rest of the continent. But despite widespread support for this idea, the ancestry of the earliest Americans is still debated because, among other things, the facial features of the oldest American skeletons don’t look much like those of modern Native Americans. Thus, in terms of the Native American origins debate, the Hoyo Negro story is significant. And to this, the genetics contingent of the team had something to say about Naia………….

Haplogroups and Beringia

As is often the case, interdisciplinary scientific teams often turn to the best technical experts when it comes to finding answers to their questions. So it was that the Consortium approached Applied Paleoscience founder and Director James Chatters, a forensic anthropologist, archaeologist, and paleontologist. It was Chatters who played the lead role in the study of the famous Kennewick Man, and it is Chatters who is now leading the examination and analysis of Naia’s remains, resulting in a recently-published study that has catapulted Naia front-and-center on the stage of Native American origins research. 

For the genetic examination component, Chatters recruited Brian Kemp of Washington State University to take the lead. Using the latest techniques, Kemp, a molecular anthropologist with expertise in the field of ancient human genetics, has sequenced DNA from ancient human remains found in both North and South America, including human coprolites — ancient excrement — from archaeological sites in Oregon and the American Southwest. His task was to tease out small amounts of Mitochodrial DNA (mtDNA), found in the energy-generating structures of cells, of one of Naia’s teeth. Isolating and sequencing mtDNA is common among studies of ancient human remains, as organisms contain many more copies of it than chromosomal DNA. And because it is inherited from the mother, one can determine maternal ancestry.

Extracting and analyzing Naia’s DNA was challenging, to say the least. “There is very little DNA preserved in a skeleton of such great antiquity,” said Kemp.

But because mtDNA is so much more plentiful than chromosomal DNA, and because scientists have now clocked years of experience sequencing mtDNA from ancient bones, it was a feasible undertaking. Using molecular biological techniques to amplify the DNA, Kemp was able to make hundreds of billions of copies of Naia’s original DNA material. “Once so many copies are made,” said Kemp, “it is pretty straightforward to study the DNA.”

The study results were enlightening. Kemp had determined that Naia shared a common ancestry with many of today’s Native Americans — a genetic signature that is derived from an Asian lineage but is only found in the Americas today. Called Haplogroup D1, it is found throughout North, Central and South America. It is thought that about 11% of Native Americans exhibit this haplogroup — especially in South America, where it shows up in about 29% of indigenous Chileans and Argentinians. Geneticists theorize that this lineage arose in Beringia after the ancient Beringian population separated from other Asian populations.

Kemp then asked two other expert colleagues to conduct similar tests on Naia’s tooth to verify his results. Deborah Bolnick, assistant professor of anthropology at the University of Texas at Austin, came up with essentially the same results. University of Illinois anthropology and Institute for Genomic Biology professor Ripan Malhi, focusing on regions of the mitogenome that mutate more slowly than other parts of the genome (and thus could be more reliable markers of genetic relatedness through time), also corroberated Kemp’s results. 

Three separate labs had confirmed the same results. Naia, a Paleoamerican with physical characteristics distinctly different than today’s Native Americans, shared the same ancestry — an ancestry that pointed to ancient Beringia — and by extension to Asia.

The finding was not altogether startling. Previous studies have indicated similar results. But this is the first time that a nearly complete skeleton of this age has been genetically linked to today’s Native Americans, and protractively to Beringia and Asia. “Because she exhibits distinctive Paleoamerican skull and facial features,” says Bolnick, “the study shows for the first time that Paleoamericans with these distinctive features can have Beringia ancestry.”

Yet, what accounts for these different physical characteristics?

It could be evolution, suggests Bolnick and colleagues. “It seems likely that differences between Paleoamericans and Native Americans of today are due to evolutionary changes that occurred in Beringia and the Americas over the last 9,000 years,” she says. To support her suggestion, Bolnick points to evolutionary changes that have occurred over the last 10,000 years in other populations — a comparatively short time period on the evolutionary time scale, traditionally measured in millions of years. “There have also been studies of the skeletal morphology in later remains discovered in the Americas over the last 10,000 years. We certainly do see changes in the skull shape, in the facial features over that period of time.”  She points additionally to Native American populations in South America who live at high altitudes, who evolved genetic and physical characteristics that have facilitated their survival in these regions.

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The upper right third molar of Naia, which was used for both radiocarbon dating and DNA extraction. The tooth is held by ancient genetics expert Brian Kemp of Washington State University, who led the genetic research on the skeleton. Photo credit: James Chatters

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The 10,000-Year Homeland

The Naia story and its evolutionary element have important implications in connection with other recent studies related to Native American origins. Some of them revolve around one emerging theory that has drawn the focus and attention of researchers from all over the world. Known as the “Beringia Standstill” theory, it was first suggested by two Latin American geneticists in 1997 and then refined or corroborated by a University of Tartu research team in Estonia in 2007. From a sampling of mitrochondrial DNA from more than 600 Native Americans, they found that mutations in the DNA pointed to the likelihood that a group of their direct ancestors from Siberia was isolated from their Siberian origins for at least several thousand years, during the time period from 25,000 (if not earlier) to 15,000 years ago (when ice-free corridors developed), before their descendants moved into the Americas. Evidence from recent paleo-ecological research suggested that this isolation most likely occurred in Beringia, a land mass that once covered the present-day Bering Strait between northeast Asia and Alaska. 

“A number of supporting pieces have fallen in place during the last decade, including new evidence that central Beringia supported a shrub-tundra region with some trees during the last glacial maximum and was characterized by surprisingly mild temperatures, given the high latitude,” said John Hoffecker of CU-Boulder’s Institute of Arctic and Alpine Research, who is the lead author of a related Perspective article that appeared in the Feb. 28 issue of Science magazine. 

This is an important aspect within the overall geographic context of the area, as the last glacial maximum reached its peak about 21,000 years ago with the development of massive ice sheets across North America and Europe, essentially blocking access to North America from northeast Asia until about 15,000 years ago. Thus the ice sheet barrier, along with distance from Siberia, would have created a geographic basis for the gap suggested by the genetic data.

But combining the genetics with the recent paleoecological research, which involved analyzing fossil pollen, plant and insect material taken from sample sediment cores from the now submerged landscape, has been the key.

“The genetic record has been very clear for several years that the Native American genome must have arisen in an isolated population at least by 25,000 years ago, and the bulk of the migrants to the Americas really didn’t arrive south of the ice sheets until nearly 15,000 years ago,” said co-author and University of Utah anthropologist Dennis O’Rourke. “The paleoecological data, which I think most geneticists have not been familiar with, indicate that Beringia was not a uniform environment, and there was a shrub-tundra region, or refugium, that likely provided habitats conducive to continuous human habitation.” 

Scott Elias, an article co-author and a professor with the Department of Geography at Royal Holloway, elaborated: “We believe that these ancestors survived on the shrub tundra of the Bering Land Bridge because this was the only region of the Arctic where any woody plants were growing. They needed the wood for fuel to make camp fires in this bitterly cold region of the world. They would have used dwarf shrub wood to get a small fire going, then placed large mammal bones on top of the fire, to ignite the fats inside the bones. Once burning, large leg bones of ice-age mammals would have burned for hours, keeping people alive through Arctic winter nights.”

On the genetic side of things, the theory that humans inhabited Beringia for as much as 10,000 years “helps explain how a Native American genome (genetic blueprint) became separate from its Asian ancestor,” said O’Rourke.

“At some point, the genetic blueprint that defines Native American populations had to become distinct from that Asian ancestry,” he explains. “The only way to do that was for the population to be isolated. Most of us don’t believe that isolation took place in Siberia because we don’t see a place where a population could be sufficiently isolated. It would always have been in contact with other Asian groups on its periphery.”

“But if there were these shrub-tundra refugia in central Beringia, that [would have] provided a place where isolation could occur” due to distance from Siberia, O’Rourke says.

In contrast to the genetic and paleoenvironmental evidence, however, the archaeological record has been lacking. This would be explained by the suggestion that, according to a number of scholars, the archaeological evidence was submerged under the rising sea levels that resulted in today’s geography of the region. “These shrub-tundra areas were likely refugia for a population that would be invisible archaeologically, since the former Beringian lowlands are now submerged,” maintains O’Rourke. The suggestion that rising sea levels subsequently covered the evidence of human migration into the Americas has also been a long-standing theory among researchers studying the model that advances the notion that early Native Americans moved south along the Pacific coast as the glaciers receded and sea levels rose. 

In addition, Hoffecker suggests that the Beringia inhabitants during the last glacial maximum could have made successful hunting forays into the uninhabited steppe-tundra region to both the east and west of central Beringia, where drier conditions and more grass supported a plentiful array of large grazing animals, including steppe bison, horse and mammoth.

There is now solid evidence for humans in Beringia before the last glacial maximum, as geneticists first predicted in 1997, according to Hoffecker. After the maximum, there are two sets of archaeological remains dating to less than 15,000 years ago. “One represents a late migration from Asia into Alaska at that time,” he said. “The other has no obvious source outside Beringia and may represent the people who are thought to have sheltered on the land bridge during the glacial maximum. If we are looking for a place to put all of these people during the last glacial maximum, Beringia may be the only realistic option.”

Hoffecker, O’Rourke and colleagues say new archaeological sites must be found in Beringia if the long human layover there is to be confirmed. Although most such sites are presumed to be underwater, they are hopeful that some evidence of human habitation in shrub-tundra areas might remain above sea level in low-lying portions of Alaska and eastern Chukotka (in Russia).

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Map of ancient Beringia, showing the outlines of modern Siberia (left) and Alaska (right) with dashed lines. The area in dark green is the portion of Beringia (now submerged by the ocean) near the end of the last glacial maximum, a period that lasted from 28,000 to 18,000 years ago when sea levels were low and ice sheets extended south into what is now the northern part of the lower 48 states. Photo Credit: Wlliam Manley, Institute of Arctic and Alpine Research, University of Colorado. Illustration below: credit Julie McMahon

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Moving Forward

Although the Hoyo Negro findings have added significant additional support for the Beringian origins theory, the scientists caution that there is much more work to do before any far-reaching conclusions can be drawn. After all, Naia has shown that she is related to only 11 percent of all Native Americans. What about the other 89 percent? The findings thus far suggest that at least some modern-day Native Americans can trace their roots to an ancient Beringian population. But scientists have learned, over and over again, that the more they discover about human prehistory, the more they realize they don’t know. Reconstructing the past often turns out to be far more complicated than we initially suppose. And this, the Hoyo Negro scientists would likely agree, makes the whole journey of discovery all the more exciting.

“The dating and genetic analysis of Naia is a first step in what we hope will be a long productive investigation of Hoyo Negro,” says Chatters, looking ahead. “Our next steps will focus on attempting to sequence Naia’s nuclear DNA, determining the ages and genetics of the cave’s other animal skeletons and reconstructing the environment in which they and Naia lived.”

But it will all take a village of experts, just as it has to this point, employing a variety of disciplines and skills to re-create the evolving picture. “Balancing all of these elements has been a wonderful challenge,” says Pilar Luna from INAH. “On the one hand, we have a large team of renowned scientists who have been extremely generous sharing their knowledge and experience to analyze, understand, and interpret the findings, in order to produce the knowledge we have gotten so far. On the other hand, there are the cave divers, who have taken specialized courses to gather data for archaeological purposes, to properly record the general context taking samples, measurements, photos, videos, etc., and following with extreme care all requests made by the experts, in spite of the complexities and dangers of the cave itself. Also, we have had the support from engineers and technicians who have developed extraordinary methods to be used in this project for the first time, with significant results, mainly in the fields of software and photography.”

The combined efforts will extend to protecting the cave itself from would-be looters and other environmental dangers, so that research can continue as long as possible. “We feel obligated to protect this incredible resource,” says Nava. “So we are working with INAH and other Mexican authorities to control site access and make sure Hoyo Negro is preserved for future generations.”

Time will tell the ultimate fate of the Black Hole. In the meantime, what is collected of Naia’s remains will rest enshrined within laboratory walls and, perhaps in time, behind museum glass. 

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 Article Supplement

Naia’s World of Gomphotheres

When University of Arizona archaeologist Vance Holliday and colleagues began uncovering large fossilized bones at the site of El Fin del Mundo in the Sonoran Desert of northwestern Mexico in 2007, they weren’t sure what kind of animal they were unearthing. 

“At first, just based on the size of the bone, we thought maybe it was a bison, because the extinct bison were a little bigger than our modern bison,” said Holliday, who has been researching geoarchaeology at Paleoindian sites across the U.S. for years.

Then, in 2008, they discovered something that clinched it for them.

“We finally found the mandible, and that’s what told the tale,” Holliday said.

It was a gomphothere.

Actually, two of them.

About the same size as a modern elephant, but smaller than their extinct cousins the mammoths, gomphotheres were once widespread in North America but were thought to have disappeared from the fossil record long before humans arrived in North America some 13,000 to 13,500 years ago.

Until now.

Radiocarbon dating of charcoal flecks and burned bone found within the context of the fossils indicated a reliable age of 13,390 years. This made these two gomphotheres the last known gomphotheres in North America.

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Gomphothere mandible in place, upside down, at El Fin del Mundo excavation site. The fossil was fully prepared at the INAH zooarchaeology lab in Mexico City. Image courtesy of Vance T. Holliday.

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The fully excavated and prepared gomphothere mandible. Courtesy Joaquin Arroyo-Cabrales/Instituto Nacional de Antropología e Historia

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These sculptures, made by Mexican artist Sergio de la Rosa, show three elephant ancestors: (from left to right) the mastodon, the mammoth and the gomphothere. Courtesy Sergio de la Rosa

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But there was more.

As they excavated among the bones, they also uncovered human artifacts—Clovis artifacts, to be specific—including 7 projectile points, some stone cutting tools and 21 flint flakes from stone tool-making. The position and proximity of the Clovis fragments relative to the gomphothere bones at the site suggested that humans did in fact kill the two animals there. Of the seven points found at the site, four were in place among the bones, including one with bone and teeth fragments above and below. The other three points had eroded away from the bone bed and were found scattered nearby. This suggested that the gomphotheres were likely hunted and thus constituted a Clovis prey species, along with mammoths, mastodons, and bison, already known to have been hunted by the Clovis.

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A clear quartz Clovis point found near the bone bed at El Fin del Mundo. Although very difficult to shape into a tool, quartz was used by Clovis tool makers at several sites.  Courtesy INAH Sonora.

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“This is the first Clovis gomphothere, it’s the first archaeological gomphothere found in North America, it’s the first evidence that people were hunting gomphotheres in North America, and it adds another item to the Clovis menu,” Holliday said.

The Clovis culture, today considered the oldest clearly defined and recognized Paleoindian culture in the Americas, is characterized by its distinctive stone tools, particularly the fluted projectile points. The first examples of this culture were discovered by archaeologists near Clovis, New Mexico, in the 1930s. The El Fin del Mundo site, along with the Aubrey site in Texas, is now among two sites that show the earliest solid evidence of Clovis hunting in North America, indicating that the earliest widespread and recognizable group of hunter-gatherers were already in place 13,390 years ago in the North American Southwest.

Holliday and colleagues suggest that the finds support the model of an American southwestern origin for the Clovis material culture. As they conclude in the study, published this week in the Proceedings of the National Academy of Sciences:

“These data expand our understanding of the age range for Clovis, Clovis diet, raw material preference, and the late Pleistocene megafaunal assemblage of North America, and provide evidence for a southern origin of the Clovis technocomplex.”*

Holliday and the study team report that the radiocarbon ages from El Fin del Mundo were made based on testing the site’s charcoal, shell, and organic matter at the Arizona Accelerator Mass Spectrometry Laboratory.

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*Article #14-04546: “Human (Clovis)–gomphothere (Cuvieronius sp.) association ~13,390 calibrated yBP in Sonora, Mexico,” by Vance T. Holliday et al.

In addition to Holliday, authors of the PNAS paper include: lead author Guadalupe Sanchez, who has a doctorate in anthropology from the UA; UA alumni Edmund P. Gaines and Susan M. Mentzer; UA doctoral candidates Natalia Martínez-Tagüeña and Andrew Kowler; UA master’s student Ismael Sanchez-Morales; UA scientists Todd Lange and Gregory Hodgins; and Joaquin Arroyo-Cabrales at the Instituto Nacional de Antropología e Historia.

The dig at El Fin del Mundo, a joint effort between the U.S. and Mexico, was funded by the UA School of Anthropology’s Argonaut Archaeological Research Fund, the National Geographic Society, the Instituto Nacional de Antropología e Historia and The Center for Desert Archaeology in Tucson.

Source: Some material for this article was adapted and edited from a University of Arizona press release, Meet the gomphothere: UA archaeologist involved in discovery of bones of elephant ancestor

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Travel and learn with Far Horizons.

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This richly illustrated issue includes the following stories: Recent findings shedding new light on the whereabouts of the remains of Philip of Macedon, father of Alexander the Great; how an archaeologist-sculptor is bringing bones of the dead back to life; archaeologists uncovering town life at the dawn of civilization; an exclusive interview with internationally acclaimed archaeologist James M. Adovasio about what makes the Meadowcroft Rockshelter prominent in the ongoing search for the first Americans; what archaeologists are finding at the site of the ancient city of Gath, the home town of the biblical Philistine giant, Goliath; and how scientists are redrawing the picture of human evolution in Europe.  Find it on Amazon.com.

It is a cave within a cave, dark and silent.

But when the scientists are there, it is artificially lit, revealing a chamber just four meters wide, created by nature at the terminus of a natural vertical 13-meter subterranean karstic shaft. Spelunkers and scientists call it the “chimney”, and for good reason—it’s a bit like negotiating down the interior of a house chimney. Entering the shaft can only be done by traversing through yet another, much larger cave—Cueva Mayor—part of a larger karstic mountain complex in northern Spain called Atapuerca. Atapuerca is important because here in the caves of these mountains scientists have found fossils and stone tools of some of the earliest known humans in Western Europe, and this small cave at the end of the vertical shaft is arguably the star of the show. Here, over a period of 30 years, teams of archaeologists and other specialists have excavated nearly 7,000 human fossils of at least 28 individuals, individuals who lived more than 400,000 years ago. Aptly named Sima de los Huesos (“Sima”), or “Pit of Bones”, it is to date the largest single assembly of early human fossil remains ever found at any single location………

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A team at work inside the cave: The Sima de los Huesos site. Image courtesy Javier Trueba / Madrid Scientific Films

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Game of Thrones

For Juan Luis Arsuaga, it has been perhaps the quintessential journey of his career. Currently a professor in the Paleontology Department at the Universidad Complutense de Madrid, since 1982 he has been a member of a research team investigating Pleistocene deposits in the Atapuerca Mountains. Most recently, the revelation precipitated by his most recent findings at the Sima de los Huesos (‘Sima’) site has stirred the scholarly world of human evolution studies. It centers on 17 skulls.

“An astonishing collection of skulls (up to 17) belonging to a single population, or paleo-deme, of a fossil hominin species has been recovered,” says Arsuaga. “This incredible discovery of skulls is unprecedented.” Dating to 430,000 years ago and among the most complete fossil skulls recovered from any site, “this unique collection has to be compared with the much more fragmentary and generally isolated remains yielded by other Middle Pleistocene sites in Europe and out of Europe.” 

It offered Arsuaga and his team a unique opportunity to study a scene on the stage of the evolution of Middle Pleistocene humans. These 17 skulls, according to Arsuaga and colleagues, represented a single population of a hominin (early human) species. Although some of them had been studied before, seven have been presented anew, and six are now more complete than ever before, after many hours of painstaking assemblage in the lab. With these mostly intact samples for study, the researchers have finally been able to more clearly define the common features of this population.

Most striking, the examiners found that the fossils exhibited a mosaic of physical characteristics that could not be wholly attributed to any single, recognized human species to date. The skull samples showed clear Neanderthal features in the face and teeth. The researchers suggested the ‘Neanderthal-derived’ features were functionally related to mastication, or chewing. “It seems these modifications had to do with an intensive use of the frontal teeth,” Arsuaga said. “The incisors show a great wear as if they had been used as a ‘third hand,” typical of Neanderthals.” But elsewhere, the skulls showed characteristics that diverged from the Neanderthal model. The braincase itself, for example, still showed features associated with more primitive hominins.

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Professor Juan Luis Arsuaga holds newly excavated early human fossil inside Sima de los Huesos cave. Image courtesy Javier Trueba / Madrid Scientific Films

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Location of skull 17 at the Sima de los Huesos site. Image courtesy Javier Trueba / Madrid Scientific Films

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So what did these findings mean?

“Morphologically, the Sima de los Huesos skulls exhibit some (generally incipient) Neanderthal traits,” said Arsuaga. “These traits have been scrutinized to understand how the Neanderthal specializations developed……..and it is now clear that the full suite of Neanderthal characteristics did not evolve at the same pace.”

However, according to Arsuaga,”we think based on the morphology that the Sima people were part of the Neanderthal clade, although not necessarily direct ancestors to the classic Neanderthals.”

“One thing that surprised me about the skulls we analyzed,” Arsuaga continued, “is how similar the different individuals were. The other fossils of the same geological period are different and don’t fit in the Sima pattern. This means that there was a lot of diversity among different populations in the Middle Pleistocene.”

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Reconstruction of skull 15. Image courtesy Javier Trueba / Madrid Scientific Films

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Reconstruction of skull 17. Image courtesy Javier Trueba / Madrid Scientific Films

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Reconstructed skull 9 of the 17 skulls found at the site. Image courtesy Javier Trueba / Madrid Scientific Films

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Skull 15. Image courtesy Javier Trueba / Madrid Scientific Films

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Skull 17. Image courtesy Javier Trueba / Madrid Scientific Films

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In other words, because other European Middle Pleistocene fossil specimens found in Europe have not exhibited the combination of features seen in this fossil group, the researchers have concluded that more than one evolutionary lineage appears to have coexisted during the European Middle Pleistocene, with that represented by the Sima sample being closer to the Neanderthals. 

“We propose what we call the “Game of Thrones theory” to explain hominin evolution in Europe and a part of Asia in the period known as the Middle Pleistocene (780,000 years ago to 130,000 years ago),” said Arsuaga. “According to this scenario, hominin evolution was not a peaceful and “boring” process of very slow change throughout time and across an immense land: the European continent and Western and Central Asia. There was not simply one kind of hominin living in that big territory and quietly evolving everywhere towards the “classic” (Late Pleistocene) Neanderthals.” This early human landscape, at least according to this team of scientists, featured something of a potpouri of morphologically and perhaps culturally distinct human groups or populations. To complicate the picture yet more, the team has found that a DNA study of a bone sample from Sima indicates that these humans shared a common ancestor with the Denisovans (another extinct archaic human species that likely lived in Asia) about 700,000 years ago. “The fact that the mtDNA (mitochondrial DNA) of the Sima de los Huesos hominin shares a common ancestor with Denisovan rather than Neanderthal mtDNAs is unexpected since its skeletal remains carry Neanderthal-derived features”, says Matthias Meyer, a key scientist with the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who led the genetic study. Another possibility is that the Denisova genes were introduced from another group of hominins into the Sima hominin population, or their ancestors.

In any case, Arsuaga’s “Game of Thrones” scenario and the genetic study results didn’t fit neatly into the traditional picture of Neanderthal evolution. It shook the applecart—a model that long suggested that Neanderthal traits all evolved in somewhat linear fashion, in unison from an earlier single species, theorized to be Homo heidelbergensis.

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The Sima de los Huesos team. Image courtesy Javier Trueba / Madrid Scientific Films

Changing Paradigms

For those who follow the paths of thinking about how humans became what they are today, the discoveries at Sima are among a number of recent developments in the field and in the labs that are effectuating a new pattern of making and remaking evolutionary paradigms and calling into question some long-standing constructs.

One case in point: Researching a Late Pleistocene site in China, researchers Xiu-Jie Wu, Wu Liu and Song Xing of the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, Isabelle Crevecoeur of PACEA, Université de Bordeaux, France, and Erik Trinkaus of Washington University in St. Louis, re-examined a circa 100,000-year-old archaic human skull originally found during excavations 35 years ago at the Xujiayao site in China’s Nihewan Basin. The skull belonged to a human species not classified as Neanderthal, but based on their micro-CT scans of the interior configuration of the temporal bone, they found that the inner-ear formation closely resembled the formation long identified exclusively with Neanderthals.

On the surface, this appears to be a small thing. But in the worlds of paleoanthropology and paleontology, especially as they relate to human evolution, the implications are significant, because the temporal labyrinth has been broadly considered a key characteristic that has set Neanderthals apart from earlier and modern humans.

“We were completely surprised,” Trinkaus said. “We fully expected the scan to reveal a temporal labyrinth that looked much like a modern human one, but what we saw was clearly typical of a Neandertal. This discovery places into question whether this arrangement of the semicircular canals is truly unique to the Neandertals.” Moreover, he said, “the discovery places into question a whole suite of scenarios of later Pleistocene human population dispersals and interconnections based on tracing isolated anatomical or genetic features in fragmentary fossils. It suggests, instead, that the later phases of human evolution were more of a labyrinth of biology and peoples than simple lines on maps would suggest……It shows that human populations in the real world don’t act in nice simple patterns.”

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The Xujiayao 15 late archaic human temporal bone from northern China, with the extracted temporal labyrinth, superimposed on a view of the Xujiayao site. Courtesy Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Science

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The Dmanisi Illumination

David Lordkipanidze has been excavating at the site near the town of Dmanisi in the Republic of Georgia for decades. He is a  paleoanthropologist and Director of the Georgian National Museum. He is best known for the news-making discoveries he has been making at the site, known now in the popular literature and press after its namesake town. It was here, in 1991, when he and his team of researchers first uncovered a remarkable fossilized mandible (the lower jaw). It appeared to be human—but quite different from a modern human mandible. It was more like what scientists had been finding for decades only in East and South Africa—fossils of what paleoanthropologists know to be some of the earliest members of our kind—genus Homo—the genus that includes modern humans and species closely related to them—human ancestors that lived between 1 million and 2 million years ago. Scientists began scratching their heads. Geographically, this fossil appeared to be several thousands of miles off course. And the associated stone tools resembled the earliest known stone tool technology, the Oldowan, found, until now, only in Africa.  It challenged the long-accepted models of a significantly later first exodus of early Homo out of Africa, more advanced Homo erectus groups who carried with them a more sophisticated technology, the Acheulean, a 1.5+ million-year-old tradition forged in Africa and best known for its hallmark, distinctive oval and pear-shaped handaxes.  

What followed was a series of incredible discoveries that, together, proved to shake up the world of human prehistory almost without parallel. Three other remarkably complete and well-preserved fossil skulls, including an assortment of other fossils, were unearthed and pieced together during the ensuing years.

But the latest find brought yet new surprises.

In 2005, Lordkipanidze and his team uncovered yet another well-preserved early Homo fossil. Designated as specimen ‘D4500′, it turned out to be the matching cranium to another fossil find uncovered 5 years earlier—a complete mandible they designated ‘D2600’. These two fossils were discovered alongside the remains of the four other early Homo fossil skulls, animal fossils, and the simple Oldowan-like stone tools. Designated Skull 5, the new cranium together with its mandible displayed a relatively small braincase with a long face and large teeth. Other similar early Homo fossils, all found in African contexts, were either incomplete, or adolescent or younger individuals. According to Lordkipanidze and his associates, this new find constituted “the most complete adult skull known from Early Pleistocene Homo.“* Moreover, like some of the other finds, it was associated with other parts of the body that exhibited characteristics akin to Homo erectus body engineering, which closely approximates that of modern human morphology. In other words, this species had a very human-like body.

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The Dmanisi early Homo cranium (D4500) in situ. Courtesy Georgian National Museum

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The Dmanisi D4500 early Homo cranium and a large rodent tooth in situ. Associated fauna, such as the tooth from a rodent species that lived 1.8 million years ago, helped to date the find. Courtesy Georgian National Museum

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The Dmanisi D4500 early Homo cranium and ancient herbivore fossil remains in situ. Courtesy Georgian National Museum

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The complete Skull 5. Courtesy Guram Bumbiashvili, Georgian National Museum

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Skull 5 was another first. But the biggest revelation came not with the latest fossil, but with what the fossils collectively had to say about human evolution. Now, with fossil finds that represented five distinct individuals, it was the first time that such an assembly of Early Pleistocene Homo fossils were found together within the same time and space context. No other site, in Africa or elsewhere, could boast of such a collection. The Dmanisi discoveries meant that scientists could study a range of variation in human species within the context of one place and time range, a relative mother lode of information that could potentially clarify, and perhaps even revolutionize, how we see these earliest of Homo human ancestors. 

The researchers set to work. And the result of their study was startling. Lordkipanidze and his colleagues summarize it well in the following words in a recent report published in Science: 

Geometric morphometric analysis and re-sampling statistics show that craniomandibular shape variation among the Dmanisi hominids is congruent with patterns and ranges of variation in chimpanzee and bonobo demes [a population of one species](Pan troglodytes troglodytes, P. t. verus, P. t. schweinfurthii, and P. paniscus) and in a global sample of H. sapiens. Within all groups, variation in cranial shape is mainly due to interindividual differences in size and orientation of the face relative to the braincase. The Dmanisi sample, including skull 5, thus represents normal within-deme variation, ranging from small-faced relatively orthognathic (typically female and/or subadult) individuals to large-faced relatively prognathic (typically male) individuals.*

In other words, after examining the remains, the research team concluded that the differences among these fossils vary no more than the differences between five modern humans or five chimpanzees. 

“Thanks to the relatively large Dmanisi sample, we see a lot of variation,” said Christoph Zollikofer from the Anthropological Institute and Museum in Zurich, Switzerland—a co-author of the Science report. “Had the braincase and the face of Skull 5 been found as separate fossils at different sites in Africa, they might have been attributed to different species.”

Historically, variations among Homo fossil finds in Africa and Asia have also been found, but these differences have normally not been found within the same spatial and time period context, and thus scientists have classified the various finds as belonging to separate species. But now, according to Lordkipanidze and his colleagues, what has previously been thought to be separate ancient human species — Homo erectus, Homo habilis, Homo rudolfensis, and Homo ergaster, for example — may actually be variations or sub-species of one and the same species.

The upshot: Researchers now need to re-adjust their thinking and proceed more cautiously when determining how early Homo fossils are classified. 

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Dmanisi Skulls 1-5 (left to right), showing the individual variations, and a Dmanisi landscape. Courtesy M. Ponce de León and Ch. Zollikofer, University of Zurich, Switzerland

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Redefining the Ape/Human Transition

It was Raymond Dart who started it all. Back in 1925, publication of his findings in the journal Nature of the skull of a 3-year-old, now famously known as the Taung Child (named after its initial recovery from the Taung lime quarry in South Africa), led to a human fossil-hunting regime that has continued and expanded to this day. His discovery launched the worldwide effort to uncover the first ‘proto-human’ species, a genus that, for the evolutionists, stood somewhere between the apes and humans on the traditional time-line of evolutionary progression within the Primate Order. Dart assigned his find to a new genus, Australopithecus (“Southern Ape”).  Discoveries by other scientists at various sites across Africa followed, leading to new species classifications—Australopithecus (Au.) afarensis, Au. anamensis, Au. bahrelghazali, Au. garhi. Scientists on the Australopithecus trail were beginning to come to some arguable consensus about what the direct ancestor to the Homo line would look like—until Malapa.

It dominated science headlines when the news was first released. The discovery of the remains of a new species of ancient hominin (human ancestor) revealed a candidate that sported a mosaic of features both ape-like and human — an unprecedented 2-million-year-old hybrid called Australopithecus sediba (Au. sediba). First stumbled upon in 2008 by Professor Lee Berger of the University of the Witwatersrand and his then 9-year-old son Matthew at the fossil bearing site of Malapa in South Africa, the finds, consisting of remarkably complete skeletal remains as well as other well-preserved fauna and flora, instantly became the subject of perhaps the most intense and thoroughly studied hominin fossils ever documented.

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Above, the Malapa site, September 4th, 2008, at the moment of the discovery of the fossilized female skeleton MH2. Photo courtesy of Lee Berger and the University of the Witwatersrand, under Creative Commons attribution license.

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The U.W. 88-50 (MH 1) cranium. The cranium forms part of the holotype skeleton of Australopithecus sediba from the Malapa site, South Africa. This image relates to an article that appeared in the April 9, 2010, issue of Science, published by AAAS. The study, by Dr. L.R. Berger of University of the Witwatersrand in Johannesburg, South Africa, was titled, “Australopithecus sediba: A New Species of Homo-like Australopith from South Africa.” Photo by Brett Eloff courtesy of Lee Berger and the University of the Witwatersrand.

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 SKELETAL LAYOUT OF MH1 AND MH2, the first two skeletal specimens unearthed at Malapa by Lee Berger and his team.; Image produced by Peter Schmid taken from Berger et al 2010. Australopithecus sediba: A new species of Homo-like australopith from South Africa, Science 320:195-204. 

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The team, led by Berger and composed of South African and international scientists from the Evolutionary Studies Institute (ESI) at the University of the Witwatersrand (Wits) and 16 other global institutions (totalling more than 100 researchers from around the world), examined the anatomy of Au. sediba based on its skeletons catalogued as “MH1” (a juvenile skeleton) and “MH2” (an adult female skeleton), as well as an adult isolated tibia catalogued as “MH4”. Dispersed among separate studies, the examinations determined, more than any other Australopithecus findings to date, the essence of how this hominin looked, walked, chewed and moved, complicating and changing perspectives on the evolution of the australopith forerunner genus in human ancestry:

Au. sediba’s teeth

The first study, led by Professor Joel Irish from the Research Centre for Evolutionary Anthropology and Palaeoecology at Liverpool John Moores University in the United Kingdom, examined dental traits in the fossils.

In this study, Irish, Debbie Guatelli-Steinberg of the Ohio State University and their colleagues examined the teeth from sediba and compared them to eight other African hominin species, which include modern humans from Africa, and extinct species of Homo, Australopithecus, and Paranthropus. In all, the researchers examined more than 340 fossils and 4,571 recent specimens. They also examined teeth from 44 gorillas for comparison.

Based on the examination, Irish and his colleagues suggested that the species is distinct from east African australopiths, but is close to Au. africanus, thus forming a southern African australopith “clade” (a group consisting of an ancestor and all its descendants).

The latter, in turn, shares a number of derived states or physical characteristics with a clade comprising four fossil samples of the genus Homo. This surprising result has significant implications for our present understanding of hominin phylogeny (the evolution of the species), and alludes to the possibility that Au. sediba, and perhaps Au. africanus are not descendant from the Au. afarensis lineage, represented most prominently by the famous “Lucy” skeleton discovered by Donald Johanson in Ethiopia in 1974, as had been widely hypothesized.

“Our research on teeth can’t definitively settle if either sediba or africanus is more closely related to humans than the other species,” Guatelli-Steinberg said. “But our findings do suggest that both are closely related to each other and are more closely related to humans than afarensis.”

Irish noted that even though the results of this study were surprising and were bound to be viewed as controversial given the long held hypotheses relating to the origins of the genus Homo, the genus more directly ancestral to humans, he would have come to the same conclusion. “The extreme age and rarity of these fossils naturally draws enhanced interest in and scrutiny of any new findings”, he said. “Based on the evidence, I would have come up with the same conclusions whether the samples were three million or 30 years old.”

But the surprises were not over regarding the teeth. A later laser scanning study on the teeth by researchers at the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany and the University of *Colorado Boulder showed that sediba had a diet unlike other known early human ancestors. That study indicated that sediba ate harder foods than other early hominids, focusing on trees, bushes and fruits, in contrast to most  other ancient human ancestors fossils examined from Africa, including Paranthropus boisei, also known as “Nutcracker Man” because of its massive jaws and teeth, which targeted grasses and sedges.

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Au. sediba skull showing teeth. Courtesy Paul Sandberg, University of Colorado

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Sediba’s jaw 

Professor Darryl de Ruiter of Texas A&M and the Evolutionary Studies Institute at the University of the Witwatersrand, and his colleagues, examined the mandibular (lower jaw) material from the Sediba MH2 individual.

The study concluded that the mandibular remains share similarities with other australopiths, but differ from Au. africanus in both size and shape, as well as in their growth trajectory.

“These results add further support to the claim that Au. sediba is taxonomically distinct from the temporally – and geographically – close species Au. africanus. Where the Au. sediba mandibles differ from those of Au. africanus, they appear most similar to representatives of early Homo,” says De Ruiter.

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Reconstruction of the MH1 skull, highlighting lower jaw and teeth. Reconstruction by Kristian Carlson, courtsey of Lee R. Berger and the University of the Witwatersrand.

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Sediba’s upper limbs 

Professor Steven Churchill of Duke University and the Evolutionary Studies Institute at University of the Witwatersrand examined the upper limb elements of Au. sediba. They noted that with the exception of the hand skeleton (which exhibited a suite of features that may signal enhanced manipulative capabilities compared to earlier australopiths), the upper limbs of Sediba were largely primitive in their morphology (physical characteristics). Au. sediba thus shared with other australopiths an upper limb that was well-suited for arboreal or other forms of climbing and possibly suspension, and perhaps even more than had been previously suggested for any other member of the australopith genus.

Churchill adds that “it is possible that the climbing features in the skeleton of Australopithecus sediba and other australopiths are functionally unimportant primitive traits retained from a more arboreal ancestor. Even so, it is curious that these features persist unchanged for several million years, only to abruptly disappear with the emergence of the genus Homo.”

That question perhaps awaits answers through future finds and research.

Sediba’s rib cage 

Dr. Peter Schmid and colleagues at the Evolutionary Studies Institute at the University of the Witwatersrand and the University of Zurich studied the remains of Sediba’s thorax, or rib cage. Their findings revealed a narrow upper thorax, much like that of the large-bodied apes, and unlike the broad, cylindrical chest characteristic of humans. In conjunction with the largely complete remains of the shoulder girdle, Schmid notes that “the morphological picture that emerges is one of a conical thorax with a high shoulder joint that produces in Au. sediba an ape-like “shrugged” shoulder appearance, and thus a configuration that is perhaps uniquely australopith, and that would not have been conducive to human-like swinging of the arms during bipedal striding and running”.

Their research, however, showed that the less well-preserved elements of the lower rib cage suggested some degree of human-like narrowing to the lower thorax, a surprising feature that is not like that of Homo erectus or modern humans. 

The vertebral column 

Dr. Scott Williams of the Center for the Study of Human Origins at New York University and colleagues examined the vertebral column of Au. sediba. They described a remarkably articulated lumbar vertebral region that showed a human-like curvature of the lower back. Williams noted that “the adult female is the first early hominin skeleton that preserves an intact terminal thoracic region and this provides critical information on the transition in inter-vertebral joints, and, by inference, mobility of the lower back”.

The study also demonstrated that Au. sediba had the same number of lumbar vertebrae as modern humans, but possessed a functionally longer and more flexible lower back. In addition, the morphology of Sediba’s lumbar curvature suggested that Au. sediba was more similar to the Nariokotome Homo erectus (the famous “Turkana Boy“ discovered by Kamoya Kimeu and Richard Leakey) skeleton than to the australopiths.

Sediba’s gait 

Dr. Jeremy DeSilva and colleagues at Boston University and the Evolutionary Studies Institute at the University of the Witwatersrand examined the lower limb anatomy of Au. sediba. 

“The female Australopithecus sediba preserves a heel, ankle, knee, hip and lower back—all of the ingredients necessary to reconstruct how she walked with remarkable precision. Even the famous Lucy skeleton only preserves two of these five (ankle and hip)”, said DeSilva.

The combination of, and the individual anatomies of, the heel, mid-foot, knee, hip, and back are unique and curious, said the team. This defined an australopith that walked with a ‘hyper-pronating gait’ (compared to the modern human gait, feet that pronated or rotated differently during the gait cycle).

“The implications of this study are that multiple forms of bi-pedalism were once practiced by our early hominin ancestors,” added Berger about the DeSilva conclusions.

Sediba’s brain

The exceptionally well-preserved cranium of the MH-1 juvenile was scanned at the European Synchrotron Radiation Facility, the most powerful facility in the world for scanning fossils, under the direction of Dr. Kristian Carlson from the University of the Witwatersrand.  By doing this, he and his colleagues were able to develop a precise map or image of the impressions on the interior surface of the cranium, or brain case, producing an endocast (or 3-dimensional image) of the area where the brain, long decayed into nothing, would have been.

Said Carlson, “the actual brain residing within a cranium does not fossilize. Rather, by studying the impressions on the inside of a cranium, palaeontologists have an opportunity to estimate what the surface of a brain may have looked like. By quantifying how much volume is contained within a cranium, palaeontologists can estimate the size of a brain.”

The results revealed that the brain was human-like in shape, yet much smaller than brain volumes recorded in Homo species. In fact, the size was not significantly more than that of a modern chimpanzee. However, the orbito-frontal region of the brain, which is behind the eyes, showed characteristics of neural (nerve cell structure) reorganization. The researchers suggested that this indicated a “re-wiring” of the frontal lobe’s neural constitution to a pattern more human-like. This questioned the generally-accepted theory of brain enlargement during the transition from Australopithecus to Homo as the most essential consideration and supported the alternate theory that a neural reorganization in the orbitofrontal region made it possible for A. sediba to be more “human-like” with the smaller cranium. Thus, size may not matter more than brain tissue structure and organization.

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Image shows a reconstruction of the skull of MH1 (partially transparent) with the brain endocast depicted in green. Dentition also visible and the specimen is viewed from slightly above and anterolateral. Courtesy University of Witwatersrand and Kristian Carlson

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Sediba’s hands for toolmaking  

Tracy Kivell of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a team of colleagues studied the hand fossils of MH-2, the adult female Au. sediba. They determined that the hand exhibited a strong flexor capability, good for tree-climbing. However, the hand also featured a long thumb and short fingers, a clear requirement for precision gripping, or gripping that involves the more refined use of the thumb and fingers and not the palm. This suggested that Au. sediba had the mechanical capability to make tools.

According to Kivell:

“The hand is one of the very special features of the human lineage, as it’s very different from the hand of the apes. Apes have long fingers for grasping branches or for use in locomotion, and thus relatively short thumbs that make it very difficult for them to grasp like a human.

Au. sediba has, in contrast, a more human-like hand that has shortened fingers and a very long thumb. Although at the same time, it appears to have possessed very powerful muscles for grasping. Our team interpreted this as a hand capable of tool manufacture and use, but still in use for climbing and certainly capable of human-like precision grip.”

These findings pair Au. sediba with Homo habilis, the famous “handy man” Australopithecine discovered by Louis and Mary Leakey in the 1960’s, as a hominin or early human candidate capable of making tools. It also suggests that there were more than one ancient hominin species that may have produced tools, either during different time periods, overlapping time periods, or concurrently in time.   

To date, no stone tools have been found in association with the Malapa cave fossils. But excavations will continue, and it is anticipated that much more data will be forthcoming.

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All of the right hand bones of Au. sediba rearticulated. Courtesy Peter Schmid

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Sediba’s pelvis

Dr. Job Kibii of the University of the Witwatersrand and associates examined the partial pelvis of MH-2 and found that it also exhibited features that combined primitive elements more akin to that of earlier hominins and apes along with elements more characteristic of humans. They observed that the size of the joint that connects the sacrum with the vertebral column and the length of the front portion of the pelvis is like that of earlier hominins and apes, but the overall shape of the pelvis is short and broad, creating a bowl shape like that of humans, with an s-shape along the top of the blades, another human characteristic. Indeed, simply placing the reconstructed pelvis next to that of an ape and an earlier hominin is very telling. The pelvis clearly appears more human-like than ape-like. 

Said Kibii, “It is surprising to discover such an advanced pelvis in such a small-brained creature because of previous ideas as to the origin of the shape of the human pelvis.”

The generally-accepted theory is that broader pelvises evolved, at least in part, in response to the enlarging brains of hominins, on the assumption that the more human-like pelvis shape more easily accommodated the larger-brained hominin infants in childbirth. The new findings turn this on its head, suggesting that there was another, perhaps more important evolutionary reason why the pelvis changed:

The change in the pelvic morphology accommodates a more bipedal, or erect, gait – a salient hallmark of being human. Says Steven Churchill of Duke University, one of the co-authors of the paper detailing the pelvis study, “What’s cool about sediba is their pelvises are already different from other australopiths, and yet they’re still small-brained… It’s hard to imagine that there’s no change in locomotion behind all this.”

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The reconstructed pelvis of MH2. Left side is a mirror image. Image created by Peter Schmid, courtesy Lee R. Berger and the University of the Witwatersrand

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Sediba’s feet and ankles

Dr. Bernhard Zipfel of the University of the Witwatersrand and colleagues examined the feet and ankle fossils of MH-1 and MH-2, finding them to consist of a mix of both primitive and modern characteristics unique to Au. sediba as a species. As the ankle fossils represented a very rare and opportune find in that they constituted one of the most complete hominin ankles ever found, and in an articulated position or association, it was feasible to perform a study and reach reasonably sound conclusions about the characteristics of the fossils and their implications. The ankle joint and foot bones were constructed much like a human’s, with some evidence for a human-like arch and a well-defined Achilles tendon. It was also clear that the distal tibia or leg bone had to contact the anklebone perpendicular to the vertical shaft of the leg bone. These are all requirements for habitual bipedal locomotion, or upright walking.

However, the heel and shin bone exhibited more ape-like qualities, suggesting that Au. sediba was also a tree-climber, much like its ape cousins. 

Their overall analysis suggested that Au. sediba likely practiced a unique form of upright walking, not quite like that of humans, along with some degree of tree climbing.

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Ankle of Australopithecus sediba. Red is adhering matrix. Image created by Kristian Carlson, courtesy of Lee R. Berger and the University of the Witwatersrand.

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The investigations at the Malapa site have seen the discovery of more than 300 early human ancestor remains, including parts of skeletons still encased in rock. Though there may be much more to come out of Malapa, the finds and their interpretation will likely have a salient impact on how human evolution, particularly at the transition from “ape-man” to Homo, or early human, actually unfolded.

“What these papers suggest is that sediba probably doesn’t come from the east African species that Lucy comes from, Australopithecus afarensis, and it may be considered the best candidate as an ancestor for the genus Homo“, said Malapa project leader Berger……”Such clear insight into the anatomy of an early hominin species will clearly have implications for interpreting the evolutionary processes that affected the mode and tempo of hominin evolution and the interpretation of the anatomy of less well preserved species.”

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Composite reconstruction of Au. sediba based on recovered material from MH1, MH2 and MH4 and based upon the research presented above. As all individuals recovered to date are approximately the same size, size correction was not necessary. For comparison, small-bodied female modern H. sapiens on left, Male Pan troglodytes on right. Photo by Lee Berger, courtesy of the University of the Witwatersrand

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The New Synthesis

Educated at Harvard University, paleoanthropologist Rick Potts has spent much of his career as an academic and administrator, having taught at Yale University and curated archaeological/anthropological materials at Yale’s Peabody Museum. He now serves as the director of the Smithsonian’s Human Origins Program and curator of anthropology at the National Museum of Natural History.

But this is only part of his world. When he’s not attending to his program and curatorial duties, one would find him in the field, along with teams of other scientists, at locations in the East African Rift Valley or in southern and northern China. These are places where new evidence is now transforming an old paradigm of human evolution in a big way. 

In a recent study published in the journal Science, he and co-author colleagues Susan Antón, professor of anthropology at New York University, and Leslie Aiello, president of the Wenner-Gren Foundation for Anthropological Research, report results from comprehensive research on shifting paleoclimates, ancient stone tools, isotopes found in teeth, and cut marks found on animal bones in East Africa. The findings support an emerging new consensus that suggests a rethinking of long-held assumptions about human origins and evolution. 

It begins with questions surrounding early hominin diversity.

“Soon after our genus (Homo) first evolved by about 2.4-2.3 million years ago there were at least three different species of Homo walking the earth: Homo habilis, Homo rudolfensis, and slightly later Homo erectus,” explains Briana Pobiner in an article published by the National Museum of Natural History. Pobiner is a Smithsonian paleoanthropologist and researcher who works closely with Potts. “In fact, these species overlapped in time with each other and even with Australopithecus and Paranthropus, two other genera of early humans. Some of their fossils have even been found at the same prehistoric sites. Anatomical traits and behaviors were mixed and matched, with some more human-like features found in some species, and others in different species, which according to the authors indicates that each species utilized a different strategy to survive.”**

In other words, the traits that made us human evolved at different times and in different places. From the beginning, if a clear beginning could be simply defined at all, there was no single evolving package and line of evolution. 

And climate variability was a key driver, maintains Potts. “Unstable climate conditions favored the evolution of the roots of human flexibility in our ancestors. The narrative of human evolution that arises from our analyses stresses the importance of adaptability to changing environments, rather than adaptation to any one environment, in the early success of the genus Homo.”***

It was therefore adaptability to change, not the long-held notion of specialization, that was the ultimate key to human evolution, say the researchers. Potts has called this the variability selection hypothesis, an idea he has actually been advancing for years. It challenges the long-held “savanna hypothesis”, which has suggested that our genus, Homo, emerged and evolved at least in part due to adaptations (such as walking upright, dietary change, a larger brain and body, and making tools) as a result of a major, gradual climate change from a warmer, wetter forest environment on the African continent to a cooler, drier one that resulted in the spread of a savanna grassland.

It has helped to define a new synthesis, at least according to these scientists, that explains a major engine of human evolution, one that ultimately revolves around the ability of our species to adapt, innovate, and even modify the world around us. And this, most scientists of human evolution can agree, is what Homo sapiens have done best, and why they are the lone survivor among an originally diverse line of hominins.

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Between 2.1 and 1.8 million years ago, Homo, exhibited diverse traits. These genus included the Kenyan fossils KNM-ER 1470 (left) and KNM-ER 1813 (second from left). By 1.8 to 1.9 million years ago, Homo erectus evolved in Africa and spread to Eurasia. Early populations of this species are represented by the Kenyan fossil KNM- ER 3733 (right) and the Georgian fossil Dmanisi Skull 5 (second from right). The three lineages — the 1470 group, the 1813 group, and Homo erectus — overlapped in time for several hundred thousand years. From press release of the Smithsonian Institution, Smithsonian scientist and collaborators revise timeline of human origins, 3 July 2014 Credits: Kenyan fossil casts – Chip Clark, Smithsonian Human Origins Program; Dmanisi Skull 5 – Guram Bumbiashvili, Georgian National Museum

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Hominin evolution from 3.0 to 1.5 Ma. Green: Australopithecus, Yellow: Paranthropus, Red: Homo. The icons indicate from the bottom the first appearance of stone tools at ~2.6 Ma, the dispersal of Homo to Eurasia at ~1.85 Ma, and the appearance of the Acheulean technology at ~1.76 Ma. The number of contemporaneous hominin taxa during this period reflects different strategies of adaptation to habitat variability. The cultural milestones do not correlate with the known first appearances of any of the currently recognized Homo taxa. Text and Image courtesy of Antón et al., Science/AAAS 2014 and related press release.

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Evolutionary timeline of important anatomical, behavioral and life history characteristics that
were once thought to be associated with the origin of the genus Homo or earliest H. erectus. Image courtesy of Antón et al., Science/AAAS 2014

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Going forward, Potts and his colleagues know that nothing stays the same when it comes to building theories and models—with each new discovery, a new twist can be introduced, or an entire theory can be called into question. But in science, everything is open to question. It is the “nature of the beast”, so to speak. 

In any case, what they are finding may have implications for the present and the future of human survival. Global warming comes to mind, here. Potts and his colleagues at the Smithsonian’s Human Origins Program have eloquently summarized the challenge well:  

The question ahead is how well our sources of resilience as a species will succeed as our alterations of the landscape, atmosphere, and water interact with the tendency of Earth’s environment to shift all on its own. This is an ‘experiment’ just now unfolding, one that has never occurred before. The intensity of environmental change seems likely to create entirely new survival challenges for the lone hominin species on the planet, and many other organisms as well.****

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* David Lordkipanidze, et al., A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo, Science 342:326-331, 2013.

** Pobiner, Briana, The traits that make us human evolved at different times, Unearthed, National Museum of Natural History, Smithsonian Institution (website page)

*** From a press release of the Smithsonian Institution, Smithsonian scientist and collaborators revise timeline of human origins, 3 July 2014

****From Climate Effects on Human Evolution, the Smithsonian Human Origins Program website: What does it mean to be human?