June 2011, Daily News

Chimps, Humans Share a Key Evolutionary Development, Study Says

By Dan McLerran   Thu, Aug 11, 2011

There is something a little more human-like about the appearance of an infant chimpanzee as compared to its fully grown counterpart. A new study shows that there is even more to that beneath the surface. It confirms that infant chimpanzees, much like infant humans, are born with immature forebrains, otherwise scientifically known as the prefrontal cortex, allowing for the time and plasticity needed to develop the more complex social functions, knowledge and skills that distinguish humans and chimpanzees from other primates and life-forms.  It also suggests that the condition existed in a common ancestor of both humans and chimpanzees before they theoretically split apart some 6 - 7 million years ago in the evolutionary bush.  

The study, conducted by a Japanese team from the Kyoto, Toyama, and Chubu Gakuin universities in Japan, involved an analysis of a progressive series of magnetic resonance imaging (MRI) scans of the brains of three developing chimpanzees from six months old to six years, the point at which chimps generally reach pre-puberty. It was the first study to actually track the development of the chimpanzee brain over a time horizon and then compare the same to that of humans.

"One of the most marked evolutionary changes underlying human-specific cognitive traits is a greatly enlarged prefrontal cortex," said team member Tetsuro Matsuzawa. "It is also one of the latest-developing brain regions of the cerebrum (the largest, most superior area of the brain)." Just as in humans, the longitudinal study showed that chimpanzees experience a delayed development of the white matter (WM) content of the prefrontal cortex during infancy. White matter is critical to nerve cell connectivity and function in the brain and throughout the nervous system. It is key to the development and functioning of the prefrontal cortex, responsible for the "executive" functions of the brain, such as planning, decision-making, problem-solving, creativity, and social control and interaction. The delayed development, according to many scientists, allows for an extended period of plasticity, permitting life experiences and exposure to catalyze and shape the development of complex cognitive processes and behaviors that are trademarks of humans, and, to a lesser but equally distinguishing extent, chimpanzees. Similar studies that have been done on other primates, such as the rhesus macaques, have shown much less similarity.

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Human/Chimpanzee (pan troglodytes) brain comparison. The cerebral cortex constitutes the front third portion of the brain. Courtesy Todd Preuss, Yerkes Primate Research Center. Wikimedia Commons.

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Primate skull series showing the comparative brain volumes among key primates.  Notice differences between human, chimpanzee, and macaque, the subjects of the study. Courtesy Christopher Walsh, Harvard Medical School; a derivative work: Gary2863, Wikimedia Commons. 

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"Although the developmental pattern of prefrontal WM in macaques differs markedly from that in humans", writes Sakai, et. al., in the study report, "this has not been explored [until now] in our closest evolutionary relative, the chimpanzee. The present longitudinal study of magnetic resonance imaging scans demonstrated that the prefrontal WM volume in chimpanzees was immature and had not reached the adult value during prepuberty, as observed in humans but not in macaques". [1] 

But there are some important differences, as well.

Although both chimps and humans experience the same white matter development delay, the rate of prefrontal white matter volume increase was slower in chimp infants than in humans. In addition, the total volume increase of the prefrontal cortex is significantly, and predictably, greater in humans than that of chimpanzees.

What does all of this mean?

Matsuzawa and his team maintain that the study may have implications for human evolution and the evolutionary relationship between humans and chimpanzees, considered to be close biological relatives based on comparative DNA makeup. Apart from the close genetic relationship, both humans and chimpanzees enjoy close relationships between infants and parents, they both use tools, they both exhibit relatively complex social relationships and behaviors, and they both exhibit self-awareness and novel behavior.

And they likely had a common ancestor.

Concludes the report:

......our results suggest that, as groundwork for the reformatting of these expanded brain regions in humans, the prefrontal reciprocal connections to the posterior brain regions became enhanced during the course of evolution. This enhancement likely occurred through an extension of the period of prefrontal connection maturation in the common lineage after the last common ancestor shared by chimpanzees and modern humans split from macaques. However, the lineage leading to modern humans has undergone substantial evolutionary modifications, resulting in the rapid development of the prefrontal connections during infancy. This likely facilitates the development of complex social interactions and the shaping of neuronal connectivity through the acquisition of experience-dependent knowledge and skills. [1]

The study details and results are published in the August 11th issue of Current Biology, a Cell Press publication.  

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[1] Sakai et al., Differential Prefrontal White Matter Development in Chimpanzees and Humans, Current Biology (2011), doi:10.1016/j.cub.2011.07.019.

Cover Photo, Top Left: Chimpanzee, Courtesy Aaron Logan, Wikimedia Commons. 

Photo, Second from Top, Right: Animated illustration of the cerebrum, which includes the prefrontal cerebral cortex. Database Center for Life Science, Wikimedia Commons.

By Dan McLerran

As Founder and Editor of Popular Archaeology Magazine, Dan is a freelance writer and journalist specializing in archaeology.  He studied anthropology and archaeology in undergraduate and graduate school and has been an active participant on archaeological excavations in the U.S. and abroad.  He is the creator and administrator of Archaeological Digs, a popular weblog about archaeological excavation and field school opportunities.