The progressive evolution of human brain size

By RACHEL HUANG | March 8, 2018

PUBLIC DOMAIN Through evolution, modern humans have acquired larger brain sizes than those of their ancestors.

There are a number of differences between Homo sapiens and our closest living relatives, the chimpanzees and bonobos. Perhaps the most striking is the variation in physical appearance, but evolution has led the three species to differ in many ways. 

In a recent study published in the Proceedings of the Royal Society B: Biological Sciences, scientists discovered that the size of human brains are more than three times larger than those of chimps and bonobos. 

The study looked at 94 hominin fossils and revealed that the evolution of larger brains occurred separately in populations of individual species. This occurrence, along with the sudden introduction of larger-brained species and the extinction of smaller-brained species may have contributed to the disparity in brain sizes among the species.

According to Andrew Du, a postdoctoral scholar at the University of Chicago and one of the researchers in the study, brain size is a defining human trait, one that distinguishes humans from other similar species. 

“[Brain size] is related to cultural complexity, language, tool making and all these other things that make us unique,” Du said, according to University of Chicago (UChicago) Medicine. “The earliest hominins had brain sizes like chimpanzees, and they have increased dramatically since then. So it’s important to understand how we got here.”

Du and his fellow researchers analyzed fossil specimens from a range of 13 different hominin species. 

The team compared information about the Australopithecus genus, which consisted of the earliest unambiguous human ancestors, to the Homo erectus species, whose brain size was more similar to that of present-day humans.

Looking at biological groups that descended from a common ancestor, the general trend observed was that the average brain size increased gradually over the course of three million years. 

This increase was attributed to both the direct evolution of larger brains as well as the extinction of smaller-brained species. It was also discovered that the rate of brain size evolution within hominin lineages was considerably hindered in comparison to today. 

Bernard Wood, a senior author of the study, introduced Du to researching evolution through brain sizes when Du was a graduate student at the George Washington University. Du continued studying this topic throughout his time at the university and, together with his fellow students, helped co-author the paper. 

People generally believe that large brains had evolved out of a series of step-like increases, with each step making human ancestors smarter. However, there seems to be no distinct correlational or causational relationship between brain size and behavior.

Du equates the reasoning behind the evolutionary increase in brain size to the way in which a football roster is built. 

In order to recruit the best players, an option would be to transform the entire team as a whole, therefore increasing their fitness, whereas another option would be to cut certain players and replace them with better candidates. 

“That’s exactly what we see going on in brain size,” Du said. “The dominant process is like the players hitting the gym. They’re evolving larger brains within a population. But we also see speciation events adding larger-brained daughter species, or recruiting bigger players, and we see extinction, or cutting the smallest players too.”

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