All the skulls of the great apes were equally large. Until homo sapiens got fifth

Talk about the human evolution is talking about a gigantic puzzle of which we have completed a large percentage, but whose last pieces resist us. There are many who continue trying to put these pieces together, and each new fossil studied brings us one step closer to the goal… or to rethink everything. One of those questions was at what rate the hominid brain evolved compared to that of the great apes, and the conclusion of a new study It has been devastating.

Double.

The hypothesis. The researchers at University College London, led by the Spanish Aída Gómez-Robles, started from a well-known premise: current humans have brains about three times larger than those of our closest ape relatives. And not only a different sizealso a noticeably different cranial structure. While most great apes have forward-projecting faces and small brains, humans have a flatter face with a larger head and rounded.

The exception among the apes would be the gibbons and their heads were rounded, but with much smaller brains. The hypothesis they used was that these craniofacial adaptations evolved at an accelerated rate in humans thanks to the advantages of having a large brain, but also that social factors would have influenced this accelerated transformation.

The study. The team examined virtual models of skulls of several species of modern primates. Thus, they analyzed the skulls of seven species of “great apes” including humans, two species of gorillastwo of orangutans, chimpanzees and bonobosas well as nine species of hylobatids or “lesser apes”, like the aforementioned gibbons.

To do this, and using a technique that allows landmarks to be mapped onto anatomical structures, the researchers divided each skull into four sections. They analyzed the markers of the upper face, lower face, front and back of the head and compared between all the skulls analyzed.

As a control group, they used hylobatids, since the species separated from hominids about 20 million years ago and they realized something: while gibbons are very similar to each other, hominids are very different from each other. And, among them, humans are the ones that evolved the most. At an astonishing speed, too.

Face + neuroskull. The conclusion is that the human brain It evolved twice as fast as that of other hominids. Studies have already been done on additional factors driving accelerated changes in the brain and skull, but this study is the first to quantify the speed at which different species evolved.

And, in addition to speed, what they found is that the human face transformed almost as quickly as the neuroskull. Brain expansion and facial flattening are related, but in other species there is not such a clear relationship between the evolution of the neurocranium and the face. Therefore, the team concluded that there was something external at play: a selective pressure caused because we started living in a society.

“The face is the interface through which we interact with other people, so a possible explanation is that the selective pressure that caused its accelerated evolution is related to how we interact with each other in a social group,” exposes Gómez-Robles.

Implications. This has not happened exclusively with humans. In the case of gorillas, the UCL team concluded that they had had the second-fastest rate of cranial evolution, probably also driven by social selection, which means a larger cranial crest is a symbol of higher social status.

Now, as we said at the beginning, although the UCL study has demonstrated the evolution of human brain growth in relation to that of other similar species, there are still pieces of the great puzzle to put together. Future studies can examine other aspects to better understand what were those biological or social factors that drove the accelerated cranial development in humans.

Images | UCL, Jacklee

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