The modern human brain developed much later than scientists hitherto believed, after the first dispersal of our ancestors out of Africa, a new study reveals Thursday.
The genus Homo, which includes many extinct species (Homo Erectus, Neanderthals …) as well as ours (Homo Sapiens), has not always had a brain evolved in a similar way to ours.
The researchers wanted to answer a question so far remained mysterious: “When did the structures of the brain that make us humans evolve?”, Summarizes Christoph Zollikofer, paleoanthropologist at the University of Zurich in Switzerland, and the one of the co-authors of the study, published Thursday in the prestigious journal Science.
“People thought until now that the brain similar to that of man had evolved at the very beginning of the genus Homo, about 2.5 million years ago,” he told AFP.
But this evolution took place much later, between 1.7 and 1.5 million years ago, conclude this new work.
To achieve this result, he and his colleague Marcia Ponce de Leon, lead author of the study, studied numerous skull fossils, from Africa, Georgia, and Java, Indonesia.
Since brains themselves do not fossilize, the only way to observe their evolution is to study the marks they have left inside the skull.
Scientists have thus “scanned” the fossils, and created a virtual image of what filled them long ago, like a cast – what is called an endocast.
What characteristics did they then look for to determine the “modernity” of a brain?
In humans, “some areas of the frontal lobe are much larger than the corresponding areas in great apes,” explains Christoph Zollikofer. One of them is, for example, Broca’s area, associated with language.
This expansion has the effect of shifting everything behind. “And this backward shift can be seen on fossil endocasts over time, when we spot impressions left by cracks in brain convolutions.”
– “Surprise” –
By studying skulls from Africa, researchers were able to determine that the oldest of them, dating back more than 1.7 million years, actually had a frontal lobe characteristic of great apes.
“This first result was a big surprise”, underlines the paleoanthropologist. This means that the Homo genus “began with bipedalism (the ability to walk on two feet, editor’s note), not with a modern brain”, he sums up, and that the evolution of the brain “has nothing to do with being bipedal “.
“From now on, we know that in our long history of evolution (…), the first representatives of our genus Homo were terrestrial bipeds, with a brain close to the great apes”, he adds.
In addition, younger African fossils, dating back 1.5 million years, have revealed characteristics of modern human brains.
This means that the evolution took place between these two dates, in Africa, according to the study.
A conclusion corroborated by the fact that it is during this period that more complex tools appear, called Acheuleans, which have the particularity of presenting two symmetrical faces. “It is no accident,” says Zollikofer, “because we know that the areas of the brain that develop during this period are those used for complex manipulations, such as making tools.”
Why did this development occur? The researchers’ hypothesis is that a virtuous circle has developed between cultural innovations and physical changes in the brain, both of which stimulate each other in return.
– Two migrations outside Africa –
The second surprising result of the study comes from observations made on five fossils of skulls found at the site of Dmanisi, in present-day Georgia, and dating from between 1.8 and 1.7 million years ago. Particularly well preserved copies.
These were found to have primitive brains.
However “people thought that there was need of a big modern brain to disperse outside Africa”, explains the paleoanthropologist. “We can show that these brains were neither big nor modern, and that groups were able to leave” this continent anyway.
Finally, the more recent fossils from Java presented modern characteristics. The researchers therefore believe that there was a second dispersion outside Africa.
In short, “you have a first dispersion of populations with primitive brains, then the modern brain evolves in Africa, and these people disperse again”, until arriving in Indonesia, says Christoph Zollikofer.
“This is not a new hypothesis, (…) but for the first time we have fossils which prove it.”