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When did our brain become "modern"?


In a study published in Science, an international team of researchers led by the University of Zurich, in collaboration with the ESRF, has been able to answer this question using computed tomography to examine the brain cases of Homo fossils that lived in Africa and Asia between two and one million years ago.

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The human brain as we know it today is relatively young. It evolved about 1.7 million years ago when the culture of stone tools in Africa became increasingly complex. A short time later, the new Homo populations spread to Southeast Asia, researchers from the University of Zurich have now shown using computed tomography analyses of fossilized skulls.

Modern humans are fundamentally different from our closest living relatives, the great apes: we are ground-dwelling bipeds and have a large brain. But when did these features emerge? The first populations of the genus Homo appeared in Africa about 2.5 million years ago. They were already bipeds like us, but their brains were only half the size of ours today. Until now, scientists assumed that these small brains had a similar modern structure as those of today's humans and thus made it possible for Homo to disperse into the Old World from Africa as early as two million years ago.

However, the theory was on shaky ground, mainly because of the dearth of fossil data. The brains of our fossil ancestors have not been preserved; their former structure can only be deduced from imprints on the inner surface of the skull left by the cerebral convolutions and furrows. Since the imprints vary considerably from individual to individual, it was not possible until now to determine unambiguously whether a particular Homo fossil had a brain that was more ape-like or more human-like. This left the central question unanswered: did the typical human brain structures actually evolve at the very beginning of the genus Homo?


Skulls of early Homo from Dmanisi, Georgia (specimen D4500, left) and Sangiran, Indonesia (specimen S17, right). Virtual fillings of their braincases permit inferences on brain organization (blue). Brains of Homo evolved from ape-like (D4500) to human-like (S17) in the time between 1.7 ­– 1.5 million years ago.

source: M. Ponce de León and Ch. Zollikofer, University of Zurich


An international team led by Christoph Zollikofer and Marcia Ponce de León from the Department of Anthropology at the University of Zurich (UZH) has now succeeded in answering these questions. “Our analyses suggest that modern human brain structures emerged only 1.5 to 1.7 million years ago in African Homo populations,” Zollikofer says. The researchers used computed tomography, combining CT scans and synchrotron tomography at the ESRF to examine the brain cases of Homo fossils that lived in Africa and Asia between two and one million years ago. At the ESRF in particular, one Dmanisi specimen - the earliest Homo populations outside Africa - was studied using beamline ID17. The team then compared the fossil data with reference data from great apes and humans. Apart from its volume, the human brain differs from that of the great apes in the positional relationships of its individual regions to one another. A human peculiarity are those areas in the frontal region that are responsible for planning and executing complex patterns of thought and action, and ultimately for language. In humans, these regions are significantly enlarged, and as a result, all the adjacent brain convolutions and furrows have been displaced backward. When this process took place in human evolution can thus be deduced from the displacement of some prominent brain furrows, which left imprints in all the fossil skulls studied.


Dmanisi cranium D4500, mounted for synchrotron tomography on beamline ID17 at the European Synchrotron Radiation Facility, Grenoble, France.

source: Paul Tafforeau, ESRF


The results of the analyses were unexpected but clear. The earliest Homo populations in Africa had evolutionarily primitive, ape-like brains - just like their ancestors, the australopithecines. The biggest surprise for the research team, however, was that the earliest Homo populations outside Africa, those from Dmanisi in what is now Georgia, had brains just as primitive as their African relatives. So until about 1.7 million years ago, early humans had neither especially large nor especially modern brains. Yet we know from other studies that they were quite capable of making a variety of tools, adapting to the new environmental conditions of Eurasia, tapping animal food sources, and caring for members of the group in need of help, so we should not underestimate the cognitive achievements of ape-like hominin brains.


Early Homo skulls from Dmanisi, Georgia (specimens D2280, D2282, D2700, D3444, and D4500) with internal braincase structures revealed by computed tomography and virtual reconstruction.

source: M. Ponce de León and Ch. Zollikofer, University of Zurich


The team went on to show that the new brain structures characteristic of modern humans first emerged between 1.7 and 1.5 million years ago, in African Homo populations. During the same period, stone tool cultures in Africa became more complex and diverse. Biological and cultural evolution - so the researchers assume - were mutually dependent, and at that time the earliest forms of human language had probably also developed. Fossil finds from Java show that the new populations were extremely successful. Shortly after their appearance in Africa, they had already spread to southeast Asia. On the way there, they probably met the descendants of the first emigrants with the more primitive brains. How these encounters might have unfolded is the next question the researchers want to investigate. 


Marcia S. Ponce de León, Thibault Bienvenu, Assaf Marom, Silvano Engel, Paul Tafforeau, José Luis Alatorre Warren, David Lordkipanidze, Iwan Kurniawan, Delta Bayu Murti, Rusyad Adi Suriyanto, Toetik Koesbardiati, Christoph P. E. Zollikofer. The primitive brain of early Homo. Science. 8 April 2021. DOI: 10.1126/science.aaz0032


Top image: Skull of early Homo from Dmanisi (specimen D4500) showing internal structure of the brain case, and inferred brain morphology. source: M. Ponce de León and Ch. Zollikofer, University of Zurich