The evolution of modern human brain organization

Endocast data suggest evolutionary changes to early brain development that were critical for the evolution of our species
The evolution of modern human brain organization

Our paper "The evolution of modern human brain shape" in Science Advances can be found here.

How to investigate evolutionary changes of the human brain when brains are not preserved in the fossil record? This is one of the first questions that people outside my research community ask. Well, actually there is a straightforward proxy: I analyse the internal bony braincase or so-called endocasts - casts of the endocranial cavity. Endocasts are mute about internal brain organization but they approximate brain size and shape and reproduce some surface details like impressions of brain convolutions. Translating evolutionary changes of endocasts to evolutionary changes of the brain is, however, a major challenge.

Modern humans typically have a large and rounded brain, a feature that endocasts document well. Being about 300,000 years old and showing key features of modern craniofacial morphology, fossils from Jebel Irhoud, Morocco, can be regarded as the currently earliest known representatives of our species Homo sapiens1,2. However, their endocasts are not round but elongated like those of Neanderthals and more ancient Homo individuals. My colleagues and I therefore wanted to find out when and how the brain achieved its modern shape. Unfortunately, the smooth surface of endocasts lacks well-defined anatomical points but three-dimensional coordinates of hundreds of geometrically homologous measurement points capture endocast shape as measured from CT scans of the original fossils (see below), a measurement protocol that I developed during my PhD project.

recent human and Neanderthal
Modern humans have rounded endocasts (left) while Neanderthals like La-Chapelle-aux-Saints (right) have elongated endocasts. (CC-BY-SA 4.0, Simon Neubauer, Philipp Gunz)

We found that H. sapiens endocasts had increasingly more modern shapes in accordance with their geologic age. Surprisingly, only fossils younger than 35,000 years show the same globular shape as present-day humans, suggesting that modern brain organization evolved sometime between about 100,000 and 35,000 years ago. We also found that brain size at 300,000 years ago falls already within the range of that of present-day humans.

So, which fossils are then "anatomically modern humans"? Fossils from Jebel Irhoud, from Florisbad (South Africa, about 260,000 years old) and from Omo Kibbish (Ethiopia, about 195,000 years old) show clear affinities with present-day human facial morphology. To me, they therefore clearly belong to the H. sapiens clade but, given their archaic brain shape (if preserved), they are not anatomically modern humans. Even fossils from Skhul and Qafzeh (Israel) and Laetoli (Tanzania) from about 100,000 years ago have not achieved fully modern brain shape, although they are usually called (anatomically) modern humans without any controversy. Therefore, "anatomically modern humans" is an outdated, rather misleading term that should be dropped. We should recognise that H. sapiens is an evolving species with deep African roots.

early and recent modern human
The still elongated endocast of Jebel Irhoud 1, an early representative of our species (left) compared to a present-day human's endocast (right). (CC-BY-SA 4.0, Simon Neubauer, Philipp Gunz)

What do endocasts tell us about brain function? Well, there is no doubt that endocast shape approximates brain shape but is it the brain that is responsible for this shape? Some of my colleagues have argued that the roundness of the brain could just be the consequence of the combination of large brains and modern, small faces. However, the fossils themselves demonstrate that changes in facial and endocranial form cannot be the driving force for globular brains: modern faces and large brains evolved long before the evolutionary brain rounding started.
Two features of the globularization process stand out: bulging of parietal and cerebellar areas. The parietal lobe is an important hub in brain organization and involved in various integration and transformation functions. The cerebellum, in addition to motor-related functions, is associated with language, social cognition and affective processing. Of course, these brain functions and behaviours did not necessarily change because of brain shape changes.

However, evidence from developmental patterns attach importance to evolutionary brain globularization. We are born with rather elongated brains. The characteristic rounded brain shape develops only within a few months around the time of birth3, a critical period for neural wiring and cognitive development. Furthermore, the gradual emergence of behavioural modernity as seen from the archaeological record seems to parallel the evolutionary brain rounding. Combined with evidence from ancient DNA for important genetic changes affecting early brain development within the H. sapiens lineage, our data therefore suggest evolutionary changes to early brain development that were important for the evolution of morphology and behaviour of our species. 

I am excited that we have revealed an unexpectedly recent evolutionary change in our own lineage. We started this project knowing that brain shape must have evolved within our own species, but we were surprised to discover just how recent these changes to brain organization were. Now I look forward to new unexpected results on brain evolution in humans and our relatives based on evidence from palaeoanthropology, archaeology, and genetics.

1. Hublin, J.-J., Ben-Ncer, A., Bailey, S.E., Freidline, S.E., Neubauer, S., Skinner, M.M., Bergmann, I., Le Cabec, A., Benazzi, S., Harvati, K., Gunz, P., 2017. New fossils from Jebel Irhoud (Morocco) and the Pan-African origin of Homo sapiens. Nature 546, 289-292.

2. Richter, D., Grün, R., Joannes-Boyau, R., Steele, T.E., Amani, F., Rué, M., Fernandes, P., Raynal, J., Geraads, D., Ben-Ncer, A., Hublin, J.-J., McPherron, S., 2017. The Age of the Homo sapiens fossils from Jebel Irhoud (Morocco) and the origins of the Middle Stone Age. Nature 546, 293-296.

3. Gunz, P., Neubauer, S., Maureille, B., Hublin, J.J., 2010. Brain development after birth differs between Neanderthals and modern humans. Curr Biol 20, R921-R922.

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