We have an extensive collection of fossils from the generations our humans have produced. A large amount of Australopithecus and early Homo skeletons follow the transition to bipedal walking and the appearance of features indicative of our current anatomy. But it is much harder to figure out what has led to the mental faculties – complex language, the almost constant use of tools, and so on – that help to discern people.
Much more difficult – but not completely impossible. Remains of skulls can help us determine the probable cranial ability of extinct species. And the brain actually leaves its stamp on the inside of the skulls so that some aspects of the brain’s anatomy can be put together. Now an international team has done this kind of analysis on a set Homo erectus from a critical point in the past of our species. They found that some earlier brain species continued into the history of our generation Homo, but that did not stop those ancestors from moving out of Africa.
Brain reconstruction
How can you find out what a brain once looked like? You need a fairly intact skull, which is relatively rare, given the fragility of the bones. After the skull has been reconstructed, it is possible to make an ‘endocast’ from the inside of the skull and capture the details of its functions, including where it meets the underlying brain. In some cases, endocastes naturally form during the deposition of material around a fossil. It can also be made after discovery and can now be done virtually thanks to our ability to scan and reconstruct 3D volumes.
Clearly, there are many things going on in the brain that are not near the interface with the skull, and endocastes will not be able to tell us about these changes. But if you look at endocastes from the brains of humans and our immediate family members, there are clear diagnostic differences. One of the most important is in an area called Broca’s cap, which is related to language ability.
Over the years, many endocastes have been made that show a clear pattern. Early relatives such as Australopiths retained the ape-like arrangement of the forebrain. More recent ancestors, such as Homo erectus, had an arrangement that was much more similar to what we have today. This led to the assumption that the modern arrangement developed at the same time as our kind Homo appeared.
The new work expands our collection of endocasts to some critical skeletons: the Dmanisi hominins, which were dated about 1.8 million years ago and were discovered in the Republic of Georgia. It is usually referred to as members of Homo erectus, but they retain enough characteristics of earlier species that this label remains controversial. The Dmanisi skeletons are interpreted as indications that Homo erectus expanded out of Africa very early, perhaps while its characteristics were still in check.
Draw the tree again
The results are pretty clear: all five Dmanisi skulls show the earlier pattern of brain structure. This has a number of important implications. This clearly means that the modern brain structure did not originate in the genus Homo but only evolved after we existed for almost a million years. In addition, the Dmanisi skeletons were found with various stone tools, so we can conclude that the modern brain structure was not a prerequisite for its development.
Finally, it also shows that our ancestors did not need the modern brain structure to spread beyond their point of origin in Africa. In fact, it suggests that the relationship between our brain and migration is extremely complex, because previous data, when included in this analysis, indicate that the modern arrangement of the brain was in place 1.5 million years ago – and virtually simultaneously from Africa to Southeast Asia.
This suggests that our ancestors left Africa in multiple waves, some not separated by much time, at least evolutionarily. And before this critical period, the size of the brain (as opposed to its arrangement) gradually increased. (Although with some serious outliers like the Indonesian hobbits and Homo naledi, who was small of brain, but very recently.)
To complicate matters further, the researchers note that during this time there were much larger changes in facial morphology, probably mainly through diet. But there is no clear connection between what goes on with the face and jaw and with the brain structure.
Although the new study clarifies many questions and overturns a large assumption, there are limitations to how much it can tell us. Although the brain region we are looking at here is related to language, there is no way to see if its appearance is in line with language use. Tool technologies have changed around the same time as the transition between brain structures, but it is impossible to see if the two are related. And we can only guess at the selective pressure that drove the changes in the brain.
But one thing is clear: our ancestors’ ability and desire to wander around the world was long before our current brain structure was in place.
Science, 2021. DOI: 10.1126 / science.aaz0032 (About DOIs).