New analysis of ancient fossils could upend decades of thinking on when the last common ancestor of humans and chimps…
One of the major questions in human evolution has been when, exactly, the last common ancestor of humans and chimpanzees lived. Modern chimpanzees (genus Pan) are the most similar to humans (genus Homo) . Debates over when our respective genera split from one another have varied over the decades, as new fossils (and techniques for dating those fossils) have been discovered. The process has been hampered by evidence that there was no clean single break. In addition, the earliest members of the Hominina and Panina clades may have diverged, hybridized, and then diverged again in multiple progressive cycles over several million years.
Estimates of when this divergence took place have varied widely, from as recently as four million years ago, to as far back as 13 million years. Estimates of where it happened, however, have been much less contested, with the majority of anthropologists believing that the split occurred in Africa. There’s good practical reason for that — East Africa is where many of the oldest (and most ape-like) hominid fossils have been found. The “Out of Africa” theory is well supported by the fossil record.
Scientists working with fossils from one particular common ancestor between great apes and humans, Graecopithecus freybergi, have found evidence that could upend what we thought we knew about the earliest divergence between Pan and Homo. This species is known from one fossilized jaw, recovered in Greece, and an upper molar found in Azmaka, Bulgaria. Their findings suggest that the last common ancestors between humans and chimpanzees may not have been African at all, but European.
The bulk of the paper, available here, is concerned with modeling the types of climate that existed in the Mediterranean millions of years ago, as climate adaptations are believed to have driven many of the evolutionary differences between humans and great apes. In this case, Graecopithecus freybergi ‘s teeth show clear evidence that the roots of the premolars are fused. That’s significant, because it’s one of the traits that separate humans and apes, even if you go back to Australopithicus, which lived ~3.2 million years ago.
But just as interesting is the idea that our respective lineages split at a time when the Mediterranean climate is believed to have conformed to what is known as the Savannah hypothesis. As humans and apes diverged, we lost the ability to grasp things with our feet as nimbly as we do with our hands. Our pelvis and spinal columns are adapted to bipedal walking, whereas those of the various ape species are not. Much of this has to do with the environments in which we evolved, and savannahs — mixed woodland / grassland regions in which trees do not grow closely enough together to form a canopy — could have been instrumental to the emergence of the earliest evolutionary traits that would one day separate us from our current closest kin.
But even if the last common ancestor of chimps and humans did emerge in the Mediterranean, this would only raise further questions. Why (and when) did these species spread south and east, into the Great Rift Valley and other parts of Africa known to be rich in ancient hominin fossils? What part did climate change, as opposed to the different requirements of different geographical areas, play in species differentiation? Claims that our last common ancestor diverged in Europe rather than Africa don’ t upend the idea that humans evolved in Africa for millions of years thereafter, but they put an interesting twist on a story that previously seemed clear-cut.
