May 23 2017

Graecopithecus – Possible Early Human Ancestor from Europe

graecopithecusFinding fossils is like finding pieces to a jigsaw puzzle, although we don’t know what the final picture is, or where the edges are, and the pieces themselves are damaged or partial and so it is not always clear if they fit. A piece may seem to fit in one location, but it actually goes somewhere else. Sometimes one section of the puzzle can come together, but you are still not sure where it fits into the greater puzzle. But eventually a clear picture can emerge.

For the last century paleontologists have been trying to piece together the puzzle of human evolution. It’s hard to say how close we are getting to having a reasonably complete picture, because again we don’t know how big the puzzle is or where the edges are. One way to get a sense of where we are is this – with each new fossil find is the puzzle getting bigger and more complex or are we filling in known gaps? It’s definitely some of both, but mainly the puzzle is still getting bigger. We don’t know yet how much we don’t know. We’re not just connecting the dots, we keep adding new dots.

A recent analysis of one potential hominin makes the picture more complex still. Graecopithecus is known from a lower jaw and an upper pre-molar. That is not much, which is why the papers on this fossil all describe is at a “possible” early human ancestor. If Graecopithecus turns out to be legit, then it would be the oldest human ancestor after the split with chimpanzees, and it would move the likely location for that split from sub-Saharan Africa to the Mediterranean.

First a little background. The recent paper of Graecopithecus takes the time to conveniently define the terms we use to define apes and human species:

“In the present study, we define ‘hominoid’ as ‘apes’; ‘hominid’ as ‘great apes and humans’; ‘hominine’ as ‘African apes and humans’; and ‘hominin’ as ‘humans and their non-ape ancestors’.”

Graecopithecus, therefore, would be a hominid, hominine, and a hominin. Hominins are what we are most interested in here – the split with chimps and the line from that common ancestor to modern humans. Here is a quick sketch of what we know about that lineage:

Sahelanthropus tchadensis from 7 million years ago is likely a close relative of the last common ancestor between hominins and Pan (chimps and bonobos). It may in fact be the last common ancestor but we don’t have enough evidence from that time period to know. Sahelanthropus is known only from a skull, so we cannot be sure if it was bipedal or not, but the skull suggests it may have been. We cannot rule out that Sahelanthropus is a species from after the split with Pan, but that would complicate our understanding of the relationship of Australopithecus with later humans.

Orrorin tugenensis from 6 million years ago is the first species more clearly from after the split with Pan, and so may be currently the oldest known hominin. Orrorin is known from 20 fossils including both cranial and limb bones. It was bipedal, and the shape of its femur may be more similar to the Homo genus than is the later Australopithecus afarensis. If this turns out to be true, then Australopithecus may not be in the direct line to humans but be a side branch.

Ardipithecus ramidis from 4.4 million years ago and A. kadabba from 5.6 million years ago are also likely early bipedal hominins. They also probably were arboreal some of the time. While they are likely hominins, they are also likely not in the direct line to humans, but are rather a side branch.

Australopithecines, most notably afarensis, lived from 3.9 to 2.9 million years ago. A. afarensis is considered a likely ancestor to the Homo genus, but it is still possible that the entire Australopithcus genus are cousins and not direct ancestors to Homo. They were the most widespread hominins in that time period in Africa, but it is possible that a smaller branch of hominins with yet undiscovered members (perhaps descended from Orrorin) eventually lead to Homo.

Kenyanthropus platyops lived from 3.5-3.3 million years ago, likely descended from Australopithecus, and may also be a direct ancestor to Homo.

The early Homo genus is also getting more complex. H. habilis is the best contender right now for the earliest member of the Homo genus. This is when a more sophisticated stone tool kit emerges. H. habilis likely evolved into H. ergaster and then H. erectus. However, the recent finds of H. naledi cast doubt on that simple lineage as well, showing a combination of primitive and derived features.

It is pretty clear, though, that H. erectus is a human ancestor. They evolved into H. antecessor, which is likely the common ancestor to H. neanderthalensis and H. sapiens.

There are other species in there as well, all likely side branches, but that is the probable line to humans. As you can see, the hominin clade was a complex branching bush. It was not a straight line to modern humans. There are lots of different features that define different groups. Analysis may be complicated by homoplasy – when different groups independently evolve similar looking features. There was also likely lots of interbreeding going on. Species divisions are fuzzy and not always clear.

Graecopithecus

Into this picture now comes Graecopithecus. Analysis of the tooth shows that the roots are partially fused. Apes have separate roots, while hominins have fused roots. Therefore the partially fused roots of Graecopithecus may be transitional, placing it close to the split between hominins and chimps. Of course, the closer you get to this split the harder it is to tell if a specimen is on one side or the other. There was also probably a 1-2 million year period where chimp and human ancestors were interbreeding.

Genetic analysis also places the split between humans and chimps to about 7 million years ago, so that fits nicely with the fossil evidence.

Another interesting aspect of Graecopithecus is that it is from Europe, not Africa. Up until now it was assumed that hominins evolved in Africa. However, at this time, 7 million years ago, southern Europe was a savanna and likely had more continuity with northern Africa. There were giraffes, zebras, and rhinos in southern Europe, so why not hominids?

It is even possible that this geographic split is what caused the lineage split between humans and chimps. Perhaps human ancestors migrated to Europe while chimp ancestors stayed in Africa. As the climate further changed, those human ancestors may have then moved back to Africa.

As we try to draw the lines of what evolved into what, we may also have to figure out where. Hominin lines may have move around, splitting off, then interbreeding. It’s likely a mess, and will defy any attempt at drawing a nice, clean lineage.

Clearly we need more fossils, especially from the 8-4 million years ago time, which currently has a very sparse fossil record. We just don’t have any really good windows into this time period in Africa. Although if the implications of Graecopithecus turn out to be true, maybe we need to be looking more in the Mediterranean region.

The thing that is most clear at this point is that the puzzle is still expanding. The picture is getting more complex, and each new find seems to suggest that there is more that we don’t know. Given how evolution tends to proceed, however, that is expected. Evolution results in a complex branching bush of adaptive radiation, not single lines or ladders. That old image is long dead.

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