Friday, December 10, 2010

What happened to Sahelanthropus tchadensis? (Part 2: Conclusions)

Two weeks ago, I finished my Post regarding the (almost non-existing) controversy about Sahelanthropus tchadensis with a more or less cryptic statement, which I thought should deserve a separate post to elaborate.

I ended my post asking the question whether or not the exclusion of Sahelanthropus would diminish this fossil from its scientific value. Well, the answer itself is pretty easy: Of course it wouldn’t do it. The more interesting question is: Why?

To answer this question we should take a closer look at the situation on the base of human evolution:

Timeline showing the estimated age of the earliest putative homids. As indicated by the red circle, these fossils almost perfectly fall inside the timeframe of the Chimpanzee/Human divergence based on molecular-clock estimations. (pictures taken from Johanson & Edgar, 2006; Suwa et al. 2009)

As we can see on this picture, we’re more or less in the right time frame when it comes to finding putative hominid ancestors, since we’re pretty much inside the estimated date of divergence between humans and chimpanzees (as indicated by the red circle). On one hand, this is pretty great because now we have much better resources to actually build up and test hypothesises about human origins. On the other, we got the problem that it’s actually pretty hard to classify these fossils in a proper way. The reason why there is this problem is because the closer we get to the actual date of divergence between humans and Chimpanzees the more the fossils we find will resemble the most recent common ancestor (MRCA). This means that these fossils to a great extant would still reflect the ancestral condition of the MRCA and would not have developed a huge amount of derived characters which could provide us with enough information to classify them properly.
Samuel Cobb (2008) came to the same conclusion, when he tried to reconstruct the facial morphology of the MRCA:

„In light of the problem summarized above and the paucity of the fossil evidence of the face in the hypodigms of these four taxa [Sahelanthropus tchadensis, Orrorin tugenensis, Ar. ramidus, Ar. ramidus kadabba -my addition], it is not possible to determine with any confidence whether any of them is the LCA , or a stem taxon in either lineage, or a member of an extinct, and until now unrecognized, hominid lineage.” (Cobb, 2008; p.482)

So, in the end we’re stuck in a situation which looks something like this:

Possible classification of the earliest putative hominid fossils. Because of their (probably) very ancestral condition, they could be either stem hominids, on the lineage to the Chimpanzee/Huma LCA, stem Chimpanzees or stem Gorillas.

On the first hand this picture might look a little bit depressing (at least I found it to be depressing), but, and now I finally come to my original statement, even if we’re not able to classify these early putative hominids with absolute certainty, they’re still providing us with lots of valuable information. Each fossil from this specific timeframe helps us to reconstruct the ancestral morphology of the Chimp/Human common ancestor, to reconstruct the original ecological niche of the MRCA and they help us to build up and test hypothesises on how exactly and under which circumstances the divergence of the Chimpanzee/Human clade took place.
All these things actually are much more important then the exact classification of the fossils we use dot draw conclusions from. Sure it might not sound very spectacular if someone publishes the description of a new putative hominid and states that he isn’t sure where to exactly place It., but it’s probably much closer to reality then all those ground breaking discoveries we encountered in the last 10 years.

Ok, this doesn’t sound very optimistic, but I still think that we, even if it seems futile, should still try to classify each fossil we find. I also want to add that this might look futile, from a present day perspective. But right now we also have to face the fact that there is one huge, but very critical gap in the fossil record. I’m of course speaking about the almost non-existent fossil record in the Chimpanzee and Gorilla lineage. Right now we have actually no Ideas how and in which way these genera evolved since they split from our lineage. Usually the lack of a Chimpanzee/Gorilla fossil record is explained by the fact that rainforests don’t provide the circumstances for good fossilisation. Right now this sounds like a sorry excuse, at least in my eyes.

In the last century we were able to get a very good coverage about the whole timeline of human evolution. We got this coverage, because there were huge efforts put into the search for fossil hominids. And right now, I’m pretty sure that, if we put even a small bit of this effort into the search for Chimpanzee and Gorilla ancestors, we will find them. On the other hand it’s pretty clear that if we do not search explicitly for these kinds of fossils we pretty sure won’t find any.

Just to show that I’m not the only one with this opinion and since it’s always good to support one’s argument with some famous words, here’s one of my favourite quotes from the paper of Esteban Sarmiento (2010), whose critique on the classification of Ardipithecus I start to like more and more:

“(...) it is curious that in a century-old race for superlative hominid fossils on a continent currently populated with African apes, we consistently unearth nearly complete hominid ancestors and have yet to recognize even a small fragment of a bona fide chimpanzee or gorilla ancestor.” (Sarmiento, 2010 p.1105b )

Cobb, S. (2008). The facial skeleton of the chimpanzee-human last common ancestor Journal of Anatomy, 212 (4), 469-485 DOI: 10.1111/j.1469-7580.2008.00866.x

Sarmiento, E. (2010). Comment on the Paleobiology and Classification of Ardipithecus ramidus Science, 328 (5982), 1105-1105 DOI: 10.1126/science.1184148


Johanson D., Edgar, B. (2006). From Lucy to language. Simon and Schuster, New York
Suwa G., et al. (2009). The Ardipithecus ramidus Skull and Its Iimplications for Hominid Origins. Science 326, 68.