21 September 2008

Mystery Chart Solved

Mike Hanson pretty much got it (after Malacoda's initial close guess).

Click to see full size.


Each dot represents a known individual. Vertical distribution is, of course, geochronological. Horizontal distribution is meant to be more or less morphological, but it's completely subjective. If it were more rigorous, the vertical distribution would be more stratified, but this is a good approximation.

Some interesting things to note:
  • Wherever there's a gap, it's due to age and/or a rainforest habitat. (Note that the only definite chimpanzee fossils we have, which are all teeth, are from a savannah environment.)
  • Genetic data indicates that the chimpanzee-human split occurred around the Ardipithecus level. In fact, Ardipithecus kadabba (the earlier Ardipithecus population) shares a possible synapomorphy with chimpanzees (the canine cutting complex) not seen in earlier fossils, so it could be a very early stem-chimpanzee.
  • The earliest populations all have some vague indications that they may have been more habitually bipedal than chimpanzees are. Perhaps instead of humans arising from a chimpanzee-like ancestor, we both arose from an ancestor that was sort of in between. (Even today, chimpanzees are certainly not completely quadrupedal.)
  • If you look closely near the dark splotch that is our species, you'll see a dot lying between it and Neandertals. This represents a fossil of a child, which lived after all known Neandertals, with some traits of both species. A hybrid?
  • We have a damn good fossil record.

10 comments:

  1. Utterly fanstastic.

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  2. Incredibly cool graph!

    A few notes though: hybrids are a very difficult thing to "prove" in the fossil record. A recent review of the evidence of Neanderthal-H. sapiens hybrids by Schwartz and Tattersall pretty much demolished the case of the existence of a hybrid child at Lagar Velha. They found that pretty much all of the supposed Neanderthal-features fall within the range of variation observed within our own species.

    Additionally, given that there is so much difficulty finding hybrids, it is surprising so many supposed "hybrids" are recognized in the fossil record. At one time or another, the sample from early Aurignacian modern humans in Romania has considered to be representative. Given the overall fragmentary state of the fossil record (despite the results of your graph), the chances of finding a non-hybrid should be MUCH larger than finding a hybrid.
    I am therefore very cautious when people make these claims.

    And last: considering the supposed hybrid is an immature individual, it is probably by no means representative of a fully adult population. This caution is frequently noted with regards to the H. antecessor sample from TD 6, Atapuerca, and especially regarding the partial face of the immature individual. The face of this individual is argued to be so generic that the species of H. antecessor is argued to be the LCA of modern humans and H. heidelbergensis, and eventually Neanderthals. However, adults show a diverging morphology, countering the previous hypothesis.

    Anyway, keep up the good work! ;-)

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  3. Ah, perhaps I should remove that one dot, then.

    As for hybrids, I'm not so sure about them being rare. I kind of depends on your definition of "hybrid", i.e., how long do two populations have to be separated before their intermingling is considered "hybridization"? (Or perhaps, how different do the populations have to be, morphologically? Or genetically?) Some of the recent work on the chimpanzee-human split posits that the populations took a long time to split up, making us descendants of hybrids, in part.

    As Rosen (1978) said, the ability to interbreed is a plesiomorphic character.

    Interesting about H. antecessor. It does seem to be in the wrong place geographically to be the LCA, for what that's worth (probably not much). What about the adult morphology is divergent? How unlikely is reversal? Is it terribly far off from "the main path" (so to speak)?

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  4. Hybrids may not be that are in the current record, and with this, I agree with you. However, the discovery of a hybrid in the fossil record is a completely different challenge. Palaeoanthropology is renowned for the problems about assigning species and subspecies to specimens. Different standards have lead to quite an interesting history of lumping and splitting up to a point that we are not exactly sure what to do with all the material. Some argue for one continuous lineage from H. erectus onward to H. sapiens, although others view the same lineage as one of experimentation and variation, with the existence of several species and subspecies. The recognition of hybrids in the fossil record is therefore a big challenge, because it is not sure what kind of "variation" we are picking up. Is it on the genus-, species-, sub-species, or part of the individual variation?

    In regards to your question about the TD6 hominins, I have checked the original literature. I was mistaken in saying that the adult anatomy was divergent. However, facial remains are still limited to the immature individual. Considering that facial anatomy changes considerably during our growth, it is uncertain whether the same feature is still present ini adulthood. In addition, I would like to point out that, as you note, it could represent a reversal, or, assuming it is also present in adults, something that was developed convergently. I do not know the "phylogentic strength" of this feature, but at least the evolution of apes has been one of mozaic evolution, similar to that of birds. To simply consider it to be part of single lineage leading to H. sapiens might be a bit to easy at this point for this reason, and I can't wait to see the adult facial anatomy of this hominin species.

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  5. David Marjanović04 October, 2008 09:44

    Some argue for one continuous lineage from H. erectus onward to H. sapiens, although others view the same lineage as one of experimentation and variation, with the existence of several species and subspecies.

    Part of the problem is that the fossil record is good enough that most people believe they can just see the phylogenetic tree in the fossil record. Phylogenetic analyses are almost never done (and the one I've seen had way too few characters and taxa). In other words, science is almost never done in fossil hominid phylogenetics.

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  6. This is true, however I would also point out that this is one case where a tree (which is all that a cladistic analysis can create) might not be the best model. The network of relationships might be more complex.

    That's no excuse for not doing cladistic analyses, though -- it just means that other types of analysis might be needed as well.

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  7. Various phylogenetic analyses have been performed. The majority of these are done by David Strait in collaboration with Fredrick Grine. They largely conform to the current notion how we structure our history based on the fossil evidence.

    However, the problem with these analyses is that they apply parsimony not only to the analysis but also to the specimens under consideration. For example, both H. habilis and H. erectus are probably undersplit given both the variation in the specimens included in their respective sample and the temporal scale. Opinions vary on the amount of species contained within this sample, but I would argue for a greater diversity than hiherto recognized. Not as much as suggested by Tattersall and Schwartz, but not as few as commonly published.

    Furthermore, the human fossil record keeps turning up neat little surprises. An example is a recently published paper about the ramal morphology in P. afarensis. An examination of its morphology allied this species to paranthropines rather than Homo. This was not previously considered as far as I know and is therefore testament to these surprises.

    In conclusion I would argue that, despite the relative completeness of the current human fossil record (as opposed to other lineages, e.g. chimps and gorillas), we still have much to learn. Science might be a way forward, but I would only be a first step. The various recognized species established in our fossil history need to be re-examined and clear types identified for them.

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  8. An examination of its morphology allied this species to paranthropines rather than Homo.

    Correct me if I'm wrong, but wasn't this predicted by those who support Kenyanthropus (not to mention Orrorin!) as a basal member of Clade(Homo sapiensParanthropus robustus)?

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  9. Science might be a way forward...

    Did you mean to say "cladistic analyses" instead of "science"?

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  10. David Marjanović09 October, 2008 01:41

    That's no excuse for not doing cladistic analyses, though -- it just means that other types of analysis might be needed as well.

    Agreed.

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