April 7, 2013

Brain shape favors 'Toumaï' as a human ancestor

The seven million years old Sahelanthropus tchadensis (i.e. Sahel man from Chad), nicknamed Toumaï, already had a hominin structure in its small brain:

... though Toumaï’s brain was apelike in its small size, it was apparently homininlike in other ways. In a presentation given on April 2 at the annual meeting of the Paleoanthropology Society, Bienvenu reported that the endocast shows strongly posteriorly projecting occipital lobes, a tilted brainstem, and a laterally expanded prefrontal cortex, among other hominin brain characteristics.

Other human-like traits in Toumaï are small canines and the forward position of its foramen magnum (where the backbone connects to the skull), which suggest upright walking, which is now argued to be related to brain reorganization. 

This clearly confirms, as I have insisted so much, that the Pan-Homo divergence date is necessarily older than 7 million years, because Toumaï  was already closer to us than chimpanzees and bonobos, our closest living relatives.

13 comments:

  1. But couldn't it also mean that some traits that we (perhaps chauvinistically) think of as derived human traits are actually ancestral traits for African great apes? After all, there is some evidence that Pan and Gorilla evolved their methods of quadrupedal locomotion independently -- the ancestral mode may have been more bipedal than often thought. (Just look at gibbons.)

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    1. I don't think so. At least all I read, even among initial skeptics like John Hawks, implies that Toumaï's brain (plus other details like bipedal locomotion) was much more human-like than ape-like in everything but size.

      The issue of bipedal/quadrupedal locomotion may be more blurry because orangutans and bonobos do sometimes walk (or gibbons, as you say), especially on branches or when they carry stuff but still I doubt they have the high specialization we do, indicated here by the forward position of the foramen magnum. Of course a key evidence would be the hips but we don't have those... yet.

      Whatever the case the brain shape alone is very telling.

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    2. There is some modern thinking that bipedalism first evolved via walking on horizontal tree branches, long before the split-off of the human line. It may have been rudimentary, and not (much) further pursued by the Pan and Gorilla lineages. But when conditions where right and the geology changed and isolated the human lineage from the others in a more savanna-like environment, they (presumably many attempts in parallel) were ready to take advantage of this. Between older Australopitheci and Australopithecus sediba and homo habilis and erectus, it appears that in particular, a lot was going on in the 3 million to 2 million years ago time frame.

      My bet is that there were a lot of Australopithecus side branches, and we haven't found the ones yet that were (most?) responsible for the habilis and erectus lineages before 3 millions years ago.

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  2. What are your reasons for a Homo split well before 7 million years ago?

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    1. For example Langergraber 2012, who establishes a Pan-Homo divergence date of 8-13 Ma.

      This is in agreement with what I have been saying, based on other papers since 2008, for example:

      http://leherensuge.blogspot.com.es/2008/04/new-paper-ofn-chimpanzee-and-bonobo.html

      http://leherensuge.blogspot.com.es/2010/09/bonobo-mtdna-and-some-human.html

      http://forwhattheywereweare.blogspot.com.es/2010/11/chimps-and-humans-divereged-some-eight.html

      http://forwhattheywereweare.blogspot.com.es/2012/06/bonobo-genome-sequenced.html

      All them suggest or demand vehemently a Pan-Homo split at least 8 Ma old. I used to say 8-10 Ma before the Landgraber paper but now I assume 8-13 Ma indeed, what essentially multiplies all your usual "age estimates" by at best x1.15 and at worst x2.3.

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  3. What are your reasons for a very old split like more than 7 million years ago?

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  4. One thing with tchadensis is that it can be only dated at 6-7 million years; 7 million years is the upper limit. It's age could be much closer to 6 million years than 7 million years and give more room for divergence from the common ancestor with chimpanzees and bonobos that way.

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    1. It's still much closer to modern humans than to chimpanzees.

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    2. Well... to all humans, not just modern ones (ahem).

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  5. "Toumaï was already closer to us than chimpanzees and bonobos, our closest living relatives."

    What if Toumaï was also a common ancestor of chimps and bonobos, who diverged more than us?

    Some studies actually conclude that chimps evolved more than humans, when examining genetic signatures of selection and whatnot. And I believe some people still argue that we even have some ancestral anatomical states that would suffice for making us closer to orangutans than to chimps or gorillas on anatomical grounds. Some people actually argue that such evidence actually means really that humans are more closely related to orangutans, I think that, if the anatomical comparison is correct, it would more likely mean just that chimps and gorillas evolved more dramatically to an "easier" niche, while humans didn't have the same "luck" and, well, ended up becoming humans.

    Some people argue that gorillas and chimps evolved from bipedal hominids.

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    1. Is extremely unlikely: traits like the foramen magnum or brain shape do not seem like the kind of stuff that de-evolves to exactly the same shape it had before. It's possible that in some aspects human-specific evolution borrowed some stuff from the genetic set closer to orangutans but not these: these are too specifically human (hominid).

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  6. Another thought: a few years ago there was a paper putting forward the theory that chimpanzees and humans had first split 5 million years ago, and then hybridized as recently as 1 million years ago. Couldn't such an hybridization event (whatever would be the real date) help creating an illusion of a more recent split?

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    1. That "theory" is almost 100% certainly wrong. Don't believe all you read: there are many "theories" (more like working hypothesis) popping up all around but only some of them seem somewhat solid, when more and more evidence coalesces around them and discards competing models.

      It's not only this item which supports an >8 Ma divergence of Pan and Homo branches but much other evidence from genetics, including the age of divergence of bonobos and chimpanzees, which was sudden and caused with all likelihood by the formation of the Congo basin c. 1.8 Ma ago. See also:

      http://leherensuge.blogspot.com.es/2010/09/bonobo-mtdna-and-some-human.html
      http://leherensuge.blogspot.com/2009/11/molecular-clock-two-to-six-times-slower.html
      http://leherensuge.blogspot.com/2010/03/mutation-rate-is-less-than-half.html
      http://forwhattheywereweare.blogspot.com/2010/11/chimps-and-humans-divereged-some-eight.html

      etc.

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