Apr 20 2011

A New Hominin – A. sediba

Following the branching bush of human evolution is getting increasingly difficult. When I studied human evolution in college, things were much simpler. There were a few Australopithecus species followed by a few Homo species, leading to modern humans. It was recognized at the time that these fossil species probably did not represent a nice clean straight line to Homo sapiens, but it seems the family tree has become much bushier than was imagined at the time.

Here is a recent representation of the hominin family tree. We have added more species of Australopithecus and Homo, plus new genuses of Kenyanthropus and Paranthropus (not even including older genuses that predate Australopithecus).

Now researchers have announced the discovery of yet another species of early hominin, about 2 million years old – likely a late species of Australopithecus named A. sediba. They discovered four individuals – two adults, a child and an infant, who likely fell into a “death trap”  in a cave in what is now Malapa, South Africa.

Each bit of fossil evidence is like a piece to a complex puzzle. As more pieces fit into place, however, the picture becomes more complex and more questions are generated. We are still at the stage where new evidence generates more questions than answers, and we have no idea how complex the final picture that emerges will be.

The new discovery is no exception. A. sediba has a mixture of modern (Homo) and primitive (Australopithecine) traits. It has a small brain like a primitive Australopithecus, but has pelvic structure and hand features that are more modern than other members of the genus.

It should also be noted that the first members of the Homo genus arose about a million years before the age of these specific specimens – so these individuals do not represent a population that in ancestral to our genus.

As always, there are multiple ways to interpret this data. It is possible that A. sediba is the ancestral Australopithecine species that led to Homo – either directly, or closely related to that species (yet to be discovered). In this case, these individuals would be later representatives of that species. Species often persist, even for millions of years, after other species branch off from them. So it is always possible to find representative of an ancestral species that are more recent than species that evolved from them.

It is also possible that A. sediba is a separate line of Australopithecines that did not lead to Homo, but developed some similar features. In this case the “modern” features in A. sediba would be analogous to (similar to, but not ancestral to) the modern feature, rather than homologous to (related through evolutionary derivation) the modern Homo features.

Another possibility that was not mentioned in the Science article that I linked to is that these individuals, and possibly A. sediba as a species, or perhaps just one breeding population, represent the results of interbreeding between Homo and Australopithecus species. In this case modern features would have literally mixed with the more primitive features together in A. sediba.

This adds a new layer complexity to our picture of the human family tree (or any family tree). When species divide the separation is not clean, and later remixing of genes is not only possible but probable. There is genetic evidence, for example, of later mixing of genes between human ancestors and chimpanzee ancestors after the split. So it’s not a stretch to think that hominin populations were at least occasionally interbreeding .

I suspect there are many more hominin species and subspecies to be discovered. The picture that is emerging is fascinating, if it is becoming increasingly difficult to keep track of it all. I’ll just have to muddle through.

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20 responses so far

20 Responses to “A New Hominin – A. sediba”

  1. SARAon 20 Apr 2011 at 12:04 pm

    So you suppose our species killed off the rest of them? I think its odd that many of these hominin species co existed and but now none do.

    But far more interesting – Will some new species be spawned from us and wipe us out? Now that is an interesting thought.

  2. eeanon 20 Apr 2011 at 1:23 pm

    I wouldn’t suppose that. Sapiens were only comtemporary with a few other hominids and often on other continents. Probably just environmental change etc

    I wonder what would’ve happen if another hominid had survived to now. we’d either being warring nation states or more likely enduring enslavement of the less clever species. Would make a good alt-history. :)

  3. locutusbrgon 20 Apr 2011 at 2:18 pm

    Pardon my ignorance, but how do they piece together a separate species based upon a single group’s fossils? DNA, or is it just accepted in anthropological circles that small groups can justify a different species. My point is, in the distant future will Shaquille O’Neal, and Gary Coleman be classified as different species, if they were found? The article spoke about further review or debate, but did not really enlighten me as to how.

  4. kakaydinon 20 Apr 2011 at 2:34 pm

    eean,
    You might appreciate “Hominids” by Robert J Sawyer (link below). Not an alt-history so much as a simultaneous history embracing the “many-worlds” interpretation of quantum mechanics.

    The story, which juxtaposes the cultures of modern humans and a race of neanderthals that have progressed to roughly equal technological ability, exposes and explores many idiosyncrasies of humanity that we often take for granted.

    http://www.amazon.com/Hominids-Neanderthal-Parallax-Robert-Sawyer/dp/0312876920

  5. Jim Shaveron 20 Apr 2011 at 3:22 pm

    Thanks for the article, Dr. Novella, and congratulations regarding your appointment as Senior Fellow and Director of the JREF’s new Science-Based Medicine project! I hope this project and your association with it continues to increase public exposure to the truth about So-Called Alternative Medicine.

  6. eeanon 20 Apr 2011 at 4:30 pm

    @locutusbrg from what I understand, anthropologists waste/spend a lot of time arguing if such-and-such find is a new species or not (when of course the concept of species is just an abstraction for nomenclature and not a real entity). Anyways thats a real problem, and certainly the fact a slight tweak in genes and you have Gary Coleman doesn’t make things easier. Some scientists were claiming essentially that for Homo floresiensis.

    @kakaydin thanks for the rec!

  7. HHCon 20 Apr 2011 at 5:45 pm

    The interbreeding of hominin populations with chimpanzees makes sense based on Jane Goodall’s work with chimpanzees. She documented chimpanzee reasoning skills and a need for dominance to be similar to humans and maintained their gene pool was shared.

  8. locutusbrgon 21 Apr 2011 at 10:08 am

    eean
    Thanks! Appreciate the feedback.

  9. HHCon 23 Apr 2011 at 11:25 am

    The hominin representation is tiny and not clear upon magnification. Could you enlarge/ clarify it?

  10. BillyJoe7on 23 Apr 2011 at 5:19 pm

    Actually, it is easily readable on magnification.
    It might be your eyes or you screen.

  11. Jeremiahon 23 Apr 2011 at 6:27 pm

    Add Hominin?

  12. ccbowerson 23 Apr 2011 at 6:59 pm

    I wonder how many “species” of dogs we would conclude from the varieties of dogs that we have today if we took the same approach and had no/few modern examples.

  13. eeanon 24 Apr 2011 at 6:42 am

    @ccbowers situation isn’t totally analogous since dogs don’t make tools.

  14. ccbowerson 24 Apr 2011 at 9:38 am

    @eean

    If something were “totally analogous” it wouldnt be an analogy. Not to mention that I didn’t intend it only as an analogy, but just a related but separate question

  15. HHCon 24 Apr 2011 at 12:03 pm

    ccbowers, Wild dog was also found at the Malapa anthropological site.

  16. colluvialon 26 Apr 2011 at 7:01 am

    It would be great to see more fossils from before the common ancestor with chimpanzees. Why do we hear so little about that?

  17. Michael Meadonon 26 Apr 2011 at 9:43 am

    Steve, I might just be mis-reading your post, but you seem to have forgotten that A. sediba was announced about a year ago. (You talked about it on SGU, if I recall!). What’s new here is Berger et. al.’s revelation that they found two more individuals at the same site.

  18. Michael Meadonon 26 Apr 2011 at 9:46 am

    (By the way: while it’s correct to say A. sediba is a hominin, it’s more specific to say it’s a hominan. The terminology has changed quite a bit of late. Hominan means what hominid used to mean – i.e. the “human” branch of the apes).

  19. brian7777on 30 Apr 2011 at 8:07 pm

    Interesting article. But I have a question regarding branching species over evolutionary time. This passage frames the issue:
    “When species divide the separation is not clean, and later remixing of genes is not only possible but probable. There is genetic evidence, for example, of later mixing of genes between human ancestors and chimpanzee ancestors after the split.”
    I don’t understand how we determine where one species ends and the other begins if there is still interbreeding that results in fertile offspring. If two populations become reproductively isolated from each other, when do we consider them separate species? I would have thought that if they demonstrated interbreeding with fertile offspring we would by definition consider them a single species.
    I’m a fairly avid consumer of science writing aimed at the science enthusiast. This is an issue I’ve wondered about many times.
    Thanks for the blog. Long time reader, first time poster.

  20. BillyJoe7on 02 May 2011 at 6:09 am

    brian7777,

    “I don’t understand how we determine where one species ends and the other begins”

    The answer is that it is actually impossible to do so.

    It’s as impossible to determine when speciation occurs as it is impossible to determine when a child becomes an adult. Sooner or later, however, the child DOES become an adult. And sooner or later, a new species IS formed.

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