Evolving Thoughts

New ape fossil

The National Geographic and the news services are touting a new ape fossil found in Ethiopia as “forcing a rethink on human evolution”. As usual, the headlines are hyperbolic. This ape is fragmentary, and so far only teeth and a jaw bone have been found, and the teeth are similar to gorilla teeth. Gorillas are thought on molecular grounds to have split off from the chimp-human clade about 7 million years ago, but this specimen is 10 million years old. What gives?

I can think of a couple of options. One is that, as I have reported previously, teeth are not great diagnostic material for phylogeny. They are very adaptable, and if a species finds itself in a similar environment with a similar diet, the teeth will tend to evolve rapidly to a particular morphology similar to that of relatives who later evolve in the same situation.

Another is that the ancestral state (called a plesiomorphy, from the Greek for “neighbour” + “form”) may have had teeth rather like this, and the chimp/human dental formula is the derived one. Non-human ape fossils are rather rare, so we may not know this. If it is true, then this species may have diverged millions of years before the chimp/human lineage split from the gorilla lineage.

One of the things that greatly bothers me about how the media frame this sort of thing, is that every revision to what is, after all, a historical claim rather than a theoretical one about the process by which we or any other species evolved, is called a “major reformulation”. It’s as if the origins of the United States was found on the basis of new evidence to have derived from German-speaking settlers rather than English speaking ones; the story about the origins of the USA would change, but not the idea that the USA arose in a historical fashion. Creationists of course leap onto these headlines as evidence that evolution isn’t true, when in fact this is further evidence that the evolutionary model is fruitful and sensitive to evidence. The USA did not exist for all time, and neither did the present ape species.

History is hard to grasp if the evidence is not there. Finding out that Washington never chopped down the cherry tree ought not cause people to deny that Washington existed, and finding out that things occurred somewhat differently to earlier hypotheses in human and ape evolution doesn’t undercut the idea of evolution itself. New evidence is welcomed in biology, if not in religiously motivated ideologies.

Late note: John Hawks has a nice review of the paper here.

Comments

  1. #1 chris y
    August 23, 2007

    I’m increasingly confused about the usage of “hominid”. I had thought that these days it essentially meant “the clade including Homo and Pan“, but Hawks seems to be using it to mean “the first species in the clade including Homo to diverge from the clade including Pan, and all its descendents”. Can you explain which is the better usage these days, and why?

    Also, if you run with the “Homo and Pan +” definition, does this imply “LCA of Homo and Pan and all its descendents”, or “the first species in the clade including Homo and Pan to diverge from the clade including Gorilla, and all its descendents”? If the latter, Sahelanthropus etc. could presumably be hominids without necessarily being ancestral to either people or chimps.

  2. #2 John Wilkins
    August 23, 2007

    From here:

    “When the classification system changed to include apes in the human lineage (Hominidae), the term Hominid came to include apes and humans. Today, when talking about the human lineage and its ancestors, we use the term Hominin. Older publications that use the term Hominid are usually refering to the human lineage only.”

    This means that “hominid”, being the vernacular of Hominidae, includes great apes and humans. Wiki agrees.

    As to how clades are specified – the diagnostic criterion can be “stem based” (first species that split from a sister clade) or “crown based” (the LCA of two or more extant species), but the reality they both are intended to refer to is the first species of the clade and all descendants. I think of clade names as being inherited from that first species, whether or not it had the typical characteristics of the clade.

  3. #3 Laelaps
    August 23, 2007

    Although I’m not suggesting that the teeth are not genuine, when I saw some of the headlines the first thing that popped into my mind was Osborn’s Hesperopithecus.

  4. #4 RBH
    August 23, 2007

    I sent an email to National Geo News a few minutes ago:

    A recent headline: “New Fossil Ape May Shatter Human Evolution Theory.” Oh really?

    Has National Geographic News been bought by Rupert Murdoch and converted to “all tabloid all the time”? Did the headline writer do his internship (after graduating from a Bob Jones University journalism course) writing headlines on alien abductions and Bat Boy for the Weekly World News? With “friends” like you, the creationists are guaranteed a rich supply of quote mines.

    Is there an editor in the house?

  5. #5 lunartalks
    August 23, 2007

    Headlines (in my day) used to be written by sub editors, not by the article author. Subs are often not expert at anythng, except being subeditors – hacking copy down to length and thinking up a snappy, attention grabbing headline (also drinking, smoking and being short tempered). You would hope that in a mag with kudos of NG that this would not be the case.

  6. #6 John Wilkins
    August 23, 2007

    So you worked at the same newspaper I did? I was the copyboy in the subs room, and saw first hand what went on. But they had an excuse – a daily deadline. NG doesn’t, and you’d hope the subs there were at least mildly literate in science.

  7. #7 amphibious
    August 24, 2007

    I’ve only just foud this site, very interewsting thus far.
    How does the Aquatic Ape theory grab you? Not the popularised drivel put up to knock down but the relatively brief sojourn on the littoral?

  8. #8 John Wilkins
    August 24, 2007

    The Aquatic Ape hypothesis as published includes selectionist explanations of things that a littoral (shallow wading) version need not explain, like adipose tissue and streamlined hair follicles. I think it is selectionism gone mad. Sure we’ll eat marine food if we can get it. I doubt it had much to do with human evolution, though.

  9. #9 David Marjanovi?
    August 26, 2007

    I’m increasingly confused about the usage of “hominid”. I had thought that these days it essentially meant “the clade including Homo and Pan“, but Hawks seems to be using it to mean “the first species in the clade including Homo to diverge from the clade including Pan, and all its descendents”. Can you explain which is the better usage these days, and why?

    Harr! Classifications are “not even wrong”. “Hominidae” is defined as “the family to which Homo belongs”, and “family” is not defined.

    The trend is to enlarge Hominidae; I’ve seen classifications where it included all apes, even the gibbons (as subfamily Hylobatinae). But again, this is an esthetic judgment; there’s no science about it. Nomenclature is a convention, not a science.

    Also, if you run with the “Homo and Pan +” definition, does this imply “LCA of Homo and Pan and all its descendents”, or “the first species in the clade including Homo and Pan to diverge from the clade including Gorilla, and all its descendents”? If the latter, Sahelanthropus etc. could presumably be hominids without necessarily being ancestral to either people or chimps.

    Wow, you’re asking for a phylogenetic definition! As far as I know, Hominidae has never got one. And if it has, nobody is obliged to use it: the ICZN doesn’t recognize phylogenetic definitions, and the PhyloCode has not yet been implemented. So, if you want an unambiguous meaning for Hominidae, wait.

  10. #10 Jim Thomerson
    September 6, 2007

    So far as I can see the code does not say anything about how families are defined. The general thinking among zoologists is cladistic (people working on humans and their relatives seem not to be up to speed, so to speak). To put humans and chimps in different families is simply bad taxonomy. End of discussion. I have seen the Hominidae defined phylogenetically, as indeed I think it should be. I think it should include all the great apes. This does require a shift in thinking about what to call the members of the human only lineage because Hominidae is more broadly redefined. Incidentally redefinition of taxa because of increased knowledge of a group can be confusing to anyone who does not follow that particular literature closely.

  11. #11 John Wilkins
    September 7, 2007

    I know that Phylocode isn’t obliged to follow Linnaean priority rules, but so far as the Linnaean classification goes, since Linnaeus himself included the great apes under Homo (a genus, but still), shouldn’t we?

  12. #12 Jim Thomerson
    September 10, 2007

    Linnaeus included the chimpanzee under Homo, but not the bonobo, gorilla or orang. ICZN in 1988 put chimpanzee in Pan. One could include the chimp + bonobo in Homo and still have a monophyletic group. My own thinking is that genera ought to be homogenious and diagnosable, and respect history (whatever that means). If we were talking fish, I would put humans in one genus and chimp + bonobo in another. I think the morphological diferences are great enough to justify that, regardless of genetic similarity. I’m not evolved enough in my thinking to abandon the Linnaean system.

    We have a situation in killifish where two DNA studies agree amazingly well on a monophyletic group. However the morphological diversity in that dozen or so species is about the same as the range in the suborder. Interestingly enough, this correlates with very long limbs in the DNA trees for that group. We have placed sister species in different genera based on morphology, and I’m OK with that. Morphological analysis of the group splits up that particular DNA lineage all over the place. It is, I think, a stellar example of lack of correlation of morphology with relationship.

  13. #13 John Wilkins
    September 10, 2007

    Jim, Linnaeus didn’t discuss the bonobo, as it wasn’t identified until the 20th century. The gorilla wasn’t discovered until the mid-19th. And he did include the orangutan under Homo troglodytes, along with the chimp (p33, vol I of the 12th edition).

    Pan was created as a genus by (I think) Blumenbach around 1790 because the idea that there was a genus that we shared with an ape or two was objectionable on religious grounds. Granted that we have a lot more great ape species now, given that he named a genus in which the great apes as known then were included, Hominoidea ought to include them.