Saturday, June 05, 2010

Casey Luskin on Ardipithecus

Casey Luskin thinks that Ardipithecus is "washed up." He writes:
In some ways, the career of a missing link mirrors the career of the celebutante. They break onto the scene with much fanfare and hype. Everyone is wowed—or at least, everyone pretends to be wowed so nobody can be accused of ruining the party. Besides, she’s useful for advancing lots of agendas. After a little while, people realize that the star doesn’t have all the talent everyone hoped for.
It is a little discouraging for a writer to use the phrase "missing link" when the idea has been so discredited for decades in the world of science. Palaeontologists have known for years that there is no such thing as a missing link. Recent phylogenetic systematic studies have further solidified this idea. Casey Luskin knows better. Onward. He continues:

Last fall “Ardi” came onto the missing link scene with a bang. The journal Science called her the “breakthrough of the year.” So did Time Magazine. We covered a few lone dissenters to the Ardi hype here on ENV.

But now Time Magazine is starting to go over apex of the hype curve. In an article titled, “ Ardi: The Human Ancestor Who Wasn't?,” Time notes, “Two new articles being published by Science question some of the major conclusions of Ardi's researchers, including whether this small, strange-looking creature is even a human ancestor at all.” Likewise, Nature reports, “Ardi may be more ape than human.”
White defends his analysis of Ardipithecus thus:
In an e-mail exchange with TIME, he says, "Dr. Sarmiento's views appear to be uniquely his own. Most notable in Dr. Sarmiento's comment is his refusal to recognize as significant the multiple and independently derived features of the Ardipithecus cranium, dentition, and postcranial skeleton. These features uniformly align this primate with all later hominids to the exclusion of any other ape — living or fossil. Has Dr. Sarmiento shown how the Ardipithecus evidence better fits his interpretation than the one we published? Not here."
White is not quite correct here. John Hawks also has similar misgivings, at least about the pelvis. He writes, in his blog:
During the seven months since I first detailed what I see as weak points in the pelvic description, I've become less and less persuaded that the pelvic features reflect any hominin-like locomotor adaptations in Ardipithecus. There are many unresolved functional issues, which obscure the phylogenetic relations between living and fossil apes. Ardi makes every tree less parsimonious, no matter which branch we put her on. Shoe-horning her into the hominins doesn't solve many problems, and creates some intractable ones.

I find myself calling her an ape.

Luskin, therefore, is not overstating his case. He concludes:
Discover Magazine is now saying "The bones of our ancestors do not speak across time with ultimate clarity." That's an understatement--but given how everyone previously fawned over Ardi's "missing link" status, could it be that there is more than mere science driving the promotion of these missing links?
This statement is neatly countered, however, by the last paragraph of the Time article:
While Sarmiento regards the hype around Ardi to have been overblown, Cerling says he still feels the discovery and re-creation of the ancient specimen to be a monumental breakthrough. But, he says, the science was in the evidence collected by White and colleagues, and not in their conclusions. "Many students will thoroughly examine the data and will come to their own independent evaluations," he says. In other words, science works a bit like evolution, and asking whether Ardi will survive as a major advancement is rather like going into the distant past and asking what the fate of her species would be: Only time will tell.
There is obviously considerable uncertainty in determining exactly what kind of ancestor Ardipithecus was to later hominids, if she was at all. As with every hominid species that has been discovered, only through painstaking analysis of the fossils will we be able to tell where she fits. I am reminded by the argument by Marcelin Boule that the Neandertal remains of La Chapelle-aux-Saints represented those of a pathological specimen of modern human. Given the wealth of Neandertal discoveries since then, we now know that Boule was wrong. It is still too early to tell if White and colleagues are right or wrong about Ardipithecus.

However, despite the fact that Ardipithecus may not be the hominid ancestor that we thought she was, she was at least an ape with derived characteristics toward the hominid line. While Ardipithecus may not have the "star power" we thought she would, she is a very important find, nevertheless.

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1 comment:

  1. Is it just me, or is there a peculiar lack of actual cladograms and actual parsimony analyses in the discussions of Ardi? I.e. reports of number of steps for the most parsimonious trees, CI and RI values, bootstrap supports, and Bremer supports, etc.

    IIRC The White et al. super-domination issue of Science advanced quite a major reinterpretation of human evolution, based on the idea that Ardi was on the human branch. And yet no quantitative cladistic analysis was presented. And ditto with both sides of the recent back-and-forth. Are all these people arguing like Mayrian evolutionary systematists using overall similarity and expert opinion about "important characters" to classify things? If they just ran a cladogram, the degree of support for various hypotheses could just be measured directly, if different parts of the body give different results well then that's the way it is until there is more data, or if it's unresolved then that's the way it is. If you have a bunch of closely-related forms with some missing data, lack of resolution shouldn't be shocking.

    Or am I missing something somewhere?