Monday, October 5, 2009

Ardipithecus redux

By now, you've probably already heard about the collection of papers on Ardipithecus ramidus published in Science. Ardipithecus is a fossil hominid, with skeletal features that indicate that it could walk upright or climb trees. Ardipithecus is dated to about 4.4 million radiometric years ago, and the authors of the Science articles interpret as the ancestor of Australopithecus, which in turn were the ancestors of our genus, Homo. Ardipithecus is considered to be somewhat close to the last common ancestor of humans and chimpanzees (which is supposed to have lived a couple million years earlier).

It's kind of strange that all these papers are being published now, fifteen years after the initial fossils were announced in Nature (see White et al. 1994). Usually, the follow-up studies are published in specialized journals, rather than more generalized journals like Science and Nature. Even more unusual is that each of the technical articles is accompanied by a brief, nontechnical summary. Perhaps in this anniversary year of Darwin, the Science editors saw this as an opportunity to educate the public about human evolution? That would explain the AAAS ad banners on the Ardipithecus website that proclaim "1 in 3 Americans doesn't accept evolution.... Join us. Together we can make a difference."

In any event, two creationists already commented on Ardipithecus more than a decade ago, so this commentary is going to be easy. In Mere Creation, Sigrid-Hartwig Scherer comments on the systematics of Ardipithecus in terms of basic type biology. In a paper published in Creation Ex Nihilo Technical Journal, Kurt Wise tried to contextualize the finds in terms of creationist geology. I'll begin with the biology.

The basic type model posits that species are classified in "basic types" that are identified primarily by hybridization: species that can cross (or cross with the same third species) belong to the same basic type. According to Hartwig-Scherer (1998, p. 215), "Basic type biology assumes that basic types originated by design.... A single population of the ancestors of each basic type gave rise to all known extant and extinct species." She recognizes at least two basic typesof hominids: Australopithecinae (all the Australopithecus species and some of the habilines) and Homininae (H. erectus/ergaster, modern humans, and Neanderthals).

On the subject of Ardipithecus, she wrote (p. 222),
Whether Ardipithecus ramidus should indeeed be included into the basic type Australopithecus or whether it belongs to a separate basic type has yet to be shown.

She does, however, consider it at least "possibly" (p. 221) a member of the australopithecine basic type. So let's go with that as a tentative conclusion: Ardipithecus ramidus is a member of the same basic type (i.e. baramin) as the Australopithecus species.

Hartwig-Scherer's assessment is consistent with Wise's (1994): preliminary suggestion is that A. ramidus is an ancestor of the group of apes called Australopithecines, and is in no way related to humans.

Wise interpreted Ardipithecus as post-Flood, the argument for which is more than I want to cover in this post (for a recent discussion of post-Flood sediments, see Whitmore and Garner's paper in the 2008 ICC Proceedings). Wise argued that the immediate post-Flood period was a time of rapid biological innovation, as a large number of new species began to emerge from the few survivors of the Flood. He called this intrabaraminic diversification, meaning the production of diverse species within a created kind (basic type or baramin).

Wise wrote,
At the same time as species were undergoing explosive intrabaraminic diversification, explosive intrabaraminic diversification, the earth was reeling from the recent global catastrophe through several centuries of residual catastrophism. Episodic, local, geologic catastrophes would be fossilizing snapshots of this biologic change as it was occurring, providing for us, in the Cenozoic, a motion picture of several hundred years of post-Flood biologic change. The result of this earth history scenario is that the Cenozoic sediments are likely to show:
(a) a lack of interp-specific transitional forms;
(b) species stasis;
(c) rather common species series with morphological trends which may be parallel among unrelated groups (because of similar selection pressures);
(d) high species diversity in fossil series; and
(e) relative rarity of non-stratomorphic-series stratomorphic intermediates.

I'm not going to rehash all of Wise's discussion of Ardipithecus (for that you should read the original paper), largely because it applies far beyond just Ardipithecus. Instead, I'll just leave you with a final quote from his follow-up letter from 1995:
As I indicated in my earlier paper fossil series are often characterised by a rather high number of confusing species, each showing mosaic and homoplasous characters.... Such high levels of species diversity, homoplasy, and mosaic characters continue to deepen the challenge to evolutionary models and strengthen the idea of high rates of intrabaraminic, post-Flood diversification along the lines of pre-programmed, latent morphotypes.

What does that mean? Homoplasy is an anatomical (or otherwise) similarity that is not the result of common ancestry (i.e., it's not homologous). Homoplasy makes it difficult to infer an evolutionary tree, since many trees can be consistent with the same set of characteristics. If a lot of new species originate very quickly, then we would expect lots of homoplasy, and we do see that in the australopithecines (and in Ardipithecus also, not to mention the horses and just about every other Cenozoic mammal series).

Long story short: Ardipithecus is a post-Flood animal species that is a member of the australopithecine created kind. There's a LOT more details to be filled in to this story, but that's the basic outline that we're working with now. Aren't fossils fun?

Hartwig-Scherer. 1998. Apes or Ancestors? Interpretations of the hominid fossil record within evolutionary & basic type biology. In Dembski, ed. Mere Creation. InterVarsity, pp. 212-235.

White et al. 1994. Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371:306-312.

Whitmore and Garner. 2008. Using suites of criteria to recognize pre-Flood, Flood, and post-Flood strata in the rock record with application to Wyoming (USA). In Snelling, ed. Proceedings of the Sixth ICC, pp. 425-448.

Wise. 1994. Australopithecus ramidus and the fossil record. CENTJ 8(2):160-165.

Wise. 1995. A note on new australopithecines. CENTJ 9(2):167.