If you’ve been following the evolution blogs, you’ll know that the wonderful primate fossil Darwinius masillae was touted by its discoverers (and by the book The Link) as a key “transitional form” in human evolution: a link between the two major branches of primate evolution, the anthropoids on one hand and the lemurs and lorises on the other. Many bloggers who read the initial paper were dubious, asserting that the phylogenetic analysis was too sketchy to put Ida in this position (see here, here, here, and here). I’m not a paleontologist, but agreed with the criticism that Ida’s placement as a “missing link” was premature.
Well, it looks as if the bloggers were right and the scientists who wrote up Ida were wrong. A paper in today’s Nature describes a new fossil (Afradapis) from Egypt that appears to be in the same clade (single-origin group of related species) as Darwinius. The paper is full of technical detail, but the upshot is this: the dental and jaw features of Darwinius (and Afradapis) that made her discoverers group her with anthropoid apes are probably not homologous to those of anthropoid apes, but convergent (i.e., the traits don’t show that Ida belongs with the anthropoids; instead, these features evolved at least twice in two unrelated groups). As the authors say:
It has long been known that some adapiform lineages evolved derived morphological features that are also seen in living and extinct anthropoids (for example, fused mandibular symphyses, upper canines with mesial grooves, enlarged and spatulate upper and lower incisors, short and tall rostra)16. The phylogenetic significance of these features has been a source of ongoing debate for decades. Of all known fossil prosimians (including Darwinius), Afradapis provides perhaps the most detailed examples of derived anthropoidlike adaptations in its dental and mandibular morphology. As isthe case for many of the morphological features that some have argued link adapiforms to anthropoids, however, the anthropoid-like features of Afradapis (fused mandibular symphysis with transverse torus, deep mandibular corpus, deep masseteric fossa, large upper molar hypocones, absence of P2/2 and presence of an enlarged P3 with a honing facet for the upper canine) are not present in the most primitive undoubted fossil anthropoids, such as Biretia and Proteopithecus, indicating that the features are likely to have been acquired through convergent evolution.
In the end, both Afradapis and Darwinius appear to belong to the group of primates known as adapiforms, a group that went extinct without leaving descendants. For all its beauty, then, the Darwinius fossil is not a link to any living species.
There is lots of publicity about this new finding: see the Times online commentary by Brian Switek, another Times piece by Brian Henderson, and a piece in The Guardian. It’s refreshing to see the refutation of hyped-up claims about science given as much space as the original hype. Let’s hope that American papers such as The New York Times follow suit.
Curiously, today two “formal corrections” appeared in the PLoS paper. The first says this, in part:
The following subsection should be added beneath the Methods subsection “Terminology”:
The electronic version of this document does not represent a published work according to the International Code of Zoological Nomenclature (ICZN), and hence the nomenclatural acts contained herein are not available under that Code from the electronic edition. A separate edition of this document was produced by a method that assures numerous identical and durable copies, and those copies were simultaneously obtainable (from May 21st 2009) for the purpose of providing a public and permanent scientific record, in accordance with Article 8.1 of the Code. The separate print-only edition is available on request from PLoS by sending a request to PLoS ONE, 185 Berry Street, Suite 3100, San Francisco, CA 94107, USA along with a check for $10 (to cover printing and postage) payable to “Public Library of Science”.
Presumably PLoS One is not considered a valid venue for publishing descriptions of new species. Does this have something to do with its less-then-complete peer review?
The second correction is this:
The authors have supplied an updated Competing Interests statement, which reads as follows:
The authors wish to declare, for the avoidance of any misunderstanding concerning competing interests, that a production company (Atlantic Productions), several television channels (History Channel, BBC1, ZDF, NRK) and a book publisher (Little Brown and co) were involved in discussions regarding this paper in advance of publication. However, to clarify, none of the authors received any financial benefit from any of these associations and these organizations had no influence over the publication of this paper or the science contained within it. The Natural History museum in Oslo will receive some royalty from sales of the book, but no revenue accrues to any of the scientists. In addition, the Natural History Museum of Oslo purchased the fossil that is examined in this paper, however, this purchase in no way influenced the publication of this paper or the science contained within it, and in no way benefited the individual authors.
This is a tad disingenuous, since “benefit” to scientists includes far more than money: it includes (or included) all the hype and buzz around the initial description of Ida as a “missing link” — publicity that of course redounds to a scientist’s career. And, of course, money that goes to the Museum of Oslo also benefits any author associated with that Museum, even if he/she doesn’t get the dosh directly.
h/t: Greg Mayer and Daniel Matute
Erik R. Seiffert, Jonathan M. G. Perry, Elwyn L. Simons & Doug M. Boyer. 2009. Convergent evolution of anthropoid in Eocene adapiform primates. Nature 461:118-1122.