Artist rendering of Darwinius.
Image: Julius T. Csotonyi
Last year's publication of the fossil primate Darwinius masillae claimed it to be the oldest haplorhine primate ever discovered and a multimedia blitz campaign touted the find as the ultimate "missing link" (an erroneous term that should forthwith be forbidden to all science journalists). Brian Switek at Laelaps (who has an excellent review of this paper) made headlines for challenging the way that this fossil primate was rushed to market, and it seems that his concerns were more than justified.
According to Brian's Op-Ed in the Times of London:
Over the past two years they have worked with Atlantic Productions to launch a media blitz heralding Ida as one of our early relatives. With a scientific description in the journal PLoS One, a book, two documentaries, a website and even a Twitter feed prepared beforehand, Ida burst onto the scene as the "holy grail" of evolution, the "ancestor of us all".Ida is undoubtedly a spectacular fossil. A nearly complete fossil primate, with a body outline and stomach contents, she is the sort of discovery palaeontologists dream about. It may come as a surprise, then, that Ida does not change everything we thought we knew about human evolution. Indeed, she may tell us more about the origins of lemurs than our own species.
Now, researchers at Duke University, University of Texas at Austin, and the University of Chicago have demonstrated that Ida is in fact more similar to other fossil strepsirrhines, the primate group that includes lemurs and lorises.
As the researchers wrote in the Journal of Human Evolution:
The lack of clear synapomorphies linking Darwinius to living and fossil haplorhines, the undisputed positive evidence that it is an adapiform, and the detailed evidence that adapiforms are stem strepsirrhines, suggests that Darwinius has little relevance for understanding haplorhine evolution.
To translate some of this jargon for you, what it means is that in order for Darwinius to have been an ancestor of Homo sapiens (or, at least, a member of our suborder) there would need to be shared derived characters between Darwinius and multiple taxa in the haplorhines (the new world and old world monkeys and apes) that don't appear in the strepsirrhines.
These shared derived characters are different from simply "primitive" characters (otherwise known as symplesiomorphies). For example, humans and many salamanders have five toes while horses have only one. This can't be used as evidence that humans are more closely related to salamanders than to horses. The common ancestor of all terrestrial vertebrates had five toes and both humans and some salamanders have retained this number. However, derived characters are ones that only exist in a given lineage. Hair is a feature that exists only in mammals and is therefore characteristic of our class.
Franzen et al. in their analysis of the fossil in PLoS make clear that Darwinius was a member of the primitive primates known as the adapiforms. What this latest paper has done is show that there is greater evidence to suggest that the adapiforms have more derived characters with the strepsirrhines than with the haplorhines, therefore making it unlikely that Darwinius is part of our lineage. None of this changes the fact that this is a gorgeous fossil and that it helps to answer key questions about primate evolution. Naturally, if we go back even further, this fossil shares derived characters that connect both suborders into the order primates. We're still related, but less directly.
This latest chapter in the Darwinius drama brings with it an important lesson. One should be very careful about announcing that a given fossil is the oldest human ancestor until other paleoanthropologists have had an opportunity to interrogate their evidence. Even then it's best to maintain such conclusions as tentative.
Reference:
Williams, B., Kay, R., Christopher Kirk, E., & Ross, C. (2010). Darwinius masillae is a strepsirrhine--a reply to Franzen et al. (2009) Journal of Human Evolution DOI: 10.1016/j.jhevol.2010.01.003
Life Science
Comments
Thanks for the link (no pun intended), Eric! I should also note that we have had solid evidence that Darwinius really was a strepsirrhine since last October (see this post for details).
Even more exciting, though, is that some of the researchers behind the new study have a summary of early primate evolution coming out in PNAS soon. Now that will be interesting.
Posted by: Laelaps | March 3, 2010 2:11 PM
It is unfortunate it got so much attention as a human ancestor but the paleoanthropologists had debated the place of adapiforms and resolved it to strepsirrhine a long while back. This just reaffirms it. One of the lead authors of the PLoS One paper was a major proponent of the other theory, that is why it was rehashed. I got an earful from my supervisor the day the PLoS One paper came out, I'm just glad the community (scientific and otherwise) has something to site that correctly positions Ida. Though it is to bad it may get twisted.
Posted by: Lfulvipes | March 3, 2010 4:13 PM
@Laelaps: Yes, I hadn't heard about the October news concerning Darwinius so I was excited to learn about this. Plus I know Richard Kay pretty well. He was in my department and was one of the main instructors for human gross anatomy. We would chat about primate evolution and the politics of the NSF while separating the fascia from a levator muscle.
Posted by: EMJ | March 3, 2010 9:02 PM