A few thoughts on "Uncovering Our Earliest Ancestor"

I just watched the BBC's documentary on "Ida" (Darwinius masillae), "Uncovering Our Earliest Ancestor", and to be honest I was not very impressed. Rather than dissect the show second-by-second, though, I will only discuss some of the main points that occurred to me while watching it. Heaven knows I have spent plenty of time on Ida already...

While I surely appreciate the show's efforts (however fleeting) to describe the paleobiology of Ida, the show's real hook is the conclusion that Darwinius is one of our earliest primate ancestors. This is a tantalizing hypothesis, but is it true? The documentary does not make a very convincing case.

Like other mass media sources, the documentary states that there are three traits that place Ida closer to anthropoid primates (monkeys and apes) than to lemurs: the absence of a grooming claw, the lack of a tooth comb, and an ankle bone (the talus/astragalus) that is almost exactly like our own. I will follow the pattern of the documentary in considering the absent traits together and the ankle bone separately.

So what of the grooming claw and tooth comb? Modern sterpsirrhine primates (lemurs, lorises, bush babies, and pottos) have both, but Ida does not. Since Ida does not possess these traits clearly she cannot be closely related to lemurs, or so the documentary tells us. The problem is that the program compares a 47 million year old fossil with living primates. We would not expect Ida to precisely match living lemurs, so the question is whether the ancient ancestors of modern strepsirrhines possessed these traits.

This leads question underlines a major program with the BBC program and this entire farrago over Ida in general. Outside of efforts to determine whether she was a new species of primate, Ida was not carefully compared to other fossil primates that lived during the Eocene (~56 to ~34 million years ago). Why this should be so puzzles me. It is true that the primate fossil record is largely fragmentary, but there are some species and genera that are known from more than just a few teeth and limb fragments, some of which come from the Messel site. (Indeed, these fossils of relatively complete legs and hips of other primates from Messel were important in determining Ida's sex. One of them is shown briefly in the program but is not mentioned by name. It is acknowledged that Ida lived alongside other primates but they are not mentioned in any detail.)

Believe it or not, Ida even has a close relative that is nearly completely known (and has been so since about 1920). I am speaking, of course, about Northarctus, a genus of lemur-like primate that lived in North America about 50 million years ago. It was an adapid, and the adapids are the group of fossil primates to which Darwinius also belongs.

If the adapids were not actually ancestral to lemurs they appear to be very closely related to them (It may be that adapids did not lead to modern lemurs but are a side-branch, the actual ancestors of lemurs having an earlier ancestor), but as Daniel Gebo wrote in his 2002 review of adapids for The Primate Fossil Record the difficulty is that the adapids exhibit relatively few definitive "lemur" traits. While aspects of their teeth and limbs link them closely to lemurs, for instance, they lack the grooming claw and tooth comb.

The lack of a grooming claw and tooth comb in other adapids means that Ida's lack of these traits is not really special. She would not be expected to have these traits. If Ida should be placed closer to anthropoids because of this lack, then why not Notharctus or other adapids?

In fact, the authors of the PLoS One paper describing Ida suggest moving all adapids to our side of the family tree (or making them haplorhines, in technical terms) based upon Ida alone. The difficulty with this is that no detailed comparison of Ida with other fossil primates, even relatively complete ones, was undertaken. You simply cannot shuffle the primate family tree around without doing so, much less on the description of only one genus.

If adapids are so important to determining Ida's evolutionary relationships, why were they left out of the show? I can only imagine that the adapids, and Notharctus in particular, were omitted because 1) they are not familiar to the viewing public, and 2) other relatively complete skeletons might somewhat diminish Ida's special position as the most exceptionally preserved fossil primate. In terms of point #2, I think Ida's status is secure, but I would imagine that she seems all the more spectacular compared to a handful of teeth than to a nearly complete skeleton of a similar animal.

So the tooth comb and the grooming claw do not hold up as evidence. What about the talus? According to the documentary it is so similar to our own that the idea that Ida may have even walked bipedally is briefly floated. As with the tooth comb and grooming claw, however, the ankle bone is not a slam dunk.

Given that Ida's skeleton is articulated on a flat slab her talus is somewhat obscured, but the documentary is very confident that Ida's talus is just like ours (even if the graphic that showed up at this point in the documentary didn't seem to bear this out). As Afarensis has recently pointed out the paper that describes Ida offers a few more details, but there is a bit of a problem. The talus is Ida also looks like the same bone in treeshrews, colugos, and the extinct plesiadapiformes, close relatives to true primates.*

*[I use the term "true" primates because there is some debate as to whether the plesiadapiformes should be considered primates or not. If they were some of the earliest primates, though, then the adapids were one group of the early "true" primates, or euprimates. The treeshrews and colugos are non-primates but closely related to the group primates as a whole.]

This means that the characteristics of Ida's ankle could have either been inherited from an earlier ancestor or might be a case of evolutionary convergence. I sincerely doubt, given the changes in the methods of locomotion our ancestors have exhibited during the last 47 million years, that the talus would remain entirely unchanged. To suggest as such would take some pretty compelling evidence, and even then it is not wise to place Ida closer to us than other primates on the basis of just one trait. Such a strategy may have worked in paleontology at the beginning of the 20th century, but it does not fly now. A more thorough analysis of Darwinius as compared to other primates is essential to figuring out Ida's place in the primate evolutionary tree.

All of this makes the BBC program quite bothersome. It would seem that, until the discovery of Ida, paleontologists working on early primates were essentially groping in the dark for something, anything, more than a few teeth. This is a false view, and one that robs Ida of her proper evolutionary and scientific context. If we are truly going to understand Ida we must compare her to other fossil primates, not just the present members of lineages that separated well over 50 million years ago. This sort of omission borders on dishonesty, and I would have expected better from the BBC.