The exceptionally preserved skeleton of Darwinius, known popularly as “Ida.” From PLoS One.
Almost ten months ago an international team of researchers introduced the world to an exquisitely-preserved primate from the 47 million year old oil shales of Messel, Germany. Dubbed Darwinius masillae, and nicknamed “Ida” and “The Link”, the fossil was touted as one of our earliest primate ancestors in a massive media campaign worthy of a Hollywood blockbuster. Yet the trouble was that there was no solid evidence that Darwinius was one of our ancestors. Despite the marketing blitz promoting the fossil the team of scientists who described it did not provide sufficient evidence that the lemur-like primate was anywhere close to our ancestry, and it would take the description of a related fossil primate several months later to put “Ida” in her place. Darwinius was not one of our ancient progenitors, as had been proclaimed, but instead belonged to an extinct branch of early primates which were more closely related to living lemurs and lorises.
Now another team of early primate experts has published a new analysis of the famous fossil. Writing in the Journal of Human Evolution paleontologists Blythe Williams, Richard Kay, Christopher Kirk, and Callum Ross have independently confirmed that the original description of Darwinius which appeared in the journal PLoS One was deeply flawed. Understanding why, however, requires a bit of background.
For over a century scientists have known that there are two major divisions among living primates: the haplorrhines (“dry nosed primates”) and the strepsirrhines (“wet nosed primates”). The former group contains tarsiers plus anthropoid primates (monkeys and apes) while the latter is composed of lemurs, lorises, and bush babies, and independent lines of evidence have confirmed that these groups diverged tens of millions of years ago. Because of this we know that fossil primates from at least the past 55 million years must have fallen into one group or another, but the description of Darwinius aimed to throw a monkey wrench into the scientific consensus of early primate relationships.
As recognized by the scientists who described it, Darwinius was an adapiform primate which belonged to a now-extinct group of lemur-like species. Although adapiformes had once been contenders for the role of anthropoid ancestors during the 1970’s and 1980’s they have since been recognized as strepsirrhine primates, meaning that there was no way they could be the ancestors of monkeys (and hence apes). Despite this change, however, the authors of the Darwinius paper attempted to rehabilitate the status of adapiformes by proposing that Darwinius possessed more traits in common with haplorrhines than strepsirrhines. This would not necessarily make it one of our ancestors, but it would place it one our side of the family tree and make it more reasonable to think of it as a close relative of the earliest anthropoid primates.
This hypothesis was refuted several months later when an independent team of researchers described another 37 million year old adapiform they named Afradapis. Though not nearly as complete as Darwinius, there was enough of Afradapis to determine its relationship to other fossil primates, and the authors of the new study used this as an excuse to parse the primate evolutionary tree by comparing 360 traits across 117 living and extinct primates. Not surprisingly, the results of these comparisons were more consistent with the established consensus of early primate relationships, and the scientists ascertained that the confusion about the relationship of Darwinius to other primates may have been caused by the fact that it possessed independently evolved traits seen in later anthropoid primates (but not early ones). If you were to compare Darwinius to only living primates you might think Darwinius was a close relative of anthropoids, but the actual earliest known fossil anthropoids did not share the same features.
A simplified evolutionary tree of primate relationships showing the placement of Darwinius in relationship to other groups. From Williams et al., 2010.
The new paper by Williams, Kay, Kirk, and Ross contributes to this understanding by considering how early primates are identified as either haplorrhines and strepsirrhines. Early representatives of both groups can look awfully similar to each other and lack many of the tell-tale characteristics that can be useful in telling their living relatives apart. In order to assess the relationships of Darwinius the authors of the new study provide a rundown of tell-tale characteristics that can been seen in its skeleton and what those traits mean for its relationships to other primates.
One of the most important features in determining the place of Darwinius can be seen in the skull, or rather, it can’t. Haplorrhine primates have a plate of bone behind the eye which strepsirrhines lack, and Darwinius does not appear to have this trait. And, curiously enough, Darwinius possesses a different trait that is missing in early haplorrhines. The original description of Darwinius stated that its two lower jaw bones were partially fused, but though this trait is seen in living haplorrhine primates it is not seen in early haplorrhine primates. The fusion of the lower jaws in Darwinius cannot be taken as a sign of affinity to haplorrhines because it is indicative of convergent evolution, not a close genetic relationship.
A more detailed primate family tree with Afradapis and Darwinius underlined in red. Lemurs are underlined in blue. Anthropoids are underlined in green. From Seiffert et al., 2009.
Combined with other traits from the skull, teeth, and limbs, the new analysis paints a rather damning picture of the original Darwinius study. The only way that Darwinius could be considered a haplorrhine (and thus a potential anthropoid ancestor) would be to consider it entirely out of context from other fossil primates. It was clearly a part of a radiation of strepsirrhine primates, and as the authors of the new paper conclude:
The lack of clear synapomorphies [shared derived characteristics] 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 some such statement might seem superfluous. After all, didn’t the description of Afradapis already put Darwinius in its place months ago? Yes, but it is still important to have independent teams of researchers appraise the same data. Even though the hypotheses presented in the Afradapis paper confirmed what many scientists had suspected they still required independent confirmation, and now Williams, Kay, Kirk, and Ross have supplied it. (Plus, as shown by the dates supplied on the paper, the new study was submitted almost four months before the publication of the Afradapis paper so I certainly cannot blame the researchers for submitting a paper that came to convergent conclusions.)
What I am still waiting for, however, is a thorough analysis of the relationships of Darwinius by the team that originally described it. One of the ringleaders of the media circus surrounding the primate, paleontologist Jorn Hurum, promised that the initial description was just the first of many papers that would put Darwinius in context. To date no additional papers have appeared, and given the inadequate interpretation of Darwinius in the original paper I cannot help but wonder how Hurum and his colleagues can conceivably cast the adapiform as one of our close fossil relatives. I guess we will just have to wait and see.
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