This past May a 47 million year old fossil primate named Darwinius masillae, better known as "Ida", burst onto the public scene. The lemur-like creature was proclaimed to be the "missing link" and the "ancestor of us all", but the actual science behind Ida was drowned by a tide of media sensationalism. Press releases and documentaries proclaimed that Ida would "CHANGE EVERYTHING", but despite such promises the sky remained blue, my cats continued to wake me up at 5:30 AM, and the primate evolutionary tree did not suddenly restructure itself.
So what was Darwinius? According to the descriptive paper published in PLoS One, Darwinius belonged to a group of extinct lemur-like primates known as adapids and may or may not have been related to early anthropoids, the primate group to which monkeys and apes (including us) belong. The public announcements about Ida were far less reserved. A book, a pair of documentaries, and news reports proclaimed that Ida was definitely an ancestor of anthropoids and hence one of our early primate ancestors. (Jorn Hurum, the scientist who had purchased Ida from a fossil dealer for a sum close to $1,000,000 even went as far to say that Darwinius was "the closest thing we can get to a direct ancestor" of humans.) The fact that access to the scientific description of Darwinius was tightly controlled until after the media frenzy was initiated by Atlantic Productions meant that science took a backseat to hype.
Indeed, paleontologists who specialize in the study of early primates were not impressed by Darwinius. The fossil primate bore very little resemblance to the earliest known anthropoids, and critics soon found themselves fighting a battle on two fronts. The initial description of Darwinius, despite being much more reserved than the media hype, did not provide solid support that this primate was closely related to anthropoids. Much of the media coverage, by contrast, simply parroted unsubstantiated claims that Darwinius was one of our ancestors. Both the "strong" and "weak" interpretations of Darwinius had major flaws, and it was tricky responding to both versions of Ida's story.
Yet the public unveiling of Ida was hardly the last word on whether or not we could count her as a close relative. Quite the contrary; with the publication of her description real scientific debate had only just begun. The exchange of ideas which is the lifeblood of science continues today with the description of one of Ida's close relatives recovered from the 37 million year old rock of Egypt.
Published in Nature, the new study was led by Erik Seiffert of Stony Brook University and draws explicit comparisons between the new fossil primate and Darwinius. Named Afradapis longicristatus, this fossil primate is known from less complete remains than those that represent Darwinius, but the teeth and jaw fragments the team of scientists found allowed them to make some detailed comparisons between Afradapis and other primates.
Contrary to what you might see in Hollywood movies paleontologists rarely find complete skeletons of fossilized animals. This is especially true for mammal and primate specialists. Thanks to their enamel covering, though, teeth are much harder than bone and become preserved more readily. As such the record of early primate evolution as we presently understand it is primarily made up of teeth. Fortunately for scientists, mammal teeth are very distinctive. If you wanted to figure out the evolutionary relationships among a group of extinct mammals teeth would probably be better guides than fragmentary parts of the rest of the skeleton which lack so many tell-tale characteristics.
The recovery of nearly all the elements of the lower jaw of Afradapis gave the researchers quite a bit to work with. The teeth of Afradapis identify it as an adapid, or the group of lemur-like primates to which Darwinius also belonged. The lower incisors, for instance, were flat and stuck out a little bit of an angle, a condition seen in other adapids. This arrangement is even somewhat reminiscent of the full "tooth combs" seen in the living lemurs, lorises, and galagos which are the closest living relatives of the adapids.
Even so, Afradapis shared some interesting traits with another group of primates; anthropoids. Its molars were high-crowned, a good shape for chewing on leaves, and it was entirely missing the second premolar. The loss of this tooth is something that had previously seemed unique to some anthropoid groups (like the living "Old World Monkeys" or cercopithecines), and it seems that even early anthropoids did not lose this tooth until about three million years after Afradapis lived. Some would take this feature to mean that the adapids really were close to the ancestry of early anthropoid primates, but the authors of the new study suggest that the actual picture is a lot more complex.
When Afradapis is compared to the earliest known anthropoid primates a surprising (and perhaps even counter-intuitive) pattern emerges. Many of the "anthropoid" traits in Afradapis, such as the loss of the second premolar and the fusion of the two halves of the lower jaw (called the mandibular symphysis), are not seen in closest relatives of the earliest anthropoids like the recently-discovered genus Biretia. This means that these traits were acquired through convergent evolution, perhaps as a result of a life chewing on fruits and leaves in the trees. The "anthropoid" traits in adapids like Darwinius do not mean they were ancestral to anthropoids, but that they probably shared a similar lifestyle.
The real test of these relationships, though, involves make a listing of all the relevant fossils, catalog their traits, and comparing them to each other through cladistic analysis. This is something that was not done when Ida was first announced, even though many of the bold claims being made about the fossil hinged upon just this sort of analysis. To test the different hypotheses about the relationship of adapids, Sieffert and his co-authors compared traits possessed by Darwinius and Afradapis to 117 different living and extinct primates. If adapids fell close to anthropoids, certainly it would show up in the analysis of this large sample.
What the scientists found supported the present consensus about the relationship of adapids to other primates. Despite living 10 million years or so apart, Darwinius and Afradapis fell close together within the adapids, and they were almost as far from anthropoids as it was possible to get. Darwinius and Afradapis were even fairly distant from the ancestors of living lemurs, lorises, and galagos, meaning that they represented a group of specialized adapids that died out and have no living descendants. The animals that are the closest relatives to the anthropoids are odd, large-eyed primates known as omomyids (extinct) and tarisers (both living and fossil).
It should be remembered, however, that this paper is not just about putting Darwinius in its proper place among primates. Afradapis is quite interesting on its own. Indeed, it appears that Afradapis was a very large adapid that lived alongside early anthropoids and may have been in competition with them. The adapids might not have been ancestral to monkeys, but at least some occupied "monkey niches" in the Eocene forests of Africa. With any luck more material from Afradapis will soon be found so that we can better understand what this unusual primate was like.
Nor does this study end all discussion about Darwinius. The evolutionary tree published by Sieffert and colleagues is a hypothesis that will continue to be tested against further evidence. I think they make a pretty strong case that the adapids were not ancestors of anthropoids, but I have no doubt that we will eventually see some rebuttals from those who believe otherwise. Ultimately, what branch Darwinius occupied in the primate family tree will not be determined by who can produce the snappiest press release but by the evidence. That is how science works, and I have no doubt that the relationships of adapids, early anthropoids, and other archaic primates will continue to be debated for some time to come.
Seiffert, E., Perry, J., Simons, E., & Boyer, D. (2009). Convergent evolution of anthropoid-like adaptations in Eocene adapiform primates Nature, 461 (7267), 1118-1121 DOI: 10.1038/nature08429
The loss of this tooth is something that had previously seemed unique to anthropoids, and it seems that even early anthropoids did not lose this tooth until about three million years after Afradapis lived.
I thought plenty of New World monkeys retain that tooth to this day???
Wow. Sorry, I can't resist.
*points at John Hurum*
Ed Yong makes a great point, too: there's still a book and a documentary floating around about Ida, forever erring in their advancement of certain ideas. No such massive publication is being dedicated to Afradapis, so the public at large will continue to be misinformed about this issue.
David; You're right. It's primarily the cercopithecines where it is absent among extant monkeys. Thanks for reminding me. I will make the correction.
The Nature tree is a cladistic hypothesis. In cladistic hypotheses (at least the kind I know about) there are no actual ancestors, only an hypothetical ancestor at each node. A lot of human origin thinking is still non-cladistic, with concerns about "missing links" and "is this an ancestor?". Neither of these concepts occur in cladistic thinking.
@Jim, actually, cladistic analsyses can reveal possible ancestors. If X is retrieved as Y's sister group, X has no autapomorphies, and X occurs earlier than any member of Y, then X might be an ancestor of Y. This doesn't show up on the cladogram, but the information is generally present in the study.
You can also reveal ancestor-descendant relationships between, e.g., species by using individuals as OTUs. If the members of species X form a paraphyletic series of outgroups to the members of species Y, then species X is ancestral to species Y. I've seen this happen in one hominin study (the paper that named Homo cepranensis).
Note that, given the above tree, there is no way Darwinius is even close to being a strepsirrhine ancestor. A paraphyletic Anchomomys, however, does include strepsirrhine ancestors.
Addendum to above: They also seem to find Donrussellia as a genus containing strespirrhine ancestors.
It's worth pointing out that cladistic thinking (or even a replicable phylogenetic analysis) is pretty rare in many fossil primate studies - for instance the recent Ardipithecus issue of Science only included a "Darwinius-style" table of apomorphies. Kudos to Seiffert and colleagues for moving at least one corner of paleoprimatology forward a little!
I am a fish taxonomist. I don't understand how X can be recognized as an entity if it has no autapomorphies. Additionally, if X fits all the criteria we think an ancestor would have, then it might be, as you say, an ancestor. I think this hypothesis is untestable in most cases, and it would be better, or at least consistant, to consider it as a plesiomorphous sister group.
I'll have to think about the paraphyletic individuals defining an ancestral species. Does this apply to comparing present day species? I follow the Hennigian convention of thinking that the speciation event which produced two sister species produced two new species, not an ancesteal and a descendant species. There are situations where this does not work too well; stickleback fishes for example. It also doesn't work for species of hybrid origin.
Maybe we should bury this whole "missing link" thing and bury it deep, because it's a red herring that gives a distorted, simplistic idea of how evolution works. Let's celebrate these finds on their own merits, and drop the breathless hyperbole that inevitably accompanies them.
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