Planet Earth

Breaking the Link - Darwinius revealed as ancestor of nothing

Not Exactly Rocket ScienceBy Ed YongOct 21, 2009 5:00 PM


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Cast your mind back to June, when a stunning fossil animal called Darwinius (alternatively Ida or "The Link") was unveiled to the world to tremendous pomp and circumstance. Hyperbolic ads declared the day of Ida's discovery as the most important for 47 million years. A press release promised that she would "change everything", headlines proclaimed her a "missing link in evolution" and the scientists behind the discovery billed her as "the closest thing we can get to a direct ancestor".

And according to a new study, none of that is true. Mere months later, Erik Seiffert from Stony Brook University has done a comprehensive analysis of the bones of 117 primates, both living and extinct, which throws Ida's supposed direct line of ancestry to humans into serious doubt.

Central to this new work is a new fossil called Afradapis, a member of the same group of extinct primates - the adapids - that Darwinius belonged to. The two were closely related but separated by around 10 million years. Like its more famous cousin, Afradapsis's jaw and teeth contain features that are similar to those of anthropoids - monkeys, apes and humans. But far from being a sign of direct ancestry, Seiffert thinks that these features represent convergent evolution - the two groups evolved them independently.

His team compared and contrasted 360 features in the bones of over 117 living and extinct primates. Among them were 24 adapids, including Darwinius, Afradapis and eight other that had not been previously analysed. This comprehensive set of data revealed the group's family tree, charting their relationships using their overall anatomy as a guide.And it clearly shows that adapids (and Ida among them) were more closely related to modern lemurs than to anthropoids (monkeys, apes and humans). The two groups sit on a different branches of the evolutionary tree.

The analysis also reveals that even though the adapids were a successful and widespread group, they left no living descendants. For all the hype, Ida turns out to be the ancestor of bugger all.

New evidence

To those who followed the criticisms of the Darwinius hype, this volte face shouldn't come as a surprise. The the paper describing the fossil was criticised for juggling the structure of the primate family tree to shift Ida's branch closer to ours. To recap, there are three groups vying for position as the ancestors of the anthropoids: the bizarre, large-eyed tarsiers, the related and extinct omomyids, and the equally extinct adapids. The general consensus places the first two groups closest to us; Ida's discoverers think the adapids should be there instead.

To support that view, they looked at 30 traits that might help to settle the question and noted whether Ida had them or not, and concluded that placed the adapids next to the anthropoids on the basis of this single species. That approach seems positively minimalist compared to the one that Seiffert took, which included 12 times as many anatomical features and 117 times as many animals!

Seiffert's tree places the tarsiers and omomyids as the closest relatives of the anthropoids - this is the so-called haplorrhine group. The adapids, however, are part of the strepsirrhine dynasty, the group that includes lemurs, lorises and bushbabies. This is the sort of analysis that was sorely lacking in the Darwinius paper.

There is no doubt that Idais a beautiful fossil, but Seiffert questions its worth in understanding the evolution of primates. Not only was she a growing youngster, but most of her bones have been crushed or distorted in ways that obscure important body parts. Much was made of the fact that Ida lacked a toothcomb (a set of flattened, forward-facing incisors) and a grooming claw (a special ankle bone). These are two features that modern lemurs possess and modern anthropoids don't - their absence in Darwinius was presented as evidence of a close tie to anthropoids but not lemurs. But Seiffert thinks that these body parts - the ankle and teeth - have been damaged enough that analysing them is difficult.

Afradapsis, ironically, poses no such problems. While most of its skeleton has yet to be recovered, its teeth and jaws are in excellent condition. Like those of Darwinius and some other adapids, these teeth bear a suite of features typically found in living and extinct anthropoids. The joint between the two jawbones is fused and the part of the jaw containing the teeth is deep, as is the crater in the jawbone where the chewing muscles attach. The main cusp of its upper molars - the hypocone - is very large. It's missing the second premolar, but the third has become bigger with an edge that sharpens its matching canine.

But this doesn't mean that Afradapis is an ancestor, or even a close relative, of the anthropoids. For a start, the most primitive fossil anthropoids, such as Biretia and Proteopithecus, lack these traits. If adapids were their ancestors, the early anthropoids must have jettisoned these adaptations, only to re-evolve them at a later stage. The more plausible explanation, and certainly the one Seiffert subscribes to, is that both groups evolved independently, and happened to converge on the same adaptations.

The price of hype

The arrival of a paper like this was almost inevitable given the interest that Ida stirred up. Obviously, Seiffert's analysis isn't the final word on the subject (although his study looks more convincing to me) and I'm sure that there will be a healthy debate for days to come. But what of the public impact?

Jorn Hurum, one of the key ringleaders in the Ida circus, famously said, "Any pop band is doing the same. We have to start thinking the same way in science." The key differences, of course, are that pop music is impossible to analyse objectively and its quality depends on personal taste. The same cannot be said of scientific truth, and that changes the extent to which you can use marketing tactics to promote a discovery.

 Hurum and his colleagues have played a dangerous game - they may claim to have been marketing science but they were, in fact, marketing their opinions and ones that may not stand the test of time. It's debate by media, and it's fantastically dangerous.

Consider the fact that for all the interest that the new paper will undoubtedly instigate, there will still be a book, website and documentary out there firmly enshrining the increasingly dubious view that Ida is our direct ancestor. Consider also that contradicting that view now makes the scientific establishment look like buffoons, given all the publicity and to-do a few months back. When John Hurum makes grandiose statements, he gains in the eyes of the public. When those statements are later shown to be dodgy, it's science as a whole that takes a beating.

It's also worth noting how the different publishers handled the two papers. This time, Nature made the paper available to reporters several days ahead of its publication, giving us time to analyse the paper, prepare our stories and, if necessary, contact experts for their views. The situation with the original Darwinius paper couldn't have been more different.

As Mark Henderson notes, select journalists were allowed to see the paper at a specific location and under non-disclosure contracts that prevented them from seeking further opinions. PLoS ONE admitted to rushing the publication of the paper in time for John Hurum's press conference, and indeed, it became publicly available mere minutes before said conference kick-started a blitzkrieg of media attention. In rushing the publication of the paper, the journal allowed itself to be held hostage to hype and actively hindered science writers who were trying to do their job responsibly.

Update: Already up on the Times blog is an excellent interview with Seiffert. Brian Switek has his typically thorough take on the new paper too.

Reference: Naturedoi:10.1038/nature08429

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