The course of the news was slightly perturbed recently by the description of the 55-million-year-old fossil primate Archicebus achilles, which was published in Nature. (DISCLAIMER: I was the handling editor, and steered the paper from submission, through peer review, until the time when it was brought blinking into the light.)
Here is the BBC TV news report by Pallab Ghosh, and here is the extended audio version of the interview we did together. Pallab and I worked hard over a period of several days to get the story just right, emphasizing that Archicebus was a primitive haplorhine (the group that includes tarsiers, monkeys, apes and humans), and more closely related to tarsiers than anything else. We avoided saying that this particular species, let alone this particular fossil, was ‘the ancestor’ of humans, or a ‘missing link’.
As I said in the interview, the species might be described as ancestor of humans, but one could never know this to be true – it’s an unfalsifiable hypothesis. The most one can say is that Archicebus is close to the emerging lineage leading to humans, and possibly even be what one would expect the distant ancestor of humans to look like.
Even though Archicebus is an offshoot of the lineage leading to humans, and not a direct ancestor or ‘missing link’, it is much more closely related to us than 47-million-year-old Darwinius, another early primate concerning which there was much noise and fanfare about its relationship with the human lineage. Darwinius is an adapid, a group usually placed in the strepsirrhines, whose modern representatives include lemurs, lorises, bush-babies and the potto – primitive primates only very distantly related to humans.
The original paper, perhaps significantly, did not include a phylogeny. In its place was a long discussion about the place of adapids and their possible relationship with haplorhines rather than strepsirrhines. The paper discussed several anatomical features which might have aligned adapids with haplorhines rather than strepsirrhines, bolstering the much-publicized idea that Darwinius was close to the human lineage. Phylogenies, like so many pictures, are worth a thousand words, and a formal phylogenetic treatment showed that Darwinius and other adapids are better considered as strepsirrhines. The features in Darwinius that were suggestive of haplorrhine relationship turned out to be convergent.
But I digress.
As is typical of such things, some news reports described Archicebus as an ‘ancestor’, which as a scientific term is horribly loose – and even as a ‘missing link’. In my view, this term is simply not permissible in any discussion about evolution. Its use reveals that the writer is still wallowing in a progressivist view of evolution that’s a cross between a Kiplingesque just-so story and the pre-Darwinian Great Chain of Being than anything informed by a modern understanding of science.
Worse, it implies that evolution has a course that’s somehow plottable, or knowable in advance, a notion in which science is misread as the accumulation of facts – of certainties – rather than what it is in truth, which is the delimitation of doubt.
Another matter of concern is the description in the popular prints of Archicebus as the earliest known primate fossil. Archicebus – at 55 million years old – is pretty old. Its position as an early representative of the tarsier lineage means that tarsiers must have become distinct from anthropoids (monkeys, apes and humans) before that, and the divergence between haplorhines and strepsirrhines must have happened earlier still. This is exciting, as it shows that primates as a whole were already highly diverse at such an early date. The implication of Archicebus‘ existence – that the anthropoid lineage was distinct just ten million years after the dinosaurs’ demise – is thrilling.
But nowhere in the paper did the authors claim that Archicebus was the earliest primate fossil. What it said was that it was the earliest-known substantially complete primate skeleton. On reflection, though, Archicebus is very close to being the earliest fossil of any primate of modern aspect. There are older fossils of primates, but these consist of teeth and jaw fragments. Perhaps the earliest known primate is Altiatlasius, which lived 57 million years ago in what is now Morocco. An early primate skull from China, belonging to the genus Teilhardina, was described in 2004, and is around 55 million years old.