No, it’s not an ancestor either (probably)

In addition to the “missing link” trope that is being dished out about the new primate fossil, is another one, more subtle and insidious: it’s the ancestor of all primates. How do they know that? Consider a biologically realistic scenario: at the time there were probably hundreds of species of small bodied mammals with tails and feet like that. One of these species may be the ancestor of all primates, but what are the odds that a specimen from that species is the one that was preserved? Just as all primates now look remarkably similar overall, but one may be the common ancestor of a group in 50 million years or so without being the one that is fossilised, the characters of this species may in fact be shared primitive (in the sense of “came first”) traits of its group. So-called plesiomorphic traits, or underived traits, are no indicator that the specimen is a member of an ancestral species, only that it is a member of a group of species, one of which was the ancestor. We don’t even know what extant species is the ancestor of Darwin’s finches, and we have access to their biogeography, molecular properties, development, behaviour and mating systems. How can we be sure this was “the” concestor (Dawkins’ term for common ancestor) of all primates?

We’ve been bitten by this mistake many times before. Archaeopteryx was supposed to be the ancestor of birds. Neanderthals (now spelled without the “h”) were supposed to be “primitive” (i.e., came first) humans. Both are regarded as side branches of the lineage leading to birds and humans, but they show many traits that would have been shared with other species of their group at the time. And we rarely have reason to think we have a sufficient record of all species, as the Hobbit shows (it is regarded as not even a descendent of the H erectus hominids we know were in Asia around the right time).

History loses information. To make claims about history one needs positive evidence, ruling out, or at the least making extremely unlikely, alternative hsitories. Phylogenetics does not rule out all alternative histories, just some subsets. Phylogeny can rule out that a species is an ancestor, but it cannot rule it in. “Ida” may be our greatn parent, but equally it may just be a long lost cousin.

5 thoughts on “No, it’s not an ancestor either (probably)

  1. Good post John. It’s a shame to see so many organizations that should know better, as well as news outlets who can be expected to over sell, jumping on “ancestor of all primates” bandwagon. It’s not just a case that the framing is wrong or that they are taking the wrong angle on the story, but that the story itself is probably incorrect.
    This episode will probably come back to haunt the paleontologists and evolutionary biologists for years to come as creationists use it as an example of “Darwinists” misleading the public.
    This is a shame since it is a beautiful fossil, eminently newsworthy in its own right (though perhaps not with so many headlines), and could be a great example for explaining evolution by natural selection to the general public. Instead of this they’ve gone for a big splash, whuich will no doubt shortly be followed by the story sinking like a stone.
    I just hopoe that the documentary discusses the find and the various claims made about it in an more balanced and less excitable way.

  2. Very very true. Of course Ida isn’t the ancestor of anybody as the specimen is a juvenile and therefore never bred!

    I think the link metaphor pulls people away from the “tree-thinking” so important for understanding evolution. Darwinius masillae is no doubt an important find but it is very unlikely to be a direct ancestor but rather a an early branch on part of the primate family tree that leads to us. Ida doesn’t have to be a direct ancestor to be an important part of demonstrating our common ancestry with the prosimian primates and other mammals but it hard for most people to understand that with all the “link” talk. I’ve read the original PLoS paper and know the authors understand this. It will be interesting to see how the BBC documentary handles this, apparently it does a good job.

    In your Darwin’s finch example, no extant species is the ancestor of the Darwin’s finches. Molecular phylogenetics supports the conclusion that in fact they are not finches at all but nested within the tanagers. Darwin’s finches share a more recent common ancestor with the tanagers than compared to other songbirds but that is not the same thing as saying their closest tanager evolutionary relations are their ancestors.

  3. I think you’re taking too literal the metaphor. Do you think anyone really cares, do you think it actually really matters, do you think it has anything to do with the issue, that this might have ONLY BEEN one of a number of species of a group which included whatever one species it was that was the link at that time between what was and what is now?

    I think the hype sucks, and was wrongly directed, and was predicated on the egotism of on relatively unimportant guy, but I also think that most of the scientific community is missing why this could really be a very fascinating find, not for the well educated, but for the every day guy and their kids. Who’s lives couldn’t otherwise be actually affected by anything in paleontology, at all.

  4. I actually think it is pretty important to at least try to convey this stuff to the public. Understanding evolutionary trees and common ancestry is very important to understanding evolution and the notion of links pushes us back to a more linear, progressive, ladder-like thinking.

    It’s just a point to be made but I don’t think by any means this makes the whole publicity push a waste of time. When stuff like this reaches such a high profile I think more often than not the net result is good.

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