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Is a new adapid a “Missing Link”?
May 10, 2009Posted by on
I know I said I’d be on a brief hiatus, but a friend sent me this article from the Daily Mail. Apropos of my previous post/exam question, I thought I’d clear up a few of the misconceptions hidden (or not so hidden) in the article, which just so happens to feature a species of adapid that was recently discovered in the Messel Shales of Frankfurt, Germany.
A good starting place is the term “missing link.” The term dates back to the medieval concept of the Great Chain of Being, or the Scala Natura. The Great Chain represents a hierarchy, with each “link” in the chain being higher than the one that preceeded it. Rocks are down at the bottom, and humans are at the top. Angels and God are even further up. “Missing links” are the links in the chain that have gone extinct. So why don’t modern evolutionary biologists like to talk about “mssing links”? Apart from the idea of the Great Chain being atiquated, it implies that we are linking some known, lowly form with a known, higher form. But that isn’t always the case when we find fossils. We don’t always know the animal that preceeded our new find, and we don’t know which animal succeeded it. We can know that an animal represents a transitional phase between two different, general kinds of animals, though, so we usually use the term “transitional form” nowadays. To illustrate my point a little more clearly, I’ll use the famous whale example. We know that the ancestors of whales were once terrestrial animals, and that they evolved into the aquatic animals that we all know and love. We know that Ambulocetus is a transitional form between the animals which were fully terrestrial and those which are fully aquatic. What we don’t know is which particular terrestrial animal evolved into Ambulocetus, and whether or not Ambulocetus eventually evolved into, say, an Orca.
Okay, on to something a bit less general. As I said before, the article is about a species of adapid which was recently discovered in Germany. The generally accepted theory is that the adapids are the precursors to modern-day strepsirrhines (lemurs, bushbabies, lorises) because of certain shared anatomical traits. They have long, projecting snouts like lemurs, smallish eyes like lemurs, and shared a number of features of the wrist and ankle with lemurs and their close cousins. Happlorrhiness such as ourselves, the other apes, monkeys and tarsiers evolved from a different primate that was around at the same time. This primate was probably an omomyid, and we can say that because omomyids share a certain number of features with modern happlorrhines: they have short, broad faces, huge eyes, and some even have a fused tibia and fibula like modern tarsiers.
The actual journal article hasn’t been published yet (or, I can’t find it if it has been!), but it seems that they are suggesting that their new fossil, dubbed Darwinius masillae, may be a stem Happlorrhine, even though it’s an adapid! They say this because the fossil lacks a tooth comb, which is a highly specialized set of lower incisors used for grooming, and a also lacks a toilet claw, which is a retained claw on the first digit that is also used for grooming.
Hmm. Absence of Strepsirrhine traits doesn’t a Happlorrhine make. I will have to wait for the journal article before I can say anything more on that subject.
It sounds like they’ve got an exciting fossil, but not quite for the reasons stated in this Daily Mail article. It’s not that this fossil is possibly a human ancestor- it’s much, much broader than that. This fossil might be the common ancestor to monkeys, apes, humans, and tarsiers- or it might not be. A rather annoying graphic shows our new little fossil evolving directly into an ape, skipping all the really interesting and diverse animals in between. Animals like Aegyptopithecus, Eosimias, Proconsul, and Oreopithecus.
I understand the emphasis on the human connection, but in the effort to make sure that that angle of the story was represented, the rest of the story became convoluted and confusing, and in many places inaccurate (Humans did not evolve from tarsiidae! We just have a more recent common ancestor with tarsiers than we do with lemurs.). I have no doubt that David Attenborough will present the story much more elegantly and accurately. And I REALLY want to read the article by Phillip Gingerich and Jorn Hurum!