Tuesday, January 22, 2013

Primate Origins Revisited

Artists reconstruction of C. simpsoni about to nosh on a fruit on while grasping a terminal branch.  Illustration by Doug Boyer. 

There is a new review paper out in the American Journal of Primatology on competing ecological explanations for the origins of primates.  The title "Rethinking Primate Origins Again" is a reference to the classic 1974 paper by Matt Cartmill entitled "Rethinking Primate Origins", in which Cartmill introduced the Nocturnal Visual Predation hypothesis for primate origins. Cartmill's big idea stemmed from the observation that many of the features shared by all living primates --- especially features of the visual system such as convergent eye orbits --- are also found among nocturnal visual predators such as felid carnivores and owls. In this view, the earliest primates are hypothesized to have been small nocturnal visual predators of insects.

A felid carnivore exhibiting orbital convergence.
The authors of the new review paper favor a slightly different view of primate origins.  They believe that the evolution of flowering plants (angiosperms) is the key to understanding primate adaptations.  Rather than nocturnal predation as the driving force, Sussman and colleagues hypothesize that the earliest primates evolved to exploit newly available resources of flowering plants (flowers, fruit, and insects attracted to them) in a fine branches setting. This idea is known as the Angiosperm Coevolution hypothesis.

This should be easy to resolve, right?  We should just look at the fossil record of the earliest primates and see whether they are insectivores --- possessing visual adaptations for nocturnal visual predation --- or whether they lack these features and instead possess dietary adaptations for plant resources.  

Well, its not so easy as it turns out. The problem is that we can't all agree on which fossils are the earliest primates.

The debate centers around a group of mammals known as plesiadapiformes, and one species in particular, Carpolestes simpsoni. Now, pretty much everybody agrees that the first primates of modern aspect arrive on the scene about 55 million years ago, and that many of these guys ate lots of insects.  However, plesiadapiformes are an earlier group of mammals, sharing some features with living primates, but do not have all of the derived features characterizing living primates.  C. simpsoni is one plesiadapiform that has grasping hands and feet and ate fruit but lacks the visual specializations characterizing modern primates.

So....if you think C. simpsoni is a stem primate, then it is of HUGE importance to primate origins because is shows that primates acquired some of their modern features related to fruit eating in terminal branches BEFORE they acquired the visual features that Cartmill claimed were related to nocturnal visual predation of insects.  This would support the Angiosperm Coevolution hypothesis.  However, if you think C. simpsoni has no special relationship to modern primates, then this critter simply has nothing to add the debate on primate origins.

The affinities of plesiapiformes is an OLD (like....paleo-old) debate. It is a complicated question that brings up a lot of difficult issues:

  • how to distinguish features inherited from a common ancestor from features evolved in parallel
  • how to deal with the fact that living primates represent only those species that have survived extinction, and thus...
  • the taxonomic question of how to define primates based on anatomical characteristics with a limited sample of living primates
This new review paper won't be the end of the debate by any means, but it is a nice summary of the competing ideas.  I look forward to the replies from the other side of this debate, and to the new fossils that will, no doubt, be brought to bear on this question.

Thanks to Brett Nachman for pointing out this article.