May 10, 2008

The 'us' in platypus

The daily cladogram.

Biologist PZ Myers has written a terrific post on the draft of the platypus genome: an important post, since the popular press was so busy playing up the "weird little composite creature" angle that they missed the central point of the study. A comparative analysis of the genomes of multiple organisms, with emphasis on the newly revealed data from the platypus [writes Myers] "reveals unique signatures of evolution." [In fact, that's the paper's title: "Genome analysis of the platypus reveals unique signatures of evolution."]

From Myers' post:
Over and over again, the newspaper lead is that the platypus is "weird" or "odd" or worse, they imply that the animal is a chimera — "the egg-laying critter is a genetic potpourri — part bird, part reptile and part lactating mammal". No, no, no, a thousand times no; this is the wrong message. The platypus is not part bird, as birds are an independent and (directly) unrelated lineage; you can say it is part reptile, but that is because it is a member of a great reptilian clade that includes prototherians, marsupials, birds, lizards and snakes, dinosaurs, and us eutherian mammals. We can say with equal justification that we are part reptile, too. What's interesting about the platypus is that it belongs to a lineage that separated from ours approximately 166 million years ago, deep in the Mesozoic, and it has independently lost different elements of our last common ancestor, and by comparing bits, we can get a clearer picture of what the Jurassic mammals were like, and what we contemporary mammals have gained and lost genetically over the course of evolution.
Every organism is going to be a mix of conserved, primitive characters and evolutionary novelties — a mouse is just as "weird" as a platypus from an evolutionary perspective, since each is the product of processes that promote divergence from a common ancestor, and each are equidistant from that ancestor. It's just that we primates share more derived characters with a mouse than with a platypus, because we are more closely related, and the mix of characters in the mouse are more familiar to us.

[M]odern echidnas, elephants, and emus are all products of different evolutionary trajectories through history, and no one by itself is a representative of the ancestral condition. We derive the ancestral state by comparison of multiple lineages. And that is the virtue of this paper, that it adds another lineage to the data set, one that diverged from ours over 160 million years ago. It is a lens that helps us see what novelties arose in that 160 million year window … on both the eutherian and monotreme sides.
Read the whole post at PZ Myers' blog Pharyngula.

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