There was a time when the publication of the entire sequence of a genome--any genome--was exciting news. I don't have any particular passion about Haemophilus influenzae, a microbe that can cause the flu various infections. But in 1997 it was the first species to have its genome sequenced. It became immensely fascinating, simply because we could now, for the first time, scan all of its genes. Now the global genome factory is cranking away so quickly--with over five hundred sequences published and over two thousand in the pipeline--that a new genome is not necessarily news. There has to be something striking, biologically speaking, for it to light up the radar. My personal radar lit up last month, with a monkey genome, because of the clues it offered to our own deep primate history, and to the question of how new genes evolve. And today, the radar lights up again, with the genome of an opossum.
As a kid I always found it strange that along with the raccoons and the squirrels and the deer and the foxes and all the other placental mammals I saw here in the Northeastern U.S., there was a single species of mammal with a pouch. The opossum wandered through our lives, waddling across a road or scurrying along a branch, giving us a blank, primordial glare. It was the lone representative in my own experience of one of the three great lineages of mammal evolution, the marsupials.
The oldest lineage of mammals are the monotremes, which include platypuses. They still lays eggs like reptiles and birds. The ancestors of marsupial and placental mammals split off from monotremes, after which they evolved the ability to develop inside their mothers without an egg. The ancestors of marsupials and placental mammals then diverged. Paleontologists have found fossils of marsupial-related mammals dating back 135 million years; the split between the forerunners of today's marsupials and placentals must have occurred earlier still, perhaps as long ago as 180 million years. There are many features that separate marsupials and placental mammals, but the most obvious one has to do with how they are born. Placental babies spend a long time developing in the womb before being pushed out by their mother. Marsupial babies squirm out of their mothers much earlier and continue developing in a pouch, where they can nurse.
