In platypus genome, an odd map of evolution

The platypus, a web-footed, venom-storing mammal that lays eggs, occupies an isolated branch on the evolutionary tree. (Nicole Duplaix/National Geographic via REUTERS)

WASHINGTON - When the British naturalist George Shaw received a weird specimen from Australia in 1799 - one with a mole's fur, a duck's bill, and spurs on its rear legs - he did what any skeptical scientist would do: He looked for the stitching and glue that would reveal it to be a hoax.

"It was impossible not to entertain some distant doubts as to the genuine nature of the animal," Shaw wrote of the creature, which he eventually named "platypus."

Now, more than 200 years later, scientists have determined the platypus's entire genetic code. And it turns out the platypus continues to strain credulity, bearing genetic modules that are in turn mammalian, reptilian, and avian.

There are genes for egg laying - evidence of the animals' reptilian roots. Genes for making milk, which the platypus does in mammalian style despite lacking nipples. Genes for snake venom, which the animal stores in its legs. And five times more sex-determining chromosomes than humans.

"It's such a wacky organism," said Richard Wilson, director of the genome center at Washington University in St. Louis, who led the effort described online yesterday in the journal Nature.

Yet in its wackiness, Wilson said, the platypus genome offers an unprecedented glimpse of how evolution made its first stabs at producing mammals. It tells the tale of how early mammals learned to nurse their young; how they matched poisonous snakes at their own venomous game; and how they struggled to build a system of fertilization and gestation that would eventually, through relatives that took a different tack, give rise to the first humans.

The animal's complete genetic code, or genome, turns out to have 2.2 billion molecular "letters" of DNA, or about two-thirds as many as the human genome, and contains 18,500 genes, about the same as humans.

Finding the order of all those letters was grueling, scientists said, because no similar animal has ever been sequenced. The platypus inhabits an isolated branch on the evolutionary tree with just one other close cousin, the echidna, also of Australia.