a team of scientists has determined the platypus’s entire genetic code. And right down to its DNA, it turns out, the animal continues to strain credulity, bearing genetic modules that are in turn mammalian, reptilian and avian.

There are genes for egg laying — evidence of its reptilian roots. Genes for making milk, which the platypus does in mammalian style despite not having nipples. Genes for making snake venom, which the animal stores in its legs. And there are five times as many sex-determining chromosomes as scientists know what to do with.

“It’s such a wacky organism,” said Richard Wilson, director of the Genome Sequencing Center at Washington University in St. Louis, who with colleague Wesley Warren led the two-year effort, described today 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 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.

“As we learn more about things like platypuses,” Wilson said, “we also learn more about ourselves and where we came from and how we work.”