The duck-billed platypus looks and acts like an animal with an identity crisis. It swims a bit like a duck and walks like one, too (on those webbed little feet), but it's certainly no bird. It lays eggs, as if a bird or reptile, yet it nurses its young like the mammal that it is. Now, it turns out the animal looks like a hybrid even at the most minute level, its DNA. Along with today's announcement that the platypus's genome has been sequenced comes the revelation that its genes reflect its odd ancestry.
It's not that the platypus has mixed ancestry, so it's not a true hybrid. Rather, it's thought that the ancestors of the platypus evolutionarily diverged from the rest of the mammals about 166 million years ago, so it retains some of the genetic and physical characteristics that other mammals may have had at that time but have since lost.
In a press release issued by the National Human Genome Research Institute, one of the sponsors of the platypus genome study, lead author Richard K. Wilson of the School of Medicine at Washington University in St. Louis was quoted as saying: "The mix of reptilian, mammalian, and unique characteristics of the platypus genome provides many clues to the function and evolution of all mammalian genomes...Now, we'll be able to pinpoint genes that have been conserved throughout evolution, as well as those that have been lost or gained."
One set of genes that platypuses and more typical mammals share are those that produce the casein proteins that make up milk, Wilson's team found. That finding suggests that the basic genetic machinery for producing milk has remained largely the same almost since the first mammal began to nurse.