Neil Shubin recently published a book celebrating the discovery of a life form that clearly constitutes a transitional life form: tiktaalik, a fish that crawled out of the water by use of its rudimentary limbs. In a post in which I described Shubin’s book (Your Inner Fish: A Journey Into the 3.5 Billion Year History of the Human Body (2008)), I argued that every life form is a transitional life form. We’re all on the way to something else, at least those of us who will leave biological offspring. But some life forms are more obviously transitional than others and no currently living animal is more obviously transitional than the platypus.
The May 8, 2008 issue of Nature (articles available online only to subscribers) announces: “Top Billing for Platypus at End of Evolution Tree.” The article starts out by describing the platypus, one of nature’s oddest creatures:
Seemingly assembled from the spare parts of other animals. The semi-aquatic monotreme is a venomous, duck-billed mammal that lays eggs, nurses it’s young and occupies a lonely twig at the end of a sparse branch of the vertebrate evolutionary tree.
The Nature article describes the findings of a study analyzing the genome of the platypus. It was conducted by Wesley Warren of Washington University in St. Louis, Missouri. The study found that the platypus genome contains both reptilian and mammalian elements.
The study also explains the emergence of the many odd features of the platypus. For instance, the platypus does produce true milk (although it doesn’t have true nipples). The new study shows that the platypus has the genes for caseins (milk proteins) which map in a way that corresponds to the protein mapping in humans.
This is a sign that one of the genetic innovations that led to the development of milk occurred more than 166 million years ago, and after mammals first split from the lizard-like sauropsids that gave rise to modern reptiles and birds.
The platypus results from a mix of reptile and mammal genes. “The genomic features of what are now two separate lineages can coexist in the genome of a single organism.”
