The middle ear that endows mammals with their sense of hearing is one of the masterpieces of evolution. And it now seems that this is an instrument so nice, nature invented it twice.
The three delicate bones of the mammalian middle ear--the malleus, incus, and stapes--work in concert to amplify and internally direct sound waves so that they can be converted into nerve impulses. But before they became specialized for hearing, the bones of the middle ear were part of the jaw of synapsids, reptile-like ancestors of mammals. The fossil record shows that these bones migrated from the jaw to the middle ear over the course of evolution. Because all modern mammals have the same middle ear, and because its jawbone origins are so peculiar, scientists have long assumed that the middle ear arose just once in the course of mammalian evolution.
But the discovery of a well-preserved fossil jawbone on the southern coast of Australia has cast doubt on this classic story. The bone was found in 2003 by a team led by husband and wife paleontologists Thomas Rich and Patricia Vickers-Rich, at Museum Victoria and Monash University, respectively, in Melbourne, Australia. The pair immediately recognized that it belonged to Teinolophos, a distant ancestor of the platypus and other egg-laying mammals, but not of marsupials and placental mammals.
After consulting with James Hopson, an evolutionary biologist at the University of Chicago, they realized that the jawbone still carried the tell-tale imprint of the three "middle ear" bones, which had not yet migrated. Therefore, the middle ear must have evolved at least twice: once in the lineage that gave rise to the platypus, and again in the lineage that gave rise to marsupials and placentals like us. The researchers report their findings in today's issue of Science.
This evolutionary twist is a surprise to most, says Thomas Martin, a paleontologist at the Senckenberg Research Institute in Frankfurt, Germany. And he says it gives vindication to embryologists who, when comparing the way the middle ear forms between egg-layers like the platypus and other mammals, "noticed certain minor differences."