EVOLUTION can repeat itself 鈥� even in producing complex structures like the mammalian middle ear. A fossil jawbone from an early Australian mammal proves that the hearing of at least two different groups of mammals developed independently in almost exactly the same way.
The discovery raises both philosophical and practical questions. It challenges the claim made by the late biologist Stephen Jay Gould that replaying the evolutionary 鈥渢ape of life鈥� would produce different results. The finding that the same trait evolved more than once implies the contrary: that natural selection tends to converge on a limited range of options. Simon Conway Morris of Cambridge University, who famously clashed with Gould over the subject, described the findings as 鈥渕usic to my ears鈥�.
What is more, it questions a key premise made in drawing up evolutionary family trees 鈥� that all animals which share a trait have a common ancestor. If this assumption should prove to be untrue, then reconstructing evolutionary relationships becomes much harder.
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The middle ear of mammals is an ideal test case for Gould鈥檚 ideas. Its three tiny bones, the malleus, incus and stapes or hammer, anvil and stirrup, are connected to form an exceptionally sensitive acoustic receiver. This complex structure is present in all living mammal groups 鈥� placentals, marsupials, and the egg-laying monotremes, which include the echidna and duck-billed platypus.
Its origins can be traced to three bones attached to the lower jaw of mammal ancestors that shared the planet with dinosaurs. These bones transferred vibrations from the lower jaw to the eardrum. As the jaw evolved, the three bones shrank and eventually detached, migrating with the eardrum to the middle ear. The process was so complex that mammal experts assumed that it must have occurred only once, before monotremes split off from the other mammals more than 150 million years ago.
Now Thomas Rich at the Museum Victoria in Melbourne, Australia and his colleagues have found evidence that a 115-million-year-old monotreme called Teinolophos had not yet developed modern hearing. The shape of its teeth showed it was a monotreme, but its jaw had a trough to accommodate the three bones, which were still attached.
鈥淭he advantages of the middle ear are so great it was inevitable it should evolve twice in two groups with similar constraints鈥�
So both monotremes and other mammals must have developed middle ear bones independently after the evolutionary split (Science, vol 307, p 910). A few other cases of independent evolution are known, but all are simpler innovations such as the molar teeth in mammals.
Fossil mammal expert Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh thinks the advantages of the mammalian middle ear are so great it was 鈥渋nevitable鈥� that it should evolve independently in two groups with similar design constraints.
