Monday, March 8, 2010

Written in the Rocks

On page 49, Jerry Coyne suggests the theory that whales descended from terrestrial animals. He says that the transitional form between the terrestrial and aquatic mammal could possibly be a hippopotamus. Supposedly, this evolutionary sequence occurred after land mammals had evolved from fish, particularly a group of "lobe-finned fishes" as described on page 36. So if humans potentially evolved from aquatic animals (fish), then wouldn't it be contradictory to say that aquatic whales evolved from terrestrial animals? What evidence is there that proves that one event happened before the other or that it was possible to move from water to land and then back to land again?

1 comment:

  1. Evolution isn’t always smooth and it’s not unidirectional. This is exemplified by the evolution of the whale, the dolphin and the porpoise from terrestrial mammals. Consequently, whales and dolphins are ectotherms, give birth to live young and have mammalian glands that produce milk.
    Whales also breathe air with lungs, rather than with gills as do osteichthyans and chondrichthyans, and they have a blow-hole that is lined with hair, a remnant of the heat-insulating and protective fur present in the mammalian ancestor (Coyne, 49).
    The Archaeocetes were the oldest whales that evolved into two lineages of cetaceans by the late Oligocene. These two lineages became the modern whales which are subdivided into Odontoceti, toothed whales, and Mysteceti, baleen whales. (“Cetaceans”)
    With modern advancements in biotechnology, the chronological progression of the evolution of the whale has been confirmed. The “vestigial traits” of the whale are “a feature of a species that was an adaptation in its ancestors, but that has either lost its usefulness completely or, as in the ostrich, has been co-opted for new uses” according to Coyne. Coyne claims that the rudimentary pelvis and hind legs are remnant traits of its terrestrial ancestors. As the wings of the ostrich, these traits are no longer useful to the whale but are evidence of the macroevolutionary transition from land to water.
    DNA evidence of the mammalian ancestry have been studied extensively and confirmed that the hippopotamus may be the closest relative of the whale and representative of the transition from land to water. The mesonychian hypothesis originally suggested that the whale’s closest relatives were a group of hoofed animals from the Northern Hemisphere but genetic studies led some scientists to propose that the whale’s closest ancestors were the artiodactyls such as pigs and llamas. (“Whale Origins”)
    Norihiro Okada of the Tokyo Institute of Technology examined noncoding DNA, so called short and long interspersed elements, which are useful in determining the course of evolution. The genetic evidence has confirmed that the hippopotamus shares four SINEs not present in other artiodactyls. (Monastersky)
    Coyne attributes the evolutionary transition from terrestrial to aquatic mammals to the sudden disappearance of aquatic dinosaurs such as “mosasaurs, ichthyosaurs and plesiosaurs”. These would have otherwise competed with the whale ancestors for food and resources. When aquatic habitats were suddenly devoid of dinosaurs, it allowed for the dispersal, “the movement of individuals away from their area of origin or high population density”, of mammals into aquatic habitats (C & R, 1152). This dispersal would otherwise be limited by “biotic factors” such as competition and predation. One theory suggests that the transition into water was spurred by terrestrial predation and that the water offered “raoellids…refuge against danger”. (Thewissen, 287)


    Campbell, Neil A. Reece, Jane B.. Biology, AP Edition 8e. 2008.

    “Whale origins”.


    Why Evolution is true. Jerry Coyne.

    Richard Monastersky. the whale’s tale. Science News 2006.