A similar question was previously answered by Meredith F. Small, associate professor in the anthropology department at Cornell University.
This time we asked Michael J. Dougherty, assistant director and senior staff biologist at Biological Sciences Curriculum Study in Colorado Springs, Colo., to offer his opinion.
From a biological perspective, there is no such thing as devolution. All changes in the gene frequencies of populations--and quite often in the traits those genes influence--are by definition evolutionary changes. The notion that humans might regress or "devolve" presumes that there is a preferred hierarchy of structure and function--say, that legs with feet are better than legs with hooves or that breathing with lungs is better than breathing with gills. But for the organisms possessing those structures, each is a useful adaptation.
Nonetheless, many people evaluate nonhuman organisms according to human anatomy and physiology and mistakenly conclude that humans are the ultimate product, even goal, of evolution. That attitude probably stems from the tendency of humans to think anthropocentrically, but the scholarship of natural theology, which was prominent in 18th-and 19th-century England, codified it even before Lamarck defined biology in the modern sense. Unfortunately, anthropocentric thinking is at the root of many common misconceptions in biology.
Chief among these misconceptions is that species evolve or change because they need to change to adapt to shifting environmental demands; biologists refer to this fallacy as teleology. In fact, more than 99 percent of all species that ever lived are extinct, so clearly there is no requirement that species always adapt successfully. As the fossil record demonstrates, extinction is a perfectly natural--and indeed quite common--response to changing environmental conditions. When species do evolve, it is not out of need but rather because their populations contain organisms with variants of traits that offer a reproductive advantage in a changing environment.
Another misconception is that increasing complexity is the necessary outcome of evolution. In fact, decreasing complexity is common in the record of evolution. For example, the lower jaw in vertebrates shows decreasing complexity, as measured by the numbers of bones, from fish to reptiles to mammals. (Evolution adapted the extra jaw bones into ear bones.) Likewise, ancestral horses had several toes on each foot; modern horses have a single toe with a hoof.
Evolution, not devolution, selected for those adaptations.