|MadSci Network: Evolution|
This is a very popular question, so I'll start by giving you a chronological list of links to other "Mad Scientists" in the Evolution area who have answered similar questions before:
Since it is impossible to actually know exactly what humans will look like in a million years, I feel as capable at hazarding a guess as the others. The two main considerations in future human evolution are: what pressures are acting on humans that might effect changes in the future; and what has the rate of human evolution been in the past? To address this last question first, humans (Homo sapiens) are a very young species, which evolved from Homo erectus (or an intermediate species) less than 200,000 years ago. Before this, Homo erectus, its predecessor Homo habilis, and Its predecessor Australopithecus afarensis, each seem to have existed for a little over a million years. Assuming a constant rate of evolution, this would suggest that 1 million years from the present should see the emergence of a new species.
However, a constant rate of human evolution is not really a valid assumption, especially if we look at the evolutionary pressures confronting modern humans. I have seen models of future human evolution suggesting that the next species of Homo will have a big head (fer thinkin'), dark skin (from the ozone hole), a willowy body (??), less body hair (based on past trends), and no wisdom teeth or little toes (based on perceived uselessness?). Unfortunately, few of these changes would seem to confer any real reproductive benefits: for instance, a bigger head would require a wider pelvis for birth (as seen in Neandertals), which would adversely affect walking, and certainly go against the "willowy body" idea. Similarly, with the recent discoveries of clothing and sunscreen (not to mention houses), it's hard to imagine how darker skin would increase fitness, even in the face of increased exposure to ultraviolet radiation. On the contrary, forcing people to avoid exposure could result in lighter skin to maximize vitamin D production. Furthermore, since most mortalities from UV exposure are caused by malignant melanomas that occur late in life, and with fewer people dying from melanomas, the impact on reproduction would be negligible.
Wisdom teeth, little toes, tonsils, and other organs that are sometimes referred to as "vestigial" or "useless" are another matter. In each of these cases, the organ or limb being considered is formed as part of a system of other organs, and generally changing one aspect of the system requires changes to the rest of the system. For example, losing the wisdom teeth would require changes in the arrangement of the toothbuds in the jaw and mouth; just as losing the little toe would require changes in the development of the skeleton and musculature of the rest of the foot. So these would require genetic alterations that could impact adversely on the other components of the system. It is doubtful that the cost of these rearrangements would be offset by any advantage of not having these organs.
Actually, the most important selective force on modern humans is sexual selection. Even if big-headed, hairless, and toeless were advantageous for environmental reasons, the question is, "who would marry him?" The changes in bodyshape due to mate selection have been profound in the West over just the past century. Arguably, for modern humans, attractiveness and wealth are much greater advantages to selection than paying fewer visits to the dentist. Thus, any changes in human physical apperance over the next million years will probably be driven by changes in popular body images, with little actual input from environmental pressures. That is not to say that humans may not evolve in other ways unrelated to physical shape: although with modern transportation and telecommunication, it is difficult to imagine any population existing in isolation long enough for genetic drift to allow for speciation.
To summarize my theory of future human evolution, I think humans could look somewhat different than they do now, or completely the same, based on the whims and trends of societal views of beauty over the next million years, and not based on perceived past evolutionary trends or environmental pressures.
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