|MadSci Network: Evolution|
This question brings up several distinct issues, which I will try to point out and address individually.
First, it is not impossible that alien intervention influenced the evolution of our species, but that's not saying much. Many conjectures that qualify for the ``not impossible'' rating are so improbable as to be not worth contemplating, or have no value in organizing knowledge or predicting the future. The alien-intervention hypothesis does not fill any gaps in our understanding of the evolutionary process, does not provide any organizing principle clarifying existing knowledge, and does not provide any means of making predictions that can be tested by future investigations. Therefore this hypothesis has nothing to do with science. It is a fun hypothesis, and might make the basis for an interesting science-fiction story, but I wouldn't try to push it farther than that.
Second, to the extent that established facts can be brought to bear on the alien-intervention hypothesis, indications are that such intervention is quite unlikely. Over the past 30 years, a lot of smart people have spent a lot of time looking for any kind of solid evidence of extraterrestrial life, and the results have been disappointing. Do you remember the excitement generated a couple years ago by the discovery of microscopic mineral deposits that kind of looked like bacteria on a meteorite that looked as if it might have come from Mars? That incident illustrates the eagerness of the search. Should we believe that intelligent beings went to the trouble of traversing interstellar space to come to Earth, did some breeding experiments, and disappeared so thoroughly that we cannot find their artifacts on earth, their germs on earth, or their communication signals in our scans of the heavens?
Third, one need not invoke aliens to explain the evolution of men from ape-like ancestors; the familiar hypothesis of natural selection is well suited to this task. A good exposition of the evolutionary process is Richard Dawkins's book The Blind Watchmaker.
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