The team used computational reconstruction of ancestral gene sequences, DNA synthesis, protein engineering and X-ray crystallography to resurrect and manipulate the gene for a key hormone receptor as it existed in our earliest vertebrate ancestors more than 400 million years ago. They found that over a rapid period of time, five random mutations made subtle modifications in the protein's structure that were utterly incompatible with the receptor's primordial form.This seems to indicate that once evolution in a species has proceeded down a particular path, it doesn't go back. Once upon a time, the ancestors of humans had four premolars (bicuspids, if you are a dentist). We now have two because the first two have been lost along the way. I remember my advisor Fred Smith remarking once upon a time that once a trait has been lost evolutionarily, it never comes back. A species might follow a different path than the one it originally took to get to a similar place, however. This explains why whales and icthyosaurs don't look the same, even though they are both exploiting an aquatic niche. Read the whole thing.
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