err just answering the title here...
when a species evolves, who ever said that the original disappeared? One species just becomes 2... and the living fossil "proof" becomes irrelevant.
When you have a kid with a birth defect (say one less finger... it happens), do you yourself die? can't you have other normal kids? yet your 4 finger kid can have kids too and should the 4 finger be due to a genetic mutation, he may pass it on and on and on... and in the end you have 2 populations, one with 4 fingers and one with 5.
now in real life, birth defects were not operable for most of human history and were seen as curses or other disgracefull traits, thus lessening the chances of the aflicted person to have a descent...that said a very good example of this is the Vadoma tribe in africa ( http://en.wikipedia.org/wiki/Doma_people ) article even says it's seen as helpfull to climb trees in that tribe (so not a defect for them, an advantage)
Also, the simpler the organism, the less changes it is subject to from one generation to the next and the more chances it has to still be there very very long in time.
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