Sometimes people think that evolution is a progressive and straight lined affair, with one species gradually improving generation by generation. Not so, it's just as likely that within a generation or two that a species becomes disrupted and becomes less fit; when the environment changes suddenly for example. It's also possible that species diverge for a few millennia and then converge again, current thinking is that Human evolution was like this. For example, there's significant DNA evidence that more modern Hominid groups interbred with archaic (older) species (like Neanderthals) around 40,000 years ago, resulting in ancient DNA popping up in populations living today. Like most things in Biology, sex is anything but simple, we should just be thankful we're here at all, the number of possible people far outweighs the number of actual people that have ever lived.
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