Because they are all baby humans. They all live to be millions of years old and they eventually become one of us!! I don't know, but it was a good question! (and good answer pieman!)
Most scientists agree that birds evolved from dinosaurs/reptiles, yet - though the dinosaurs have gone - we still have lots of ancient reptiles. Just because one element branches off on its own doesn't mean that the original 'trunk' has to disappear.
And the element we derived from was just in the right place at the right time for change. There's evidence that human's and other animals are still evolving, eg. less people have wisdom teeth. Evolution is such a slow process it can hardly be perceived.
And while we're on the subject, evolution isn't an inevitable, upward process. It happens when it needs to, in response to selection pressures at the time, but not in any particular direction. So humans are not in any way 'superior' to a 'lesser' organiser such as a woodlouse. We could say that a woodlouse is superior because it gets by perfectly well without evolving the need to develop nuclear weaponse, for example. I just had to get that off my chest. Carry on.
It is comparable to the horse, zebra, donkey etc who all evolved from one common ancestor.
And, well said PieMan, I have a great respect for ants in that way. As an indivual they possess very limited capabilities, but enough to become a formidable force as a colony.
atkins may I suggest that for some basic-level - but very comprehensive and easily understandable - answers to your question, you should read A Short History of Nearly Everything by Bill Bryson?