Intermediates

As RAZD has already said, it's populations that evolve, not individuals. You can't evolve the ability to breathe underwater - a population of which you're a part eventually could, in theory, though you'd be long dead before the feat was accomplished. As you said - it takes a long time to make major changes.But, to the more general question of why there aren't still populations living in the ancestral ways of modern populations - there are!Before our ancestors successfully made the transition to the land, they would have been lobe-finned fish, with lungs, that probably lived in shallow, poorly oxygenated water. Some of these fish evolved into the tetrapods that make up all of today's land vertebrates (as well as a bunch of aquatic verebrates). But not all of them. Some of them are still out there today, happily getting along as lobe-finned fish with lungs in shallow waters - often places that dry out often. These are the appropriately-named lungfish. They aren't the same as our fishy ancestors, because they've been evolving over the last 400 million years too, but just as some populations of humans taking to the water wouldn't require the land humans to be wiped out, nor did some lungfish taking to the land require the original lungfisgh to be wiped out.The natural world's full of this sort of intermediate. Ancestral mammals evolved away from laying eggs to laying live young, but some of those laying eggs didn't - the ancestors of echidnas and platypuses carried on laying eggs and never evolved nipples. Wasps evolved to be social insects, living in vast hives - but not all of them. Plenty of wasps still live the solitary lives of their ancestors.Of course, we shouldn't expect to find animals living as every animal has ever lived, for the simple fact that most things become extinct. There are colossal global catastrophes like the meteor impact that wiped out the dinosaurs (and pterosaurs, and mosasaurs, and ammonites and all sorts of other animals); or the much more devastating extinction 250 million years ago at the end of the Permian.And there are smaller, more local reasons for creatures to go extinct. We can see all sorts of species on the verge of extinction today; for varying reasons usually to do with the environment around them changing in some way. All the great apes today are believed to be endangered. If they went extinct within the next 200 years, then the family tree of apes would, in an infinitesimally short period of time on geological scales, go from this: to this: It's easy to see why we can't find a living creature representing each intermediate stage in an evoltuionary sequence.