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Just read this really interesting article in Nature magazine ' Human evolution: The Neanderthal in the family '.
I recommend a read, although the cover image is a little strange :-)
So, we all have Neanderthal and Denisovian DNA, yet there are no more Neanderthals or Denisovians walking around on Earth. From an evolutionary perspective, are there any instances in the animal kingdom in which a precursor animal (e.g. wooly mammoth) and the current species (elephant) exist at the same time?
The paradox: if we evolved from monkeys and apes, why are there still monkeys and apes? If we evolved from Neanderthals and Denisovians, shouldn't they still exist as well?
"The society that makes a great distinction between its scholars and its warriors will have its thinking done by cowards and its fighting done by fools." - Thucydides
The monkeys and apes that exist today are not the same as the monkeys and apes that existed several million years ago. Humans and chimps both evolved from the same primate family, they just evolved in different ways. Evolution can take many directions, most do not include progression towards intelligence. Sometimes a new species can evolve from an earlier species while the earlier species remains in tact(a subset of the population broke off and changed over millions of years). Sometimes the earlier species splits off in several different directions, so while the separate species that evolved from it may be related to one another, they are not descended from each other. Humans and chimps are related because we evolved from the same species, but we are not descended from chimps. For example, perhaps there is a bird species today that will evolve millions of years from now an intelligent species of bird...that does not mean that other non-intelligent birds will cease to exist. So really it is not a paradox at all.
As for your other questions. I already explained that a precursor animal and a current species can exist at the same time. Mastodons gave rise to the mammoth as well as elephants, but they existed at the same time. It doesn’t happen often, but it does happen. Humans, however, did not descend from Neanderthals. Neanderthals and Humans shared the same ancestor, but are two separate species descended from Homo Heidelbergensis, approximately 300,000 years ago. The reason why there are no more Neanderthals and Denisovians is probably two-fold. #1, they had to compete with modern humans coming into their environments. Humans were more intelligent, better hunters(with better tools), and had a better birth rate and mortality rate with birth making them more numerous. Neanderthal and the Denisovans couldn’t compete and were likely outright hunted down by humans in many instances. The second situation is that it is obvious from DNA coding that there was some interbreeding going on. Interbreeding was probably difficult, but was possible since the species were so close to eachother on the evolutionary chain. Those who didn’t die out from competing with humans, were interbred into the population to the point that the dominant species traits took over and now their DNA is just a small % of modern human DNA.