Your Hitler quote is obviously inauthentic. They didn't have "reset" buttons on computers back then.
Seriously, I'm not aware of much evidence for what you suggest. BTW, humans aren't "remarkably similar" (compared to the average large mammal). It's easy to argue that they are highly variable. What you may mean to say is that this variation is not sorted out into distinct races or subspecies to the extent it tends to be in other species.
The Flood was a big reset button. Worked well, didn't it?
It's closer to a Darwin quote than a Hitler one. In Darwin's Descent of Man he explains the role of warfare in human evolution. Schools embrace Darwin's ideas on natural selection but ignore Darwin's book on human evolution, especially the key role tribal warfare plays in it.
Humans are the most genetically complex animal ever. Obviously something much faster than natural selection was going on. That something is our war making disposition. It's very important to understand the true nature of man, as leftist beliefs may have you exiting the gene pool.
In the animal world there are many variations on a theme, such as birds and fish. In humanoids there is only one species still alive. That's because we killed off in war all similar competitors long ago, the Neanderthals being the last. There are very little variations of a theme in modern humans, we are all remarkably the same. War is the reason behind this.