Well, if you wanna get serious about it, the wild difference in appearance between people is the only proof needed of those in-betweeners.
Facial structure, height, width, hair colour and type etc. Every difference that sets us apart is proof we are the end result of the mass, indiscriminate fucking that occured in the past.
What? No, the physical difference in ethnicities is not due to different ethnicities having romped around with different irritation of prior human species, it's almost entirely epigenetic.
Different ethnicities with the same basic genes were subjected to different environments. This interaction with the environment over long periods of times caused these same genes to express themselves in different ways.
Your claim is coincidentally (I hope unintentionally)based on debunked "science" like phrenology, who's practitioners liked to propose that non white races were less evolved/mixes of modern humans with proto humans.
My claim is based on being completely not serious despite my saying so and waiting for someone else to correct me, as that is an expected rule of the internet.