Animal hybrids may hold clues to Neandertal-human interbreeding

It has long been thought that lineages between species were rigid lines that do not connect with other living species and that mating between species is impossible.  Recent studies show that human evolution may have been a river stream of  inter connected lineages that interbred rather than the conventional theory that our lineage was a tree with individual lines leading to each species.  After comparing modern day non-African human DNA with DNA from Neandertal fossils, it was found that 1.5 to 4 percent of Neandertal DNA still exists in our genome.  There is also evidence of interbreeding between humans and another ancient hominid known as the Denisovans.  Evidence of these interbreeding relationships can be found in human skulls that have been found in caves that share a resemblance to the skulls of our ancient hominid relatives.

Drawing comparing human skull to Neandertal skull.

Some scientists believe that these ancient skulls may be the evidence of hybrids that are the product of human and hominid mating.  Species that diverged from a common ancestor within a few million years still have the capacity and similar DNA to still be able to reproduce.  To further observe this, scientists have experimented with hybridizing species that exist today and comparing the results they get with those of the cave skulls they have found.  Yellow and olive baboons were interbred to see how the resultant generations would appear and what traits would be exhibited.  The results showed that hybridization blurs the lines between species and makes it harder to detect differences among the species.  Due to this, a theory has come up that Neandertals were not physically pushed to extinction by being out-competed by humans, but rather they slowly began to fade away by interbreeding with humans and leaving their mark in our DNA.