How can genetically prescribed selfless behaviour arise by natural selection, which is seemingly its antithesis?[1] In humans, I have proposed that eusociality could be the result of a two-species society, and that many of the traits that exist today evolved in a two-species context, with Paranthropus as a companion-species, similar to dogs, cattle and horses today.
The gene pool may not allow certain hierarchies to form, and a natural hierarchy between Australopithecines and Paranthropus as a companion-species could have supported the evolution of caste-based organization and Theory of Mind (ToM).
The interbreeding event 6 million years ago
As Popadin et al showed last year[2], the Paranthropus lineage resulted from interbreeding between ancestors of gorilla, and the common ancestor of humans, chimpanzees, and Paranthropus, 6 million years ago.