Habitat:

The major graben (rift) faulting in the rift left the Afar drainage basin much weaker and consequently the amount of forestation receded still further, causing smaller waves of hominids leaving the ancestral homeland. Each exodus appears in the fossil record as if a giant step forward has occurred - or "punctuated equilibrium" in the jargon - but really, back in Afar, evolution was moving ahead slowly but surely with "phyletic gradualism." I think this, or perhaps a series of hybridisations, is the explanation for the way head size is taken to have grown bigger in great steps when really the observation is probably just due to an incomplete fossil record. See my Floating Head Hypothesis  for details about this idea. 

It is predicted that with the threat of crocodile predation gone and the habitat shrinking there would be tremendous competition between tribes living in the area. It is in this kind of fierce competitive environment that one would expect complex traits to evolve - as a result of some kind of intra-specific 'arms race'. It is suggested that some of the major components of language evolved in this environment. Specifically the ability for young people to invent symbolic sounds that are new and different and to quickly learn to understand these sounds in a social environment of mutual trust.

The model predicts that due to this extreme competition, some groups are forced to swim out to sea where they encounter their distant cousins the seasiders. These, you may recall, had been descendants of the original swamp apes that had remained in coastal niches and gradually evolved as rather aquatic marine apes.

They were by now quite salt tolerant and had evolved mechanisms to excrete excess salt through sweat and tears. Their breathing control was as good as an aquatic animal and I expect they had some primitive kind of vocalisation that we'd call singing. However, they lacked the quick-thinking and simple symbolic language of the riversiders.

The big idea in the model is that an interbreeding event between the two resulted in a new type of hybrid hominid with varying mixtures of their parental features. Quickly, after a few generations, natural selection would favour those hominids with the best features from the two parental groups. The excellent breathing control of the seasiders plus the syntax checker in the brain of the riversiders gave rise to the phenomenon of full syntactical language and the Homo sapiens line. This model may also explain a number of other anomalies: For instance why we sweat and cry, but not sufficiently to be explained as a salt-excretory mechanism. It also neatly explains why our chromosome number fell from 48 to 46 and consequently why from this point on Homo sapiens could not successfully interbreed with the other extant hominids. See my paper on chromosome numbers of hominoidae.

With full syntactical language, humans had a massive advantage over all the existing hominids in the world, but first would face a huge battle to overcome the fierce riverside tribes still living on the mainland.

Meanwhile, in Europe, it is assumed that some Homo erectus populations would have become adapted to their new, much colder climate, leading to Homo hiedlebergensis and then ultimately to Homo neanderthalenis.