Habitat:

Extensive drying of East African habitat reduced the forestation and drove river apes (Australopithecus afarensis ) to seek new habitats. Paradoxically, as it got drier, some (Homo ancestors) became more dependant on aquatic environments, whilst other (the ancestors of Pan) became less so. The homo ancestors settled by the great lakes of the rift valley, some stayed by the larger rivers, whose gallery forests persisted, and some moved down to the coastal areas where it is assumed mangrove forest habitats could have still sustained them. It is predicted that at the start of this period, even during the driest spells, there would still be sufficient fresh water from the Afar drainage basin to sustain several large rivers and therefore support large gallery forests and a population of river apes.

The time spent in this habitat is characterised by what can only be described as a war with aquatic predators, particularly the crocodiles. The battle for survival had a number of consequences. Hominid swimming ability became strongly selected for in addition to the fast wading that they already had. They began to lose their body hair and became far more streamlined. Our ancestors were very smart and began to fight back. Counter-predation against crocodile nests would have been easy in gallery forest habitats because they lay their eggs on dry land and the trees would have given the hominids good protection. It is proposed that these 'crocodile-hunters' would have worked in groups and developed complex vocalizations in order to survive. The model predicts that in the Afar region at least, the man-eating crocodile species were eventually overcome and driven to extinction. This is borne out by the observation that there are no man-eating crocodiles in the Awash basin. (Johanson 1981. p 151)

Once this occurred, with one of the major causes of death effectively removed, there would have been a population explosion of riverside hominids that would have been unsustainable in the rift valley. This, together with a series of major rift valley faulting in the Afar region around 2-1.5 mya caused another diaspora of hominids spreading out across Africa.  Baker (1972) suggests that major graben faulting occurred in Afar in this period and that it would have significantly affected drainage patterns on the rift shoulders. In other words the drainage basin of Afar would have been severely weakened reducing forestation still further at the same time of the hominid population explosion. 

The model assumes that these exiled Afarian hominids would, to some degree, have interbred with African hominids that had left before them, leaving the resulting populations of Homo ergaster looking more primitive than the ones who survived back in the ancestral home. Others who migrated east, crossed into Arabia (perhaps swimming across from the Horn of Africa) and then Asia for the first time. As there'd be no hominids already present there, no interbreeding with more primitive forms took place making the Asian hominids Homo erectus appear more 'advanced' in the fossil record.

All over Africa and Asia these hominids found hundreds of new niches to survive in. However, their dependence on water forced them to stay close to rivers and lakes. Meanwhile, back in the gulf of sea in Afar, their ancient cousins, the marine hominids Homo maritimus, continue to evolve into excellent divers with superb breath control and a cluster of features which help them to cope with the salinity of sea water, including salt-tears and salt-sweating. 

Socio-ecology:

The naked ape versus crocodile phase would have led to pressure for monogamy but once the threat had gone this constraint was lifted and other sexual strategies would have returned. A successful, expanding population would have favoured reduced inter-birthing intervals. 

Technologies:

In the war with the crocodiles I suspect that hominids learned to make a large number of weapons. I predict that groups of hominids would have been able to scare of crocodile females guarding their nests simply by throwing boulders from trees down onto them.  I suspect they would have learned to make sharp spears during this phase too.

This stone tool industry would have been taken with them as part of their culture during the diaspora.