The Pthirus gorillae host switch around 3.3 Ma^[1] points to that Au. afarensis would have been in contact with ancestors of gorilla. The Danakil-CHLCA (chimpanzee-human-last-common-ancestor) hypothesis posits that geographical isolation, from when the Afar rift opened up and separated Danakil from Ethiopia^[2] , would be the cause of speciation of Au. afarensis, and that Au. afarensis adapted to forage from the sea floor, similar to how otters are water-adapted mustelids (Morgan, 1990).

The discovery of Chororapithecus abyssinicus^[3] (9 Ma, Chorora formation) in 2007 shows that ancestors of gorilla lived on the southeastern Ethiopian highlands, and would have undergone speciation by adapting to a high-elevation environment as the highlands formed.

Within a scenario where the descendants of Chororapithecus were the origin of the Pthirus gorillae host switch, Au. afarensis would have migrated to the highlands, away from Danakil, for whatever reason. The pliocene flood basalts, a tectonic rift event between 1–4 million years ago, could have provided a land bridge to the mainland, and also made the south-western coastline of Danakil uninhabitable as a result of earth quakes and volcanic eruption.

Increased tectonic activity around 3 million years ago, and a land bridge to the mainland on the south, would be a reason to leave Danakil and seek shelter in the highlands.

The "lack of food" problem, and the Turkana population as those interbreeding with gorillae

The "beach apes" would not be adapted to the highland region, and so may have not been able to find food. Another possibility is that the Au. anamnesis population at lake Turkana, that developed into Kenyanthropus platyops (3.5 - 3.2 Ma) were the ones that made contact with gorillae and that spread Pthirus gorillae to extant humans. Today, mountain gorilla live in the Virunga mountains, and lake Turkana is in between those and the Chorora formation.

Turkana is also where a lot of Paranthropus are found, and Kenyanthropus platyops is occasionally suggested as a direct ancestor to humans, which means they would have been carrying Pthirus pubis.

References

1. Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice

2. Tectonic history of Danakil and the origin of Pliocene vertebrate fossils in the Afar region

3. A new species of great ape from the late Miocene epoch in Ethiopia