should one accept the utility of the molecular-genetic clock as a means to determine species divergence, that non-critical changes in DNA accumulate more or less in a defined time-dependent manner, then one can measure time by counting the number of genetic differences, and then infer a time of divergence.
Modern apes have lice. Their bodies are more or less uniformly covered with hair, and their lice are largely of a single collection of very related species (more or less).
Modern humans have lice. Their bodies are not uniformly covered with hair. Their lice are of two distinct varieties (more or less), head lice and pubic lice.
These are rather distinct genetic populations (head and pubic lice). One can use molecular-genetic clock modeling to predict when these populations diverged, and use that information to infer when it was that pubic and head circumstances became largely distinct ... when we lost body hair. It seems we lost much of our body hair back in H. erectus days. Analysis of H. erectus remains and circumstance indicates an almost strict animal protein diet. We were hunters. The hypothesis could well be that we simply could chase large game (all wrapped in their full body hair)until they failed (heat exhaustion) and that while we were a bit knackered, our ability to cool via surface evaporation proved decisive.
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