Which factor probably played a large role in human evolution?

Answer 1

This is a very good question. In fact I think it is one of the most important questions we might ask.

Of course nobody on the planet can answer it because we simply do not know. A whole field of science, called paleoanthropology, has set out to do so for the past 150 years since Darwin's "Origin of Species".

The following is only my opnion. I am no authority (not that this should really impress anyone too much if I was) but I have been studying the subject for ten years. I have a master's degree from UCL (London) with a distinction and have completed almost 6 years of a PhD at UWA (Perth, Australia).

The orthodox view of human evolution held by anthropological authorities is still very much that a shift in climate towards greater aridity is the major thing that made us human. The old 'Savannah theory' which claimed that as Africa got drier, the trees shrank back and our ancestors were forced out onto the open plains, is still pretty much the paradigm that students all over the world are taught. It is true that the idea has been somewhat discredited since Kaye Reed's paper showed that Savannah, as we know it, is a relatively recent phenomenon - too late to have driven much of ape-human divergence and it is true that a significant minority of anthropologists have never really believed the 'Savannah theory' - but it is still pretty much the main idea we have.

A small minority of interested people, however, including myself, have the view the a shift to more open habitats does not explain the differences between humans and apes. If you go for the "full on" Savannah theory it begs the question why are no other Savannah mammals bipedal, large brained, small-toothed, fat, naked and able to use some form of language. If you go for the much more modest, "it was a shift to slightly more open woodland" it begs the question "isn't this the sort of place most chimps live today?"

A far better set of explanations for the characteristics of ape-human divergence can be found amongst ideas which I label "waterside hypotheses of human evolution". Basically they posit that selection from moving (i.e. wading, swimming and diving) through water affected our phenotype more than the great apes' since the last common ancestor.

We're bipedal because wading upright in fast flowing shallow water during a flood could save your life. We're naked because it reduces drag in water whilst swimming. We're fat because it aids buoyancy and thermoregulation in water. We've got large brains because of a switch in diet to shellfish and fish - high in energy and rich in nutirents necessary for brain growth. We've had dental reduction because eating such foods do not require large teeth. We became tool users because there are many pebbles on the beach and lots of shellfish to eat if you use them in a minimal way. We speak because breath control is essential for swimming and diving and it happens to be a pre-requisite for speech which we have and apes lack.

I could go on but instead I suggest you read any of Elaine Morgan's books on the subject. The later ones (e.g. "The Naked Darwinist" and "The aquatic ape hypothesis") are the best.

Answer 2

climate changes that caused existing primates to seardh for new food sources