why did our ancestors migrate?

Our earliest ancestors were nomadic hunter and gatherer types, always on the move, and not much different in appearance and habits from the apes. The reasons that finally forced so many of them to leave their equatorial homelands to find new places to inhabit are very similar to the reasons that modern migrants give. Either there is a cataclysmic event - such as a flood or earthquake - or a social event such as a population explosion.

When populations reach a certain size over and above that which can be called comfortable and manageable, tribes tend to develop and with this phenomenon comes tribal warfare over all manner of issues - particularly food, space, mating and leadership.

In the vast and dense rainforest region of Equatorial Africa, it took millions of years before our ancestors had developed sufficiently to reach a population size that could be called crowded, but when that point was reached migration became a matter of survival.

Bearing in mind that our early ancestors were competing for food and space not only with each other but also with all manner of predatory or dangerous animals, the rainforest may have been a fecund area but it was also an extremely dangerous place to live.

Faced with depleting opportunities to find food, a mate, a safe place to sleep or merely congenial neighbors, most of our ancestors were literally forced out of their equatorial homeland. Others, more likely to be the migrants who traveled furthest - and from whom we owe our more direct evolutionary ancestry - had an adventurous or entrepreneurial streak and probably left long before they were pushed.

As animal species also evolved in all parts of the world, the overcrowded conditions in the rainforest caused the animals to explore new pastures, too; and in this scenario it's possible that our ancestors followed them as a source of food. With the hunters in pursuit, the animals moved further and further away from the rainforest until there was no going back for either of them - even if seasonal migration was their pattern.