The cause of the move from forests to seas was the hot, dry Pliocene period. The forests over most of Africa died out to only a few isolated areas; competition within the forests was too high, and man's ancestors were not suited to the hard life on the plains. So, the theory goes, we moved instead to the sea, where leopards and other predators were loathe to follow us.

A few more points backing up the theory:

After the Pliocene came the relative abundancy of the Pleistocene. Man then spread out back into the interior, using his newfound bipedalism, tool-wielding skills and intelligence to dominate the scene.

This is all very well and good, but the theory seems a little complex and unlikely - it involves a change of habitat from trees to sea to plains, all within a few tens of millions of years. All just special favours for us, the acme of evolution? It would stand up more if there were a few other species in which the same process can be seen. And there are.

  • The pig is another not-very-hairy mammal. Its relatively weak legs, fat, bouyant body, and wide snout suggest that at some time it took to the swamps, and evolved there. The body of a pig is similar to ours... there is the story of a former cannibal race who, of all western imports, enjoyed spam the best because of its similarity in taste to human flesh!

  • The rhino is yet another hairless mammal. While not as water-loving as the hippopotamus, it still enjoys long periods of river wading, and eats water plants.

  • Of all similar creatures, the elephant is the most striking. Its evolution is really quite remarkable - it is descended from a small pig-like animal, but then over the ages, grew to be the largest land animal in its era. It is very easy to compare it to another mammal whose size swelled remarkably over the ages: the whale. In water, large mass is not the problem it is on land. Indeed, it is a benefit, as larger creatures lose heat much more slowly.

  • Its anscestors also had peculiar tusks. Some had spade-shaped ones, perfect for digging in soft, waterlogged soil, but not much good in the plains.

  • The early ancestors of the elephants showed a movement of the nose towards the top of the head. This would have been uncalled for on land, but excellent in the water. Nowadays, of course, they have a trunk. And what use is a trunk? It's inefficient for grass eating (a long neck would be better), and unnecessary for tree browsing. But it makes a pretty good snorkel. Not to mention its use for picking water plants.

  • Elephants can swim very well, perhaps the best of all land animals. A whole group has been known to swim for six hours straight, and one particular individual has crossed distances of sea of over a mile. Elephants' feet are also webbed, although this is not particularly noticeable anymore.

  • Elephants have been known to cry when under emotional stress. Hardly any land creatures cry, and hardly any sea creatures don't. Notice that this indicates that they were past sea-dwellers like us, not just river-dwellers like the rhino or swamp-dwellers like the pig. Also notice the surprising case of parallel evolution; elephants and humans, both rare in that they are tear-producing land animals, cry in the same situations! It's not as if we are related - in fact we use completely different glands to produce the tears!

  • Elephants are more intelligent than similar land creatures. This makes them adaptable enough to live in a wide range of habitats. They also have a highly developed vocal signalling system - a 'silent' herd of elephants is usually chattering away in the infrasound range. As stated before, very few mammals have a sophisticated vocal language, and those that do are almost always sea-dwellers.
(I know I've talked about elephants as much as, or more than, humans in this writeup. However, I thought it was closely related enough to our own evolution to mention in this node)