This still does not explain many of our features, but it is at least a lifestyle that a human could be expected to survive, and even to thrive in, if not be perfectly adapted to.
This is the concatenation of several theories that I have heard bandied about, and various parts may be left out if wanted. It is of course biased, since I wrote it. Anyone want to rewrite it?
It has been suggested that this habitat could be split up into several types:
Water/swamp - Crocodiles, hippos Heavy tree cover - Monkeys Medium tree cover - Protohumans Light tree cover - Leopards or similar No tree cover (savannah) - Canids, felids, etc.
Since there was an ample nearby water supply in most of these habitats we can assume that there was no problem getting as much water as was necessary to maintain sweat-cooling. Because of this, things such as dilute urine, tears, etc were not wasteful.
Grubs and insects made nice additions to our diet, as well as crabs, frogs, shellfish, and any other poorly defended, slow animals.
Big game hunting seems to have begun a few million years *after* the split from apes. It is therefore probable that small game was our target.
Co-operative hunting techniques as have been seen among monkeys may have made the capture of smaller animals considerably easier.
AFAIK (not that I have checked this, or anything), there is no race of human that hunts without weapons. Even rabbits are beyond us. Maybe a lucky dive might get you one, but we are very poorly adapted to such techniques. I think everything upwards of a mouse is too fast for a human.
It is possible that we were tool users to a limited extent, in that we chucked things at the animals, or thwacked them.
First, we were reluctant to come down from the trees, but as they became scarcer, we did shamble about on the ground, and we held onto things like nearby branches to keep ourselves stable, as we were used to in the trees.
We found that when we did venture out into the sun this was a good preadaptation to coping with the heat, which we then built upon with sweat glands, nakedness, subcutaneous fat, and so forth.
Bipedality left the hands free to pick up large branches to thwack our prey with. this is not in itself tool use, since we did not craft the tools, but it is _implement_ use, the grabbing of an object and using it as an implement that does the job just fine without any pre-forming beyond maybe snapping it off a tree. This was a behavioural prerequisite to our later tool use.
This bipedality was reinforced by sexual displays, causing sexual dimorphism: enlarged breasts and penis, different silhouettes due to different fat depositions, different hair depositions, different heights, etc.
Another advantage of the bipedalism was that it allowed food to be carried or dragged back to the home base more easily.
It is possible that the protohominids would only venture out onto the more open savannah to hunt when the sun was at its peak, when all the predators were having their siesta, and hence a much safer time. The prey would also feel less like running away at these times, becoming more easily exhausted in the chase than the humans.
This also gives a more logical reason for the wierd heat- regulation system (bipedalism is only really an advantage at midday, we sweat far too much, but then at midday we would have to, we have hair on our heads which would only protect us at midday, etc)
There was also plenty of fruit and water near the trees (since the trees would tend to be around water).
If leopards came towards the trees, lookouts could scream, everyone would jump into the trees and pelt the leopard, and the leopard would be driven away without danger to the protohominids.
Snakes are a pretty constant problem wherever you are in Africa, and it can be assumed that they, as well as felids, canids, crocs, hippos, other apes and simians, scorpions, and so forth, all took their toll on the protohominids.
A leopard looks interested in the humans, but a few thrown stones make it give up. It is too hot, and has already eaten today.
Then, near the water, at a ford where animals tend to congregate, a group of small game are surounded, and clubbed to death. They are then slung over the shoulder and carried back triumphantly to the trees.
Meanwhile their dutiful mates have been collecting fruits and smaller animals, looking after the children and keeping lookout for predators.
A snake is found by a child, who screams and points. The women and children gather round and beat it with sticks.
The larger hunters return, and make their bipedal displays, causing the women hand over the fruit. The hunters tuck in, magnanimously sharing the feast with their mates, reinforcing the pair bonding.
Later, as the day cools, a lookout screams a warning as leopard approaches. The protomominids leap into the trees and pelt it with sticks and other things, but it is undaunted, climbing a tree after them. They climb to the outer branches and swing or leap to other trees, easily evading him. The leopard is persistent, as a few weeks previously, he had caught a juvenile protohominid too slow to jump to another tree. But this time there is no such luck, and it retreats, somewhat bruised.
At night, the tribe retires back to the trees, with lookouts still keeping an eye on things.