Researchers long wondered what distinguishes hominids from apes. The first theory was that hominids had a bigger brain. This theory was refuted when in the 1920s Raymond Dart discovered a hominid with the brain of the size of apes . The second theory was that bipedalism distinguishes hominids from apes. The question remained why apes descended from trees and became bipedal. There is a spectrum of subtheories of that :
All these theories have that shortcoming that they do not offer a compelling cause of why apes descended from trees. The cause had to be such that they could not have not descended. Such a compelling cause could be emergence of a species of apes that learned to use fire for protection and to warm themselves. Like chimps they probably lived both in trees and on the ground and were able to build nests. Chimps build nests both on the ground and in trees, where it is safer . The same should have been the case with those apes. But as soon as they learned to sustain fire for protection against predators, they had to abandon trees and build nests on the ground only, as fire couldn't be kept in trees. So, the learning to use fire as protection against predators compelled the apes to descend from trees completely and form villages of nests protected by fires on the ground.
After this happened the immense task of constant gathering dry wood to keep fire alive emerged. No doubt the wood was gathered and carried to the nests by hands. Although the apes that just descended from trees were, of course, quadrupedal, they could walk upright, as, for instance, chimps walk upright when carry heavy loads. Just performing the duty of gathering and carrying wood to fires did not require bipedalism but it was a definite advantage. Bipedal apes could carry more wood, faster, and from a farther away. That is why bipedal apes evolved from those apes that descended from trees.
The next steps of my theory of evolution from apes to humans are not different from others: after becoming bipedal the brain of hominids started growing, etc. Thus, not a bigger brain and not bipedalism distinguishes apes from hominids, but the capability to control and sustain fire. This capability turned apes into the first hominids which still remained quadrupedal. This capability caused them to descend from trees (as keeping fire in trees was impossible) and live in villages of nests build on the ground.
So, the real missing link in humans' evolution is those apes that became capable of using fire for protection (and to warm themselves) and learned to sustain and control it. Finding their fossils would be a great achievement.
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The preliminary version of this article was published as a comment to the article
"Did Neanderthals Die Off Because They Couldn't Harness Fire?"
on the Live Science web site.
The comment can be viewed here.