Roughly five million years ago, humans split off from chimpanzees, who had in turn split off from gorillas five million years earlier.
There is much more diversity in chimp DNA than in human DNA, so probably a small band of human ancestors wandered off from a much larger group of common ancestors. Isolated from the main group, they began to evolve separately.
Chromosomes 12 and 13 combined to form human chromosome 2; this is why humans have 23 chromosome pairs while all other apes have 24.
The common ancestors, like chimps, probably walked on their knuckles, climbed trees, and lived in shaded forests. It's good guess that the group that split off and evolved into humans lived in the dry, flat, sunny savannahs. Why? Because we started walking upright to make it easier to walk long distances; we grew hair on our heads to shade ourselves from the sun; and we began to sweat and grew and veins in our scalps to radiate away the equatorial heat.
And, male and females began to divide up labor, with men hunting the big game on the savannahs, and women gathering plants and seeds. This led to more pair bonding. We know this because the ratio of male-to-female size decreases steadily in the fossil record.
And, as Matt Ridley explains, "long pair-bonds shackled each ape-man to its mate for much of its reproductive life: quality rather than quantity was suddenly important."
So males started looking for mates who would produce a lot of healthy offspring for a long time, and so they became attracted to health and youth. All of which explains why today women spend so much money on anti-aging creams and on face and breast lifts.
- "Genome" 2006 book by Matt Ridley, pages 24-35