We’ve been through them, and here we are.
Five million years ago a common ancestor of both pan troglodytes (chimpanzee) and homo sapiens lived in wooded (one guesses) parts of Africa. An area where woods met open grasslands enticed species members living there to spend more time on the ground.
Whithin a couple of million years their skeletons featured “feet” and “hands” instead of “four hands.” The hips shifted shape to better accommodate upright, two-legged locomotion, a change correlating to having feet and longer legs.
About two million years ago, by now genus homo species erectus et al. endured the first of about ten drastic shifts in weather, climate, flora, fauna, etc. such that only the most adaptable survived. At the same time, cranial size grew; the connection is easy to grasp.
The last one was truly severe; only a handful of hominids, proto sapiens, got through it. Among them one female line, as seen from mitochondrial DNA, out-reproduced or out-survived all others. A later funnel about a third that long ago, namely seventy to seventy-five thousand years, was the funnel for the male lineage. One Y chromosomal line, hence some Adam who had it first and whose many male descendants inherited the identical, un-mutated copy, out-reproduced and out-survived all other male lines.
Man, I hate those funnels! (cartoon caption.)