On a more gradual scale the real drama began about two million years ago. The ingredients for this “perfect storm” scenario begin with a forest/open landscape that made it optimal to spend part of the time on the ground foraging. Once the early homo species in that area had specialized for a life less and less dependent on the safety of trees, i.e. their feet, legs, and pelvis began to do a better job of supporting weight while in an upright stance, they found the trees too limiting.
THEN the weather changed, a lot. Every couple of hundred thousand years “the rules changed” and the ability to work things out in the evolutionary equivalent of real time rather than rely on instinct encouraged growth in brain size.
After long series of those two hundred thousand year shakeups, HSS had the ability to travel on two legs while using hands/arms to hold onto things like weapons, food stores, children.
And while the wildebeest migrates thousands of miles back and forth annually, just about every other species turned out to be vulnerable to being run down. HSS wasn’t terribly fast, but could go at a decent clip for a LONG distance. And with a larger brain, homo became a top-tier predator as well as scrounger and omnivore.
By the time modern HSS survived one last horrible weather period – about two hundred thousand years ago most large species came very close to dying out – things like ice and altitude had become quaint, interesting problems to solve. Where the air is thin, the local HSS population, having lived there for a hundred centuries or so, adapted to that. Likewise where the summer itself is a brief ice-free interlude, that HSS population evolved a couple of metabolic tricks for that. The folks who live on the mile-high plateaus in the middle of the African continent have small hands and feet – which are optimal for both conserving body heat and for long distance running. Kenyans do really well at marathons, don’t they?
Optimizing in small ways over some handfuls of tens of centuries isn’t that big a deal. And to the folks who say we’re about to undo a lot survival genes by making the world so user-friendly via cars and air conditioning, baby don’t look back, because evolution is gaining on you! Lots of things are making us die sooner, so there is both a fabulous chance for good genes to crop up that offset modern chemical etc. stressors, and a fabulous chance for them to spread.