We have this idea humans will either go extinct, or evolve into something more sentient, like how extraterrestrial humanoids are often depicted in pop culture, with humungous, bulbous heads, enormous eyes, tiny faces, ears, mouths, noses and small, lithe bodies.
But perhaps both of these scenarios will miss the mark, and we’ll survive, but devolve into something more base, more primitive.
After WW3, and the collapse of civilization, some of the elite, despite bearing much of the responsibility for precipitating events, will go into hiding, cowering in underground fortresses they’ve already pre-emptively built in preparation for such a catastrophe.
At some point, after nuclear fallout, they may decide to emerge, or they may remain, if they’re comfortable, growing accustomed to subterranean life.
How may they evolve, and what about the few humans who manage to survive above ground?
Many will have to turn to cannibalism, or hunting and gathering whatever plants and animals remain post-nuclear fallout.
Perhaps a few will manage to eke out a subsistence agricultural existence.
Radiation is interesting, on the one hand it’s very destructive, but on the other, it might speed up mutations and thus, evolution in either direction, towards or away from sentience, a high death rate might also speed things up, and humans now being isolated from one another in small pockets might increase divergence, giving rise to new races and eventually, after thousands or millions of years, new species.
Since sentience will’ve gotten us into such a mess in the first place, and since humans will have to rely more on brute physicality and instincts to survive, we might grow dumber, and more fierce.
Humans may even play an active, conscious role in the reduction and scaling back of their own consciousness, culturally and instinctively, preferring stronger, simpler mates, turning away from science, since it was science that got us into such a mess, and away from religion too, since the messiah, the Maitreya, Jesus, whatever Gods you pray to, never came back as promised.
Perhaps some humanoids will evolve claws, a bit of a snout and sharp teeth, a keener sense of smell and hearing, but reduced vision, but keep many of their human characteristics, like lack of hair/fur, being able to walk upright, and having a little more intelligence and sociability than your average carnivore, living together in large packs, occasionally using simple tools.
They might develop dense bones and massive musculature, more strength, durability, agility and speed, but at the expense of manual dexterity and stamina.
They might also have reduced facial expressions, capacity for language, and smaller brains.
Their women may have larger litters than our women do, and more teats for suckling.
Others may take a more herbivore path, with more complex stomachs, and living together in large herds.They may grow some kind of horns or bony armor plates like many herbivores do.
And others may become more gracile, due to the tough times ahead, where food is scarce.
They may develop large eyes and ears, and have a really anxious, nervous disposition, huddling together in broken down, dilapidated buildings, or in caves, scavenging for food.
Perhaps after millions of years of devolution, some humanoids will even learn to walk or crawl on all fours, developing paws or hoofs, but still look vaguely human.