is not too surprising if you consider that other animal species have had similar patterns before their current forms developed.
It seems that after Homo erectus evolved and moved out of Africa, HE evolution outside of Africa continued in various pockets of gene pools, developing their own traits. But, meantime, HE in Africa also continued evolving new changes up to early Homo sapiens, some of whom also moved out of Africa, met up with their HE cousins, and interbred with them, spreading HS around the globe, with traits added from their mating with Neanderthals, Denisovans, and whoever else we might yet discover, to produce the "new and improved" model of HSS (Homo sapiens sapiens) that we know today as....us.
Many driving forces were behind the changes that our ancestors went through, e.g. mutations, adaptations through natural selection, isolated gene pools for long periods, etc., but enough similarities remained smong various forms of HE descendants to make interbreeding between some of them possible.
But a significant factor in our ancestral migrations that allowed the variations to develop is not recognized often enough, IMO. Ancient climate changes. Initial glaciation, followed by alternating warming and glaciation periods, changed habitats, causing human ancestors to move around, following prey and vegetation.
Glaciation affected the entire planet, not just the regions covered by ice and snow. In Africa, for example, forest areas diminished in size, grasslands and deserts increased in some areas. Insufficient resources for large groups pressured some to move on. Favorable conditions caused some regions to increase in population. The need for adaptations favored increasing intelligence, ingenuity, use of local resources, and cooperation and sharing within groups as well as sometimes trade between groups.
Today, climate changes again require ingenuity, cooperation, and adaptations and changes. Our ancestors were up to it. I hope we are today.
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