The way that current anthropological orthodoxy seemingly has it, all of the evolution that went into the development of modern humans took place in Africa. Which suggests that either --
1. human evolution doesn't take place outside Africa, or
2. if it does, then all of human evolution outside Africa was abortive, leading to varieties of human that later became extinct (Neanderthals, Denisovans and the Flores Island hobbits perhaps).
The first is unconvincing to me for biological reasons. Reproductive isolation and introduction into new ecological niches are drivers of evolution. Early hominins expanding their range into Eurasia would seem to have increased both conditions. So one would expect the pace of human evolution to pick up after these populations left Africa. (Like Darwin's finches evolving new species as they colonized new islands in the Galapagos.) One wouldn't expect the pace of evolution to cease as they spread out.
The second seems unconvincing to me too, simply for reasons of likelihood.
The evolution of human beings is convoluted. I suspect that the fact that modern humans evolved in Africa is partly due to favorable environmental conditions, and also due to the contingency of having the "right" kind of ape ancestors there, which is more or less accidental.
Primate evolution has been traced back 65 million years, after the mass extinction event that wiped out the dinosaurs. There were some primate-like species in North America, some in China, but many have been found in Eurasia and Africa. One theory has it that the early primates that eventually involved into modern apes migrated south from Europe to Africa, or else came across from western Asia. From there, the species diversified, producing the great apes and human beings. Several species of human beings then migrated out of Africa to various other places. It appears that
Homo sapiens out-competed
Homo neanderthalensis (which some argue is a subspecies of
Homo sapiens) for a similar ecological niche, leading to the eventual extinction of the latter species.
The
Hominoidea (apes) superfamily, including the
Hominidae family of which modern humans are a species, diverged from the
Hylobatidae (gibbon) family about 15-20 million years ago. African great apes (subfamily
Homininae) diverged from orangutans (
Ponginae) about 14 million years ago. Humans, chimps and some extinct bipedal species diverged from gorillas about 8-9 million years ago. Chimps diverged from humans and their biped ancestors about 5.6-7.5 million years ago.
Bearing in mind that
Homo sapiens is a very young species in evolutionary terms, and that human geographical diversification has taken place only very recently, with a lot of genetic mixing of regional groups, it is not particularly surprising that human beings have, for the most part, not split into distinct species since leaving Africa.
I think that we do see fairly distinct biota appearing on different continents.
Absolutely we do, particularly where there is geographical or ecological isolation.
Even today, Australia serves as a refuge for marsupial populations that are unusual elsewhere. (It's also a refuge for human types like JamesR and Bells.)
The marsupial populations of Australia are not just unusual. They are natively found nowhere else in the world. Think kangaroos, koalas, wombats, echidnas, platypus, antechinus, bilby, quokkas ... the list goes on and on.
But every continent has species that are unique to that continent.
On a smaller geographical scale, genetic isolation can happen in lots of different ways, leading to species whose ranges are restricted to very small areas indeed. For example, using an Australian example again, if you're a mountain pygmy possum then you can't survive the temperatures that exist below a certain level on the mountains where your species evolved. That makes it very difficult, if not impossible, to spread your population even to the next-door mountain range, let alone to a different continent.
The variable that seemingly determines how unique a continent's biota is, is how isolated that continent is.
Also worth mentioning is that how isolated the continent has been in the past is also relevant. The continents aren't in the same places today that they were 100 million years ago, for instance. You have to take continental drift into account. Also, with continental drift comes changes in climate, which can be fatal to many species. Antarctica used to be near the equator.
Well, human beings do have so many anatomical similarities that common ancestry is strongly suggested. We do seem to derive from the same ancestral interbreeding population (not necessarily the same individuals in that population, so I'm skeptical about the 'genetic Eve' idea that has all of us the decendants of one particular woman) and that population does seem to have been located among the rift valleys of east Africa. That's millions of years ago.
There is some evidence of genetic bottlenecks in
Homo sapiens. Our DNA as a species is actually less diverse than we would expect if we had maintained largish populations over the whole lifetime of the species.
If early anatomically-modern man first appeared in Africa and spread into Eurasia roughly 100,000 years ago in a new out-of-Africa episode, did they replace the variants who were already living in Eurasia, by driving them to extinction? Or did these populations interbreed, forming a hybrid modern type?
Those are good questions, and the subject of ongoing research, I believe.
These are questions that are hugely moralized and politicized in our current climate, touching as they do on race.
I don't really see how matters of human evolution over millions, or even hundreds of thousands of years, could possibly involve the somewhat arbitrary modern idea of "race".