How do we explain how different ethnicities ended up looking so different from each other in such a short amount of time, if Adam and Chava only lived less than 6000 years ago? Did Hashem make it that way after the tower was built so He could disperse people even more?
Note, the same question can be asked about how Jews, who all descend from Avraham, Yitzchak and Yaacov, are now of many different races. The traditional answer:
The whole world is a revolving globe. Some of the earth's inhabitants are above and some below. The races indigenous to the various climes differ in appearance due to the differing environments of their native lands. When one part of the world is in sunlight, the other part is in darkness. There is an inhabited latitude where daylight sometimes prevails for almost the entire twenty four hours.
The mechanism for this epigenetic effect seems to be based on what the mother sees, as we learn from Yaacov in Bereshit 30:37-39:
And Jacob took himself moist rod[s] of trembling poplar and hazelnut, and chestnut, and he peeled white streaks upon them, baring the white that was on the rods. And he thrust the rods that he had peeled, into the gutters in the watering troughs where the animals would come to drink opposite the [other] animals, and they would come into heat when they came to drink. And the flocks came into heat by the rods, and the animals bore ringed, spotted, and striped [young].
They would come into heat — The female animal saw the rods, it was startled at the sight of them and recoiled; its mate then pairing with it, it afterwards gave birth to young marked similar to the rods
Rabbi Yochanan, known for his exceptional beauty, would sit outside the mikva so women would see him and as a result have beautiful children. Berachot 20b:
Rabbi Yochanan would go and sit by the entrance to the ritual bath. He said to himself: When Jewish women come up from their immersion [after their menstruation,] they should see me first so that they have beautiful children like me
The modern understanding of this is likely linked to epigenetics. Commonly, it is thought everything about us is hard coded into our genes. This is partly true, but there is variability in how the genes are expressed. This is a famous example. It seems something in the mind of the parents, especially the mother, can affect gene expression, which includes the shape into which their children's bodies develop, as well as personality traits.
As for when humans stopped looking like Adam and we ended up with a diverse range of racial body types, I am not sure. It seems likely that this happened before the tower of bavel, as the 70 nations were all in place by that point. There are sources that indicate that even the 3 sons of Noach each had a different skin colour. See Pirkei D'Rebbi Eliezer 24.
The interesting piece was this snippet:
Human evolution has not only been recent and extensive, it has also been regional. The period of 30,000 to 5,000 years ago, from which signals of recent natural selection can be detected, occurred after the splitting of the three major races, so represents selection that has occurred largely independently within each race. The three principal races are Africans (those who live south of the Sahara), East Asians (Chinese, Japanese, and Koreans), and Caucasians (Europeans and the peoples of the Near East and the Indian subcontinent). In each of these races, a different set of genes has been changed by natural selection.
This is not a subject matter I am familiar with, but from the article, your premise of needing more time than the Torah allows for races to be created is not necessarily in line with science. (Note that the author, Nicholas Wade, has a Wikipedia page discussing how many scientists disagree with his research.)
Interesting to note that 30,000 to 5,000 years ago means that the races we can analyze are all post-flood. Not surprising to those who take the Torah literally.