system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If all homo sapiens originated from the African continent, and homo sapiens is the only human species left, then why do we have different races, colors and features?

Why do we have skin colors and drastically different features if we all came from the same region? Was it a result of mixing with other human species?
gpt
Evolution applies within a species, too.  Dogs are all the same species, and all descended from domesticated wolves, but they come in a massive variety of shapes, sizes, and colourations that are better-suited to different environments.  That's mostly *artificial* selection, but all that does is accelerate the process, so the same principle applies to humans.

Let's take the example of skin colours:

In areas where there's a lot of sunlight, and particularly where UV levels are high, melanin is an advantage, so people with more melanin (and therefore darker skin) will tend to live longer and have more children survive to adulthood, and before long you find the local population is quite dark-skinned.  Meanwhile, in places where there's much *less* UV, having more melanin is a slight disadvantage because it inhibits vitamin-D synthesis and has a slight associated energy cost, so there's a small advantage to being paler.  It doesn't have to be huge; over large numbers of generations even a small advantage adds up.

Combine all of this with some groups of humanity being geographically isolated from others, so that the cultures don't mix and therefore start to diverge, and cultural beauty standards will result in an effective selective-breeding programme, which further differentiates people.

All these differences exist, but humans are similar enough to each other and can still produce fertile offspring when they're from different ethnic groups, so we call them a single species.