How does evolution explain the spread of humans across the globe?

Question

how long would it take for a species separeted from its other varieties to eventually form a new species.

and how come there is such variety of humans across the globe but despite of developing completly different apparances to each other we are still compatible... did we all come from one place then spread across the world to develop our different characteristics

or did our ancestors spread across the globe but we all evolved at the same rate in order to remain the same species.

because i marvel at how we are all very very different in features, and spread across the globe but yet remain compatible with each other and can easily form new breeds such as mixed race people...

Answer 1

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Answer 2

It doesn't.  

Answer 3

There are a couple reasons.

1)  It hasn't been that much time since we 'separated' ourselves into various continents.  Only a few hundred generations.

2)  Intercontinental travel has been around for thousands of years, allowing intermixing of DNA from different populations.

Answer 4

We've only been separated for around 60,000 years.  For many populations much, much less than that.

That's about 3000 generations.  Not long at all.  For comparison, that many generations of bacteria can grow in a little more than a month.

In addition, we faced a population bottleneck around 70,000 years ago, which greatly limited  our genetic diversity.

So, we started with a small gene pool and haven't been separated for very long.  Two things that greatly limit speciation.