[MarianaM (OP)]

Paul is wondering...

There may be small mutations, but generally a human is born as a human. Why?
If humans evolved from apes, who going back evolved originally from microbes or bacteria, why did we stop at humans? Why did apes stop at apes, or alligators as alligators, etc?

Any thoughts?

[alancalverd]

Stopped? Homo sapiens hasn't been around very long in evolutionary terms, and has already formed several distinct subgroups. There is no absolute definition of species and whilst it is possible that modern caucasians have traces of Neanderthal ancestry, for instance, it is much less likely that any other racial group does - we have branched in evolution. The only reason we are all assigned to a single species is that we are fully interfertile and can fully coexist with each other in any of the ecosystems that each other inhabit, but there is nothing to prevent the emergence of a new species that dominates and succeeds in some corner of the ecosphere.

[evan_au]

Why did apes stop at apes?

Two of our closest living relatives are the chimpanzees and bonobos (both apes).
It is thought that the formation of Congo River, about 1.5-2 million years ago led to the formation of 2 species from a single ancestor population, since these apes cannot swim well (humans have built-in "floaties").
See: https://en.wikipedia.org/wiki/Bonobo

So geographical isolation is one prerequisite for speciation.
- But as all of human history has shown (predating Alexander the Great, Ghengis Khan, Christopher Columbus, James Cook and Amelia Earhart), humans don't let geographical obstacles get in our way.
- So today we don't have the necessary isolation to form new species
- If, however, we manage to establish a self-sustaining colony on Mars, or send off multi-generation spaceships, and then Earth suffers a civilisation collapse so that space travel is no longer possible, speciation may well occur in a couple of million years.
- Or, sooner, if astronauts are genetically engineered to be more radiation-resistant, survive long periods in zero gravity or high-G acceleration without ill-effects, and have biological interfaces to computers, this might be seen as a new species.

Another driver is a different environment.
- Darwin's observations of birds and turtles in the Galapagos Islands had slight geographical separation (a group of islands), but it also had small environmental differences due to rainfall, soil and vegetation
- This produces small biological differences between these creatures on different islands - it took a keen eye to spot them!
- Even differences in diseases can produce immunological changes in populations - and this doesn't change on the skeletal structure which appears in fossils

or alligators as alligators

Alligators in fossils look somewhat like alligators today.
- One assumes that there has always been some part of the Earth that has had the wet, swampy ground that they enjoy
- One theory for why they outlived the dinosaurs is because they can survive with a low metabolism
- But we should not assume that alligators today are the same species as during the dinosaur era - there will be dramatic DNA changes on a timescale of millions of years
- And in fact, not all alligators today are the same - many a tourist visiting Northern Australia has ignored clearly posted warnings, assumed alligators were pretty harmless, went swimming - and rapidly transformed into an alligator.