In message .com,
Boltar writes

Because it is correct to do so. "Boltar's evolution" would be correct
grammatically, therefore "a million years' evolution" is also correct.

Actually its not. Years is a plural , its not in the genetive case

It is very clearly a (plural) genitive - note spelling of genitive

in that instance otherwise it would mean "a million evolution of year"

No. The plural genitive means, in this case, "a million years of
evolution".

It is much the same as expressing the phrase "four weeks of drought" as
"four weeks' drought".

Some of us may be thinking you missed a crucial lesson in grammar
Anyway, I have no intention of being rude, but see if you can spot the
difference between these two clauses:

The boys' coats are over there.
The boy's coats are over there.

As I'm sure you spotted, the difference between s' and 's totally
changes the meaning of the expression.

--
Paul Terry