Question

how evolution occured by a single ancestor linkage?

Answer 1

Genetic mutation.
Like blonds for instance, natural blondness is a genetical mutation which was allowed to spread because most men perferd blonds over brunettes. Eventhou if i remember correctly dark hair gene is more dominant.

When it came to pre-humans, it was more likely it was traits that benifited to surviviability. Like a opposable thumbs.

Source: http://www.whyevolution.com/chimps.html

Answer 2

Exuras is absolutely correct: cavemen prefered blondes.