Scientists say they can fill a blank in the history of Central Africa's pygmies, whose past is one of the most elusive of any community in the world.
At a key period in the human odyssey, these hunter-gatherer tribes shunned interbreeding with Bantu-speaking communities who were early farmers, according to a gene analysis.
The two groups first met when the Bantu groups, having acquired farming technology some 5000 years ago, started moving out of the region of Nigeria and Cameroon into eastern, central and southern Africa.
The pygmies may have traded pottery, tools and ideas with the newcomers, but not their genes, said the study in the journal Nature Communications.
The evidence comes from a reading of the DNA of some 300 individuals - pygmies and Bantu-speakers from Gabon, Cameroon, Uganda, the Central African Republic and the Democratic Republic of Congo.
"This result suggests that social relations established since the two groups first met were quickly followed by a strong taboo against inter-marriage that is to some extent still observed today," co-author Etienne Patin, a geneticist at France's Institut Pasteur, said on Tuesday.
The latest findings challenge the accepted science that genetic diversity is closely correlated with geographic distance between human groups.
Central African pygmies are a case in point - there are only about 200,000 individuals in total, yet their genetic diversity far exceeds that of their sedentary neighbours, said the study.
The Batwa pygmies of Uganda, for example, are genetically quite distinct from the Mbuti pygmies who live a mere 500km away in the DR Congo.
The researchers further found that the pygmy genome could contain as much as 50 per cent DNA inherited from people of Bantu origin.
And their height is directly proportional to the amount of non-pygmy DNA inherited - "the less one is pygmy, genetically speaking, the taller one is," said Patin.
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