It’s all history now
How DNA studies help to trace the origins of modern humans
Following individual genes down more than a few human generations is next to impossible. Egg and sperm are each made in a specially orchestrated cell division, and their paired genes – one copy from each parent in the previous generation – are thoroughly shuffled before their single sets of chromosomes are assembled.
These then combine to create a new person’s genome. That happens in every round of reproduction, so clear lines of descent get very blurred. Both sexes, though, contribute portions of DNA that escape this mixing. Only males have a Y chromosome, and they keep only one copy of the genes on it. Everyone has lots of copies of the mitochondrial genes, which are only passed on from the mother’s egg. That means there are two sets of genes that are normally passed cleanly from one generation to the next.
Analysis of both kinds has been used to track the history of human migration out of Africa. More recently, it has been extended to more detailed tracking of who may have been where and when. More subtle traces of our past can also be found in the bacteria in our guts, and sampling shows they differ from place to place.
Genome analysis of samples of a single species recovered from two different human populations can indicate when the populations became separated, assuming the bacteria are undergoing a roughly constant rate of mutation. One study traced the lineage of Helicobacter pylori (the bug that causes stomach ulcers) from European, African and Asian populations. The researchers established similar patterns of migration for the bugs as for their human hosts, indicating that people and H. pylori have probably been living with each other ever since the first modern humans left Africa.
However, using stomach bugs is not particularly convenient. In 2014, German and Chinese researchers showed that bacteria in saliva samples also contained genes that might be useful for tracing our ancestors’ movements.Lead image:
Wellcome Images CC BY NC ND
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