Could two people actually repopulate Earth?

Adam and Eve might have managed it in the Bible, but scientifically speaking, would two people be enough to repopulate our world from scratch, despite the inevitable health issues associated with inbreeding and a limited gene pool? 

First of all, let’s deal with the obvious problems. The first ‘new’ generation would obviously all be brothers and sisters; the second, all cousins.

Multiple studies have shown that when these first or second degree relatives have children together, the results aren’t pretty.

One report looking at Czech children born to closely related parents between 1933 and 1970 found higher rates of infant mortality, and physical and mental disabilities.

Then there’s the widespread colour blindness on the island of Pingelap, caused after a devastating typhoon left just 20 survivors available to populate the community again.

Over at BBC Future, Zaria Gorvett suggests European royalty – and the strategic marriages therein – as another case in point. Charles II of Spain, to take the best known example, was born with a litany of physical and mental disabilities, something a team of Spanish scientists attributed to his high “inbreeding coefficient”.

In other words, he inherited a lot of identical genes from both parents.

There are plenty of other studies, but the underlying issue is the same across all examples: a small gene pool.

Rare inherited diseases (including the aforementioned colour blindness) are usually caused when two copies of a gene are passed on by the mother and the father.

If those parents are also brother and sister, their genes are much more likely to be similar. The problem then propagates through the generations.

And that’s not all. Genetic diversity allows species to overcome problems and get around changes in the environment, and that’s precisely what’s lost when close relatives reproduce. And sperm qualityand sperm quality is also known to be affected by inbreeding.

“With a small population size everyone is going to be related sooner or later, and as relatedness increases inbreeding effects become more important,” Bruce Robertson from Otago University in New Zealand told BBC Future. Robertson is part of a team trying to protect the remaining population of kakapo parrots from becoming extinct.

So far so bad, but there is hope for a future Adam and Eve.

The history of human civilisation shows that several pockets of small survivors have managed to grow in number and overcome the mathematical probabilities behind the genetics: take the Hutterite community of North America, descended from just 18 families.

“The evidence for the short-term effects of low genetic diversity is very strong, but all these things are probabilistic,” says anthropologist John Moore, who has been investigating how human beings might colonise other planets in partnership with NASA.

“There are stories of incredible journeys back from the brink – anything is possible.”

A version of this article was first published in November 2015.

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