So let’s fast-forward 100 years. Humanity’s endeavours have gone horribly wrong and a robot uprising has wiped us off the face of the Earth – a fate predicted by Stephen Hawking in 2014. Just two people made it. There’s no way around it: the first generation would all be brothers and sisters.

Sigmund Freud believed incest was the only universal human taboo alongside murdering your parents. It’s not just gross, it’s downright dangerous. A study of children born in Czechoslovakia between 1933 and 1970 found that nearly 40% of those whose parents were first-degree relatives were severely handicapped, of which 14% eventually died.

Recessive risks

To understand why inbreeding can be so deadly, we need to get to grips with some genetics. We all have two copies of every gene, one from each parent. But some gene variants don’t show up unless you have two exactly the same. Most inherited diseases are caused by these “recessive” variants, which sneak through the evolutionary radar because they are harmless on their own. In fact, the average person has between one and two lethal recessive mutations in their genome.

When a couple are related, it doesn’t take long for the mask to slip. Take achromatopsia, a rare recessive disorder which causes total colour blindness. It affects 1 in 33,000 Americans and is carried by one in 100. If one of our post-apocalyptic survivors had the variant, there’s a one in four chance of their child having a copy. So far, so good. After just one generation of incest, the risk skyrockets – with a one in four chance of their child having two copies. That’s a 1 in 16 chance that the original couple’s first grandchild would have the disease.

This was the fate of the inhabitants of Pingelap, an isolated atoll in the western Pacific. The entire population is descended from just 20 survivors of a typhoon which swept the island in the 18th Century, including a carrier of achromatopsia. With such a small gene pool, today a 10th of the island’s population is totally colour blind.