The last man on Earth is a common trope in fiction – but what if it actually happened? How many people would it take to save our species?
By Zaria Gorvett
This story is featured in BBC Future’s “Best of 2016” collection. Discover more of our picks.
The alien predators arrived by boat. Within two years, everyone was dead. Almost.
The tiny islet of Ball’s Pyramid lies 600km east of Australia in the South Pacific, rising out of the sea like a shard of glass. And there they were – halfway up its sheer cliff edge, sheltering under a spindly bush – the last of the species. Two escaped and just nine years later there were 9,000, the children and grandchildren and great-grandchildren of Adam and Eve.
No, this isn’t a bizarre take on the story of creation. The lucky couple were tree lobsters Dryococelus australis, stick insects the size of a human hand. They were thought to be extinct soon after black rats invaded their native Lord Howe Island in 1918, but were found clinging on in Ball’s Pyramid 83 years later. The species owes its miraculous recovery to a team of scientists who scaled 500ft of vertical rock to reach their hiding place in 2003. The lobsters were named “Adam” and “Eve” and sent to start a breeding programme at Melbourne Zoo.
Bouncing back after insect Armageddon is one thing. Female tree lobsters lay 10 eggs every 10 days and are capable of parthenogenesis; they don’t need a man to reproduce. Repopulating the earth with humans is quite another matter. Could we do it? And how long would it take?
The answer is more than a whimsical discussion for the pub. From Nasa’s research on the magic number of pioneers needed for our move to another planet, to decisions about the conservation of endangered species, it’s a matter of increasing international importance and urgency.
The average person has between one and two lethal recessive mutations in their genome
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.
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…