The alien predators arrived by boat. Within two years, everyone was
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.
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.
Even with these hideous risks in mind, if the survivors had enough
children the chances are at least some of them would be healthy. But
what happens when inbreeding continues for hundreds of years? It turns
out you don’t have to be stuck on an island to find out, because there’s one community that just can’t get enough of their close relatives:
European royalty. And with nine generations of strategic marriages
between cousins, uncles, and nieces in 200 years, the Spanish Habsburgs
are a natural experiment in how it all adds up.
Charles II was the family’s most famous victim. Born with a litany of physical and mental disabilities, the king didn’t learn to walk until he
was eight years old. As an adult his infertility spelled the extinction
of an entire dynasty.