How two people couldn't have populated the world: The bitter truth

The Doomsday Clock is a symbolic representation of how close we humans are to wiping ourselves out of existence. It isn't meant as a kind of accurate countdown, but is a metaphor for the general peril humanity is facing at any given time, with midnight representing our total annihilation. The clock was originally set in 1947, not long after the end of the Second World War when things were looking fairly dicey. Two years previously, atomic bombs had been dropped on the Japanese cities of Hiroshima and Nagasaki, and nuclear tensions were continuing to rise. To reflect the dire situation, the Doomsday Clock was set at seven minutes to midnight. In the intervening decades, the time on the clock has been reset 24 times to reflect the differing existential danger humanity has found itself in. Today, thanks to the continued threat of nuclear war and the rapid advance of climate change, combined with what doomsday scientists call cyber information warfare, the clock is set at 100 seconds to midnight. That means that never before in human history, or at least since 1947, have we been closer to our own destruction as a species.

That's why it's never been more relevant to ask ourselves the totally not ridiculous question: could two people, sole survivors of some great global catastrophe, repopulate the earth? Maybe it'll be an asteroid strike, perhaps nuclear war, or rogue AI. Whatever the cause, if you were to find yourself as one half of the last couple on earth, would it be possible for you and your new best friend to rebuild the human race from scratch, assuming all the important bits survived the apocalypse in full working order? And after the inevitable "I wouldn't sleep with you if you were the last man on earth" jokes had ceased being funny, making the first generation ought to be no problem at all. If you wanted to get things off to a particularly good start, you could even have a go at breaking the record for the most number of children produced by a single couple. That's 69, by the way, set by Randy Russians, Mr. and Mrs. Vasilyev, back in the 18th century. Though I think that was less about saving the human race and more to do with the fact that they somehow produced 16 pairs of twins, seven sets of triplets, and four sets of quadruplets, as well as the fact that condoms of the day were typically made out of animal intestines and were unsurprisingly not all that popular.

So far, so good then. But as you may have predicted, things are going to get pretty tricky from here on in. There's no way of getting around it. That first generation of post-apocalyptic survivors, your children, will all be brothers and sisters. And that means making generation number two is going to require some pretty serious incest. At least you're only going to have to give the dreaded birds and bees talk once. It was legendary founding father of psychoanalysis, Sigmund Freud, who named incest as one of the two universal taboos. I guess Cersei and Jaime didn't get the memo on that one, but Lannisters aside, the incest taboo turns out to hold remarkably true in every single human society around the world practices it in some form or another. Though the exact rules on which family members it's okay to get jiggy with change from culture to culture, some scientists believe this inbreeding avoidance behavior is essentially hardwired into us. That we're simply born with an instinctive and deep aversion to the idea of incestuous relationships. And that would make sense, because whilst I'm confident that everyone reading this finds the thought of sexual relations with their close relatives utterly repulsive, that Isn't the only reason you should be avoiding it, inbreeding is also downright dangerous.

To understand why we need to take a look at our genes. The Human Genome Project was an international research effort to determine the sequence of the human genome and identify the genes that it contains. Amongst its many achievements, it's estimated that we humans have between 20 and 30,000 genes apiece. We have two copies of each of our genes, one inherited from our father and one from our mother, and mutations in some of these genes are responsible for nasty inherited diseases like cystic fibrosis, sickle cell anemia, and Tay-Sachs disease. But the majority of the worst inherited diseases are recessive. That means that you'll only actually have the disease if you inherit the same gene mutation from both your mother and your father, which makes sense when you think about it. If a dominant trait caused very serious defects, it would most likely die out. By having children with a close relative, say a sibling with whom you share about half of your DNA, you dramatically increase the chances that your offspring would have two copies of any potentially damaging genes that happen to be knocking around in your DNA. The average person has around two of those, by the way. So that's a quick look at the science. But what happens when we see inbreeding in action in the real world?

The Island of Pinglab is a tiny atoll in Micronesia. Back in 1775, it was devastated by a huge typhoon which left only 20 survivors. Of those survivors, the island's king, whose name I will not butcher in this article, suffered from a very rare syndrome called complete achromatopsia which leaves those who have it completely unable to see colour. Today, one in 10 people on Pinglap suffers from the disease. That might not sound like much, but just one in 33,000 people suffer from it in the general population. This is an example of what is known as the founder effect - the loss of genetic variation that occurs when a new population is established from only a small number of initial founders.

Now I have no doubt that complete color blindness sucks - you can forget about a career in snooker for a start - but inbreeding can have far, far deadlier effects than that. Remember, there were 20 survivors of the typhoon on Pinglab, which means nobody was being forced to have babies with their sister or brother in order to repopulate the island. If they were, things would probably have been much worse because the more closely related two people are, the more dangerous inbreeding becomes. One study from Czechoslovakia carried out between 1933 and 1970 found that of children who were born to parents who were first-degree relatives, that is, parents or siblings, 40 percent were severely handicapped, and 14 of those died young. It's also thought that inbreeding was responsible for killing off one of Europe's great royal dynasties, the Habsburgs.

The Habsburgs ruled over Austria, not to mention half of Europe at one time or another, for almost 650 years. But like many people who have a taste of power, they were very, very keen to hang on to it. In a truly extreme version of "what happens in the family stays in the family," the Habsburgs were notorious for arranging strategic marriages within their own family tree. Uncles married nieces, cousins married other cousins, and just about everyone committed incest like it was going out of fashion. Despite all our riches, lands, and wealth, the Habsburgs weren't the prettiest bunch.

Image from #google

Take a look at one of The many portraits that were painted of their endless kings, and you'll probably notice most had one thing in common - a chin of truly epic proportions. The condition which caused this facial abnormality is known as prognathism or more fittingly, Habsburg's jaw, and it was extremely widespread in the family due to inbreeding. But generations of cousin-on-cousin action caused much more than just incurable ugliness, and the chin, like a sledgehammer, stepped forward. Charles II of Spain, also known as Charles the Bewitched, aside from being the son of royalty, Charles's life didn't get off to a great start. For one thing, his mother was also his father's niece, and his grandmother was also his aunt. In fact, thanks to generations of consanguineous marriages (that's inbreeding to you and me), Charles II was genetically speaking more inbred than the offspring of two siblings would be. As well as that distinctive Habsburg's jaw, which was said to be so severe that Charles had trouble eating and speaking, poor Charles is forced to have suffered from several genetic conditions, including combined pituitary hormone deficiency and renal tubular acidosis. He didn't learn to speak until he was four years old, and by the time he was six, he had measles, chickenpox, rubella, and smallpox. As an adult, it's said that he was so spectacularly ugly that he scared his own wife. But Charles was the last male of his line, so the Habsburgs were naturally keen to make sure he left an heir. Unfortunately, neither of his two marriages produced a child, and this, despite Charles carrying out his doctor's advice to sleep in the same bed as the corpse of his father in order to improve his fertility. How that master plan failed to work, I guess we'll never know. But try as they might, nowhere ever arrived. Having said that, being as Charles's autopsy would eventually reveal that he had only a single, atrophied testicle that was, and I quote, "black as a lump of coal," that probably shouldn't come as too much of a surprise. Ultimately, Charles was to be the last of the Habsburgs monarchs in Spain, and his death in November 1700 led to the War of the Spanish Succession as half of Europe fought over the scraps of the world's greatest inbred empire.

Looking a little further back in history, we meet the Neanderthals, one of our closest human relatives who last walked the earth about 40,000 years ago. For a long time, it was thought that we, or at least our ancestors, were responsible for killing off our broad brown cousins. Why? Anthropologists commonly assume that despite having smaller brains, us Homo sapiens were a far smarter bunch and were able to outwit Neanderthals, using intelligence and planning to steal both their food sources and their lives. But whilst that may be partly true (we are the earth's biggest bullies after all), recent evidence suggests that Neanderthals might simply have been unable to avoid inbreeding due to their relatively small population size, and hence they shacked themselves out of existence.

So what does all of this incest-based doom and gloom mean for our post-apocalyptic couple, bravely plugging away in their attempts to repopulate planet earth? Sadly, chances are, it's all going to end in tears and atrophied testicles. Now I'd forgive you for thinking that all of this is a completely pointless and hypothetical question to ask in the first place. The Doomsday Clock may well suggest that catastrophe is nigh, but it seems pretty unlikely that such an Event would conveniently leave some kind of neo-Adam and Eve behind to get busy starting the world's most disturbing family, but actually the threat to our existence from imminent catastrophe is the very reason all of this stuff is relevant. It's fair to say we're pretty much all in on planet Earth. We haven't just put all of our human eggs in the blue planet basket; we've stashed the chickens, the henhouse, and a whole damn farm in there, too. If something does go wrong on Earth, an asteroid strike, for example, we as a species are essentially gone, and if we really are, according to the Doomsday Clock, just 100 seconds away from such a disaster, it would be a pretty good idea for us to spread the hell out as soon as possible. Nuclear war might well wipe out life on Earth, but if we could establish a handy couple of colony planets out there amongst the stars, we'd have an insurance policy in place. The problem is, there aren't any other habitable planets nearby, and by habitable, I mean Earth-like. There's Mars, of course, but that will require revolutionary terraforming technology that we don't yet possess if we want to actually live on the surface that is, and not in artificial bubble-esque environments. The nearest star to our own, Proxima Centauri, is 4.2 light-years away. Just to put that number into the kind of perspective our tiny human minds are capable of understanding, that's around 25 trillion miles. The current furthest man-made object from Earth is the Voyager 1 space probe that was launched by NASA in 1977. The probe streaked clean out of our solar system back in 2012, and as of March 2020, it's made it a respectable 13.8 billion miles from Earth. Unfortunately, that is only about 1/800th of the way to Proxima Centauri. At that rate, it would need another 77,000 years or so to make it the rest of the way.

Technology has improved since the days of Voyager 1, and NASA's Juno probe that swung by Jupiter in 2016 got up to about 165,000 miles an hour. That kind of speed would bring our trip to Proxima Centauri down to about 17,000 years, and assuming this kind of technology continues to advance rapidly, it may not be too long before we're ready to send a new colony spaceship out into the stars. But such a ship would undoubtedly need to house its precious cargo of interstellar explorers for many, many generations, which is exactly why scientists need to know how many people it would take to maintain a healthy population with a sufficiently diverse gene pool that could one day repopulate an entire planet. It's a question that might one day secure the survival of our very species. So what's the answer? Well, according to astrophysicist Frederick Marin, for a six-thousand-year journey to Proxima Centauri in a shiny new space ark, you would need a minimum of 98 individuals, though it's worth pointing out that similar studies have reached wildly different conclusions. Some suggest we would need tens of thousands of people for such a trip to be a success.

Before I go, I want to leave you with a little thought experiment. Have you ever wondered how many ancestors you have? I'm willing to bet it's something that's never really crossed your mind, but working it out should be easy, right? You have two parents, four grandparents, eight great-grandparents, and sixteen great-great-grandparents. That's 30 ancestors so far. All we need to know is how many generations there are in Total,

And we should have the answer, but the mathematically minded amongst you may be beginning to nervously finger your calculators because the number of your ancestors appears to be doubling with every generation. And anyone who's ever heard of the rice and chess board problem will know that this kind of exponential growth can't be good. By this method, doubling as we go from one generation to the next, you would have just over a trillion ancestors after 40 generations (approximately the last millennium of human history). A trillion ancestors, for reference, is around 1,000 times more humans than have existed ever, and that's just 40 generations of an estimated 7,000.

So, what are we missing? I'm glad you asked because, as you can see, the numbers simply don't add up here. Nature must have taken a few shortcuts during your ancestral history to break that exponential growth somewhere. Well, it certainly has done, and the answer is quite simple when you think about it. Every single person alive today is related. We're all part of the same giant family tree, meaning we all have many, many shared ancestors, or to put it another way, and if we're prepared to stretch the meaning of the word just a tiny bit, you, are inbred. You're welcome.