From the zombie viruses to comet strikes, apocalyptic scenarios are all the rage on television these days. But what if Earth’s population was decimated to the point where only two people—a man and a woman—were left? Could they repopulate the Earth despite the fact that their offspring must mate with each other?
Repopulating the Earth sounds like a dream scenario, amirite guys? Well, it may seem like a XXX version of the Garden of Eden for you, but for your children and grandchildren—all that inbreeding could lead to rare genetic diseases and everyone turning out like Hodor from Game of Thrones.
It turns out, little isolated doomsday scenarios play out in nature relatively frequently, and we can get hints about what might happen to a human population if something devastates our global population, or if a settlement on Mars gets isolated for generations.
On the tiny South Pacific island of Ball’s Pyramid, invasive species wiped out an entire population of stick insects called tree lobsters—except for two individuals. The surviving lobsters, obviously named “Adam” and “Eve”, were sent to Australia’s Melbourne Zoo to start a breeding program and 9 years later, there were 9,000 offspring.
Human inbreeding is dangerous business
Despite this success story, humans are much more genetically complex than stick insects, which means we should take this scenario with a grain of salt.
The risks of human inbreeding come from inherited diseases caused by “recessive” genetic variants. Almost all of us carry these genetic mistakes, and they are harmless on their own. However, when two people with the same dangerous variant have a child, there is significant chance the child could be afflicted with the disease. Furthermore, after one generation of inbreeding, the risk increases significantly.
The consequences of generations of inbreeding can be seen in the history of European royalty. Famously, Charles II of Spain had numerous gene-related mental and physical disabilities. In 2009, Spanish researchers found the monarch was so inbred that he would have been better off if his parents were simply a brother and a sister.
There is one major counterargument to this bleak scenario, and it comes from our species emigration out of Africa. Populations of early Homo sapiens were so isolated, their genetic pools were undoubtedly very shallow, and yet we made it through that bottleneck.
That isn’t quite our one man-one woman scenario, but it does offer hope in the event of that very unlikely event.