If Hollywood has taught us anything, it’s that there are so many varied ways for humanity to be almost entirely snuffed out. We could be wiped out by an asteroid or aliens. Or perhaps our demise will be a result of a natural disaster, whether it is caused by climate change or an event like a super volcano exploding. Or there could be a virus or something else unexpected that manages to kill off the population.
In the incredibly unlikely event that two people of the opposite gender survived and they weren’t related, is it possible that they could repopulate the Earth with humans?
The short answer is yes, it is possible. The slightly longer answer is that it is likely, but not probable.
What Will It Take to Repopulate the Planet?
There is a whole set of conditions that both people have to meet – beyond their gender and genetics.
1. The Childbearing Years
Both the man and the woman need to have decades left of the childbearing years. The woman clearly has fewer years in which she will be able to have children as menopause will end her potential to continue to birth new children.
While it is possible for men to have children for most of their lives, their sperm does not hold up quite so well. Studies have shown that the older a guy is, the greater the risk of the children being born with disorders. 20% of infants die from birth defects, and men over 49 years old are significantly more likely to have children with birth defects.
The woman may be the determining factor if they can keep having children, the odds of children being born with birth defects will increase as the couple age.
It’s also important to remember that even today, childbirth has an inherent risk of death, both for the mother and the child. Younger women do have an easier time giving birth, but at every age, there is a risk that the mother, the infant, or both could die because of how intense the process is. If neither the mother or the father are medical professionals, they should definitely read up on the process, focusing on ways to deliver a baby as safely as possible.
2. The Well-documented Problems with Incest
If the two survivors aren’t related, that will at least be a good starting point, but it means that every generation after theirs will be directly related. Siblings would have to have children together to keep the population going, and their children would be first cousins.
Royalty over the millennia is a prime example of why incest is not a good idea, especially for direct relatives. The Habsburgs created a long line of people with visible deformities and children who were far more likely to suffer from hemophilia. While the Habsburgs and other European royalty tended to marry their aunts, uncles, and direct cousins, the Egyptian royalty usually married their direct relatives, with siblings frequently marrying to ensure that the dynasty kept the throne.
One of the primary problems shown by all of these relatives intermarrying is that any genetic disorders were significantly amplified within the family. For example, hemophilia is a fairly rare genetic disorder, but a significant percentage of royal families had cases., with both Queen Victoria and the Russian Romanov families having children with the disorder or were carriers of it.
When small groups of people settled in North America and Australia, this problem arose often as survival was difficult and after a couple of generations, more people were related. The founder effect is when a small group of humans creates a new colony or settlement. Since it won’t take more than a few generations for the small population to start intermarrying with cousins and other relatives, it increases the likelihood of any genetic disorders manifesting themselves. The Pingelapese is an example of this. They inhabit a small island with a small population, and roughly 1 in every 20 of the Pingelapese are severely color blind. To put this into perspective, the rest of the world has a ratio of 1 in 20,000 people with severe color blindness.
This is one example of a taboo being a taboo for a very good reason because it is incredibly dangerous for related people to have children together. Mental disorders and personality disorders can further create problems as these mutations are also amplified with a small genetic pool.
If the initial couple in our scenario manages to have about two dozen children, those children will be much healthier than the subsequent generations. This is why it is important for the original couple to have as many children as possible – and why those children need to be healthy, the odds of which will decrease as the parents age.
3. The Saving Grace and Potential Damage of Genetic Mutations
Genetic mutations are inevitable, though it does tend to take time for DNA to mutate. Some of these mutations could be very beneficial since it would add some genetic diversity. However, some genetic mutations could be deadly. Studies have indicated that everyone has at least one, and usually two, recessive mutations that are likely to be deadly.
When non-fatal mutations occur, it can help the population to become healthier as people will be less genetically similar. The problem is that this family will not have the ability to edit the DNA – an ability that is becoming increasingly possible today. If the couple had the necessary tools, they could reduce the risks of genetic problems caused by incest. However, with just two people who are almost certainly not going to be genetic experts and who will not have the necessary tools even if they were, they are going to have to hope for positive genetic mutations to help improve the gene pool.
4. Branching out Or Staying Put
If the original couple in our scenario is able to establish a lineage, those first few generations will need to stay close to the original settlement to improve the odds of survival. This means that the odds of the founder effect will probably have plenty of time to take hold, making the baseline problems for the future of humanity at greater risk of the recessive disorders carried by the first two people.
Once the population is more established, it would be best for a small group to leave and establish themselves elsewhere. This is similar to what happened during the early evolution of people, with the earliest generations of people moving out of northern Africa into southern Africa, the Middle East, Europe, and Asia. Over millennia, this resulted in different species of people in the different regions.
There are several benefits to having a percentage of the population moving away – evolution will work to change the genetics without the mutations. Although everyone alive today is a homo sapien, genetics of those other human species are still in people today, with the most obvious being the Neanderthal.
The risks of having a percentage of the population leaving is that it reduces the odds that some of the humans will survive. If all of the members of a settlement die, it means losing any genetic changes, meaning that reaching the previous population numbers will require people who are more likely to be genetically close for a longer period of time.
Branching out will be essential, but it will have to be done carefully and with some calculations to improve the chances of survival for those who leave. If they stay close to the original settlement, this could be beneficial, creating satellites within a small space, then continuing to move further away over time.
Over time, natural changes will be beneficial, potentially reducing the amplified genetic problems by the early incest.
5. Another Catastrophic Event
If there were to be an event that decimated the population of nearly 8 billion people, that small population of humans will be far more susceptible to problems, whether environmental or something else. This small population wouldn’t need a significant pandemic to vastly reduce the population. A bad flu could be devastating. The flu shot would obviously not be around anymore, so there wouldn’t be a way of early protection.
Only a small percentage of people die from the flu every year, but even if that percentage is only half a percent, when you don’t have many people, you can’t really afford to lose even a few people to illness.
With more genetic problems and potential birth defects because of the earlier incest, the population will almost certainly be more susceptible to illness. This includes mental and personality disorders. Remember, those are being amplified, and that could be horrible for the continuation of humanity. As the Habsburgs and ancient Egyptian pharaohs showed very clearly, children of several generations of incest produce children who are either unable to have children or are unable to comprehend the things that need to be done.
Amplified personality disorders could also be detrimental to the population. These communities are going to require that people look out for each other and work together. If there is a high population of people who suffer from personality disorders that make them less empathetic to others, it will be dangerous for the community as a whole. The saying that it takes a village to raise a child will be absolutely true as humans try to re-establish themselves.
This early population of humans will also be under greater threat from other predators. It’s always possible that they could live near a military facility, but the threat of the weapons on those bases could be an even greater risk to the humans. Probably the best place for the original couple to establish a new settlement would be on a plain where they could create a farm. This would give them the ability to produce their own food and keep animals. This could give them greater potential food sources, reducing the risk of food shortages since they won’t rely on just one or two types of food.
It is absolutely possible for two unrelated people to be able to repopulate the world. The problem is that there are so many risks and things that could go wrong that it is not very likely that they would be successful. Genetics is the biggest hurdle, but there are a number of outside factors that could cause significant problems.
The odds of two people being able to recreate human civilization again is probably about as likely as winning the lottery. From the very beginning, everything has to go right. If the woman dies during childbirth having her first child, the human race dies with her. It will take around two dozen healthy children, a safe settlement, good weather, a low risk of disease, and a host of other things going right for humans to step back from the brink of extinction.