The Last Man And Woman On Earth – Can Two People Repopulate The Planet?

Imagine a virus wipes out everyone on the planet except you. You are free to roam the world and do whatever you please, all in the comfort of your pajamas. No more rules and regulations. No more 9 to 5. You can pick whatever house you want, fill it with priceless artwork, and drive your favorite sports car as fast as you want. That is the concept behind the new television show, “The Last Man on Earth”.

The novelty of being so free does wear off for our protagonist, who soon suffers a level of loneliness that drives him to a suicide attempt. But just before he extinguishes the last XY chromosomes on the planet, he finds the last woman on Earth. A woman who wastes no time in eroding his freedoms, insisting that they use correct grammar and still stop at stop signs.

The lone pair faces the inevitable question:  can they repopulate the Earth? To do so, their children would have to mate with one another, or mom and dad, in order to rebuild the human race. All the incestuous taboos aside, is this even genetically possible?

If just one man and one woman are left to repopulate Earth, then their “family tree” would look more like a family pole.

Inbreeding has unfortunate genetic consequences due to the increased inheritance of recessive genes, which can result in neonatal death. Inbred children that survive are at increased risk of congenital birth defects, reduced fertility, smaller size, immune deficiencies, cystic fibrosis, and more. These defects are also likely to be passed on to their children as well.



If you’ve ever seen The Jerry Springer Show, you know what happens when two closely related individuals start dating. A whole bunch of pushing and shoving! While the show frequently pokes fun at incestuous relationships, it doesn’t emphasize the catastrophic consequences that may befall inbred children.

Some real-life examples of the consequences of inbreeding can be found in places where there are restricted breeding opportunities – for example, within monarchies, islanders, or closed societies. Hemophilia was notoriously prevalent in European royal families. Some Amish societies have a larger number of children born with extra digits on their hands or feet. Jews of Eastern European descent tend to have higher rates of a number of genetic diseases, including cystic fibrosis.

To understand why children of incestuous mating are often plagued by these rare diseases and disorders, we need to review some genetics. For each gene in our 46 chromosomes, we actually possess two copies called alleles – one came from mom, the other from dad. Alleles can be dominant or recessive, the former being expressed while the latter is not. So if you have a bad gene, it could be masked if you have a dominant allele; in other words, you would not exhibit that trait but you would be a carrier. If you mate with someone who also has a recessive allele for that gene, there is a chance your child will be born with two copies of the recessive allele. Such a child would exhibit that gene defect.

Dominant and recessive alleles at work. As a simple example, pretend the trait under study here is lactose intolerance and the bad allele is shown in yellow (the good allele is green). In this example, mom and dad are heterozygous for this lactose intolerance gene – they have one good allele and one bad. Consequently, they can enjoy all the dairy they want because they are only carriers. Their children get one allele from mom and one from dad and can be unaffected (hitting the lottery and getting two good alleles), carriers like mom and dad, or lactose intolerant (losing the lottery and getting both bad alleles).

 The net result of inbreeding is that the resulting population loses a diverse genetic portfolio, which means they are less resistant to rare diseases and deformities. The smaller the gene pool, the faster it gets dirty. Such individuals would also have less diverse immune systems, making it much easier for a single germ to wipe them all out. That would be an ironic twist of fate since there was something peculiar in the genomes of the last man and woman that kept them alive during the mass extinction!

In addition to the genetic landmines, the family would likely have a very difficult time overcoming the innate resistance most species have against inbreeding. Evolution knows that inbreeding is not good for the species, so it engineered a built-in “incest taboo” that creates a strong aversion to such behavior. A devil’s advocate, however, could argue that the biological barrier to familial sex could be overcome through artificial insemination.

What about using a sperm bank? Sperm is stored in liquid nitrogen, so it would stay frozen for a short time after the power goes out. However, you’d have to act fast because no one is around to monitor the storage tanks and top off the liquid nitrogen as it evaporates.

There are practical concerns to consider as well. The last man and woman, as well as their kids, would need to have large numbers of children and, unless one of the founders happens to be a doctor, it is hard to imagine many of these babies surviving in such a world. Even if they (and mom) survive childbirth, there are countless opportunities for them to perish in this type of environment before reaching childrearing age.

Considering the collective evidence, it seems virtually impossible that just two people could repopulate the planet. But that doesn’t make The Last Man on Earth any less fun to watch.

How many people are required to sustain a human population is an intriguing question that has not been settled. One study estimates that only 70 people who crossed the Bering land bridge 14,000 years ago successfully populated North America.

 

Contributed by:  Bill Sullivan
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Alkuraya FS (2012). Discovery of rare homozygous mutations from studies of consanguineous pedigrees. Current protocols in human genetics / editorial board, Jonathan L. Haines ... [et al.], Chapter 6 PMID: 23074070

Hey J (2005). On the number of New World founders: a population genetic portrait of the peopling of the Americas. PLoS biology, 3 (6) PMID: 15898833