How many humans would it take to keep our species alive?

What's interesting is that this has actually happened before - the original settlers of America, who crossed over a land bridge from Asia during glacier periods, were estimated to be less than 200 people. And there's a theory (not entirely agreed on, but has a lot of evidence backing it up) that massive volcanic eruptions about 75,000 years ago killed off most humans, with about a stadium's worth of people left alive. The entire human population actually has less genetic variation than the entire population of chimpanzees, possibly because of that genetic bottleneck.

 

Not off the top of my head, but you can look up the Toba Eruption (this is the volcano, which is a crater lake now).

 

I think the number would also have to take into account if it's "today humans" or "future humans" that might have access to genetic testing/fixes that could remove faulty genes.

The real problem with inbreeding isn't the lack of diversity so much as the concentration of genetic disorders. The average person has multiple recessive genetic mutations that don't affect them because you have to have a pair. The big issue with small populations is if the population has a ton of those recessive and starts inbreeding, you get more and more children that have two copies and thus the problems from those genetics. Take hemophilia in the royal families as an example. They inbred too tightly with hemophilia carriers, resulting in it being a real problem.

In theory, if we had a breeding population that was carefully screened to make sure those sorts of genes weren't present than the population would likely be much more viable. A good example is cheetahs. Cheetahs are so close genetically to every other cheetah that all of them can give each other skin grafts, etc. They're damn near clones of each other. Some 12,000 years ago, the ancestors of the cheetah were nearly wiped out, except a very small population so all cheetahs are now descended from that handful. Granted, cheetahs do have some issues but breeding programs where screening happens have ensured that the population has a decent number of healthy cubs in zoos at least.

Now, that said, I'm not saying we need to be purging humans of genetic flaws, etc. ala many, many sci-fi "horror" scenarios. Just speaking strictly from a "what kind of population to continue the species" standpoint that the fewer recessive genetic disorders carried, the smaller a population one can get away with since they wouldn't have to worry about those defects. Now, they would still need a certain level of genetic variation and such to avoid an inbreeding depression, which can result in lowered fertility, immune issues, and other such things.

 

Just curious: does that take into account just troglodyte chimpanzees or both troglodytes (aka 'common chimps') and paniscus (aka 'bonobos')? I'd believe both.

 

Many years ago in college I did a research paper that looked at minimum number in a population necessary to avoid issues from inbreeding. The focus was on gazelles. The number was 72, with selective breeding initially to maintain the variable mix in genes within the population.

With humans travelling, a group could always carry frozen sperm and eggs to ensure genetic diversity, both during the trip if a ship requires many generations. And also for once the destination is reached.

 

The statistic will have to account for cases of accidental or sudden death, Infertility cases and genetic anomalies for the first few generations.

This is not even taking into consideration a hostile environment with unknown variables that might affect our genetic make up in the long run.

I've just re-read John Scalzi's Old Man's War so those issues are just too fresh in my mind.

I don't think the settlers makes for a good parallel @Viergacht because of the sharp decline of birth rates since then. How many people do you know with a dozen or even half a dozen kids? Taking that into consideration the number has to be at the very least doubled and that's after securing the environment for the first and second generations.

 

I'm one of five kids, so that's at least one person I know with a half dozen kids, about LOL

Anywho, Janet Kagan's "Miribile" has a nifty solution for this, where the animals the settlers bring along have other species coded into their DNA, but during the journey some of the instructions were lost when the generation ship was damaged, and what with evolution continuing the settlers have to deal with things like flowers that seed cockroaches and otters that give birth to Loch Ness Monster like creatures.

 

Seveneves by Neal Stephenson goes into this question and has an interesting answer.

 

what was it. I tried reading diamond age and woke up the next morning drooling on the page. Don't even get me started on snow crash.