Can two people repopulate Earth? Is it possible?
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very very very unlikely.
Because in the second generation it leads to inbreeding.
Effects of Inbreeding,
The partners in these relationships that inbreed, simultaneously jeopardize their offspring's fitness and bring upon them great evolutionary risk. Inbreeding opens the door for prevalence of deleterious recessive alleles by increasing their chance for homozygosity and thereby, impeding the fitness of the offspring. With continuous inbreeding, the loss of genetic variation magnifies, and in turn, enables the expression of such recessive alleles following boundless negative implications.
In other word inbreeding leads to lack in mental and physical fitness. Specifically, inbreeding has been found to decrease fertility. Furthermore, such offspring are known to have a lower IQ and higher risk for mental retardation
Present day cheetahs are an example of this.
About 10,000 years ago, all but one species of cheetah became extinct due to climate changes. There was a drastic reduction of that one species’ numbers and the remaining ones were forced to breed, even though they were close relatives. Since then, all cheetahs are closely related.
This event caused an extreme reduction of the cheetah’s genetic diversity, known as a population bottleneck, resulting in the physical homogeneity of today’s cheetahs. Poor sperm quality, focal palatine erosion, susceptibility to the same infectious diseases, and kinked tails characteristic of the majority of the world’s cheetahs are all ramifications of the low genetic diversity within the global cheetah population.
When you look at two individual cheetahs, related cheetahs share about 99% of their genes, when in most species’ that number is about 80%. This lack of genetic diversity can cause a lot of problems, because it makes the species as a whole more susceptible to disease and less adaptable to new environments. This means that a single deadly virus could kill off all wild cheetahs in the world.Extreme inbreeding affects their reproductive success with small litter sizes and high mortality rates.
There has been a similar case of bottleneck in case of humans,
The Toba catastrophe theory as presented in the late 1990s to early 2000s suggested that a bottleneck of the human population occurred c. 70,000 years ago, proposing that the human population was reduced to perhaps 10,000 individuals,when the Toba super volcano in Indonesia erupted and triggered a major environmental change. The theory is based on geological evidence of sudden climate change and on coalescence evidence of some genes (including mitochondrial DNA, Y-chromosome and some nuclear genes)and the relatively low level of genetic variation in humans.
In 2000, a Molecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change. This would be consistent with suggestions that in sub-Saharan Africa numbers could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in the Late Stone Age.
TL:DR; According to our observations with animals facing extinction single pair is not enough to repopulate earth. The probability of success is very very low.
SOURCES:
RARE GENETIC UNIFORMITY FOUND IN CHEETAHS
The Cheetah and its race for survival
Inbreeding
Inbreeding depression
What are the general effects of inbreeding
EDIT: These are answers to discussion in comment section.
1)How would this answer given shed light on the non-biblical theory that we are all descendent from one modern human couple of Sub-Saharan African couple?
Ans:The sub Saharan couple mentioned by science are mitochondrial Eve and Y chromosome Adam. They were not a couple, they are supposed to have lived tens of thousands of years apart. They are the most recent common ancestor(MRCA) from whom all currently living people are descended patrilineally and matrilineally.
2) As Mickey Cashen said when a re population effort was made of various mammals (deer, squirrels, etc.) the minimum number of acceptable non-close relatives was 16(couples). Theory says with 2 people the population would die out in 70 years, 4 people 140 years so on.
One related tidbit is that I have read somewhere north america may have been populated by 70 people, I don't know about validity nor I have the source.
3)Question about Humans would have to inbreed once they evolved to existence.
Modern humans, they didn't come to existence one day, small small adaptations occurred over time but they were still fertile with previous people. For example one adaptation is loosing of hair. One individual would have been born with this mutation, but he was still fertile and produced children with similar mutation, gradually the mutation spread and all became hairless. They did not inbreed.
One present time example of adaptation is Lactase persistence in adults, some 35% of global population has evolved into little more modern humans, but they are not different in any other aspect and still fertile with others.
Scientists have coined a term called Minimum Viable Population (MVP), which is the minimum number of individual population required for a species to survive in the world. This figure is calculated using computer simulation taking into account inbreeding instability, climatic alterations, predators etc.
And the MVP for humans is estimated to be around 160. But this number is estimated considering controlled conditions, lesser environment fluctuation, basically if you're living like the rich folks in Elysium (Hell of a movie BTW).
Source: “Magic number” for space pioneers calculated
But in reality, this number is calculated for living in the wild, without heavy human interference (both good and bad). So this number would probably be much higher. In other words, risking the entire human population on just 2 people, I would say it's risky bet.
P.S - Watch Children of Men (2006) movie by Alfonso Cuoaron
Edit: Thanks Jonathan for pointing out the error
I continue to be castigated for not "answering the question." I will now state with specificity... The odds of two young adults, one male and one female, to repopulate the Earth, considering the reality of living on this planet without the convenience of modern science and environment, is less than nil. Nada. Non-existent. It would not happen. We have seen the removal of 99.9% of ALL species that have EVER existed from our gene pool. If our species, (homo-sapiens) were to achieve such a low level, you can be sure that our species would cease to exist. And it would not depend on something so esoteric as genetic drift. It would result from pain, disease, and other predators. We, as a species, are the recipients of incredible luck and good equipment. But we are not unique; others equally suited to their environment have long since disappeared.
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Since I have been accused of "not answering the question," let me be specific. Two people (preferably of opposite sexual equipment) could not (in all probability) repopulate the Earth. My answer below is intended to show that the fact that our species continues to exist is primarily one of luck.
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There was a point in time, some 70,000 years ago, when the entire population of our species, homo-sapiens, was down to less than 1,000 individuals, perhaps as few as 100. The extinction of our species was a very likely thing. The fact that the situation changed allowing our species to expand is almost a miracle.
Almost any time a species in the wild gets pared down to a limited number of individuals that species goes extinct.
Here is an article that describes what happened.
Because once in our history, the world-wide population of human beings skidded so sharply we were down to roughly a thousand reproductive adults. One study says we hit as low as 40.
Forty? Come on, that can't be right. Well, the technical term is 40 "breeding pairs" (children not included). More likely there was a drastic dip and then 5,000 to 10,000 bedraggled Homo sapiens struggled together in pitiful little clumps hunting and gathering for thousands of years until, in the late Stone Age, we humans began to recover. But for a time there, says science writer Sam Kean, "We damn near went extinct."
I'd never heard of this almost-blinking-out. That's because I'd never heard of Toba, the "supervolcano." It's not a myth. While details may vary, Toba happened.
Toba, The Supervolcano
Once upon a time, says Sam, around 70,000 B.C., a volcano called Toba, on Sumatra, in Indonesia went off, blowing roughly 650 miles of vaporized rock into the air. It is the largest volcanic eruption we know of, dwarfing everything else...
Robert Krulwich/NPR
That eruption dropped roughly six centimeters of ash — the layer can still be seen on land — over all of South Asia, the Indian Ocean, the Arabian and South China Sea. According to the Volcanic Explosivity Index, the Toba eruption scored an "8", which translates to "mega-colossal" — that's two orders of magnitude greater than the largest volcanic eruption in historic times at Mount Tambora in Indonesia, which caused the 1816 "Year Without a Summer" in the northern hemisphere.
With so much ash, dust and vapor in the air, Sam Kean says it's a safe guess that Toba "dimmed the sun for six years, disrupted seasonal rains, choked off streams and scattered whole cubic miles of hot ash (imagine wading through a giant ashtray) across acres and acres of plants." Berries, fruits, trees, African game became scarce; early humans, living in East Africa just across the Indian Ocean from Mount Toba, probably starved, or at least, he says, "It's not hard to imagine the population plummeting."
Then — and this is more a conjecture based on arguable evidence — an already cool Earth got colder. The world was having an ice age 70,000 years ago, and all that dust hanging in the atmosphere may have bounced warming sunshine back into space. Sam Kean writes "There's in fact evidence that the average temperature dropped 20-plus degrees in some spots," after which the great grassy plains of Africa may have shrunk way back, keeping the small bands of humans small and hungry for hundreds, if not thousands of more years.
So we almost vanished.
How Human Beings Almost Vanished From Earth In 70,000 B.C.
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It is important to remember that extinction-level events such as this are more common than you would think. "More than 90 percent of all organisms that have ever lived on Earth are extinct. As new species evolve to fit ever changing ecological niches, older species fade away. But the rate of extinction is far from constant. At least a handful of times in the last 500 million years, 50 to more than 90 percent of all species on Earth have disappeared in a geological blink of the eye."
Mass Extinction, Mass Die-Off Information, Prehistoric Facts -- National Geographic
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Just upvoted Robin Craig's answer to Can two people repopulate Earth? Is it possible? Makes it clear that it is possible. Thought I'd fill it out a little more with some images and hopefully draw attention to it as it seems there are many answers here being upvoted by people who are not aware of this study and say it is impossible:
The second paper he gives is particularly clear. A pair of mouflon (wild sheep)
One Mouflon Ram
Ovis orientalis - by Jörg Hempel
Mouflon Ram
Mouflon ewe Ovis musimon by Doronenko
Not those actual individuals - they are for illustration purposes. But just two wild sheep introduced to an island formed an entire population which was not only healthy but increased in genetic diversity. Unexpected heterozygosity in an island mouflon population founded by a single pair of individuals
Humans have the advantage as others have said that they would know that they are an endangered species and would decide what to do about it.
Obviously would have to have a lot of inbreeding. It's possible some humans might decide this is not acceptable and they'd prefer to be extinct. But biologically it seems feasible.
SCENARIO?
The question doesn't give a scenario. Hard to think of something realistic that would kill all humans except for one couple. Or that would lead to everyone dying a natural death except one young couple.
Even an illness that sweeps the world would leave some survivors, including for instance uncontacted indigenous people. Things like artificial life escaping from the laboratories and reproducing in the wild and replacing Earth DNA with XNA based life throughout the world would kill everyone and would still be there at the start of the plot.
I see it as science fiction rather than future possibility. So then in those science fiction scenarios where only two people are left alive, then sometimes they make it so that everyone dies except them in some plague that affects the entire world. They are, say, doing an experiment in living underground for years without any contact with anyone else, including no communication - then when the experiment is over, they come to the surface and find they are the only people left in an abandoned world. That sort of thing. So they have the technology, have books to recover their knowledge etc, but only the two of them.
Or. you can suppose that they are genuis scientists that find the solution to reversing the thing that kills absolutely everyone in the world, but too late to save everyone except themselves.
Another possibility from science fiction is the idea of an aged population where they have artificial ways of giving birth, and most people typically live to some great age like a million, but new children are born, say just two of them. The rest of their society dies of old age. These are the only ones left. And they make a fresh start.
It's one of the most overused tropes in pulp fiction. Adam and Eve Plot - TV Tropes .
Sometimes they are in a world with no technology and many dangers, in which case they probably have little chance, but sometimes they are surrounded by high technology, automated hospitals, libraries. Or maybe are in a starship with all the technology of the starship at their disposal. I'm assuming a scenario like that.
And - a bit more clarification got from a discussion here: How many people are required to maintain genetic diversity?
IDEA OF A MINIMUM POPULATION AND PROBABLE NEED FOR PLANNED BREEDING
The idea of a minimum population is based on the idea of unplanned breeding, where anyone mates randomly with anyone else of the opposite sex, and is a probabilistic thing.
If you can plan who mates with who, then - perhaps one couple has enough genetic diversity, if unrelated originally and they don't have some serious defect for their children to inherit.
They would be able to build up to a population of a few hundred, and then would be able to survive after that, except, that the entire population would be vulnerable to some virus or other condition they are not adapted to until they have survived long enough for some adaptations to arise through actual genetic mutation rather than just shuffling existing genes.
And the example of the sheep would seem to show practically that if you are lucky, then two would be enough if you are lucky with the individuals.
For another example of a species that has survived a major bottleneck, nearly all golden hamsters
originate from a single litter. Golden hamster
I know it depends on the species as well. You can't draw conclusions from humans directly from sheep or golden hamsters. But these are a few points that seem to have been left out of the top rated answers here, maybe someone with expertise can say more about it?