Or any other species for that matter?
In:
Because most inbreeding is actually fine. Even if you get two carriers for a recessive genetic disease, there’s only a 25% chance of the offspring actually *having* the disease, and only a 50% chance of them carrying it (ie still a 25% chance of them being completely unaffected).
Also, species don’t usually actually start with just 2 members. Speciation (the process that creates a new species) is a long one and it happens across many, many generations. There isn’t just a point where suddenly the generation is a new species, there’s a gradient, and it just so happens that the intermediary generations die out eventually. In species that *do* have a single common ancestor, such as humans, this occurs due to population issues, not as the actual origin of the species – ie, over time, other lineages either die our or merge into the dominant lineage.
In humans, we have a single common ancestor, called Mitochondrial Eve – every single human on earth, at least to our current knowledge, is descended from this human, who lived around 150,000 years ago. This was significantly later than the speciation of humans, however – she would have been living alongside many other humans, including other women, she just happened to be unusually prolific.
Contrast this with Y-chromosome Adam, our other single common ancestor, from whom all current male humans are descended – female humans aren’t due to the lack of any Y chromosome at all. This man is thought to have lived anywhere between 120,000 and 156,000 years ago – not at the same time as mitochondrial eve. Ie, Mitochondrial Eve and Y-chromosome Adam were not a couple, and almost certainly never even met.
It’s also worth noting that both mitochondrial eve and y-chromosome adam can change over time as matriarchal and patriarchal lineages respectively die out.
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