In what has become my modus operandi here recently, I'm going to recycle a Quora answer for this post. This time, the question was, 'would the parents of the first human grandchild have been siblings?'. My response is very similar to parts of a previous answer/post, Which Came First, the Chicken or the Egg? And a Discussion of the Fuzziness of Species, but I think I did a better job this time explaining the example. So, here's my answer, with a few minor edits, and some additional footnotes.
As many others have pointed out, there was no 'first human'. Evolution is a gradual process that occurs in populations. Trying to pinpoint the exact individual that was the first human is like trying to pinpoint the exact second when twilight becomes nighttime.
Some people have a bit of a hard time understanding this, even people who claim to accept evolution. They'll say that if we exist as humans now, while in the past there were no humans, there must be some individual that we can identify as the 'first human', even if it's an arbitrary distinction. But this still doesn't work.
For the sake of argument, let's say you want to arbitrarily define a certain set of genes as 'human', and any organism lacking those genes as not human*. It's just semantics, but let's see what would happen. Let's say that around 100,000 years ago, there was a population of hominids that was very, very close to fitting your definition of 'human', but lacking one last mutation that would give them the full set of genes to make them 'human'. And lo and behold, one day a couple has a child that acquired this last critical mutation, and now, by our semantic definition, it's fully 'human'**. How different will it look from its parents because of that one different gene? Once it grows up, how much of a problem will it have finding a mate among the rest of the population and producing children of its own. The answers are that it will look as much like its parents as any child, and it won't have a problem at all finding a mate and having children (at least, not because of that mutation). So, even though it fits our semantic definition of 'human', it's not a different species from its parents or the rest of the population using the biological species concept.
But let's take it further. Once this first 'human' finds a mate and has children, because its genes are being mixed with its mates, and because it likely only has one copy of this new gene, anyway, only around 1 in 2 of its children are going to contain this critical gene that makes them 'human', while the rest of its children are going to lack this critical mutation and be almost but not quite 'human'***. But those children will all grow up to have children of their own, and on and on. So at first, this particular gene was only present in one individual, so only one individual in the entire population was fully 'human'. Then in the next generation, the gene was present in roughly 50% of its children, so there were a handful more 'humans' in the population. Then, in the next generation, the gene was passed on to yet more children. And since we know in hindsight that this gene is necessary to be 'human', we know that the individuals with the gene will end up having slightly more surviving children than individuals without the gene, so that eventually, after many generations it will have spread throughout the entire population, and the entire population will be fully 'human'.
That's the problem with trying to define a 'first human'. Whatever genetic criteria you pick is going to be arbitrary. An individual with 99.9999% of the correct DNA wouldn't be human by this definition, but it would still be the same species and able to interbreed with an individual with 100% of the correct DNA. And at some point in time, there will be a population of organisms mixed between 99.9999% 'humans' and 100% 'humans', and a 100% 'human' could have a mix of human and non-quite-human children. But the only reason we'd be classifying these organisms any differently is because of hindsight, knowing that in their future, only one version of a specific gene is going to be dominant. In their own time, they'd look just like any other population with a mix of genetic diversity.
And even all that's using an arbitrary definition of exactly what genes are needed to be 'human'. In the example above, we could be tempted to say, alright, that 'almost-human' population of hominids is close enough - let's call them fully human. But now you've just shifted the same problem a few generations back. There will be a moment in time when their ancestral population was a mix of individuals with their same genes and very slightly different genes. Depending on which specific arbitrary traits are required for the definition of 'human', you could shift the first humans by tens or hundreds of thousands of years.
So, to answer the question, there never were only two individual parents that were the only ancestors of all of humanity. Our ancestors have always been members of large interbreeding populations. And because evolution is a gradual process, it's impossible to pinpoint any single individual as the 'first human'.
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* You'd probably be more focused on alleles than genes. Alleles are different variants of the same gene. But, since it's just a semantic definition, anyway, genes work for the discussion. It's also an over-simplified example. It's not as if the population was a bunch of genetic clones. Even among individuals in the population, there will be genetic diversity.
** Mutations happen regularly. Pretty much every person alive has at least some mutations differentiating them from their parents. Granted, a whole new gene is a much bigger change than just an allele, but it happens. More Info: Understanding Genetics - How new genes are made
***Remember that we have two copies of all of our genes. So, if this hypothetical individual had a mutation that created a new gene or new allele, it would have probably occurred on only one copy of the original gene, i.e. one strand of the double helix. Since reproduction involves random mixing of our genes for making sperm and eggs, since our individual only has one copy of this new gene, only around half of it's eggs/sperm will contain the new gene, while the other half will retain the old version.