It doesn't matter how many kids you have, you still have the same genes/DNA. The problem isn't they can't have enough kids, the problem is they couldn't have spawned all the genes we have today. 2 people can not possibly be the start of an entire race.
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You do realize every human has the same genetic code just is a different sequence. DNA ATGC mRNA AUGC auc UAG <--this is what we call a gene.