see, because of meiosis(sp check) the dna splits in half. so if the dna splitting was different in those days, where the dna would split but end up with half the dna when the process ends, then the dna would slowly reduce in size in each human to the point where it is today and spread te different dna to their offspring, but then certain humans would go even further in their splitting of dna and become faulty and die off. that way only the humans with workable dna live which is why we have the amount of dna and the process to divide them as it is today.