Creationists claim that the number of mutations required since the common ancestor of chimps lived is too large to have taken place in just 5-6 million years.
Here’s a typical claim or question.
> Given 5 million years at the rate of mutation you give,
> it is within the realm of possibility. I cannot argue that.
> I have a lot of questions regarding the Chimp to Human
> relationship. The mutations, would they need to be
> mostly benefiacial [sic] mutations (at least on the human side).
We know these things:
1. The size of the human and chimp genomes (3 billion base-pairs)
2. The amount of difference there is between those genomes (about 2%)
We can use those two pieces of data to calculate that the total number of base pair differences between chimps and humans as about 60 million. It is reasonable to assume that about half of those base-pair changes took place in the human evolutionary path and half in chimp ancestors. Therefore a total of 30 million base-pairs in human DNA occurred since the common ancestor lived.
We can estimate these things:
1. The number of years since the common ancestor of chimps and humans lived (about 5 million years)
2. The average world-wide population size (about 100,000)
3. The average life expectancy (about 30 years)
4. The average number of mutations per individual organism (about 100)
5. The amount of evolution taking place among chimp ancestors compared to the amount of evolution taking place among human ancestors (about equal).
Based on many factors, I believe those numbers to be reasonable estimates. Since they are round numbers, some may be a bit too high and others a bit too low. But they should come close to averaging out.
If 30 million base pair changes took place over 5 million years, an average of 6 base-pair changes per year occurred.
With that worldwide population and a life expectancy of 30 years, you would have about 3,333 individuals born each year. That would mean about 333,300 base-pair changes occurred in new individuals born each year.
Putting that all together, we see that only five of 333,300 base-pair changes became part of the human genome. With those numbers we find that about 99.998% of mutations are not retained in the genome.
If a particular number was off by a factor of 10 it would affect our calculation by a factor of 10. If, for example, we believe that the average world-wide population was only 10,000 (rather than 100,000) the rate at which mutations are NOT adopted would be 99.98% instead.
It is fairly difficult to imagine a set of numbers which would make the percentage fall below 99%.