How Many Adams?

Did the present human population descend from one couple or from a group of at least 10,000 ancestors?

1. Y-Chromosome Adam and Genetic Diversity
An article on the Biologos blog asserts that all living humans trace their mitochondrial DNA back to a single woman (“mitochondrial Eve” about 180,000 years ago) and that all living males similarly trace their Y-chromosome DNA back to a single male (“Y-chromosome Adam” about 50,000 years ago). On the other hand, he asserts that current genetic diversity indicates that humans evolved as a group, whose population was never below 10,000 people.

This raises the obvious question: how can all humans descend from one man and also from an initial population of at least 5000 males?

The article calls upon random drift. It gives an example of three initial men, who have sons and daughters but pass on their Y-chromosome only through their sons, like a surname. Each further generation has a combination of sons and daughters. A Y-chromosome line--or surname--dies out when no more sons are born in that line. In the simple example, after a few generations only one surname remains. Thus, although at the end all remaining male lines originate from only one initial male, the final population traces its ancestors--male and female-- to all three initial males and therefore still has much genetic diversity. The article suggests that the same effect applies also for larger initial populations.

2. Testing Random Drift
Is this reasonable? Suppose each initial male has a unique surname, corresponding to his Y-chromosome. It is well-known that the number of surnames in a population will decline over time. How long would it take for 5000 surnames to reduce to one?

To answer this question I wrote a little program in QuickBasic (see below). It generates random births of sons, assuming a Poisson distribution. A number of runs were made, using different random numbers.

The results are interesting. At first, the number of surnames does indeed rapidly diminish-- to about 16% after 10 generations and 2% after 100 generations. Beyond that, several surnames generally survive, each having a relatively large sub-population. It is not easy for only one surname to take over the entire population.

Time           Constant Population      Doubling every 1000 generations
 Initial                5000 surnames           5000                  1000
10 generations  ~800                           ~800                  145 to 175
100                  ~100                           ~100                  14 to 24
1000                 8 to 14                       14 to 21              2 to 5
2000                 3 to 6                         8 to 13                1 to 4
3000                 2 to 5                         6 to 11                1 to 3

For example, starting with 5000 males, and assuming the population stays at about 5000 males, we are generally left with 2 to 5 surnames after 3000 generations. If the population is allowed to double every 1000 generations, then the number of surviving surnames will be higher (e.g., 6 to 11 remaining surnames after 3000 generations). If we start with only 1000 males the number of surnames is sometimes be reduced to 1, but this typically takes about 2000 generations.

3. Discussion
Given that a recent estimate for the average male generation time is 32 years, it is extremely unlikely that random drift will reduce 5000 initial surnames to 1, even after 96,000 years (3000 generations). To achieve such reduction the initial male population must have been substantially less than 5000 individuals. Else, one must postulate significant selection pressure on the Y chromosome, which has as yet not been observed.

Note also that the above date of 50,000 years ago for Y-chromosome Adam is problematic, even within mainstream science. According to a recent article in the journal Science, Australian aborigines are directly descended from ancestors arriving in Australia 50,000 years ago, who were directly descended from a group that left Africa for Asia 70,000 years ago. This entails that the reduction to one surname must have occured some time before 70,000 years ago. If such reduction took X years, then Y-chromosome Adam must have lived at least 70,000 + X years ago--in terms of mainstream chronology. These age estimates are based on the number of  Y-chromosome differences between humans, and presumed fossil dates and past mutation rates.

From a biblical perspective, the entire human population descended from an initial couple--Adam and Eve. Given that the only males to survive the Flood were Noah and his sons, all present male lines of descent come via Noah, who thus corresponds to the "Y-chromosome Adam". Depending on the ancestors of Noah's daughters-in-law, the female "mitochondrial Eve" should be somewhere after Eve and before Noah. Thus we expect the date of "mitochondrial Eve" to be earlier than that of "Y-chromosome Adam".

What about the claims that the present genetic diversity of humans demands a large initial population?  It should be kept in mind that such assertions are based on idealized mathematical models, operating with various theoretical assumptions. Alternatively, one can plausibly explain the same observed genetic diversity in terms of creationist models. See, for example, the articles "Adam, Eve and Noah vs Modern Genetics" and "The non-mythical Adam and Eve", both by geneticist Dr. Robert Carter.

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