Studies of variation in DNA became possible in the early 1980s. Subsequent estimates for the emergence of modern humans from Africa using autosomal restriction fragment length polymorphisms were consistent with earlier estimates.

From the analysis of several mitochondrial DNA (mtDNA) polymorphisms, Cann et al. derived two important conclusions: the first major separation in the evolutionary tree of modern humans was between Africans and non-Africans; and the time back to the most recent common ancestor (TMRCA) of modern human mtDNA was 190,000 years (however, with a large error).

After early doubts about the statistical validity of these interpretations of the data, the order of magnitude was confirmed.

It is important to note that TMRCA is usually significantly earlier than the first archaeologically observable divergence among a set of populations.

Also, TMRCA does not necessarily coincide with the onset of population expansion.

The ?mismatch? method to analyze mtDNA, which analyzes the distribution of between sequence differences, gives estimates that are more compatible with the beginning of expansions inferred from archeology.

Y-chromosome data

Statistical analysis of Y chromosome data have been carried out using coalescent theory devised by Kingman.

Coalescent-based techniques using numerical methods to study complex likelihood functions derived from Bayesian analyses were developed subsequently and have facilitated estimation of key parameters in the Y chromosome genealogy under specific assumptions about demographic history.

Tang et al. have shown that important evolutionary properties of the Y chromosome TMRCA, which is close to 100 kya, can be derived under few demographic assumptions.

Two recent estimates of TMRCA from mtDNA have been made using different methods. From complete mtDNA sequences (excluding the D loop) in a sample of 53 individuals, 516 segregating sites were seen and a TMRCA was estimated at 171 50 kya. From a sample of 179 individuals with 971 SNPs, the TMRCA was estimated at 200?281 kya using a generation time of 25 years, and 160?225 kya using a generation time of 20 years. Corresponding estimates for the NRY-based TMRCA are 60?130 kya and 72?156 kya, with generation times of 25 and 30 years, respectively.

It is important to stress that such estimates of TMRCAs do not imply that the human population contained only one woman at 230 kya (the time of the mtDNA-based TMRCA, assuming constant mutation rates) or only one man at 100 kya (the time of the NRY-based TMRCA).

The only implication is that all human mitochondria existing today descend from that of a single woman living 230 kya, and all NRYs descend from that of a single man living 100 kya.

In both cases, it is likely that there were many more human individuals alive at the TMRCA-whether they were of the same species as Homo sapiens is hard to determine, but descendants of other species are either absent or extremely rare.

mtDNA and NRY differences

Although the reconstructed genealogies of mtDNA and NRY are broadly similar, there are some notable differences, probably owing to social differences in migration customs.

For example, patrilocal marriage has historically been more common than matrilocal, which can explain differences in mtDNA and Y chromosome data in a number of populations.

Demographic differences between the sexes, such as greater male than female mortality, the greater variance in reproductive success of males than females and possibly the greater frequency of polygyny than polyandry, may explain the discrepancy between the NRY and mtDNA dates.

These factors reduce the effective number of males and may explain the more than twofold difference between the NRY-based and the mtDNA-based TMRCA.

Another attractive alternative explanation is that mutation rates in mtDNA are very variable, and when this variation is taken into account TMRCA of mtDNA could become closer to that of NRY.

Estimates of TMRCAs from autosomal genes are higher than those from mtDNA or NRY. In theory, they should be higher by a factor of four and the estimates are in this direction, although the number of autosomal genes studied is small and estimates of TMRCAs vary considerably.

For analyses of autosomal and X chromosomes, recombination can complicate genealogies and make TMRCAs impossible to estimate. There is also the possibility of heterozygote advantage, which has the potential to increase estimates of TMRCA.

Heterozygote advantage may be widespread throughout the human genome but has been very difficult to show unequivocally, and the only fully confirmed example is sickle cell anemia, for which very large samples were required.

There is some optimism, however, that the development of techniques that can detect heterosis for some genes in yeast may lead to greater success in other organisms, including humans.

References

 Cavalli-Sforza LL, Feldman MW. The application of molecular genetic approaches to the study of human evolution. Nat Genet. 2003 Mar;33 Suppl:266-75. PMID: 12610536