The limiting distribution of the effective population size of the ancestor of humans and chimpanzees

- The variance of the effective population size of the Homo-Pan ancestor among studies is large.
- Estimation of evolutionary parameters from genomes contains minimum stochastic error.
- The mean ancestral effective population size of Homo-Pan was inferred at approximately 47,500.
- The uncertainty of the estimates was large, even under the limiting distribution.

The effective population size is a fundamental parameter for the understanding of microevolutionary process. Indeed, the consideration of population-level phenomena within phylogenies provides insight into the influence of the past evolutionary demography on the genetic diversity of living species. Although the effective population size of the last common ancestor of humans and chimpanzees has been extensively investigated by molecular evolutionists, variance in the estimates of this parameter among studies is large. However, with the availability of genome sequences, the estimation of evolutionary parameters may be conducted with minimum stochastic error, and the limiting distribution of the estimates may be obtained. This statistical property was utilized in the present study and coupled with analytical derivations from the coalescent theory to examine the limiting distribution of the ancestral effective population size of Homo-Pan. The mean ancestral effective population size of Homo-Pan was inferred at approximately 47,500, and the results showed that the uncertainty of the estimates was large, even under the limiting distribution. Further reductions of the estimates are feasible only if additional calibration information from the fossil record is provided and if a probabilistic model of ancestral generation time is envisioned.