We present a simple Monte Carlo method for estimating the age of the m
ost recent common ancestor (MRCA) of a sample of DNA sequences. We sho
w that Templeton's (1993) estimator of the age of the MRCA based on th
e maximum number of nucleotide differences between two sequences in a
sample is inaccurate, and we demonstrate the new method by reanalyzing
a sample of DNA sequences from human Y chromosomes and a sample of hu
man Alu sequences.