Figure 4.

Reversibility of the BDP implies that the evolutionary relationship between contemporary chimpanzees and the most recent common ancestor can be inverted. On the left, the most recent common ancestor of chimpanzees and humans lived at time T in the past. At a certain locus, chimpanzees have a microsatellite consisting of 2 repeats of the motif AAC, and at an orthologous locus, humans have 3 repeats of the motif. The number of repeats in the ancestor is unknown. On the right, using a probabilistic justification explained in the text, we may interpret the evolutionary relationship between chimpanzees and humans as unidirectional, while “integrating out” the number of repeats at the ancestral locus.