FIGURE 1. Searching for our universal common ancestor.

From the following article:

Human evolution:  Pedigrees for all humanity

Jotun Hein

Nature 431, 518-519(30 September 2004)

doi:10.1038/431518a

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The figure shows how the number of ancestors of two people alive today builds up in a manner that is close to exponential. Because the human population has a finite size, however, we do not need to go back many generations before we find an ancestor that is common to both people. The same applies in searching for the ancestor of all living humans (universal ancestors are represented as grey balls). In simplified models, the expected time back to this universal ancestor is log2n, where n is the population size. If we were to trace not both parents of each individual, but only one random parent for each (thick lines), we would in effect be tracing the history of gene variants (alleles). In standard models, the number of generations back to the common ancestor of a particular allele will be of the order 2n, which is much longer ago. If we trace the history of genomes, not genes, recombination would complicate matters; this genetic 'shuffling' ensures that each child does not inherit exactly the same genomic information as its siblings, and means that the genealogical relationship of different genome segments can be different.

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