River Out of Eden - All Africa and Her Progenies

All Africa and Her Progenies

Further information: Most recent common ancestor, coalescent theory

When tracing human lineage back in time, most people look at parents, grandparents, great-grandparents and so on. The same approach is often taken when tracing descendants via children and grandchildren. Dawkins shows that this approach is misguided, as the numbers of ancestors and descendants seem to grow exponentially as generations are added to the lineage tree. In just 80 generations, the number of ancestors can exceed a trillion trillion.

This simple calculation does not take into account the fact that every marriage is really a marriage between distant cousins which include second cousins, fourth cousins, sixteenth cousins and so on. The ancestry tree is not really a tree, but a graph.

A better way to model ancestry is to think in terms of genes flowing through a river of time. An ancestor gene flows down the river either as perfect replicas of itself or as slightly mutated descendant genes. Dawkins fails to explicitly contrast ancestor organism and descendant organisms against ancestor genes and descendant genes in this chapter. But the first half of the chapter is really about differences between these two models of lineage. While organisms have ancestry graphs and progeny graphs via sexual reproduction, a gene has a single chain of ancestors and a tree of descendants.

Given any gene in the body of an organism, we can trace a single chain of ancestor organisms back in time, following the lineage of this one gene, as stated in the coalescent theory. Because a typical organism is built from tens of thousands of genes, there are numerous ways to trace the ancestry of organisms using this mechanism. But all these inheritance pathways share one common feature. If we start with all humans alive in 1995 and trace their ancestry by one particular gene (actually a locus), we find that the farther we move back in time, the smaller the number of ancestors become. The pool of ancestors continues to shrink until we find the most recent common ancestor (MRCA) of all humans alive in 1995 via this particular gene pathway.

In theory, one can also trace human ancestry via a single chromosome, as a chromosome contains a set of genes and is passed down from parents to children via independent assortment from only one of the two parents. But genetic recombination (chromosomal crossover) mixes genes from non-sister chromatids from both parents during meiosis, thus muddling the ancestry path.

However, the mitochondrial DNA (mtDNA) is immune to sexual mixing, unlike the nuclear DNA whose chromosomes are shuffled and recombined in Mendelian inheritance. Mitochondrial DNA, therefore, can be used to trace matrilineal inheritance and to find the Mitochondrial Eve (also known as the African Eve), the most recent common ancestor of all human via the mitochondrial DNA pathway.