Most Recent Common Ancestor - MRCA of All Living Humans

MRCA of All Living Humans

Tracing one person's lineage back in time for a few generations in principle forms a binary tree of parents, grandparents, great-grandparents and so on. However, the number of individuals in an ancestor tree grows exponentially and very soon exceeds the population from which the ancestors were drawn. A human alive today would over 30 generations (going back to about the High Middle Ages), have 230 or about 1.07 billion ancestors, more than the world population at the time. Thus it is obvious that there is multiple counting and the individual is descended from some of these ancestors through more than one line: pedigree collapse changes the binary tree to a directed acyclic graph.

Consider the formation, one generation at a time, of the ancestor graph of all living humans with no descendants. Start with living people with no descendants at the bottom of the graph. Adding the parents of all those individuals at the top of the graph will connect (half-) siblings via one or two common ancestors, their parent(s). Adding the next generation will connect all first cousins. As each of the following generations of ancestors is added to the top of the graph, some of the relationships between more and more people are mapped (second cousins, third cousins and so on). Eventually a generation may be reached where one of the many top-level ancestors is the MRCA from whom it is possible to trace a path of direct descendants all the way down to every living person in the bottom generations of the graph.

The MRCA of everyone alive could have co-existed with a large human population, each of whom either has no living descendants or is an ancestor of only some of the people alive today. Therefore the existence of an MRCA does not imply the existence of a population bottleneck, let alone a "first couple".

The MRCA of the current population can change over time as new people are born and others die.

It is incorrect to assume that the MRCA passed all of his or her genes (or indeed any gene) down to every person alive. Because of sexual reproduction at every generation, an ancestor passes half of his or her genes to each descendant in the next generation. Save for inbreeding, the percentage of genes inherited from the MRCA becomes smaller and smaller in individuals at each generation, sometimes decreasing to zero (at which point the Ship of Theseus paradox arises), as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.