This was the subject of a talk to Hastings Humanists in February 2011. The talk was to have been given by Dr Tom Rees, but he was unable to come at the last minute, but provided some notes. I also used some notes of my own based on the sources given below. I find the concept of 'Most Recent Common Ancestor' (MRCA) confusing as it seems it can refer to ancestors of genes, or of populations.
Who Was Mitochondrial Eve?. Mitochondria. Passed from female to female. Only 37 genes. Genetic code has mutations, so you can construct a family tree. Eventually, you reach a point where all the branches join together – mitochondrial Eve. How to tell how long ago? – the Molecular clock. mitochondrial DNA accumulates mutations at the rate of approximately one every 3,500 years. Mitochondrial Eve is generally estimated to have lived around 200,000 years ago, most likely in East Africa, when ("anatomically modern humans") were developing as a population distinct from other human sub-species.
Who was Y-chromosome Adam? The Y chromosome is passed from male to male. Y-chromosomal Adam probably lived between 90,000 and 60,000 years ago in the African continent and is the male counterpart of mitochondrial Eve, although he evidently lived later than she did, possibly 50,000 to 80,000 years later! The more recent age of the Y-MRCA compared to the mt-MRCA corresponds to a larger variance of the probability distribution for a man to have living descendants compared to that of a paleolithic woman. While fertile women had more or less equally distributed chances of giving birth to a certain number of fertile descendants, chances for fertile men varied more widely, with some fathering no children and others fathering many, with multiple women. (This difference in variance was first pointed out, in the number of descendants of male versus female fruit flies, by Bateman, 1948.)
Variations in Size of the Human Population They were not the only man/woman of their time. They had parents, brothers and sisters and there were other people living in other clans in other parts of the world. Other people alive at the time will have contributed to other parts of the chromosome. It could have been a genetic bottleneck. Patrilineal and matrilineal lines are less likely to die out during population growth, and more likely to die out during bottlenecks. Between 150,000 and 50,000 years ago there was a prolonged bottleneck; at times maybe only around 2,000 humans.
The Toba Event The eruption of Mt Toba caused another bottleneck. The Toba super-eruption occurred between 69,000 and 77,000 years ago, and it is recognized as one of the earth's largest known eruptions. The related catastrophe theory holds that this event plunged the planet into a 6-to-10-year winter, which resulted in the world's human population being reduced to 10,000 or even a mere 1,000 breeding pairs. Some researchers argue that the Toba eruption produced not only a catastrophic volcanic winter but also an additional 1,000-year cooling episode. No-one really knows. But it is clear that human population size was quite small quite often!
Common Ancestors Neither Mt-Eve nor Y-Adam is the most recent ancestor shared by all humans. The identical ancestors point (IAP) is that point in a given population's past where each individual then alive turned out to be either the ancestor of every individual alive now, or to have no living descendants at all. It is estimated to be 10,000 years ago (±50%). This point lies further in the past than the population's MRCA. Even though each living person receives genes in dramatically different proportions from these ancestors from the IAP, all living people share exactly the same set of ancestors from this point back, all the way to the very first single-celled organism. The identical ancestors point for Homo sapiens has been estimated to be about three times as distant as the MRCA. What about further back in time? Last common ancestor with neanderthals although we interbred, meaning that outside Africa we are up to 4% neanderthal! Last common ancestor with chimpanzees 6-8 million years ago.
An Example of Anti-Evolution Propaganda. Adam and Eve are real! Shocking new evidence... Who Was Adam?: A Creation Model Approach to the Origin of Man by Fazale Rana, PhD, Vice President, Research and Apologetics at "Reasons to Believe". "There is astonishing new evidence to support the position that homo sapien sapiens exploded onto the scene less than 100,000 years ago from a single man and a single woman." ... "Even more astounding is that scientists now know that human origins had their beginning in a particular geographical area — the same area identified by most biblical scholars as the physical location of the Garden of Eden. Belief in a literal and historical Adam and Eve as recounted in the Bible has greater scientific credibility today than at any other time in human history." ... "The old paradigm of the Darwinian theory of biological evolution is simply no longer valid in light of today's rapidly changing technology," continues Rana. "But many in the scientific community refuse to accept a paradigm shift because until now, there has not been another model with superior explanatory power and predictive success to replace the evolutionary model."