All scientific evidence points to the fact that, if you go far enough back, all life on Earth is related through common ancestry. Turns out that applying the same sort of analysis shows that all humans alive today are descendants of one man and one woman who walked our planet thousands of years ago.
For several decades, there has been debate about when these ancestors, popularly known as Y-chromosomal Adam and mitochondrial Eve, existed. Two studies published this week find that there is a good chance Adam and Eve may have existed about the same time, evolutionarily speaking.
Two pieces of DNA are responsible for starting these debates. One is the Y chromosome, which is present in the nucleus of human cells and is passed down from father to son. The other, much smaller piece of DNA, is found not in the nucleus of the cell but in its mitochondria, small compartments in the cell that provide its power. Mitochondria are passed down from a mother to her children.
These pieces of DNA do not undergo any mixing like the regular chromosomes. This makes tracking mutations that affect them much easier, which can help reveal their ancestry.
An analysis of a child’s mitochondrial DNA can help track his maternal ancestry, whereas an analysis of a boy’s Y chromosome can reveal his male ancestry. (A girl obviously won’t have that option.) This sort of analysis, pioneered by Alan Wilson of University of California-Berkeley, can also say something about the timing of a sequence’s origins. If mutations occur with a regular frequency over time, then they can act as a molecular clock to tell us when a population last shared a common piece of DNA.
In 1987, Wilson’s team used the molecular clocks analysis on the mitochondrial DNA of about 147 people from various places around the globe. Because the subjects were considered to be a statistically diverse population, the researchers concluded that all human beings were descendants of this one woman who may have lived in Africa about 200,000 years ago. Over the past 20 years, their estimates have been challenged many times, but the rough outlines of these conclusions have withstood the scrutiny.
Similar research has been carried out on the Y chromosome of men from various places, but here the estimates of research vary quite widely—from 60,000 years to 580,000 years ago. One potential reason for this large range is that we simply haven’t sequenced as many Y chromosomes compared to mitochondrial genomes. Another reason is that there may be some very old Y chromosome lineages that surivived to the present.
Now, Carlos Bustamante of Stanford University and Paolo Francalacci of University of Sassari have separately attempted to find out when the Y-chromosomal Adam may have existed. Both their estimates, published in Science, are quite similar.
Bustamante used 69 males from seven globally diverse populations. Their estimate for the existence of humanity’s common paternal ancestor is between 120,000 and 156,000 years ago. Francalacci collected DNA samples from 1204 Sardinian men. Their estimate worked out to be between 180,000 and 200,000 years ago. These new estimates put Y-chromosomal Adam’s age at about the same as mitochondrial Eve’s age, bringing up the possibility that they may both have walked the Earth about the same time, give or take a few thousand years, which is much closer than previous estimates.
Rebecca Cann of University of Hawaii, who was involved in the 1987 study on mitochondrial Eve, writes in Science that these new analyses are “elegant and careful”. But Michael Hammer of Arizona University, an expert on the Y-chromosomal lineage of modern humans, is not convinced. His own analysis, published in March, has suggested that the Y-chromosomal Adam dates farther back in time. He said: “One reason these estimates are not as old as ours is because they didn’t sample the most divergent Y chromosome that we know about, which to date has been found only in small area of western Cameroon.”
Daniel Zadik of University of Leicester said, “We must take the numbers with a pinch of salt. Small changes in the assumptions made for the calculation can end up varying the result by a large margin.” Hammer added, “These estimates need to be interpreted with great caution with respect to what they tell us about human history or evolution.”
There is no reason to expect that the ancestor of all Y chromosomes and the ancestor of all mitochondrial genomes lived at the same time. But events like a strong population bottleneck or an isolated population would make that more likely. So, figuring out a narrower date for these two ancestors could help us understand the history of modern humans.