Question: "What are Y-Chromosomal Adam and Mitochondrial Eve?"
Answer: “Y-Chromosomal Adam” and “Mitochondrial Eve” are the scientifically-proven theories that every man alive today is descended from a single man and every man and woman alive today is descended from a single woman.
Humans have 23 pairs of chromosomes. One of those pairs, known as the sex chromosomes (because they determine gender), consists of two X chromosomes in females, and one X-chromosome, one Y-chromosome in males. Girls receive one of their X-chromosomes from their mother and the other from their father. Boys receive the X only from their mother and the Y only from their father. Therefore, the Y-chromosome is passed directly from father to son. Because of this, scientists are able to trace male ancestry.
In 1995, the journal Science published the results of a study in which a segment of the human Y-chromosome from 38 men from different ethnic groups were analyzed for variation (Dorit, R.L., Akashi, H. and Gilbert, W. 1995. “Absence of polymorphism at the ZFY locus on the human Y chromosome.” Science 268:1183–1185). The segment of the Y-chromosome consisted of 729 base pairs. To their surprise, the researchers found no variation at all. Their conclusion was that the human race must have experienced a genetic bottleneck sometime in the not too distant past. Further research was done and it was determined that every man alive today actually descended from a single man who scientists now refer to as “Y-Chromosomal Adam.”
Mitochondrial Eve takes it a step further. While Y-chromosomes are only passed down from father to son, mitochondrial-DNA is passed down from mother to both daughter and son. Because mitochondrial-DNA is only passed on by the mother and never the father, mitochondrial-DNA lineage is the same as maternal lineage. Knowing this, scientists have found that every human alive today can trace their ancestry back to a single woman who they now refer to as “Mitochondrial Eve.” While Y-Chromosomal Adam is believed to be the ancestor of every living man, Mitochondrial Eve is believed to be the mother of all living humans, male and female.
It is important to note that this does not prove that Y-Chromosomal Adam was the only man alive before he started having children. This only proves that his descendents are the only ones to have survived. Likewise, Mitochondrial Eve was not necessarily the only woman alive before having children. Rather, all we know for sure is that she is at least one of the ancestors of all living humans. While other contemporaries of her may or may not figure into the ancestry of living humans, we can at least say that none of their mitochondrial-DNA has survived.
Scientists who share the Darwinian bias naturally presume that these two were not the only humans alive during their pre-child bearing lifetimes, while Biblical creationists naturally presume that they were. As for determining when these two actually lived respectively, the conventional perspective is founded upon uniformitarian assumptions which many creationists reject, and with fair reason. So there is disagreement there too. Naturally, the Darwinian timeframe is much longer (tens to hundreds of thousands of years), presuming an Old-Earth scenario, while the Young-Earth perspective is much shorter (less than ten thousand years). What we can say with fair certainty is that regardless of time frames and alleged contemporaries, every man alive today descended from one man while every human alive today descended from one woman.
“Adam named his wife Eve, because she would become the mother of all the living” (Genesis 3:20 NIV).