All humans descended from Adam and Eve. That's the biblical story at least. The scientific community has their own Adam and Eve: Y-Chromosome Adam and Mitochondrial Eve to be more precise. They did not live in the garden of Eden but probably in Africa. They didn't live 6000 years ago, but about 60,000 years ago (Adam) and 140,000 (Eve). Who are these people, or rather, how are they defined?
Let's focus on Eve. She is defined to be the most recent common female ancestor of all humans that live today if we only consider female descend. Ok, let's explain that a bit more. Consider all living humans today. Next consider all their mothers, then all their mothers etc. The crucial point is that we only consider women and moreover that everyone has precisely 1 mother so there can never be more mothers than children. However, mothers can have more than one child (and typically will) while other females will not have any children. So, we expect that the set of our grand-grand-etc. mothers will actually shrink when we extrapolate back in time, until there is only a single mother left.
This reasoning can easily lead to confusion. For instance, it doesn't mean that Eve lived alone, nor that she was the sole female in her times. In fact, many other women lived in her times. More importantly, they may have well been our ancestors through their son's lineages. Eve is defined throught her daughters alone: to be Eve, you need to be the only female from which a path of daughters exists all the way to modern times (in fact to all humans). In contrast, female paths from all other female contemporaries will end prematurely.
Mitochondrial DNA (mitochondria are the power plants of our cells) happens to be passed along the female line and so we can claim is that everyone today inherits the mitochondrial DNA from Eve (not so for the DNA residing in our genes.) A similar effect occurs for the Y-chromosome in a man which is passed along the male line.
Perhaps, more bizarrely, the honor of being Eve changes hand continuously. Imagine that Eve lived in 140,000 BC. In 130,000 BC there happened to be two female descendants of Eve that are the two mothers of all modern humans through female lineage. One day, the last descendant of mother A will die and at that point in time, mother B will take over the title "Eve" (she is then more recent than the former Eve living 140,000 BC). Thus, the honor is only bestowed retrospectively.
The mathematics of the proof depends on the fact that it is likely that some two mothers in one generation will be the daughters of a single mother in the next generation. It is not necessary. If two populations are well separated it will take many steps of going back in time to "coalesce" the two mothers to one mother. So the "mother-set" will remain stable at two for a long time. However, if we go far enough back in time, it seems very unlikely that two species of human evolved from fish! In the extreme case, to argue for two Eve's, we would have to argue that life has started twice on earth. Not impossible, but very unlikely if we consider how much we look alike.
Then finally, we should also not confuse Eve with the "most recent common ancestor" which has all humans as descendants. This person or persons (male or female) lived much later (perhaps even 3000 years ago.) because its descends are defined through both male and female lineages. Moreover, their contemporaries may also have many descendents today (just not everyone). Going back in time one can show that there must be a point in time (the "identical ancestors point") where each person who lived then will have either everyone or no-one as a descendent today.