Microblog Memes

That's not wrong at all, but there is a much more recent Adam and Eve - an actual anatomically modern human woman and man that all of us are descended from (although, again, they probably lived about 100,000 years apart - I.e. they weren't a couple)

It should also be stressed that I'm not saying that at one point there was only one woman alive or one man alive.

Mitochondrial DNA isn't like the usual inherited mix of DNA - instead it's passed down directly from mothers to their children. If they have girls, then it'll be passed down to their children too. If they only have boys, though, then that mitochondrial DNA has come to a dead end. Basically it's matrilineal - it passes down the female line.

The same with the Y Chromosome. Fathers pass the Y chromosome to their sons. If they only have daughters, well it's come to a dead end. It's patrilineal - it passes down the male line.

DNA analysis shows that all of our mitochondrial DNA is shockingly similar - especially compared to many other animals. Rolling back the clock (by accounting for the tiny random mutations that occur over time) shows that everyone converges to a single person around 155,000 years ago. Before modern humans, as a species, had expanded out of Africa and spread across the world.

Interestingly, especially so if looking for biblical comparisons, the Y-chromosomal Adam can be traced to around 100,000 years earlier than Mitochondrial Eve - so he did appear first.

So, the next question might be, "if anatomically modern humans existed for at least 100,000 years before the Mitochondrial Eve, what happened to all the other mitochondria variants - why did we end up with only one?"

That is simply a numbers game. Compared to many other animals, humans only have relatively few children in their lifetime. This keeps small populations from exploding suddenly, but it also increases the likelihood of only having boys (and stopping that line of Mitochondrial DNA).

You can map it out on a piece of paper:

Get five different coloured pens and a black pen.

Across the top of the page, draw dot in each coloured pen, with a black one beside it. The coloured dots are female, the black ones male). These are the parents.

Now for the kids. Toss a coin twice - heads for a boy (draw a dot in black pen beneath the parents), tails for a girl (draw a dot the same colour as the mum). Do this for all.

Now pair up the offspring and carry on.

You'll find very quickly, one colour will start to dominate the paper. This represents the Mitochondrial DNA.