Last week I went to see Prof Steve Jones from University College London, giving a talk on genetics and human evolution. Very entertaining.
Let’s say my friend and I are trying to find our most recent common ancestor. Since we were both born in the same town in a time when people weren’t travelling so much, it’s quite possible that our great great great-father only lived two-three hundred years ago. Similarly, one could find a common ancestor for two people living on different continents for instance, it’s just that you’ll have to go much further back in time. And using the same logic we could eventually find an Adam, the father of all men alive today.
Biologists can actually check these things by looking at the Y-chromosome DNA from males from around the world. It turns out that indeed, there was an Adam and he lived somewhere in Africa about 60000 – 90000 years ago.
The interesting thing is that there is nothing special about this particular Adam. He is not unique. He had many friends he used to play football with and go to the pub. It’s just that all his mates failed to produce a direct link to us today. Getting your Y-chromosome to your decedents is a bit like passing on your surname. If Mr Smith has a son and his son also has a son, then a few generations down the line, there will be someone proudly calling himself Smith. But if Smith Jr has a daughter and she marries Mr Peters Jr, then the Smith name is lost. There goes his Y-chromosome...
Now what about Eve? It turns out that biologists can also track her down by looking something called mitochondria (tiny little machines that power the cell). Mitochondria genes are only passed down through females, so just like the Y-chromosome in males, they are a sort of signature useful in identifying your ancestors.
So if we do the same exercise trying to find our common Eva, it looks like we have to travel much further back in time. She might have lived something like 50000 years before Adam. And the reason is quite simple. It’s to do with the fact that men are much more variable in their sexual success than women. If we take any population we see that some men have lots of children and others have fewer or none. A few males pass on many copies of their genes, while most don’t. So very soon we find our Adam. On the other hand most women have children. It means that their genetic material is not “lost”, at leas not that quickly. And if you think that females have roughly the same number of kids you soon realise that you can’t just go back a couple of generation to find a shared ancestor, but maybe hundreds and hundreds of generations. This means that maybe our common Eva wasn’t even Homo Sapiens…
I’ll finish with a little anecdote. While teaching these things to some students in Botswana, Prof Jones asked them how do they reconcile these ideas with their religious believes.
“It’s simple!” said one of them. “You evolved, we were created!”
