[simonh]

This is about evidence of people who themselves personally came from North Africa. Not people who's ancestors came from North Africa.

The evidence is found in the oxygen isotopes in their teeth from the water they drank in their early lives, so it's nothing to do with genetics or ancestry and is purely environmental.

[JoeAltmaier]

Though I'm surprised the research didn't simply use present-day Britain DNA records?

[simonh]

With DNA there would be no way to know when the African ancestry was introduced. Bear in mind we've had extensive contact with Africa via the slave and sugar trades for about a dozen generations.

[JoeAltmaier]

I think that's exactly what you can use dna evidence for. E.g. its used to identify when Neanderthal was added etc. Or is it because the introduction was continuous? Hm.

[simonh]

We don't know from the DNA evidence itself when we received Neanderthal DNA other than the fact we know roughly when main-line Neanderthals died out.

There's a huge difference between saying these modern British people have some tiny amount of African DNA and we don't know where it came from, and saying this person right here in this grave from this historical period grew up in Africa. They tell us very different things.

[JoeAltmaier]

That's just wrong, far as I know. We can tell when DNA was introduced - but I don't know how that works. Anybody?

[simonh]

I can't reply to ema's post.

To a point, but that wouldn't be useful in this sort of case. We're talking about people who lived 80 generations ago, many of them before the Saxon and Norman invasions that both flooded the English gene pool. I'm English with no 'known' African ancestry. But it's quite possible I have some African ancestry from 1800 years ago, but I might also have African ancestry from one ancestor in medival times, and perhapse an ancestor from the West Indies sugar plantation days. How would you ever tease that out? It would only add up to a few percent of DNA but would be all over the map. Many brits probably have ancestries all over the place like this. What about ancestors who were Berbers? They have largely Visigoth ancestry, but are from North Africa.

In any case, the idea that this specific evidence isn't of value 'because DNA' is just daft.

[ema]

We inherit the genes of our parents not completely mixed but in large chunks. So someone who is first generation mixed race while have large continuous regions of their genome from one race or the other. If two such people have children they will have the same amount of genes from each race as their parents[1] but the continuous regions will be smaller. So from the average length of regions which we can attribute to different founder populations we can estimate how long ago the admixture happened.

[1] This is not completely true as we seldom inherit exactly 25% of our genome from any grandparent, but it is usually pretty close.