He said "Yes, I'm from [place]."
She said "I knew as soon as I saw you that you were my brother! I'm from [other place nearby]." (The only reason I know that these places are in Nigeria, or indeed near each other, is because the woman explained it to her friend. I think she said they were ten miles apart.)
He said "That's so amazing!"
She said "Isn't it? I'm going to phone my mum to tell her."
He said "Yeah, I should do the same."
And she pulled out her phone to call her mum right away, and as soon as he'd finished serving customers, he called a family member too.
I just don't know how she recognised him as being from that very specific part of Nigeria though. It wasn't accent, because they both had broad London accents. I've been thinking about it ever since, how bad people are at recognising ethnicities beyond broad definitions like "black", "South Asian", "East Asian", "South American". I recognise the difference between north Africans, west Africans, South Africans, Somalis (they look like Mo Farah!), and people from certain parts of the Caribbean, but that's as far as I could get. And I suspect that's better than a lot of people who aren't themselves black.
The sad thing is, I could have a reasonable stab at identifying the origins of white Europeans - but that's based on things like clothing style as well as just physical appearance. So it is obviously possible even within people who look broadly similar. Is it cultural indoctrination of a sort, recognising people who are "like us"? Probably. It's probably related to whatever it is in childhood development that makes a baby of a certain age know how to recognise an animal as a "dog", even considering how many different and strange shapes of dog there are. (Don't get me started on some of the crazier specifications of pedigree dog breeds. Just don't.)
But how can a person who is interested learn as an adult how to recognise people's ancestry as belonging to a specific ethnic origin? Not because you're prejudiced - I'm inclined to think that a prejudiced person would simply label them all as "foreign" and not bother learning the nuances - but because people are fundamentally fascinating and you live in a huge city with people of every conceivable background. I suppose that's something taught in anthropology, but it's not as if you get to measure the bones and calculate the ratios of the measurements when you pass random people in the street! Hrm.