Well, that's a big question. These "why" questions, particularly in genetics and the study of history in general, are always the big ones, the ones that are tough to answer.
这是个很重要的问题。这些“为什么”的问题。 尤其是对遗传学和人类史学来说，都一直是很重要的问题。 这些问题很难回答。