It's not about a person's occupation, but about that person's genetics.
If someone could genetically be an olympic 10.000m champion, if he doesn't join an athletics club as a child but as an adult chooses a career in IT he'll probably not become an olympic 400m champion.
But if he marries to another potential olympic 10.000m champion who is working as a lab assistant, their children probably still have the potential of becoming an olympic 10.000m champion.
If on the other hand, he marries a potential olympic shot put champion, the chance of their children becoming olympic 10.000m champion decreases, since the body types of an olympic 10.000m champion and an olympic shot put champion are not the same.
You are right in the sense that, because neither of them ever competed in athletics, they would have had no way of knowing any of this.
The question is, that if you have a group of 70 people (in all likelyhood, those 70 were probably already related to each other!), marrying the two potential olympic 10.000m champions would be a good idea for the group. Alternatively you could also marry the 10.000m champion to the shot put champion on purpose. Or marry the 10.000m champion to the person just genetically inclined to be a fat slob. In that last case, the children might not be 10.000m champions but not fat slobs either. That might actually be the most beneficial for the group because such a child could at least be a reliable hunter. I am not sure whether, if you have already a small group, just culling the fat slobs would be beneficial, or if that would just reduce the gene pool even further and risk future defects.
If you are only interested in your own bloodline, it might make sense to breed the line of the 10.000m champions (who are related to each other) into the line of medicine women (also related to each other), to create a mix of body and brains that might be better than just being good runners. But whether creating such an elite is a good idea for the group is yet another question.
But all of this assumes a scientific breeding program, discounting things like love, attraction and family life. Which I think cannot be assumed for those first 70 Native Americans. So I wonder how they did manage that.
The number of 70 is based on genetic research rather than on archeological remains of actual people.
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I'm not a geneticist and therefore am not qualified to say something about the study either way.