Although it would super cool for the story to have a common ancestor across huge swaths of land and time on Earth, I think this has a higher likelihood of being

Although it would super cool for the story to have a common ancestor across huge swaths of land and time on Earth, I think this has a higher likelihood of being correct. There are several stars in the 5-6 magnitude range close to the limits of human vision in the Pleiades. Seven is also often a sacred number. We also have precedent for cultures such as the Greeks and Arabs using double stars to test vision, namely Mizar and Alcor[1,2]. The typical limiting magnitude for unaided vision is about 6 under perfect conditions, but some people with exceptional vision can see to mag 7. So it seems highly plausible that most people could only see six, but they knew there were about 7 because some sharp-eyed people could see them and would tell them so. [1] https://en.wikipedia.org/wiki/Mizar_and_Alcor#Test_of_eyesight https://en.wikipedia.org/wiki/Mizar_and_Alcor#Test_of_eyesig... [2] https://www.surveyophthalmol.com/article/S0039-6257(08)00119-7/fulltext https://www.surveyophthalmol.com/article/S0039-6257(08)00119...