A girls first period happens relatively early in puberty, it isn’t the conclusion of it. The age of onset of puberty for each individual girl is determined by a combination of genetics and body weight. We live in a time of plenty, so prior to the post-WW2 era, girls were hitting puberty later (12 is an average now, it’s not a rule. Similarly, 14, 15, 16 would have been average at various times in the past, but not a rule).
I think, simply, people *in general* have always known that 12 was too young, or “just got their period” was the start of maturity, not the conclusion, so wait a few years. Everyone? No. But enough that dying due to premature pregnancy was merely a risk, not something that decimated the species. Modern humans have generally been smart enough not to impregnate 12 year olds as a general rule. So there’s never been any need for our development to change drastically.
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