Another possibility is some form of epigenetic factor (epigenetic = heritable phenotype changes that do not involve alterations in the DNA sequence).
I remember seeing a study somewhere that female offspring who are born when their fathers were older were more likely to remain fertile for longer. It couldn't be "pure" genetics, because the older siblings of these offspring (who were born when the same fathers were younger) did not exhibit the extended fertility. Apparently, the fact that the fathers were older when they produced these later offspring somehow "told" the females' genetics to "stay fertile longer". I can't remember the details of the study, or if it applied only to animals and not necessarily humans, but the point is, there are epigentic factors at work in reproduction. Something can "tell" the genes that older parents are apparently still reproducing, so conditions must be good and you can plan to keep reproducing longer.
It's possible that one of the reasons females might reach menarche younger could have to do with parental factors. Perhaps pregnancies taking place in higher-stress situations (meaning stress of any kind, emotional or physical, poor nutrition, etc.), or even fathers experiencing high stress levels (maybe cortisol affects sperm) could have an epigentic effect that "tells" the offpring's genes that times are tough, plan to reproduce early because it's rough out there and you may not live that long.
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I remember seeing a study somewhere that female offspring who are born when their fathers were older were more likely to remain fertile for longer. It couldn't be "pure" genetics, because the older siblings of these offspring (who were born when the same fathers were younger) did not exhibit the extended fertility. Apparently, the fact that the fathers were older when they produced these later offspring somehow "told" the females' genetics to "stay fertile longer". I can't remember the details of the study, or if it applied only to animals and not necessarily humans, but the point is, there are epigentic factors at work in reproduction. Something can "tell" the genes that older parents are apparently still reproducing, so conditions must be good and you can plan to keep reproducing longer.
It's possible that one of the reasons females might reach menarche younger could have to do with parental factors. Perhaps pregnancies taking place in higher-stress situations (meaning stress of any kind, emotional or physical, poor nutrition, etc.), or even fathers experiencing high stress levels (maybe cortisol affects sperm) could have an epigentic effect that "tells" the offpring's genes that times are tough, plan to reproduce early because it's rough out there and you may not live that long.