[–] ProxyMusic 6 points Edited

for most of human history, menarche (age of first period) didn't happen til around age 16-17, with first birth around 19.

I believe this about the age of first birth, but not about the age of menarche. I've looked into the age of menarche cross-culturally and in history too, and I think this professor of anthropology is way off about normal age of menarche historically and in more recent times.

Normal age of menarche is anywhere from 8/9 to 15; most girls get their first periods from 11-13 - but some much earlier, some later. This has been the case for a long time. The youngest girl known to have given birth in the 20th century was 5 at the time she became pregnant still 5 when she gave birth.

The professor's mistake seems to be that she's viewing menarche as a sign that a girl is now fully and optimally ready to conceive, gestate and give birth to a baby. When it's not. Menarche simply means a girl's ovaries have matured to the point where ovulation can and does/will start occurring on a regular basis, and her uterus has developed to the point where it can build up and slough off the extra lining that one day would be needed for implantation of a blastocyst if conception occurs.

Although this professor might know anthropology, she doesn't seem to know about how human females develop in our tweens and teens, nor about all the processes that female bodies have to become capable of carrying out before a girl/woman reaches the point in physical development where she is ready to bear children.

Menarche is an important milestone in female puberty - but it's not the only marker or kind of physical development that girls have to go through during and after puberty of adolescence in order to become equipped with the capacity for healthy, safe childbearing - meaning healthy and safe for both mother and child.

There are other many other bodily changes that need to occur - such as growth of the skeleton and bones to adult size, increase in bone density, changes in the shape and width of the pelvis, the ligaments and connective tissues "learning" to become lax in response to changing female hormones - before a female human is capable of growing a healthy fetus to term, going through labor and giving birth to a fully-developed baby.

Because human female health and development are very poorly researched and understood, there are probably numerous other as-yet-undiscovered developmental changes that human female bodies and organs go through over the course of puberty of adolescence to put us in physical condition where we're ready and able to grow and birth children. Such as changes to the uterus that will allow it to expand to full size and to create the contractions involved in human labor, and changes to the kidneys that will allow them to take on the extra burdens that human pregnancy entails.

Isn't the thinking that in agrarian societies the age of menarche is lower while in hunter gatherers it's usually later? And that puberty seems to be earlier these days compared with the past?

Well you have to reach a certain body fat percentage in order to menstruate. Typically poor girls and girls who had less food would start their period at 16 or 17. Richer girls with more access to food would start at 12 or 13 like today.