Why do women mature (physically) earlier than men?

Question

All sources I've read (the first page) states that the fact that women mature (in the sense of physical maturing) earlier than man is caused by the earlier selective prunning in brain. Though, this is an explanation just for the mechanism, not for the cause.

So what is the real evolutionary reason women mature earlier (though, probably slower) than men?

Answer 1

This is a question about Neoteny and Heterochrony.

The closest field of study which will have specialists of this kind of topic is the study of the late maturation of humans compared to other species, which has had some research: 5-7 years ago there were some papers measuring the different bone growth rates of humans compared to chimpanzees and exploring the notion of natural selection through preference of immature traits: being attracted to girls with child like faces, smaller nose, large eyes, small voice, babyish, with various causes for example allowing for later brain pruning and longer brain development.

If you research in google publications you can find some research on the topic, for example:

Heterochrony in human evolution: the case for neoteny reconsidered

Sexual selection, physical attractiveness, and facial neoteny: cross-cultural evidence and implications (1995)

The origins of sex differences in human behavior: Evolved dispositions versus social roles.

It's all about evolutionary selection due to survival fitness. A complex age dynamic is at work in human evolution due to our extended lifetime, memory and brain size, amidst heavily variable social patterns (mate selection, monogamy, tribe size, bride rights, patriarchal/matriarchal systems) in paleolithic humans.

Bonobos mature at roughly the same time: 9 years, and female bonobos have a baby at 11 to 13 years. I think that illustrates the flexibility of maturation age according to the role of the adults.

Human females can have babies in between 9 and 15 years for the most part, some of them at 7, so maturation age is an evolutionary trait rapidly changeable according to fitness of the resulting offspring, so it is entirely fine tuned by survival performance.

EDIT: I wrote this answer to be helpful, but apparently my lack of sources and "errors?" deserve a mark down. so here is an additional text:

"They found that Paleolithic girls arrived at menarche - the first occurrence of menstruation - between seven and 13 years. This is a similar age to modern girls, which suggests that this is the evolutionarily determined age of puberty in girls.

'This would have matched the degree of psychosocial maturation necessary to function as an adult in Paleolithic society based on small groups of hunter-gatherers,' they write.

Disease and poor nutrition became more common as humans settled, causing puberty to be delayed. Modern hygiene, nutrition and medicine have allowed the age of menarche to fall to its original range.

However, today there is a mismatch between sexual maturity and psychosocial maturity, with sexual maturity occurring much earlier. This mismatch is a result of society becoming vastly more complex, with psychosocial maturity therefore taking longer to reach."

Note that they are talking about female adult paleolithic maturity and tasks at that age. Females compete for mates by attractiveness and fitness, and some research suggests that males compete for females by other mechanisms, even force or threat.(ref)

There is clearly a reason for which men are available to make and care for their offspring later than females. Females prefer older males by at least a couple of years with moderate adult muscle mass(ref), and in our species males of 17 to 35 years are biologically the strongest and wisest. Nature would have it that 11 /13 year old fathers clearly are counter productive to the survival of their partner and child in an ordinary setting of hunter gatherers, because of the very long gestation and complexity of human survival tasks.

The human mental maturation range has a wide variety of deviation, and between the age of 8 and 25, we are pre-programmed to be in an intense task of learning and physical adventure. Physical coordination of the male can reach an optimal level as late as the age of 22, 3/4 years after his bones have finished growing and his muscles and reflexes have had time to increase and adapt to his new frame, which is why athletes are at their physical peak between the ages of 18-27, and the "mental pruning" article suggests that their mind continues to re-organize itself well beyond the age of 30. During that stage, a human male is programmed to learn a wide array of advanced skills: flint knapping, tool making, botanic knowledge, hunting skills, clothes making, landscape exploration, orientation, memorization of food places, wells, tree climbing, metal work, net weaving, pottery, pigments, weapons, blow darts, wild animal experience (game, lions, rhinos, birds, rodents, traps, stalking, poisons, frogs) etc... All that time based survival experience reinforces the notion that a male reaches his functional optimum as a provider and protector of his family well after his sexual maturation age.

Females have a less physically intensive role to play in human society. While they can learn all the hunting/survival skills of the male, their central role is to be nourished and provide for the child(ref). Their parenting tasks rely less on individual memory because they stay socially at home more (papua new gineans, africans, aborigines, yanomami). Their task can be less intense physically, require less physical training, and can be less intensive on memory and adventure (being nourished, childcare, craftwork, leadership of the group at home and less in exploration and precarious tasks). Both the human male and female memory has a very important role for guiding the group. A female can provide guidance up till the age of 50-90 in a natural setting.

Females are generally less expendable and adventurous than males, because a male can potentially have 3 offspring every day, he is limited by the number of females, and she can only have 1 offspring every 9 months, she is more precious to the multiplication of the group.

Add to that the function of memory, hunter gatherer fitness, and social acumen, absorption of mental and survival skills

Other great apes have gestation periods arount 8.5 months compared to our 9.

Because of the difficulty to assess the paleolithic social and environmental pressures that caused the sex age maturation difference, it's a question that theorists would be unwise to wade into to any great depth, in an academically competetive field. A good anthropologist should be able to list all the predominant selection factors.