synchronised reproduction...
yes, spot the evolutionary biologist!
Humans have no "breeding season". We don't depend on the weather, prehistorically, we depend on the circumstances of the tribe. If a tribe is in a good position to reproduce, then many couples reproduce at a similar time. Have you ever wondered why so many men are so aroused by porn? It is a visual signal that others in the tribe are reproducing, and to synchronise reproduction, then this is a good time for you to reproduce as well.
So a non synchronised tribe has one baby born a year. Baby one is undernourished because mum can't make milk (breast feeding ability is not universal among humans) so baby 1 is passed around some of the tribe's grandmothers, who produce enough calories for him between them, but not enough protein or vitamins.
Baby 2 next year gets breast milk from mum, but mum can't go hunting or foraging as baby is EBF so that particular family unit has a work force at 50%
Baby 3 the following year has an anxious and unconfident mother as she has seen her cousins struggles.
OR in a tribe 3 babies are born within a couple of months. One mother can babysit and feed all 3 for half a day at a time, and all babies are fed, and two out of three mums can still forage and hunt, so the workforce is down by 1/6, not 1/2.
Incidentally, this is where the gay uncle comes in! A relatively large % of human men are gay. probably about 5%. The lower down you are in the birth order, the more likely you are to be gay.
Hello, supply of "spare" men, to hunt for and protect their nieces and nephews, without adding to the tribes burden of babies themselves. Can replace a lost father, or just supplement a living father who is not much good at hunting!
The complications and difficulties in raising human children account for the year long cycle of periods, as children are helpless for so long that being born in an abundant or austere season is meaningless, they will be infants throughout all seasons, so flexibility and the ability to synchronise reproduction mattered.
And accounts for the relatively high number of gays in the population - most other species have no life-long gays at all - and again, like the menopause, human kind would not have survived without them. Homosexuality and menopause- both provide additional carers for family/tribe infants, who are too much for the parents alone to provide for, protect and educate to adulthood