There used to be an old BBC show called "The Hairless Ape" that aired here as "The Human Animal". The show would look at the evolutionary reasons behind human behaviors...i.e., why do men like red lips and blushing? why an hourglass figure? The answer to the question was that there was no mating season because humans needed two people caring for the baby in order to insure it's survival. A libido that wasn't tied strictly to reproduction created a stronger pair bond, and dad was more likely to stick around and provide for baby. As for the timing of births, it probably was the case that more children were conceived in the fall when food was plenty, because human women don't ovulate when there isn't much food available. Ever been on a serious crash diet? Same thing.