It easier to maintain pair bonding in larger primate social groups if males can’t easily tell when females are fertile.  In turn, monogamy makes it easier to keep the peace in larger groups.  And since folks in large groups have more uses for big brains, and more resources to pay for them, monogamous social apes should have bigger brains.  So monogamy encouraged by hidden female fertility may have been the key to humans succeeding far beyond other apes.

Why might we think this?  Chimps are humans’ closest living relatives, splitting apart 5-7 million years ago.  The Ardipithecus ramidus proto-humans of 4.4 million years ago were bipeds with a broad diet in woods and grasslands, and with a brain

about the same size as a modern bonobo or female common chimpanzee brain … The less pronounced nature of [their] upper canine teeth … has been used to suggest that the last common ancestor of homonids and African apes was characterized by relatively little aggression between males and between groups.

A recent Science article persuasively elaborated this argument:

Elimination of the [upper canine teeth] in hominids is unique among all higher primates and occurred long before Australopithecus. … Available evidence now suggests [it] was, as is theoretically most likely, a social adaptation … consistent with a strategy of increasingly targeted provisioning. …. Males would benefit from enhanced male-to-male cooperation …. Foraging could be achieved most productively by cooperative male patrols … Provisioning would reduce female-to-female competition … and would improve (or maintain) social cohesion. …

A large brain is not our most unique characteristic. … The combination of [upper canine teeth] elimination, habitual bipedality, and reproductive crypsis (each in itself an extreme rarity) is unique among all known mammals. Conversely, simple brain enlargement is readily explicable in myriad ways.

They plausibly suggest that these three key uniquely human features appeared together over 4 million years ago, leading over time to our uniquely large human social groups and brains, and all else they imply.

If monogamy is this essential to human success, that does make me a bit more concerned about current trends away from monogamy.  Of course hunter-gatherer monogamy may only have been for 4+ year periods, and we are in some ways moving more toward that.  But still, it gives me pause.