Before school started up again for the year Alek Dog and I were walking the trail near one of the neighborhood schools. In from the nearby street rode 2 boys on their bicycles: "This is the trail by the school," said one.
I thought, if we humans never advanced beyond the mental and physical capacities inherent in 10 year old boys we would nevertheless be capable of maintaining a viable culture and civilization.
Of course if the girls also attained the level typical of 10 years of development that would likely raise the overall competence of humanity by a notch or so as developmental dimorphism has already set in. The developmental level of the boys would be adequate if sufficient time is allowed.
A typical 10 year old boy is observant and curious, has stable control over his body (which 5 and 50 year olds typically lack), and exhibits an adventurous mastery of language well enough developed not only to communicate facts for survival and to transmit core elements of technical skill but also to support hours of wordplay in the form of laughter and lyrics.
It is puzzling, in fact, that we ever developed 12, 16, and even 22 year old people especially in the mental realm. Hunting large animals likely requires 16 year old physicality. Extensive, animal-based agriculture may benefit from the muscularity and increased leverage of the 18 year old body. Yet in actual reality we have trouble making good use of most 15 year old minds.
There is likely a collective advantage to having such excess capacity. We have conjured vast cooperative endeavors across continents and among people who will never see each other. We have invented technologies based on invisible properties of materials whose very existence could not be observed in advance. We have even invented mathematics which allows us to be absolutley certain of truths we can not possibly know at all.
Development beyond the level of the 10 year old boy is powerful, it is remarkable, it can be astonishing. But it is not essential to the existence of human civilization. For that, the 10 year old suffices.