The most human-like primates in terms of intelligence are monkeys, not apes

(writing in progress)

Some monkeys are brainier than any apes, with the possible exception of chimps (https://www.researchgate.net/figure/Bar-graph-of-the-encephalisation-quotients-EQs-of-extant-primates-and-cetaceans_fig4_7205381 and http://www.ling.upenn.edu/courses/ling001/com_bio.html and https://brainevo.sitehost.iu.edu/publications/dissertation/Dissertationch2.htm and http://www.brooklyn.cuny.edu/web/aca_naturalsciences_anthropology/BIO-RosenbergerAlfred-PUB_2011_AR.pdf).

If you look at the graph below and imagine the straight-line regression best ‘averaging’ this relationship, you can see that there is a cluster of brainy monkeys that are farther above the imaginary regression line than any of the apes are, with the possible exception of chimps. Of course, Homo is far above this line.
 
I stress this because most educated lay persons have this wrong in their minds. They think that the natural order of things, in terms of braininess, is human > apes > brainiest monkeys.
 
In fact the apes are not a brainy as many naturalists may assume. Instead, it is certain monkeys that are the closest runners-up to humans in braininess, and these monkeys belong to two quite separate lineages, one in the Americas and one in Africa. The former is the capuchins (Cebus, https://www.inaturalist.org/observations?place_id=any&taxon_id=43429) while the latter is some of the cercopithecids, particularly talapoins (Miopithecus, https://www.inaturalist.org/observations?taxon_id=43542 and https://www.sciencedirect.com/science/article/abs/pii/S0047248405800041).
 
There is no doubt that apes are more closely related to us brainy humans than monkeys are. But this does not mean that apes are more similar to us in braininess than some monkeys are. The thing is that there has been a kind of evolutionary divergence within the apes, with us humans specialising on braininess and e.g. gorillas specialising on 'un-braininess' (https://www.researchgate.net/figure/Encephalization-quotient-EQ-plotted-against-log-body-weight-for-several-genera-of_fig3_229482899).
 
Gorillas look uncannily like humans but in terms of braininess they are only at a fairly average monkey-level. The brainiest of monkeys quite outclass gorillas in terms of EQ.
 
What this means is that extreme EQ values have tended to evolve at least three times within the primates. One time has been via apes to humans. Another time has been in African monkeys to e.g. talapoins. And a third time has been in American monkeys to capuchins. It would not be true to say that the non-human apes have evolved extreme EQ, because the true way to look at them is as only moderately brainy in the overall scheme of higher primates.
 
Another way of saying this is that, when you look into the human-like face of a great ape, you’re not so much looking at our nearest relatives in braininess as at our nearest relatives in various other features. Our nearest relatives in braininess are cheeky monkeys which we do not identify with to the same degree, because they are not closely related to us taxonomically. 
 
Any evolutionary increase in braininess as part of a kind of ‘arms-race’ with humans? I don’t see apes as evolving in this direction. What I see if anything is the opposite direction. Gorillas (https://www.inaturalist.org/observations?place_id=any&taxon_id=43579), for example, have if anything evolved to be decephalised, filling a niche far more vegetarian than any human species, and having the bulk required for this. Gorillas are adapted for their niche, and they may be continuing to adapt, but they’re not trying to adapt into anything resembling, or competing with, humans.

If you’d like to stare into the eyes of a little-known but perfectly accessible monkey group which beats most or all apes in braininess, making it more similar to us in braininess than any ape with the possible exception of chimps, try google images for any of the members of the genus Cercocebus (mangabeys, https://www.inaturalist.org/observations?place_id=any&taxon_id=43465).

Talapoins may have some connection to humans because they tend to be commensal near villages, raiding human crops and even swimming for refuge when humans chase them, but as far as I know the mangabeys keep to themselves. They are among the brainiest of Old World primates not because they have adapted to humans in any way, but merely because that is their original, wild niche.

So here is a ‘best-kept secret’ of primate braininess: that some shy forms of African monkey, which hardly any lay person has heard about and which have little to do with humans in their natural niches, may be more like Homo sapiens in braininess than any of our closest relatives among the apes.

(writing in progress)

Posted on June 16, 2022 08:55 PM by milewski