As biologist John Timmer notes at Ars Technica, some life forms appear much more intelligent than others despite having brains of roughly the same size:
Animals with very different brains from ours—a species of octopus and various birds—engage with tools, to give just one example. It seems intuitive that a brain needs a certain level of size and sophistication to enable intelligence. But figuring out why some species seem to have intelligence while closely related ones don’t has proven difficult—so difficult that we don’t really understand it.John Timmer, “Brain size vs. body size and the roots of intelligence” at Ars Technica (July 12, 2022)
As he points out, some things we might expect to be true — puzzlingly — aren’t:
One of the simplest ideas has been that size is everything: have a big enough brain, and you at least have the potential to be smart. But lots of birds seem to be quite intelligent despite small brains—possibly because they cram more neurons into a given volume than other species. Some researchers favor the idea that intelligence comes out of having a large brain relative to your body size, but the evidence there is a bit mixed.John Timmer, “Brain size vs. body size and the roots of intelligence” at Ars Technica (July 12, 2022)
Seven years ago, London School of Economics psychology prof Nicholas Humphrey, responding to a question at Edge, “What scientific idea is ready for retirement?”, responded “the bigger an animal’s brain, the greater its intelligence.” He elaborated, admitting he had been wrong about this in the past:
In particular, you’ll find the idea repeated in every modern textbook that the brain size of different primate species is causally related to their social intelligence. I admit I’m partly responsible for this, having championed the idea back in the 1970’s. Yet, for a good many years now, I’ve had a hunch that the idea is wrong.
There are too many awkward facts that don’t fit in. For a start, we know that modern humans can be born with only two thirds the normal volume of brain tissue, and show next to no cognitive deficit as adults. We know that, during normal human brain development, the brain actually shrinks as cognitive performance improves (a notable example being changes in the “social brain” during adolescence, where the cortical grey matter decreases in volume by about 15% between age 10 and 20). And most surprising of all, we know that there are nonhuman animals, such as honey bees or parrots, that can emulate many feats of human intelligence with brains that are only a millionth (bee) or a thousandth (parrot) the size of a human’s.
Biochemist Michael Denton offers some insights in The Miracle of Man (2022). Although whales (10 kg) and elephants (6 kg) have the biggest brains, primates and monkeys have a much higher than expected number of cortical neurons relative to brain size. Humans, not surprisingly, have the highest information processing capacity of any life form. (pp. 190-191)
What about making humans smarter?
Neuroscience researcher Michel Hofman describes the human brain as “one of the most complex and efficient structures in the animated universe.” Denton, noting that a cubic millimetre of human brain features sixty times as many synaptic connections as a 747 jetliner has components, goes on to say,
Many authors have concluded that it may be very nearly the most intelligent/ advanced biological brain possible. That is, its information-processing capacity may be close to the maximum of any brain built on biological principles, made of neurons, axons, synapses, dendrites, etc., and nourished by glial cells and provided with oxygen via circulation. For example, Peter Cochrane and his colleagues, in a widely cited paper, conclude “that the brain of Homo sapiens is within 10– 20% of its absolute maximum before we suffer anatomical and/ or mental deficiencies and disabilities. We can also conclude that the gains from any future drug enhancements and/ or genetic modification will be minimal.” Hofman concurs: “We are beginning to understand the geometric, biophysical, and energy constraints that have governed the evolution of these neuronal networks. In this review, some of the design principles and operational modes will be explored that underlie the information processing capacity of the cerebral cortex in primates, and it will be argued that with the evolution of the human brain we have nearly reached the limits of biological intelligence.” (p. 193)
If Hofman, Cochrane and colleagues, and Denton are right, recent proposals to produce human superintelligence “within a decade” via genetic engineering are doomed.
… what may be the most advanced brain in the “animated universe,” close to the limits of biological intelligence, has a volume of only about 1.5 liters and thus can be conveniently placed within the skull atop the upright android form of a human. How fortunate that the size of what may be close to the smartest possible biological brain— capable of the miracle of understanding the world and transforming it through technology and culture — is perfectly commensurate with the design and dimensions of the human frame and body plan. (pp. 193-194)
In short, we don’t need genetic engineering to produce smarter humans. We’d likely gain the same effect by using the intelligence we have more efficiently.
You may also wish to read: Ever wish you had total recall? Ask people who do… Recall of every detail of one’s past works out better for some people than for others. Just why some people can recall almost everything that happened to them is a mystery in neuroscience, in part because they are few in number.