Counting has changed the brain of humans. It first shows itself in a slow down ofhe growth of numerical ability acquisitions with monkeys developing quantitive reasoning abilities two to three times faster than humans. This is due in part to the slower maturation rate of human children as numerical choice behavior appears to be tied to perceptual and motor development (Primitive Concepts of Number and the Developing Human Brain 2019, Kersey and Cantion). This is an important piece of our knowledge that numerosity is, to some degree, both innate and a very old trait in primates if it is shared with monkeys.
Counting activates neurons in the Intraparietal Sulcus (IPS). Kersey and Cantion note that monkeys trained for numerical matching see neurons activate when a preferred numerical value is presented and activate much less as the number is numerically distant. Larger numbers, regardless of position, have more activation than small. The same region activates in adult humans with the added function of a strong response to sudden changes of number. This applies to both symbolic and non-symbolic numbers and the differences between the two are clearly seen within the IPS (as well as other regions) with areas that do not overlap. This is largely duet o the different roles that symbolic and non-symbolic numbers have, but the data on this is unclear.
Kersey and Cantion also found that the inferior frontal cortex was engaged by humans to process numbers. This is not used by non-human primates and seem to allow for the ability to integrate numerical representation with sequence representation. This allows for children to learn to count and for the idea of a symbolic number to exist. Indeed, in children it was found that frontal regions of the brain were used to a degree that was never found in adults and appear linked to language-based functions of numerical literacy.
But where did these unique functions come from? How does the brain have regions to count without counting? We can look to neural recycling for the answer. As we have discussed the IPS use for numerosity is an old one, but humans have remapped it for numbers and arithmetic. Ordinality also uses the IPS, though a slightly different area of it is used for this process. The proximity of this to spatial cognition suggests that some neuron resources were taken and recycled. The placement of finger-counting neurons bears this theory out, with neurons for spatial cognition, fingers, and overlearned sequences being close (Bootstrapping Ordinal Thinking 2017, Overmann et all). That was all it took to start the enculturation arms race between the use numbers and the brain changing to use them.
As neurons were recycled and brain function increased to further numerical competency more counting functions could arise. Is it coincidence that beads appear after the parietal lobe expansion? Probably not, as the parietal lobe largely handles information from our senses to interpret the world as well as hand-object manipulation and interaction. This is not seen in other humans and also coincides with increased tool use and type for early humans. This change to the human brain allowed for an increase in "prosthetic capacity", or the ability to integrate tools and symbols to our body and thoughts (The Parietal Lobe Evolution and the Emergence of Material Culture in the Human Genus 2022, Bruner et all), which is essential to the development of complex numbers. As we continued to create and count we changed our very brain structure to enable more and more.