An exciting question. Thanks for putting it into a session.
However, to begin with, the philosophy of the idea of evolution itself, would it be scientifically neutral and appropriate to posit that ‘humans evolved faster’? Is there a standard gauge to measure what is considered to be a ‘fast evolution’? How wrong would it be if I alternatively told you, by the rate of mutations and resultant evolutionary fitness, viruses (and other pathogens too) are the ones that are evolutionarily fastest?
On a lighter note, we can start our discussion by explicitly concentrating on the ‘cognitive’ evolution (and as far as I understand, this was what the session was aimed at). This is where we need to look at how our brain evolved: mostly similar in the overall biochemical mechanisms, anatomy and physiology, why are we capable of performing the ‘sentient’ and exclusively human-like activities that our primate relatives cannot? The answer might be found in the connection between developmental embryology and the evolution of the brain.
The neocortex, as the name suggests, is one of the most recently formed regions of the brain that was modified from the dorsal cortex of reptiles- gradually thicker and layered in the mammals. The neocortex is the most enlarged part of the mammalian brain, and it is the structure with enhanced information-processing and storing capacity, and it is considered to mediate consciousness in humans and possibly other mammals. Since the neural circuits in the neocortex are modifiable throughout life, it is the part of the brain that becomes tailored for relevant perceptual and behavioural abilities and enables individuals to acquire skills, personalities and memories. From a cognitive standpoint, this is very crucial. Hence, it is very likely that neocortex made all the difference in the pattern of cognitive evolution among the human vs other primates .
Evolutionarily, the brain features we share with close relatives like bonobos and chimpanzees are retained from the common ancestors. The fossil records, though not definitive on their own, suggest that the brain size relative to the individual’s body size is highly indicative of computing power and intelligence. We can look at the hominin clade that includes humans (emerged around 190 thousand years ago) and all extinct species more closely related to us than the chimpanzees, the closest living relatives. Given that all our other hominin relatives (Homo habilis, Homo erectus, for example) are extinct, we can try to see the fossil records to see if we can say anything about the last 5-8 million years of hominin brain evolution. Over the last 1.5 million years, the brain size has increased in size: modern human brains being three times as big than that of the great apes. There are also hints of proportional reduction of the primary visual cortex and reorganization of the cortical region, and also of the hemispheric asymmetries related to right-handedness and languages. It is guessed that cortical fields stabilized in size after a certain level of continual enlargement, and went through a reorganisation. This suggests that the neocortical region played a major role in making us humans what we are today. Indeed, there is extensive evidence from fMRI that the human brain houses more cortical areas (more than 150) than the brains of macaque monkeys.
Another reason to consider the importance of neocortex in setting us apart is that there is a unique trend of migration of bipolar neurons in humans that is unseen in other primates. The migratory cells (from ganglionic eminence to thalamus) rely on homotypic neutrophilic guidance, and they express DIX1 and DIX2 proteins that guide the migration. The process is anatomically linked to the association neocortex involved in higher cognitive functions like reasoning and language mastery .
There are other molecular underpinnings that point in the direction that neocortex is important for the human cognitive edge over its close relatives. One such phenomenon is the region-specific differential gene expression. There is a much greater transcriptional difference between neocortical areas in humans than there is in rodents . For instance, CNTNAP2 (contactin associated protein-like 2) is selectively and highly expressed in the orbital prefrontal cortex, the area involved in pro-social behaviour in humans. What is interesting about this is that the protein does not have any comparable analogue in rodents .
Hence, while there is a need for assessment of more fossil records and also more molecular data to be examined, one plausible answer to why humans evolved more (in terms of cognition) might be answered by looking at the evolution (and the developmental biology) of the human neocortex.
Also, joining with Shubhankar's earlier contribution, given the role of neocortex in the cognitive and meta-cognitive functions like socializing and forming relationships, the idea that Humans have something to do with the extinction of other Human-related species becomes even more plausible. As our forefathers grew more adept in performing tasks in groups- by the virtue of more complex neocortical prowess- we outdid other evolutionary relatives.