>Among terrestrial vertebrates, birds and mammals generally exceed both amphibians and ‘‘reptiles’’ with respect to cognitive abilities. Among birds, corvids and parrots stand out in this regard with behavioral flexibility, innovation rate, tool use and tool fabrication, and also with respect to truly mental abilities such as logical reasoning and mirror self-recognition––at least in one corvid species.
Several groups of mammals, like dolphins and whales, dogs, elephants, and bears (just to mention a few) show signs of high intelligence at least in some cognitive domains. Primates on average exhibit an intelligence superior to all other mammals. Among primates, there is a rather clear-cut ranking order in intelligence, from the prosimians to monkeys and to the great apes. Except a few species (e.g., capuchins), the great apes exhibit at least some aspects of cognitive and mental abilities not found in monkeys regarding tool fabrication, insight into causal mechanisms, mirror recognition, theory of mind, knowledge attribution, metacognition, and consciousness.
However, humans, even under the most critical aspects, are superior to other animals in all cognitive functions, no matter how astonishing the achievements of the latter may be. The most clear-cut differences between humans and non-human primates lies in two abilities that are interconnected: planning abilities and a syntactic-grammatical language. When we compare the cognitive-mental abilities of the most intelligent non-human animals with those of humans, then we find that they roughly correspond to the abilities of children aged 2 1/2-5. As for linguistic abilities, chimpanzees and gorillas equal a 3-year-old child, while with respect to psychosocial abilities (empathy, theory of mind, etc.) they may be equivalent to those of a 5-year-old child. In light of these empirical findings, the standard question of whether human intelligence differs qualitatively or only quantitatively from the non-human one, may ironically be transformed into the question about whether, with respect to cognitive functions, an adolescent or adult is qualitatively or only quantitatively superior to a 3–5-year-old child. Besides maturation of social competences, the most decisive feature that distinguishes humans from nonhuman animals is the appearance of a syntactical-grammatical language at age 2 1/2, which is paralleled by an enormous increase in the capacity of working memory and, consequently, intelligence, i.e., novel problem solving.
Therefore, the ‘‘rubicon’’ between animal and human intelligence seems to be the evolution of the syntactical-grammatical language, which is essentially bound to an increase in the ability to mentally manipulate processes (first actions, then thoughts, then words) in the temporal domain. Once evolved, human language served as a mighty ‘‘intelligence amplifier,’’ as was later development of writing and invention of the computer.<
The long evolution of brains and minds
Gerhard Roth (2013)