Human: The Science Behind What Makes Us Unique

Gazzaniga argues that humans are qualitatively unique, not just quantitatively smarter than other animals. He challenges the Darwinian continuity thesis—the view that mental differences between humans and higher animals are 'one of degree and not of kind'—by invoking the concept of a 'phase shift' in evolution, analogous to how gases become liquids: the same components can produce radically different properties when their organization changes . He grants that most human activities have antecedents in other animals, but insists the exponential increase in brain connectivity, combined with the left hemisphere's narrative-forming Interpreter, produced capacities no other species possesses.

The Interpreter is the left hemisphere's meaning-making function that integrates the hemispheres and modules to create a coherent narrative self—what Gazzaniga calls 'the narrative known as consciousness.' It is this capacity, he argues, that underlies our ability to construct alternate realities, simulate future scenarios, and reason about unobservable forces like gods and gravity . The Interpreter was first identified in Gazzaniga's split-brain research, where the left hemisphere would confabulate explanations for actions initiated by the disconnected right hemisphere. In Human, he elevates it to the central mechanism distinguishing human cognition from that of other primates.

Gazzaniga identifies the capacity to reason and form beliefs about unobservable forces as a key benchmark. Humans create explanations for events in terms of imperceptible things—gods, gravity, spirits—a cognitive capacity no other species demonstrates . This is distinct from consciousness, which he treats as an emergent property of neurobiology present in other animals. Robin Dunbar, reviewing in Evolutionary Psychology, highlighted this as a core claim: humans are 'the only species that can reason and form beliefs about unobservable forces, creating explanations for events in terms of imperceptible things.' The capacity manifests in both religion and science, which Gazzaniga sees as twin products of the same cognitive architecture.

Reviews were mixed but engaged seriously with the thesis. John Cacioppo and William Patrick, writing in Nature Neuroscience, noted that Gazzaniga 'clearly sides with those who see something like a phase shift having occurred' while granting that 'there simply is no one thing that will ever account for our spectacular abilities' . They found the argument forceful enough to conclude that 'Gazzaniga has made a forceful argument for its need and importance.' Robin Dunbar in Evolutionary Psychology called the book a 'tour de force' covering a wide range of disciplines. However, Dunbar found the 'chatty fraternity locker-room writing style rather irritating,' indicating the book's informal tone divided reviewers.

Gazzaniga opens by challenging the notion that more neurons alone explain more brain power. He shows that the human brain differs from nonhuman brains in more fundamental ways than sheer number of neurons—the neocortex, cerebellum, and white matter all contribute disproportionately to human brain expansion . The critical factor is connectivity: human brains have exponentially greater connectivity than any other species, and this, combined with modular organization, enables the integration that produces human cognitive capacities. As Cacioppo and Patrick summarized, Gazzaniga attributes our 'aspirations, and our capacity to travel mentally in time' to the modular brain combined with connectivity 'exponentially greater than that found in any other species.'

Gazzaniga devotes a chapter to what he calls 'making special'—artistic creation—arguing it has adaptive value and developmental benefit. He explores the intrinsic pleasure humans find in beauty, a capacity chimps seem to share to a limited degree, but which humans elaborate into symbolic art . The neural substrate includes the orbitofrontal cortex, which represents the appreciation of beauty. Reviewers singled out this chapter as particularly compelling. Cacioppo and Patrick described it as 'perhaps the most compelling and original chapter,' noting that the Interpreter is central to artistic creation as it is to consciousness itself.