This relatively slim volume (217 pages of text, excluding notes, references and index) is the latest offering from Nicholas Humphrey, renowned Cambridge evolutionary neuropsychologist and prolific author on the subject of the evolution of primate intelligence and human consciousness. As both materialist and evolutionist, Humphrey shares in this work his latest take on the evolution of human consciousness which, according to his account, has emerged through a long and complex journey shaped by the forces of selection (natural, social, sexual, etc.) and, crucially, without invoking any factors outside the realm of physics and biology (no mysterian or supernatural explanations). For evolutionists and other materialists out there, this work is a welcome scientific antidote to the rampant spate of panpsychist and neo-dualist models and theories that have pervaded the field of consciousness studies in the past couple of decades.

The book is written for a wide audience that includes motivated and interested members of the general public as well as experts and specialists in the fields of consciousness, psychology and neuroscience. Although it is rigorously referenced, this is done for the most part unobtrusively. It is both groundbreaking and well-grounded. The book consists of conversational, autobiographical and historical narrative sections interspersed with the inevitable and necessary (given the subject matter) sections discussing philosophical and abstract ideas and concepts.

Humphrey distinguishes two kinds of consciousness – cognitive and phenomenal – and considers the latter to be the one people refer to when speaking of consciousness. Phenomenal consciousness refers to what it is like to experience something, such as redness, sweetness, etc., also known as ‘qualia’. He explains that not all living organisms have the necessary neurobiological systems to support such a state or experience, so that the two-stage vision processing in a monkey (where nervous impulses pass first through the superior colliculus (optic tectum) and then to the visual cortex) is contrasted with the single-stage visual processing in the frog. In his strict adherence to evolutionary principles, Humphrey points out the straightforward fact that a private and subjective state such as phenomenal consciousness or sentience could only evolve through selection if it had explicit effects in the real world; the world of survival and reproductive success of the experiencing organisms. Otherwise, any such phenomenon would have been invisible to evolution. The question of the adaptive significance of phenomenal consciousness is conspicuously an appropriate evolutionary question, yet, discussing this issue in such a volume is a rare occurrence in the philosophical literature on human consciousness. It is a welcome scientific contradistinction to books such as Thomas Nagel's Mind and Cosmos: Why the Materialist Neo-Darwinian Conception of Nature is Almost Certainly False.

So, what selective advantage might phenomenal consciousness (and ultimately, in humans, the phenomenal self) offer to our ancestors? It seems to be the ability to model the mental states of others on one's own; to form an accurate ‘theory of mind’. Those with a well-developed phenomenal consciousness were better able to navigate the increasingly complex human social world than were others. This is particularly in keeping with the evolutionary work of Robin Dunbar and others.

Humphrey discusses at some length his early pioneering work and discoveries on blindsight in monkeys. Blindsight is a case of unconscious vision which Humphrey persuasively proposes is the visual experience that occurs in living organisms who possess intact eyes and other central nervous system circuitry but no visual cortex. He describes important neuroscientific experiments that illuminate the connection of perceptions with affect and also special positive feedback systems, possibly explaining the mechanisms and systems beyond the mere reflexive. The most primitive forms are attributable to all animals. However, mammals and birds are the only sentient life forms with varying degrees of phenomenal consciousness, according to Humphrey's model. Sentience requires the evolution of complex, sophisticated circuitry beyond that required for reflexive actions or simple algorithms, to that requiring complex affective processing. Apart from the possession of a cerebral cortex, the other trait shared exclusively by birds and mammals is the fact that both are warm-blooded animals. This surprising, and to many, unexpected factor in the discussion of consciousness turns out to have played a pivotal role in enabling the evolution of phenomenal consciousness. Warm-bloodedness increases the speed of nerve conduction and also reduces the refractory period following nerve-cell firing. It also insulates the organism from changes in environmental conditions, keeping the internal bodily environment constant, thus enabling warm-blooded animals to survive and thrive in a wider range of environments.

Humphrey's model of consciousness has wide-ranging and profound implications for the understanding of human behaviour and the behaviour of other sentient organisms. It helps us understand the significance of play and predicts which organisms will engage in this activity as opposed to those that do not. It also explains the phenomenon that Humphrey terms ‘qualiaphilia’: the active seeking, magnifying and manipulation of phenomenal experience or qualia in multiple forms, which explains the universality of visual art and music in all human cultures and also predicts which species will engage in … wait for it … masturbation!

So, has Humphrey solved Chalmer's ‘hard problem’ of consciousness? We would say not, as there are still important outstanding questions about the nature of consciousness itself and the need for an explanation as to how brute matter can give rise to such an unexpected phenomenon – although in attributing an affective component as mandatory for its development, this places the evolution of emotions centre stage. Thus, what he has achieved is to specify the building blocks required for the phenomenal self/consciousness, the possible evolutionary steps that gradually built a system capable of generating this, as well as specifying its evolutionary payoffs. Humphrey's model of consciousness based on the sound foundations of evolutionary biology also makes specific testable predictions about the traits of the organisms that are likely to possess this and those that are not. The resulting model is demonstration of what biological explanations can achieve when considered in their proper evolutionary context. For those wary of unrestrained speculation and the wilful rejection of science in the consciousness literature, it is a welcome relief. This is the kind of book we will definitely keep and read more than once.