The feature of consciousness is one of many modules, or functions, that have been added to the mammalian brain over the course of evolution. It involves a select fraction of the processes taking place in the awake brain. The processes not brought to conscious awareness are referred to as subconscious. The subconscious activity has the capacity to direct the attention of the conscious brain in a fashion analogous to what, in the language of business, is referred to as ‘‘information given on a need-to-know basis.’’ Thus, even information that impacts on our emotional life is not necessarily brought to conscious attention Presumably, consciousness is costly to operate, and only capable of handling one experience at the time; thus conveying too much information to the part of the brain responsible for consciousness could cause dangerous distractions.
Consciousness can be turned on or off, either by the power of control vested in the subconscious (as when falling asleep), by external means (as in anesthesia), or by damage to the brain (e.g., coma caused by a stroke). The various situations in which consciousness is off may be collectively referred to as unconsciousness (used here in a physiological, rather than Freudian, sense). The natural form of unconsciousness (sleep) is, however, different from anesthesia and coma in its capacity to generate dreams, and in that the subconscious retains the power to turn on awareness when needed, as when external stimuli suggest danger.
In humans, ‘‘accurate report’’ (e.g., in response to queries about a sensation or experience) may be used as a sign of consciousness (Seth et al. 2005), but in order to probe for a homologous feature in animals, we need to identify other defining qualities. A variety of neurobiological and behavioral correlates have been suggested, including: the presence of a thalamocortical complex, extensive ‘‘crosstalk’’ between dispersed nerve circuitry within this complex, a ‘‘default mode network’’ involving core activity in prefrontal and medial parietal regions of cortex, distinct sleep-wake cycling, behavioral flexibility (or behavior indicative of choice based on motivation rather than on hard-wired patterns), play behavior, signs of emotions or feelings, advanced communication, skill acquisition, and cultural transmission. The position taken here is that several of these features, but not necessarily all, should be present in order to ascribe consciousness to an organism within the vertebrate lineage.
There is reasonable evidence indicating the existence of primary forms of consciousness in mammals and birds, and possibly in reptiles as well. Taken together, these observations suggest that the trait first evolved in the common ancestor of these three classes, collectively referred to as amniotes, some 300 million years ago. Excluding the reptiles would mean that it evolved independently in birds and mammals; and a model not requiring convergent evolution is, arguably, more parsimonious.
All amniotes have a complex behavioral repertoire, and at least birds and mammals appear to have cultural transmission (Laland and Galef 2009). Moreover, the amniotes (but apparently neither fish nor amphibians) display signs of emotion, such as tachycardia and fever upon handling, an increase in brain dopamine activity (the neurotransmitter most closely associated with reward-oriented behavior), and an apparent capacity to feel pain. Compared to lower vertebrates, amniotes have larger brains, and are thus presumably capable of a more complex response to the challenges of life. While it might be tempting to explain consciousness as an evolutionary strategy aimed at facilitating computational brainpower, or as a by-product of a sophisticated brain (Rosenthal 2008), advanced behavior—for example, communication in social insects—apparently does not require consciousness (and, one might add, neither do computers). Either presumed non-conscious species such as insects and fish do not possess a sufficiently sophisticated brain, or other factors beyond mere intricacy of response are required in order to explain the origin of consciousness. I shall argue in favor of the latter.
Amniotes were the first vertebrates to adapt to life on land. It has been discussed whether the complexity of terrestrial environments spurred the emergence of more complex behavior and consciousness. One would expect, however, that early terrestrial environments were a lot simpler, harboring a considerably lower diversity of life forms, compared to the oceans. Moreover, non-vertebrate animals, including annelids, arthropods and mollusks, colonized dry land at about the same time, or shortly after, without a similar expansion of the nervous system.
Interestingly, two of the most impressive escalations of brain capacity, i.e., in the molluscan class Cephalopoda (Edelman and Seth 2009) and the mammalian order Cetacea (Marino 2007), occurred in the ocean. In fact, cephalopods are the foremost candidates for consciousness in invertebrate animals (Mather 2008; Edelman and Seth 2009). The brains of these invertebrates are profoundly different as to neuroanatomical structures compared to amniotes. To the extent that they display signs of consciousness, a closer examination may therefore suggest general principles as to the underlying circuitry. Nevertheless, the presence of anything resembling consciousness in invertebrates would require convergent evolution, and has consequently limited relevance as to delineating the evolutionary trajectory leading to consciousness in humans. The present discussion will therefore focus on vertebrates.
A capacity for feelings, or emotions, are typically listed among the defining features of consciousness; however, even if consciousness were to be defined solely by other qualities, the current evidence suggests that the two features evolved concurrently. This observation may offer a more fruitful starting point for explaining the evolutionary scenario leading to vertebrate consciousness.