Evolution of consciousness: phylogeny, ontogeny, and emergence from general anesthesia

Abstract

Are animals conscious? If so, when did consciousness evolve? We address these long-standing and essential questions using a modern neuroscientific approach that draws on diverse fields such as consciousness studies, evolutionary neurobiology, animal psychology, and anesthesiology. We propose that the stepwise emergence from general anesthesia can serve as a reproducible model to study the evolution of consciousness across various species and use current data from anesthesiology to shed light on the phylogeny of consciousness. Ultimately, we conclude that the neurobiological structure of the vertebrate central nervous system is evolutionarily ancient and highly conserved across species and that the basic neurophysiologic mechanisms supporting consciousness in humans are found at the earliest points of vertebrate brain evolution. Thus, in agreement with Darwin's insight and the recent "Cambridge Declaration on Consciousness in Non-Human Animals," a review of modern scientific data suggests that the differences between species in terms of the ability to experience the world is one of degree and not kind.

Fig. 1.

Theories of brain evolution. Ancient brain structure evolution theory of Scala Naturae showing brain development proceeding from simple to more complicated with the addition of new brain regions as evolution progressed. This erroneous view is compared with a modern understanding of brain structure evolution that reveals a basic common structure evolved in the vertebrate brain and various regions expanded to accommodate each specific animal’s needs. Modified from (42) with permission from Elsevier.

Fig. 2.

Brain structures functionally related to primitive emotional arousal and the return of consciousness following sleep or anesthesia. The primitive emotional response of air hunger shows activations in brainstem and anterior cingulate regions; thalamic changes are also seen (73). Subjective emotional arousal activates similar regions in an event-related functional MRI study of picture viewing. Reproduced with permission from (85). Midline thalamic and anterior cingulate arousal is seen with PET neuroimaging when consciousness first reemerges following sleep or anesthesia. Reproduced with permission from ( and 65). A common brainstem, thalamic, cingulate neuroanatomy associated with conscious brain activity is seen. Images used with permission.

Fig. 3.

Schematic showing relative size of frontal lobe across different species and the potential capacity for anterior–posterior information flow. The blue areas represent the prefrontal cortex, and the schematic shows how the prefrontal cortex proportionally increases in size with increasing brain size across species. Relative brain size is scaled to the human brain. Modified from (102) with permission from Elsevier.