Evolution of the thirst mechanism in Homo: The need and limitations of thirst and hydration

There is a view that the perception of thirst and actual body fluid balance may affect cognitive and exercise performance. The evolutionary evidence suggests that our survival was dependent on our ability to sweat profusely when hunting during the heat of the day (persistence hunting), so if water deficits were not tolerated, consequently the thirst mechanism would limit our persistence hunting capability. This also means that hunting and searching for water was undertaken with some extent of water deficit, and in turn suggests that performance; physical and cognitive, was conducted with a degree of dehydration. Given the current views on the maintenance of body water for performance, there is a need to evaluate the evidence relating to tolerance limits for water deficits with respect to both physical and cognitive performance. This review considers the thirst mechanism and the conditions and selective pressures under which this might have evolved. Consideration will be given to how the thirst mechanism influences our physical and cognitive performance. The review suggests that Homo developed appropriate tolerances for water deficits and thirst perception, with a safety margin that prevented detrimental declines in physical and cognitive performance to the point of inhibiting corrective action. This would have offered a selective advantage, enabling the search for water and functioning adequately during periods of water scarcity. [Display omitted] •Homo's survival was dependent on profuse sweating when hunting in the heat of the day.•Water deficits must have been tolerated so that thirst mechanism would not limit our persistence hunting capability.•Hunting and searching for water was undertaken with a body water deficit, so that performance; physical and cognitive, was conducted with a degree of dehydration.•Homo can tolerate up to 11 % body mass losses with no immediate physiological consequence, although this represents an upper limit.•Overall, evolution endowed Homo with appropriate tolerances for both water deficits and thirst perception with a safety margin before physical and cognitive performance is severely compromised.•Tolerance to thirst and body fluid deficits provided a selective advantage not only to search for water but also to continue to function adequately during periods of water lack.