In one of the first attempts to quantify the limits of human energy expenditure over time, researchers using data on athletes who competed in global endurance events report that human energy expenditure could not be sustained above 2.5 times the rate of metabolism at rest. The findings from Caitlin Thurber and colleagues suggest that all human activities share the same metabolic limits, which means natural selection that enhanced one endurance ability, such as running, may have consequently benefited many others, such as brain size expansion and gestation length. While there is considerable research interest in the physiological limits on energy expenditure for humans, studies that have explored this vary in their results, and few studies have explicitly explored how energy expenditure varies with event duration. Thurber and colleagues compiled published measurements of average energy expenditure and metabolic rate for human endurance events, including marathons, the Tour de France, swimming, arctic trekking, and pregnancy. They incorporated original data from the Race Across the USA (RAUSA), the longest event to date from which metabolic measurements have been recorded. In this grueling event, athletes run from Huntington Beach, California to Washington, D.C., covering approximately a marathon per day, six days per week, for 14-20 weeks. The researchers found that metabolic rates varied greatly based on the length of the endurance event, with high metabolic rates accompanied by decreases in total energy expenditure. This may have enabled RAUSA athletes to complete the race--they could only have sustained their Week 1 metabolic scope for half the event. However, the same metabolic ceiling applied across all events. Energy expenditure could not be sustained for any physical activity above 2.5 times the rate of metabolism at rest, meaning that activities involving different muscle groups and organ systems are all united by the same energy intake controls. It is unclear which human adaptations were initially targeted for greater metabolic capacity through natural selection.