Life expectancy is increasing in most countries and has exceeded 80 in several, as low-mortality nations continue to make progress in averting deaths. The health and economic implications of mortality reduction have been given substantial attention, but the observed malleability of human mortality has not been placed in a broad evolutionary context. We quantify the rate and amount of mortality reduction by comparing a variety of human populations to the evolved human mortality profile, here estimated as the average mortality pattern for ethnographically observed hunter-gatherers. We show that human mortality has decreased so substantially that the difference between hunter-gatherers and today’s lowest mortality populations is greater than the difference between hunter-gatherers and wild chimpanzees. The bulk of this mortality reduction has occurred since 1900 and has been experienced by only about 4 of the roughly 8,000 human generations that have ever lived. Moreover, mortality improvement in humans is on par with or greater than the reductions in mortality in other species achieved by laboratory selection experiments and endocrine pathway mutations. This observed plasticity in age-specific risk of death is at odds with conventional theories of aging.

About once a week, Jean-Christophe Terrillon wakes up and senses the presence of a threatening, evil being beside his bed. Terror ripples through him, and he tries to move or call out. But he is paralyzed, unable to raise an arm or make a sound. His ears ring, a weight presses down on his chest, and he has to struggle for breath. ''I feel an intense pressure in my head, as if it's going to explode,'' said Mr. Terrillon, a Canadian physicist doing research in Japan. Sometimes he finds himself transported upward and looking down on his body, or else sent hurtling through a long tunnel, and these episodes are terrifying even for a scientist like him who does not believe that evil spirits go around haunting people. Called sleep paralysis, this disorder -- the result of a disconnect between brain and body as a person is on the fringe of sleep -- is turning out to be increasingly common, affecting nearly half of all people at least once. Moreover, a growing number of scholars believe that sleep paralysis may help explain many ancient reports of attacks by witches and modern claims of abduction by space aliens. ADVERTISEMENT Continue reading the main story ''I think it can explain claims of witchcraft and alien abduction,'' said Kazuhiko Fukuda, a psychologist at Fukushima University in Japan and a leading expert on sleep paralysis. Research in Japan has had a headstart because sleep paralysis is well-known to most Japanese, who call it kanashibari, while it is little-known and less studied in the West. ''We have a framework for it, but in North America there's no concept for people to understand what has happened to them,'' Professor Fukuda said. ''So if Americans have the experience and if they have heard of alien abductions, then they may think, 'Aha, it's alien abduction!' '' Sleep paralysis was once thought to be very rare. But recent studies in Canada, Japan, China and the United States have suggested that it may strike at least 40 percent or 50 percent of all people at least once, and a study in Newfoundland, Canada, found that more than 60 percent had experienced it. There, as in Japan, people have a name for the condition and some scholars believe that people are therefore more likely to identify it when it happens to them. In Newfoundland, it is called ''old hag'' because it is associated with visions of an old witch sitting on the chest of a paralyzed sleeper, sometimes throttling the neck with her hands. ADVERTISEMENT Continue reading the main story Sleep paralysis seems to have been described since ancient times, and an episode appears in ''Moby Dick'' and perhaps also in the 18th century Henry Fuseli painting, ''The Nightmare,'' which shows a goblin sitting on the stomach of a sleeping woman. What is striking is that although the symptoms of sleep paralysis are generally very similar, the images in the hallucinations and the interpretation of them seem to vary. Europeans seem to have interpreted ancient sleep paralysis as assaults or abductions by witches taking them off for a forcible ride on a broomstick. Chinese called it ''gui ya,'' or ghost pressure, and believed that a ghost sat on and assaulted sleepers. In the West Indies, sleep paralysis was called ''kokma'' and meant a ghost baby who jumped on the sleeper's chest and attacked the throat. In old Japan, it sometimes seems to have been interpreted as a giant devil whose foot came down on the sleeper's chest. ''People will draw on the most plausible account in their repertoire to explain their experience,'' said Al Cheyne, an associate professor of psychology at the University of Waterloo in Canada. ''Trolls or witches no longer constitute plausible interpretations of these hallucinations. The notion of aliens from outer space is more contemporary and somewhat more plausible to the modern mind. So a flight on a broomstick is replaced by a teleportation to a waiting spaceship.'' ADVERTISEMENT Continue reading the main story Dr. Cheyne said that in a survey he had worked on involving more than 2,000 people identified as experiencing sleep paralysis, hundreds described experiences similar to alien abduction. ''A sensed presence, vague gibberish spoken in one's ear, shadowy creatures moving about the room, a strange immobility, a crushing pressure and painful sensations in various parts of the body -- these are compatible not just with an assault by a primitive demon but also with probing by alien experimenters,'' Dr. Cheyne said. ''And the sensations of floating and flying account for the reports of levitation and transport to alien vessels.'' In recent years there has been a huge increase in the number of people who insist that they have been kidnapped by alien creatures from outer space, perhaps subjected to medical experiments and then released again. These claims have been a bit of a scientific puzzle, because they strike most people as utterly wacky and yet they are relatively widespread.