People started diving into deep water and making like dolphins thousands of years ago, long before they understood exactly why they had the ability to do it. Ancient Greek divers weighted themselves down and descended to depths of 100 feet (30.5 meters) or more to pluck sponges from the sea bottom. In Japan and Korea, traditional female freedivers known as Ama have long collected edible seaweed and clams at depths of 65 feet (19.8 meters) or more [source: Hanna]. Because coastal dwellers have been freediving for many generations, you might suspect that they've developed special genetic traits that make them better at it than inlanders. But scientists who've studied them say there's no evidence of that [sources: Focazio].
All humans do share an evolutionary adaptation with air-breathing aquatic animals that makes it possible for them to dive and swim underwater for a while without taking a breath. In 1870, French physiologist Paul Bert observed that ducks had an amazing ability to slow their heart rate if they were forced to stay underwater. Scientists went on to discover that immersion in cold water -- or even simply holding your breath -- triggers something called the diving reflex, which the body uses to conserve life-giving oxygen. When the reflex kicks in, your pulse slows and your circulatory system constricts, and blood is shunted away from your muscles, skin and visceral organs, guaranteeing that your heart and brain get first dibs [source: Bove].
For years, scientists believed that humans were capable of enduring without oxygen for only a few minutes and that they couldn't dive deeper than 164 feet (50 meters), because the pressure would shrink their lungs and crush their rib cages. But in the 1960s and 1970s, divers disproved that myth. As it turns out, a phenomenon called blood shift causes the lungs to fill with plasma, preventing collapse [source: AIDA New Zealand]. Learn how freedivers put blood shift to work on the next page.