A less intrepid scientist might have stayed in the comfort of her laboratory. But not Anna Gislen.
Gislen went the extra mile. The extra 5,328 miles, to be more precise -- from her lab in Lund, Sweden, to a cluster of tropical islands off the west coast of Thailand, to study a tribe of highly skilled divers known as "sea gypsies."
It was rough. The blazing sun. The gorgeous beaches. The fresh coconuts and crystal-clear waters. But in the end, with the help of her 7-year-old daughter and a few colleagues, Gislen overturned conventional thinking about the limits of human vision underwater.
Her work offers new proof of the body's remarkable capacity for adaptation -- its ability to go beyond standard biological bounds and even physically remodel itself when novel needs arise. It could also invigorate efforts to protect the threatened sea gypsy culture.
For centuries these nomadic people have lived on the islands of the Andaman Sea, harvesting clams, sea cucumbers and other marine morsels. They are excellent divers, plucking their fare off the seabed as many as 75 feet beneath the surface. But what is most impressive is their underwater vision. Without goggles or other aids, sea gypsy children routinely spot even the smallest of shellfish -- ones that most people would be unable to distinguish from surrounding pebbles.
The human eye evolved to provide excellent vision in air, but it typically performs poorly underwater. At the heart of this limitation is a phenomenon called refraction -- the bending of light as it passes from a substance of one density into a substance of differing density.
Refraction is important to human vision because, to get a sharp visual image, incoming rays of light must land precisely upon the retina in the back of the eye. A crystalline lens inside the eye does some of the necessary bending of light to focus those images on the retina. But about 70 percent of visual refraction occurs as light passes from the air outside the eye into the more dense, fluid-filled eyeball itself.
Swimming presents a problem for human vision because water is virtually the same density as the fluid inside the eye, so underwater light barely bends as it enters the eye. The result is the blurry vision that swimmers know so well.
Fascinated by reports of sea gypsies' remarkable underwater vision, Gislen did what any good scientist would do: She told her daughter and a few colleagues to pack their swimsuits and sunblock for a working trip to the island preserves of Ko Surin, Ko Poda and Ko Phi Phi, home to a tribe of sea gypsies known as the Moken.
The first thing the team did was recruit six Moken children (ages 8 to 13) and 28 children of European tourists (ages 7 to 14) to see whether Moken children do indeed have superior underwater vision.
"We went around the beach asking people to help us," Gislen said. "It was kind of odd, like, 'Hello . . . ' " Her daughter served as a sort of ambassador, she said, putting the children and their parents at ease.
The team used a series of waterproof placards with tightly spaced black and white lines, and asked children to press their faces into an underwater headrest and report whether the lines were oriented horizontally or vertically. With each trial, they used placards with spaced lines that were more difficult to distinguish, until a card exceeded the child's resolving power and looked merely gray.
"When they start making mistakes, we know they can't see it anymore," Gislen said.
The team found that Moken children and European children have the same visual acuity on land, but the Moken have better than twice the underwater resolving power of European children -- a level of underwater acuity previously thought to be impossible in humans.
Further studies by Gislen's team showed that the Moken do this not by flattening the corneas on the front of their eyes -- a method used by some amphibious birds, fish and frogs. Neither do they rely on mere "accommodation," the use of tiny muscles to change the curvature of the lens inside the eye -- a standard means of image correction that cannot, by itself, correct underwater blurriness. Rather, they shrink the size of their pupils, the round black aperture through which light enters the eye, down to a diameter of 1.96 millimeters, or 22 percent smaller than the 2.5 millimeter minimum seen in Europeans.
It is a feat never before documented -- indeed, most people's pupils enlarge slightly underwater, in response to the lower level of light. But it makes up for the lack of air-water refraction by changing the angle of incoming light.
"This extreme reaction -- which is routine in Moken children -- is completely absent in European children," the team wrote in the May 13 issue of Current Biology.
But it is not impossible to learn. In preliminary experiments, Gislen recently found that Swedish children can be trained to constrict their pupils when diving and enhance their underwater visual acuity. That suggests the Moken learn the skill in childhood and do not simply inherit it as an inborn reflex.
The Moken's pupillary response joins a growing list of physiological and anatomical adaptations discovered in recent years, each one dismantling an old assumption about human biological limits.
Researchers long believed, for example, that the neuronal architecture of the brain was largely fixed by adulthood, with the only notable change being the gradual loss of neurons that comes with age. Yet in a series of paradigm-shifting findings, scientists have found that the brain can remodel itself significantly in response to certain stimuli or specific needs.
Researchers in London have shown that cab drivers develop enlarged hippocampuses, the part of the brain that stores spatial memory and holds mental "maps." And others have found that the part of the brain devoted to processing music grows larger in musicians over time -- and does so differently depending on the kind of instrument a person plays. In trumpet players, the neurons that respond to brassy resonances rewire themselves and expand, while the brain region that responds to string sounds expands in string players.
In string players, moreover, the part of the motor cortex that controls the hand that works the fingerboard is enlarged, but not so for the region controlling the hand that does the bowing.
Bones and muscles, too, remodel themselves in response to the demands of various workloads -- a case in point being bow-legged cowboys, said Stanford biologist Robert Sapolsky. And contortionists -- such as those who perform the signature slithery acts seen in the Canadian Cirque du Soleil circus -- are proof that if children train their joints from an early enough age (typically 5 to 8 years old) the human body can attain the flexibility of a salamander.
Gislen hopes that wider recognition of the Moken's visual adaptations may bring increased appreciation of their culture.
"These people are endangered in a sense," Gislen said. "They are kind of shunned today, and their culture is being lost."
One more point she wants to make: "Honestly, sitting in the water doing experiments in the scorching sun for hours is not that amusing," she said. "I actually think Sweden has a lovely climate compared to Thailand."