People who live farther from the equator have larger eye sockets than their tropical counterparts, a new study finds. And as people inhabited higher and higher latitudes, eye socket size grew along with the northerly or southerly extent of their migrations.

"It's never been shown before that latitude and vision are related in this way in humans," says Robert Barton, an evolutionary anthropologist from Durham University in England, who was not involved in the research.

The experimental design was quite simple. The researchers studied a museum collection of 73 healthy 200-year-old skulls from 12 different populations. For each skull, they lined an eye socket (which anthropologists call the "orbit") with laboratory film and filled it with small glass beads to measure the volume. Then they charted the location where each skull was dug up, and calculated average light levels for each locale, based on its latitude and longitude. It turned out that skull orbits obtained from higher latitudes held more glass beads: Those from nearer to the equator held 22.5 milliliters, on average, whereas the ones from inhabitants living more than 40 degrees north or south of the equator held closer to 27 milliliters.

"Days are shorter as you move away from the equator, and the duration of dawn and dusk increases," explains Eiluned Pearce, a University of Oxford doctoral student who co-authored the paper with her advisor, evolutionary anthropologist Robin Dunbar. Because of the diminished light, Pearce and Dunbar hypothesized that humans at higher latitudes would need to develop specialized visual systems to continue to be able to spot predators and recognize social cues from other humans.

The implications of the report, published in the February issue of Biology Letters, are not completely surprising. A few years ago, ecologist Robert Thomas of Cardiff University in Wales found that songbirds with larger eyes sing at lower daily light levels (such as at dawn and dusk) than do birds with smaller eyes—presumably because animals with larger eyes have larger pupils which funnel more light into the eye, thereby increasing the length of activities associated with daylight. "We all imagine people as a bit different from animals and maybe not subject to the evolutionary circumstances," Thomas says, "but here's a fantastic example of the evolutionary process working on human societies that live in different places."

However, it is risky to use eye-socket volume as a proxy for eyeball size, cautions physical anthropologist Christopher Kirk from the University of Texas at Austin. The actual eyeball takes up less than one third of the volume of the socket—the rest of it is filled with fat, muscle and connective tissue. One alternate explanation is that humans at higher latitudes require larger eye sockets to accommodate larger fat pads to act as insulation in colder weather. Other factors such as chewing muscles, the shape of the nasal cavity and the size of the frontal lobe also influence eye-socket size and shape. For these reasons and more, it would be best to measure eyeballs themselves, Kirk says.

And that is exactly what Pearce is working on now. With the help of a network of researchers, she is collecting and analyzing MRI scans from all over the world, in order to measure the eyeball sizes of modern populations and to determine how exactly they vary with latitude. At this point, it is not clear whether the eye-socket volume is genetically heritable or if it develops over an individual's lifetime. "This seems to be the first time anyone has ever really thought about this, so this paper was really about putting that hypothesis out there," Pearce says. There's still a lot of work to be done and many questions to be answered.

"For some reason, orbit size increases with latitude," Kirk says. "That's a phenomenon that needs to be explained."