DALLAS — Prehistoric shamans used to mark the transition from the real world to the spirit world, anthropologists think, by blowing pigments around their hands onto cave walls. These ghostly handprints, which still dot European caves more than 10,000 years later, now serve a less ethereal purpose — telling scientists how many of those shamans were left-handed.
New research shows that the frequency of left-handed painters — 23 percent — is the same today as it was back then.
The work is a rare look at how left-handedness has persisted for millennia, says Charlotte Faurie, the French graduate student who did the research. It suggests there is no evolutionary disadvantage to being a lefty, as some scientists had thought.
There's older evidence that lefties were at work almost as soon as Homo sapiens arose: Wear marks on stone artifacts may signify the presence of southpaws 200,000 years ago. Neanderthals also had their share of lefties; some of their fossilized teeth carry telltale marks that indicate left-handed eating practices, says Faurie.
But such evidence is rare and doesn't allow scientists to estimate the frequency of left-handedness. That's why Faurie turned to a database, compiled by Marc Groenen of the Free University of Brussels, of prehistoric handprints.
In caves stretching across France and Spain, Groenen identified 507 "negative hands," in which pigment was splattered around a hand by blowing through a tube, spitting or daubing the paint. Oddly, such negative prints are much more common than positive handprints in which the palm was painted and pressed against a surface, Groenen wrote in an e-mail.
Through careful measurements, Groenen could often identify the age and gender of the person who made the negative hand. In 343 cases, he could also determine the handedness of the artist; somebody holding the pigment tube in his or her left hand would presumably have made the imprint of a right hand. Groenen found that 79 of the prints were of right negative hands, suggesting that 23 percent of the cave artists were left-handed.
Faurie thinks the European caves represent a fair sampling of the number of lefties back then. Some of the handprints are large and some are small; others are higher or lower on the wall.
"Maybe some hands are from the same artist," she said. "But we are sure that it is at least many artists."
She decided to test the cave painting numbers against the experience of students at her university, France's University of Montpellier II.
First, she gave students an ink-blowing pen and asked them to outline one hand on a piece of paper taped to a wall. Next, she had them pick up a ball from a table and throw it at a target across the room. Finally, she asked them to identify with which hand they normally wrote.
Just as in the cave paintings, 23 percent of the students held the ink-blowing pen with their left hand and created a right negative hand, Faurie and her adviser, Michel Raymond, report in an upcoming issue of Biology Letters.
But after that, things become murkier. Only 9 percent of the students wrote with their left hand, and 8 percent threw as lefties.
The findings show how difficult it is to quantify left-handedness, says Faurie. Many people are southpaws for certain tasks but not for others.
Estimates of left-handedness range between 3 and 30 percent of the population, depending on how and where questions are asked. Studies of tool use show that the northern Inuit people are just 3 percent lefties, while the Yanomami tribe of the Amazon are 23 percent left-handed, says Faurie. About 10 percent of Americans write left-handed.
Negative hands appear in caves in other countries, including Australia, South America and Indonesia. Faurie wants to extend her study to these areas, if there are enough well-catalogued handprints there to make the findings statistically significant.
Michael Corballis, a psychologist at the University of Auckland in New Zealand who studies left-handedness, says the Australian caves could shed light on the handedness of Australian aborigines. As a group, they have been mostly isolated from other people for at least the past 40,000 years, he notes, and they are predominantly right-handed. Studying Australian cave prints could reveal whether that is a long-standing trait or something introduced by recent colonizers, he says.
"I think it's a clever paper," he said of Faurie's work.
Faurie plans to continue working to understand why left-handedness might have persisted in the same proportion for such a long time.
"I'm trying to show the possible advantages associated with being left-handed," she said. And she has no motivation beyond scientific curiosity on this: She is right-handed.