Crosby said that until archaeologists are able to substantiate the rock art dates with other well-dated materials the mystery of the painters will remain.
"There hasn't been enough archaeology done yet and not enough dating for anybody to finger some group that was known to be on the peninsula at that time," he said. "We simply have not got enough information to do that."
Crosby began exploring the region in 1967 and first published The Cave Paintings of Baja California: Discovering the Great Murals of an Unknown People in 1975. A second edition of the book was published in 1997.
He said the dates by Watchman and colleagues confirm his belief that the painters were not Cochimi, a group of native peoples who were present in the region when Jesuit missionaries from Spain arrived at the end of the 17th century.
Missionaries who were familiar with the paintings confirm this belief in their journals where they write of asking the Cochimi about the paintings and being told a widespread tale that the paintings were done by a race of giant people long past.
But the tales did not faze the late Clement Meighan, an anthropologist from the University of California at Los Angeles who in 1962 radiocarbon-dated a wooden peg recovered from one of the rock art sites as approximately 550 years old, which he used as an indicator to date the paintings to the time of the Cochimi.
But when Crosby looks at the murals he cannot imagine them as being so young.
"The vast majority of them are painted on volcanic agglomerate; it is a volcanic material that literally agglomerates from non-basaltic rock. It isn't flows of lava, it is beds of material like pumice and ash that over a period of millions of years have agglomerated into surprisingly hard material," he said.
Knowing the geology of the painting surface is important, added Crosby, to understand the pace at which the paintings deteriorate. Volcanic agglomeration, he said, breaks down over thousands of years owing to the subtle expansion and contraction forces caused by humidity and drying.
"A lot of these rock art sites have over-painting," said Crosby. "One figure is painted on top of another. In some places they have as many as five layers."
The newer layers are quite fresh appearing and visible, but the older layers are so degraded that they have almost disappeared. To Crosby this suggests that many years must have passed between the first painting and the last.
"These are older than Clement Meighan and others have suggested," he said. "There is no other way to account for that kind of difference between the oldest and newest paints."
Watchman, who is a specialist in rock art dating, obtained his dates from painted samples collected in 2001. More than 30 samples have been dated so far and several of them are at least 5,000 years old. Some go back 7,500 years, suggesting a painting culture that lasted for millennia.
"It says something about the continuity of culture over a long period of time," he said.
Rock Art Conservation
Further studies in the years to come may resolve the mystery of who painted the murals, for what reasons, and how they vary through space and time. In the meantime, the researchers believe their recent discoveries warrant protection.
In 1993, the United Nations Education, Scientific, and Cultural Organization recognized the well-documented rock art murals in the nearby Sierra de San Francisco as a World Heritage site.
The research team is asking that the boundaries of this site be extended south to include the rock art in the Sierra de Guadalupe so that it can be properly protected and preserved for future generations.
"What we are trying to do is make them aware that there are many more sites in the sierra and that this is a rare set of paintings and an environmental situation that requires protection," said Watchman.
Crosby believes that the remoteness of the sites affords them ample protection and that environmental and conservation organizations have already succeeded in educating the local populations that it is economically advantageous to preserve the paintings.
"The process of surface degradation is going to continue," he said. "But that is a slow process. Five hundred years from now we'd find the paintings relatively little changed."
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