On the beaches of southern California you can sometimes find clumps of a sticky black substance with a texture halfway between molasses and rubber. These tarballs are made of an oily substance called bitumen, commonly used today in asphalt for roadway surfaces. Prehistoric people around the world used bitumen, which seeps from the ground naturally in places, to caulk the seams of ocean-going craft and for waterproofing woven baskets to make drinking vessels as well as for making casts for broken bones and poultices for sore joints. Some of California's prehistoric Chumash peoples even chewed bitumen like gum.
     We now know that bitumen can be a source of polycyclic aromatic hydrocarbons (PAHs) - pollutants that have been linked to a number of health problems. To find out whether California's tar balls had the potential to damage the Chumash's health, Sebastian Warmlander of Stockholm University in Sweden and colleagues analysed samples taken from Californian beaches and from the La Brea tar pits in Los Angeles. They found these tars contained 44 PAHs including many known carcinogens.
     Warmlander's team then turned to the Chumash's bones to see whether the tar balls had had an effect on their health, and studies have suggested that mothers who are exposed to PAHs during pregnancy give birth to smaller than average babies, who become shorter than average adults.
     Warmlander and his colleagues measured 269 adult skulls from burials made between 6500 BCE and 1780 CE on Santa Cruz and Santa Rosa islands off California's coast. They found that, over the generations, the skulls of men decreased from 3370 to 3180 cubic centimetres. The women's skulls decreased from 3180 to 2980 cm3. Previous studies have shown femur length declined over this period as well.
     The decreasing stature of the Chumash suggests declining health, says Warmlander's team. Their work provides the first results that bitumen might have been a contributor to these declines. "It's pretty clear that health was compromised over time on the Californian Channel Islands," says co-author Sabrina Sholts of the University of California, Berkeley.
     Patricia Lambert of Utah State University in Logan, who has studied the health of the Chumash, confirms that their stature declined over time. However, she says that the idea that bitumen poisoned the Chumash lacks direct evidence. Sholts says the team accepts their idea is still a hypothesis and that it requires more evidence. That evidence may be found by analysing the bones for PAHs, says Carl Wendt at California State University-Fullerton, who was not involved in the study. "If PAHs became incorporated into actual bone collagen, we should be able to extract that."

Edited from New Scientist (1 June 2011)