Asphalt may have poisoned ancient Americans
ON THE beaches of southern California you can sometimes find clumps of a sticky black substance with a texture halfway between molasses and rubber. Could these tar balls - collected by humans for thousands of years - provide evidence that our long-standing relationship with hydrocarbons was toxic from the outset?
Long before we started asphalting roads, prehistoric people around the world used bitumen, which seeps from the ground naturally in places. Archaeological finds suggest that California's prehistoric locals, the Chumash people, eagerly collected the tar balls. They used them to caulk the seams of ocean-going craft and waterproof woven baskets to make drinking vessels, as well as for making casts for broken bones and poultices for sore joints. Some Chumash 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 (see "Poisonous ingredients"). To find out whether California's tar balls had the potential to damage the Chumash's health, Sebastian Wärmländer 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 the tar contained 44 PAHs, including many known carcinogens.
Wärmländer's team then turned to the Chumash's bones to see whether the tar balls had had an effect on their health. Most symptoms of health problems caused by PAHs reveal themselves in the flesh, but 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.
Wärmländer and his colleagues measured 269 adult skulls from burials made between 6500 BC and AD 1780 on Santa Cruz and Santa Rosa islands off California's coast. They found that, over the generations, the skulls of men decreased from 3370 cubic centimetres to 3180 cm3. The women's skulls decreased from 3180 cm3 to 2980 cm3 (Environmental Health Perspectives, DOI: 10.1289/ehp.1103478). Previous studies have shown femur length declined over this period too.
The decreasing stature of the Chumash suggests declining health, says Wärmländer's team. This has been suggested before, but this is the first time bitumen has been considered as a contributor to this decline.
"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. "I'm certainly not excluding the possibility, though - it's a very interesting idea," she says.
Sholts says the team accepts the idea is still a hypothesis that 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."
There are over 100 polycyclic aromatic hydrocarbons (PAHs) found in oil, coal and tar, which are readily released into the environment when fossil fuels are burned. Many of them are harmful to humans and wildlife.
In humans they have been shown to cause infertility by killing egg cells in the ovaries. They may also stunt the growth of fetuses by damaging their DNA.
A 2002 study of beluga whales in Canada's Saint Lawrence Estuary suggested that PAH pollution caused over a quarter of the 263 deaths reported there between 1983 and 1999, by inducing various cancers.
PAHs settle in soils and sediments. Preliminary surveys suggest that, following last year's Deepwater Horizon oil spill, sediments in the Gulf of Mexico contain higher than usual quantities of PAHs. Oil-sand mining may also release PAHs: between 1999 and 2009, the concentration of PAHs in sediments at the bottom of Canada's Athabasca river rose by 41 per cent, in line with increased mining in the region (Environmental Science and Technology, DOI: 10.1021/es104375d).
The urban environment also contains PAHs. In the US, the biggest source is the tarry gunk used to seal the surfaces of parking lots and playgrounds. Tobacco smoke is also a major source, and even burning incense can produce them.
However, PAHs may have been crucial for the first life. They contain most of the carbon found in space, and under the right conditions can be transformed into some of the complex molecules necessary for life.
COURS : CIV 106 : Civilisations paléo-indiennes
CIV 106 : Paleoindian Civilization