Two anthropologists are reporting that they may have an answer to why people use drugs even when they know how bad they are. They helped our ancestors to survive, say Roger Sullivan of the University of Auckland and Edward Hagen of the University of California at Santa Barbara.

Our ancestors were exposed to plants containing narcotic substances for millions of years, the anthropologists say. They argue in the April issue of Addiction that humans are predisposed to taking drugs because we evolved to seek out plants rich in alkaloids.

They say that consuming plants like these was likely a basic survival strategy. "Stimulant alkaloids like nicotine and cocaine could have been exploited by our human ancestors to help them endure harsh environmental conditions," Sullivan says. This is nothing new, says Sullivan.

Until recently, Australian Aborigines consumed the nicotine-rich plant pituri to help them get through the desert without food, and Andeans still chew coca leaves to help them work at high altitudes.

Archaeologists have found that drug use was widespread in ancient cultures. Betel nut, for example, was chewed at least 13,000 years ago in Timor, to the north of Australia, and the use of coca in Ecuador has been dated to at least 5000 years ago. These were potent substances: pituri contains up to five per cent nicotine, whereas tobacco today contains about 1.5 per cent.

Also, people sometimes "freebased" the drugs by chewing them along with an alkali such as lime or wood ash, which releases the free form of the drug so it can be directly absorbed into the bloodstream.

Sullivan says in Pacific cultures where chewing betel nut is still widespread, it is viewed more as a source of food and energy than as a drug. In addition, some drugs actually have real nutritional value. One example is coca - 100 grams of coca leaf contains more than the US recommended daily intake of calcium, phosphorus, iron and vitamins A, B2 and E.

The diets of people in some marginal environments may have been so poor that they struggled to produce enough neurotransmitters of their own, and consuming plants containing substances that mimic neurotransmitters could have helped make the difference, say Sullivan and Hagen.

This part of their theory could be tested by depriving animals of certain neurotransmitters and seeing if they then choose to eat food rich in substitutes, they say. Sullivan's theory is definitely plausible, says Wayne Hall of the University of Queensland, who until recently was head of the National Drug and Alcohol Research Centre in Sydney. "There is certainly evidence that plants evolved to mimic the neurotransmitters of mammals," he says. "But the problem today is that we have much larger doses of much more purified drugs."

Source: University of California at Santa Barbara; Reuters

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