Drug use linked to ancestors' habits

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http://www.newscientist.com/news/news.jsp?id=ns99992095

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Drug use linked to ancestors' habits

Exclusive from New Scientist Print Edition

If drugs are so bad for us, why do so many people use them? Because they helped our ancestors survive, argue two anthropologists.

Our predilection for psychotropic substances is usually seen as a biological accident. The conventional view is that drugs fool the brain into thinking it is getting a reward when in fact it is not.

But anthropologists Roger Sullivan of the University of Auckland and Edward Hagen of the University of California at Santa Barbara point out that our ancestors were exposed to plants containing narcotic substances for millions of years. In the April issue of Addiction, they argue that we are predisposed to drug-taking because we evolved to seek out plants rich in alkaloids.

Consuming such plants could have been 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.

For example, until recently Australian Aborigines used the nicotine-rich plant pituri to help them endure desert travel without food. And Andeans still chew coca leaves to help them work at high altitudes.

Freebasing drugs

Archaeological evidence shows 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. Artefacts date the use of coca in Ecuador to at least 5000 years ago.

Many of these substances were potent: pituri contains up to five per cent nicotine, whereas tobacco today contains about 1.5 per cent. What is more, these drug pioneers sometimes "freebased" drugs by chewing them together with an alkali such as lime or wood ash. This releases the free form of the drug and allows it to be directly absorbed into the bloodstream.

But in Pacific cultures where chewing betel nut is still widespread, it is seen more as a source of food and energy than as a drug, Sullivan says. And some drugs do have real nutritional value.

For example, 100 grams of coca leaf contains more than the US recommended daily intake of calcium, phosphorus, iron and vitamins A, B2 and E.

Larger doses

And in some marginal environments, people's diets may have been so poor that they struggled to produce enough neurotransmitters of their own. Consuming plants containing substances that mimic neurotransmitters could have helped make up for the shortfall, Sullivan and Hagen speculate.

They say 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.

Sullivan's adaptive model of drug use 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."

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