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New Scientist 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.