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Apparently, the urge to pour a stiff drink is an ancient one, stretching back thousands of years. According to a recent study, the ability to process alcohol first appeared about 10 million years ago.

Steven Benner, a chemist from the Foundation for Applied Molecular Evolution in Gainseville, Florida, believes that he is cracked the answer as to why humans and some other primates, like chimpanzees and gorillas, can process ethanol, while others. He believes that the answer lies in fermented fruit.

According to Science News, in order to break down ethanol, humans need the enzyme alcohol dehydrogenase 4, or ADH4 for short. The enzyme populates the esophagus, stomach and intestines and is one of the first enzymes that comes into contact with a drink when humans drink alcohol. However, not all primates have the correct enzyme that allows them to break down ethanol.

Benner's laboratory was able to assess what occurred by bringing to life the genetic code of the enzyme in ancient primates. He and his team estimated the genetic code. Then they built them in a laboratory and discovered the processes in order to assess how they likely changed over time. This process was performed by analyzing the enzyme ADH4 in 27 modern primates, which included apes, humans, lemurs and monkeys. By mapping the primate family tree, the researchers were able to trace exactly what the enzyme likely looked like at different portions of evolutionary history.

Benner thinks that the answer can be traced to the common ancestor of chimpanzees, gorillas and humans. About 10 million years ago, a version of the enzyme appeared that was 50 times better at metabolizing alcohol, which would be an evolutionary advantage.

At about the same time, Benner believes that the ancestor developed a tendency to walk on land. That would have allowed the ancestor to come in contact with fermented fruit. The fruit, which would have fallen from trees, would likely have a damaged skin or husk, meaning that yeast could come into contact with its sugars and cause fermentation - and the creation of ethanol. That explanation - about the preference for walking on land - would explain why humans and gorillas can metabolize ethanol, while tree-inhabiting primates cannot, because the primates who live in trees would not have a great deal of contact with fermented fruit.

The study was presented at the American Asssociation for the Advancement of Science.