Each second 26,000 cups of coffee are consumed globally. That’s over 93 million cups an hour, or an astonishing 2 billion cups a day! Why is coffee the most popular beverage on Earth? Well it might have something to do with all that lovely caffeine it contains. We are a species thoroughly addicted to caffeine; the most popular psychoactive substance in the world. Recent research into the genomics of the coffee plant is shedding some light on the evolutionary processes behind the world’s most popular drug, and revealing some of the reasons it is popular not just with humans, but with insects, too.
Caffeine is a crystalline xanthine alkaloid that acts as central nervous system and metabolism stimulant. Or, in other words, it’s a bitter compound that makes you feel alert and gives you energy. On average we consume 300 milligrams of it a day, largely in coffee. Other popular sources of caffeine include tea, chocolate and mate (a traditional South American tea brewed from the yerba mate plant). Coffee beans are produced by around 120 species of coffee plant in the genus Coffea, although most coffee on the market comes from Coffea arabica (Arabica coffee) or Coffea canephora (Robusta coffee).
A recent study by scientists at the Institut de Génomique in France has sequenced the genome of the coffee plant Coffea canephora in order to investigate the evolution of the caffeine-producing equipment that mades it’s coffee beans so addictive to humans. Researchers found a great deal of recent evolutionary changes to genes involved in the production of caffeine, that turn out to be unique to the coffee plant.
The biochemical process that eventually produces caffeine begins with a compound called xanthosine. Xanthosine is then altered through a series of enzymatic reactions, which add or remove groups of atoms and alter the structure of the molecule, finally converting it into theobromine and then caffeine. The enzymes involved belong to a group of enzymes called N-methyltransferases, which means they’re responsible for adding and removing methyl groups from other compounds, like xanthosine. These enzymes are found in all plants, and perform a mind-boggling array of different jobs from gene regulation to synthesising anti-cancer proteins and cell division. In many cases, the compounds that N-methyltransferases are involved with serve as chemical weapons against predators and parasites.
Coffee Bean Evolution
During the evolution of the coffee plant, a mutation in the gene for one N-methyltransferase allowed it to interact with and modify xanthosine. Over evolutionary time, this gene was accidentally duplicated (through another mutation), and the new duplicated versions were able to mutate and evolve further into other forms. Repeated duplications eventually produced a family of 23 different N-methyltransferases which specialise in caffeine production.
This process has occurred more than once; although caffeine in cacao beans (chocolate; Theobroma cacao) and tea (Camellia sinensis) are produced by N-methyltransferases too, these genes evolved their caffeine-producing abilities separately, in an example of convergent evolution. Researchers also found signs of recent positive natural selection on these genes in coffee, which was not found in similar N-methyltransferases in tea or chocolate. There are many examples of convergent evolution in nature, such as flight in both birds and bats, echolocation in bats and whales, fins in whales and dolphins, and so on, and research suggests the convergent evolution of plant metabolites such as caffeine may be surprisingly common. When evolution converges on the same complex trait repeatedly, it suggests it’s a very useful adaptation, and caffeine plays an important role in both defensive and offensive warfare with enemies of the coffee plant, as well as helping to attract allies.
When coffee leaves die, they fall to the ground and contaminate the soil with caffeine, preventing other plants (potential competition) from germinating. Further, while the leaves are still attached to the plant, the caffeine inside them is protecting the plant from being eaten by insects. Because caffeine is extremely toxic in high doses, insects have evolved taste receptors that tell them to avoid eating food containing it. Caffeine has another string it it’s bow, however. While it’s toxic in high doses, as we’ve come to discover, it’s rather pleasant in small doses. Coffee plants produce nectar to entice insects and other animals to pollinate their flowers, and by adding just a hint of caffeine to it’s nectar, the coffee plant is able to sweeten the deal even more. Caffeine has been found to improve memory formation in insects making them more likely to remember the coffee plant flowers and visit similar flowers in the future. So, the coffee plant uses caffeine to manipulate the behaviour of other animals, both friend and foe.
Although not deliberately, coffee plants are playing exactly the same trick on us – the positive effects of small doses of caffeine (increased alertness, improved task performance, faster reaction time, enhanced memory*) that are useful for attracting pollinators are also enjoyed by humans every time we make a cup of coffee. These effects keep us coming back to the coffee plant, again and again, just as it’s pollinating insects do. Caffeine is highly addictive, after all.
*The jury is still out on whether or not caffeine improves short- or long-term memory in humans, as it has been shown to do in pollinating insects such as bees.
Want to Know More?
- Denoeud et al (2014) The coffee genome provides insight
into the convergent evolution of caffeine biosynthesis
- Wright et al (2013) Caffeine in Floral Nectar Enhances a Pollinator’s Memory of Reward
- Pichersky & Lewinsohn (2011) Convergent Evolution in Plant Specialized Metabolism
- Smith (2002) Effects of caffeine on human behavior