THE taste of food is something that seems fundamental. In Marx’s view, expressed in A Contribution to the Critique of Political Economy (1859), a physical property like taste provides a measure of the use-value of a commodity as distinct from its exchange-value: “From the taste of wheat, it is not possible to tell who produced it, a Russian serf, a French peasant or an English capitalist.”
Today, humanity consumes nearly 800 million metric tons of wheat a year. Other foods such as potatoes, rice, and different cereal crops are essential to diets around the world.
All of these contain large amounts of carbohydrates in the form of starch molecules. These molecules were made by the plants as an energy store. When plants photosynthesise, they use sunlight to convert water and carbon dioxide into glucose molecules.
Glucose and other sugars can be used for energy to power cells, but not all of it is needed at the point of photosynthesis. Those glucose molecules are therefore joined together into larger chains — starch molecules — to be broken back down into sugars when the plant needs them.
The way plants break down starch into sugar was discovered in the 1830s. The French scientists Payen and Persoz were trying to explain why malting grain — soaking it in water and then drying it out — made it sweeter.
Something in the grain was performing a chemical reaction to break down starch, a reaction which could only otherwise be accomplished with heat and acid. The explanation had remained a mystery for decades.
Payen and Persoz wrote that they had wondered if there was “anything left to be found on this beaten path.” There was: they isolated a substance that they called diastase. Diastase was the first named enzyme, a biomolecule capable of speeding up a chemical reaction by its presence without itself being used up. (All enzymes since have been given an “ase” suffix because of their naming choice.)
The enzymes humans use to break down starch are called amylases. If you eat a piece of bread — made from wheat and high in starch — the taste will change as you chew. As you leave it in your mouth, it will become sweeter over time. There is amylase in your saliva, which mixes with the bread and starts to break down the starch molecules.
The amylase in saliva is encoded by a gene called AMY1. But when it comes to the ability to produce enzymes, it doesn’t just matter about having the gene to be able to do it. It also matters how many copies one has.
Humans on average carry six copies of this gene within their total genome. Having more copies means more amylase can be made, which increases the rate of breakdown of starch in our mouths.
Most animals are not as good at it: chimpanzees, our closest living relatives, only have two copies. When a chimpanzee chews bread, it will not taste as sweet. In Marx’s terms, the use-value of wheat is different for humans and chimpanzees. That taste difference is a sweet signature of the history of agriculture.
In 2007, scientists observed that populations with high-starch diets tend to have slightly more copies of AMY1 than those with low-starch diets. That suggests that having multiple copies of AMY1 is a genetic adaptation to the emergence of agriculture based on starchy foods.
Agriculture is believed to have first developed around 12,000 years ago in the “fertile crescent,” a region that spans from Palestine in the west up through Lebanon and Syria, then down to Iraq in the east.
In historical terms, as some people began to eat more starchy plants, it was beneficial to produce more amylase in saliva to get more sugar out. Genes in the human genome can sometimes be duplicated due to random events in the copying of the genetic information, like accidentally typing the same word twice.
People with more copies of AMY1 could get more energy out of agricultural food, so were more likely to pass these copies on to their descendants because they could extract more nutrition from their food. The result was that over the last 12,000 years, the average number of copies of the amylase gene in the human genome increased.
Last month, researchers shed more light on that evolutionary process in a study published in Nature. They looked at the number of copies of AMY1 in over 500 genomes from human remains from Eurasia, ranging from 12,000 years ago to the late 18th century. The researchers found a significant increase in the number of copies as time passed.
Another study in Science last week agrees that the number of copies has increased in the human genome over the past few thousand years, although this separate research team suggests that humans, in fact, evolved three copies of AMY1 as far back as 800,000 years ago.
More work will ascertain which team is right about the original duplication of AMY1, but the increase in copies connected to the history of agriculture seems solid.
These studies show how quickly evolution can happen. Traditionally, biologists have tended to look at single mutations to DNA happening within genes, which are easier to study but happen at a much slower rate.
Now, it has become easier to study gene duplication because of improved DNA sequencing. These recent research papers underline just how quick duplication can be: the rate of duplications of AMY1 was around 10,000 times higher than the background mutation rate.
The general principle that gene duplications can increase the production of useful enzymes has been seen elsewhere, such as in bacteria responding to antibiotics. Thanks to recent advances in the technology needed to read genomes, it is likely that more evidence of the role of gene duplication in human evolution will be reported over the next few years.
Marx, who wrote a great deal on the role of agriculture in human civilisations, would no doubt have been fascinated by what new genetic research can reveal about our past. What we taste varies, not only because of our tastebuds but because of how food changes in our mouths.
Human genetic variation continues today. Although the average number of copies of AMY1 is six, some people alive today have 11 copies. Increased numbers of copies of the gene is not wholly a good thing; it is also associated with getting more dental cavities, presumably due to the extra sugar generated in the mouth. For his part, Engels was apparently afraid of getting false teeth — but as far as we know, he never did.
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