Our food choices have interacted with our gene pool, shaped our evolution and created variation between and within populations.
Our food choices have not only determined our way of life, but they have also influenced our evolution. Cooking has made us human, and products such as meat, milk and starch have interacted with our gene-pool.
As our diet is changing again, our evolution continues.
It all started about two million years ago, when our ancestors started to put some meat on the menu. To digest meat, your guts do not need to be as long as for digesting plants. As we did not have to maintain a long gastrointestinal system anymore, our bodies could save energy and grow larger brains.
Then our ancestors did something even more revolutionary: they started to cook. Prof Richard Wrangham showed that this was a crucial step in our evolution. Cooked food is much easier and quicker to digest than raw food and provides more energy. It allowed us to spend most of our days doing other things than digesting, and to have enough energy to travel long distances, hunt efficiently, and grow even larger brains. As we did not need big teeth and jaws any more, our faces became more vertical, and the opening of our mouths smaller.
Richard Wrangham argues that most of the growth in brain size is due to cooking, but as the oldest hearths we have discovered date only from between 400.000 and 750.000 years ago, this is still a matter of debate. Other experts think that growth started because humans began to eat meat, and was only later spurred on by cooking.
The Neolithic revolution or the introduction of agriculture
About 10.000 years ago, people started to farm. Although this is called the Neolithic revolution, it was probably a very slow and gradual process, but it changed our lifestyle completely.
As farming is much harder work than hunting and foraging, we are still not sure why humans took it up. It is possible that hunting was becoming less successful because of climate changes and the extinction of many big animals. There is evidence that the human population was increasing, and the territories might have become too small for hunting, while farming allowed people to produce food on a smaller piece of land.
It is much easier to raise children if you have settled, and farmers could have much larger families. When agriculture started to create a surplus of food, some members of the group could specialise in trade, warfare or the manufacture of non-food items, and this benefitted the whole community. The economist Samuel Bowles calculated that early farming was not more productive than hunting-gathering, and concluded that these social advantages must have been the main attraction.
But farming also had disadvantages. Our diet became less diversified and of a poorer quality. Living together with domesticated animals made early farmers more vulnerable for diseases, and settlements became filthy because of a lack of sanitary hygiene. Infections could spread very quickly and eliminate whole communities. Health declined, but there was more food and the population grew.
As farming changed our diet profoundly, it influenced our genes, even though 10.000 years is a very short period in evolutionary terms.
Milk, lactase, diet changes and evolution
About 7500 years ago, some cattle farmers noticed that they could drink cow milk as adults without getting ill. To digest milk you need the enzyme lactase. It is produced by the cells of the small intestine and it is vital for babies and small children. In most mammals, including many humans, lactase activity decreases after weaning, probably to give the younger siblings a better chance of survival as they will not have to compete with the older ones for breast milk. It is also “cheaper” for the body to stop producing an enzyme unless it offers a clear advantage.
In Europe and in parts of Africa, the Middle East and South Asia however, most people never stop producing lactase and can therefore digest milk as adults. This phenomenon is called lactase persistence.
Burger and colleagues showed from DNA analysis of human remains that lactase persistence was rare in the early Neolithic farmers, and archaeological evidence suggests that dairy farming existed before the genetic variant became widespread. This makes sense, because lactase persistence could only become an advantage if there was fresh milk available on a daily basis. Why it was such an important evolutionary advantage is not completely sure, but milk was probably a continuously available energy- rich drink while crops could fail.
Consequently, lactase persistence people started to herd even more dairy cattle and imposed a selection force on the cows to produce more milk.
Starch in the diet and amylase types
To digest starch we first need to break it down in smaller components using an enzyme in our saliva, called alpha-amylase. The quantity and quality of this enzyme is different from one person to another, and depends on factors like stress and the amount of starch in the diet. There is also a genetic variation in the number of gene copies coding for amylase.
George Perry and colleagues noticed that populations with a diet traditionally rich in starch (potatoes, rice, bread…) are genetically programmed to produce more amylase than populations with a low-starch diet. High-starch diets are found in agricultural societies, while low-starch diets are typical for hunter-gathers, who favour meat, fish, and simple sugars from honey or fruit.
More amylase means that we can absorb glucose from starch more efficiently, and they concluded that natural selection must have favoured this. Abigail Mandel and colleagues also showed that it makes starch tastier, encouraging you to eat more and improve your nutritional status. Moreover, obtaining more energy from starch can be critical to survive diarrhoea, and diarrheal diseases have always been an important cause of death.
Use it or lose it: bitter taste, taste buds and evolution in humans
Many plant toxins taste bitter, and bitter taste perception prevents mammals from eating them. But when humans started to eat farmed products, they reduced the risk to almost nil.
In 2014, Xiaoxia Wang and colleagues noticed that our bitter taste receptors genes are becoming less and less active because of a loss of selective constraints.
There is a large variety between populations, which is not entirely understood. It might be due to the fact that certain bitter plants offer some protection against malaria, and this outweighs the risk of poisoning.
Human evolution today
Today, we are changing our diet again -probably faster than ever before-, and it is bound to have an influence on our gene-pool.
But as we are in the midst of it, this can be difficult to see. Moreover, any changes will be modified by other lifestyle changes, and attenuated by modern medicine. But are there any trends?
In 2015, Sean Byars and Stephen Stearns showed that natural selection is acting on women to make then slightly shorter and stouter, with lower blood pressure and cholesterol levels, an earlier first child and a later menopause, because these are the women who raise the largest families, and pass their genes on to the next generation. As the observation period was only 60 years, the researchers do not know yet if the selective forces will be sustained enough to cause a real change in our gene-pool. It is not clear either if it is due to diet or to other lifestyle factors, but it is sure that we are evolving. Evolution never stops.