A clear indication of the importance of sugar in human evolution is our ability to see colors. Evolutionary biologists believe that the primary function of color vision was to identify the ripest, sweetest and most nutrient-rich fruits.

“The fruits that are the deepest red and the deepest orange and the deepest yellow are the ones with the most sugars,” Brand-Miller says.

There’s even a theory that human intelligence leveled up through the search for ripe fruit. In a tropical forest, different fruits ripen at different times of the year. There is a distinct survival advantage for primates—including early humans—that could create a mental map of the forest and find those ripe fruit trees before the competition. In the foraging race, the smartest species gets the most sugar.

“You can see how this would be a stimulus for cognition, for a bigger brain and for a bigger memory,” Brand-Miller says.

Another clue that early humans and other ancient primates ate a lot of natural sugar in their diet is that most of them had cavities. Brand-Miller says there’s fossil evidence that primates living 30 million years ago got cavities from eating sugary foods. Human cavities really exploded after the invention of cooking, because that’s when people started eating starchy tubers and other carb-heavy foods that were previously indigestible.

Most of human evolution took place in Africa, where ripe fruit and honey were abundant and available year-round. But as humans migrated out of Africa and into colder, drier climates, sources of dietary sugar were harder to come by. In response, Brand-Miller believes human evolution continued to select for genes that improved our ability to access sugars and carbs.

A good example is milk consumption. No other animal species drinks milk as an adult, and not all humans carry the genes that allow them to digest lactose, the sugar found in dairy. But for ancient humans who lived in cold climates where fresh fruit was scarce, there was a distinct evolutionary advantage to being able to drink and digest milk.

There are even more recent genetic adaptations that greatly improved humanity’s ability to access dietary sugars and carbohydrates. Starting with the Agricultural Revolution 10,000 years ago, the human diet began to include far more carbohydrates in the form of grains and starches like rice, corn and potatoes.

There’s an enzyme called salivary amylase that kickstarts the digestion of starchy foods in the mouth, releasing their sweet taste. Brand-Miller says chimpanzees have just one copy of the gene that encodes for salivary amylase in their DNA, whereas humans have as many as 20 copies. Evolution has repeatedly selected for additional copies of the salivary amylase gene, because they allow us to efficiently break down complex carbohydrates into glucose, our favorite brain food.

“These genetic changes tell us how important carbohydrate calories were to human beings,” Brand-Miller says.