When Chimpanzees groom each other it releases a mild narcotics in the primate brain

Primates spend huge amounts of their day—between 10 and 20 percent—grooming each other. Like other grooming animals, they have to fend off an endless assault of lice and other skin parasites. Simply picking off these parasites is soothing, because touch releases mild narcotics in the primate brain. According to Robin Dunbar of the University of Liverpool, this parasite-driven pleasure took on a new importance when the common ancestor of monkeys and apes and humans moved into habitats with lots of predators about 20 million years ago. These primates had to huddle together in order not to be killed, but they then had to compete with one another for food. As social stresses emerged, the primates began to depend on the soothing sensation that comes from grooming, not for its previous function—getting rid of parasites—but as a kind of currency to buy the alliance of other monkeys. Grooming became political, in other words, and in order to keep track of the larger and larger groups, apes evolved larger brains and had to dedicate more of their time to grooming. Hominids eventually reached the point, at about one hundred fifty members to a band, where there wasn’t enough time in the day to groom one another to keep the band intact. And it was then, Dunbar claims, that language arose and took grooming’s place.

Defending against parasites could have played a part in the evolution of human intelligence in another way—an even more speculative one, but one that might be more significant. Perhaps medicine played a role. When a woolly bear is attacked by a parasitic fly and switches its diet from lupine to hemlock, it does so purely by instinct. It doesn’t pause on its leaf and think to itself, “I seem to have a maggot growing inside me, and it will leave me a hollow shell if I don’t do something.” Its tastes presumably just shift from one kind of plant to another. For most animals that engage in this protomedicine, the process is probably the same. But something different seems to be going on in primates, particularly chimpanzees, our closest relatives. Sick chimps will sometimes search for strange food. They will swallow certain kinds of leaves whole; they will strip the bark of other plants and eat the bitter pith inside. The plants have almost no nutrition in them, but they have another value. The leaves seem to be able to clear out worms from the intestines, and the bitter pith is used as medicine by people who share the forests with the chimps. When scientists have analyzed the plants in laboratories, they’ve discovered that they can kill many parasites.

Chimps, in other words, may be medicating themselves. As the years go by, more evidence accrues to the chimp-doctor theory, but it is slow to gain acceptance. It demands far more proof than a typical idea in biology, since scientists need to demonstrate that chimps are sick with particular parasites when they choose their plants, and they need to show how the plants fight the parasites. Proving this as you run to keep up with chimps racing along rainforest ridges makes for slow scientific progress. But Michael Huffman, the primatologist who has done most of the running, has indeed shown that after chimps eat certain plants, their parasite load drops and their health improves. He argues that the chimps are a lot more sophisticated in their medicine than instinct-driven woolly bears. When they select only the pith of the plant Vernonia amydalina, casting aside the bark and leaves, they are avoiding the poisonous part of the plant and taking only the part of the plant that has steroid glucosides that kill nematodes and other parasites. A hungry goat will eat too much of the plant and sometimes die.

If Huffman is right, the chimps must accrue medical lore and carry the information through time by teaching and observing one another. Huffman once watched a male chimp eat some Vernonia and throw it to the ground; a baby chimp tried to pick it up, but his mother stopped him, put her foot on the pith, and carried him away. Chimps must have some remarkable cognitive sophistication if Huffman is right. They can recognize the symptoms of particular parasites and associate eating plants with their cure. They may even eat some plants preventively, which would put the association on an even more abstract plane.

You usually hear this sort of talk—abstraction, an awareness of the potential uses of things in nature—when people are discussing one of the most important steps in human evolution: the ability to make tools. Chimpanzees can strip sticks to use them to fish out termites from nests; they can smash shells between rocks; they can even fashion themselves sandals to cross expanses of low thorny bushes. As our closest primate relatives, they may embody some of the abilities of the earliest hominids 5 million years ago. Later, as our ancestors moved out of dense forests, they evolved the ability to make more sophisticated tools by flaking stones to butcher meat. The ability to connect the shape of a tool to the job it could do brought a reward of more food. This abstract thought made it possible to make better tools, and survival became even easier. Tools, in other words, may have made our brains swell.

Conceivably, that same argument could apply to medicine as well. Could the ability to recognize how plants could fight various parasites have given hominids longer lives and more children? And could that success have driven more powerful brains in order to find better cures against parasites?
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