Evidence is emerging that medicine is not a human invention at all. In fact, we ape animals. An example is the deliberate ingestion of soil, known as "geophagy". In people, it is thought to signal mental health problems. But according to a study of chimpanzees in the Kibale National Park in Uganda, it turns out to be a remedy. Consuming a particular kind of soil, as Sabrina Krief and her colleagues at the Muséum National d'Histoire Naturelle in Paris reported recently in the journal Naturwissenschaften, increases the potency of ingested plants, such as the leaves of trichilia rubescens, which have anti-malarial properties. Her team collected earth eaten by chimpanzees, as well as leaves from young T. rubescens trees in the same area. All the soil was rich in the clay mineral kaolinite, the principal component of many anti-diarrhoea medicines.
Clays can bind mycotoxins (fungal toxins), endotoxins (internal toxins secreted by pathogens), man-made toxic chemicals, bacteria and viruses. They also act as an antacid and absorb excess fluids. The scientists replicated the effects of mastication, gastric and intestinal digestion in the laboratory and were surprised. Before being mixed with the soil, the digested leaves had no significant effects. However, when the leaves and soil were digested together, the mixture developed clear anti-malarial properties. "This overlapping use by humans and apes is interesting from both evolutionary and conservation perspectives," says Krief. "Saving apes and their forests is also important for human health."
This is far from the first example of our borrowing from other species - indeed, Prof Michael Huffman, of Kyoto University, believes humans have long looked to other animals for medicinal wisdom. In 1987, he happened to be watching a constipated chimpanzee called Chausiku in the dense rainforest of the Mahale mountains in western Tanzania. Reaching for the shoot of a noxious tree that chimps would normally avoid, Chausiku peeled it and sucked its bitter pith. Within a day, her constipation was gone. It was the first time a scientist had seen a sick chimp select an unsavoury plant known by humans to have medicinal properties, and then recover. The pith, from the tree Vernonia amygdalina, has now given up its secrets. It contains compounds active against many of the parasites responsible for malaria, dysentery and schistosomiasis. The shrub is poisonous and called mjonso - "strong medicine" - by the local people, the WaTongwe. But what is fascinating is that they use the same plant to treat the same illnesses - and take the same time to recover. Prof Huffman has found that nearly all of the ape remedies he has studied are also used by local people as medicine - echoing, he believes, the evolutionary origins of human medicine.
For more evidence, take a remarkable type of self-medication first seen among chimpanzees in Tanzania. Jane Goodall, the veteran ape watcher, spotted chimps swallowing leaves in Gombe Stream National Park in the Sixties, which she then studied with Richard Wrangham, of Harvard University. What is remarkable is that leaf-swallowing peaks in many sites about two months after the rainy season has begun - about the same time as the peak of infection with Oesophagostomum stephanostomum, a nodular worm which is linked with bacterial infection, diarrhoea, severe stomach pain, weight loss, and weakness, resulting in high mortality.
Prof Huffman has shown that individual leaves from any of 34 different plants are swallowed whole by chimpanzees, bonobos and gorillas across Africa - but that it tends to be only the sick chimps that will swallow leaves, and that they do so on an empty stomach. He was particularly excited to stumble upon a freshly deposit of chimpanzee dung filled with these swallowed leaves, in which live parasitic worms had been entrapped in the hairy folds. (When not chewed, the leaves are flushed out of the system within six hours in a laxative-like action.)
But self-medication is not confined to chimps. At Bwindi Impenetrable Park and Mgahinga National Park in Uganda, mountain gorillas chew the bark of the nondescript Dombeya tree as a food. The bark is laden with active ingredients, including antibiotics that kill common bacteria such as E. coli, and there is anecdotal evidence that the presence of bugs in gorilla dung matches the ape's Dombeya-eating patterns. In southern Mexico, howler monkeys eat figs that can fight parasite infections.
It is not just curative medicine that was invented by our animal relatives, but preventative, too. Baboons living near the city of Taif, Saudia Arabia, are known to dig drinking holes in the sand directly adjacent to the murky, algae-tainted watering sites of livestock. To ensure the water does not make them sick, they "patiently wait for the filtered water to seep through the sand," says Prof Huffman.
Monkeys also resort to aromatherapy. When capuchins rub each other's fur on the Nicoya Peninsula of Costa Rica they use citrus fruits, notably lemons, limes and oranges, during the rainy season. Either applied directly, or mixed with saliva, the citrus oils help fight bacterial and fungal infections and repel insects. Their cousins in central Venezuela, the weeper capuchins, like to anoint themselves with the secretions of millipedes, which act as an antiseptic and repel mosquitoes and ticks.
Apes may even resort to recreational drugs. They eat the seeds of Kola (cola) trees, thought to be a pick-me-up of the kind found in coffee. Two hallucinogenic plants are ingested by gorillas in Equatorial Guinea and by chimpanzees in the Republic of Guinea: Alchornea floribunda and A. cordifolia (Euphorbiaceae). The apes even resort to eating the root of Tabernanthe iboga, which contains a chemical called ibogaine which has been studied by doctors for use in detox therapies.
Some have said that human medicine is true medicine because we organise and teach it. But Prof Huffmann has seen a young chimp watch its sick mother take medicine, before trying it itself.
For Prof Andrew Whiten, of St Andrews, an expert on ape culture, it is easy to conclude that the animals can learn symptoms, medicines and dosages from their peers. Take the use of Vernonia amygdalina, the constipation-countering shrub mentioned above. Prof Whiten points out that the chimpanzees are meticulous and careful to discard all but the inner pith. The outer parts are so poisonous that they often kill domestic goats unfamiliar with the forest flora - indeed, it is known as "goat killer" by the Temme people of Sierra Leone. Learning how to take this medicine "requires close observation of what mother is doing," he says. Strikingly, self-medication is found in apes across Africa. This, says Prof Whiten, probably means "it is very ancient, culturally important and spread widely".
Given that the common ancestor of chimpanzee and humans lived between seven and six million years ago, the roots of medicine are indeed prehistoric.