What does it mean to be human? For most people, it all comes down to that extraordinary object between our ears, and how it blesses us with language, laughter and logic. But not for Ajit Varki, a doctor-cum-scientist who works in California.
For him, being human is also about a single chemical that separates us from our closest relatives, and which could be linked to many of our most debilitating illnesses.
The story began in 1984, when Prof Varki was working at the University of California, San Diego. When treating a woman with bone-marrow failure, he injected her with horse serum. The treatment carried the risk of a side effect called "serum sickness", in which the patient's immune system launches an attack on a molecule present in the serum called Neu5Gc.
Sure enough, her skin erupted with an itchy red rash. Investigating further, Prof Varki found that Neu5Gc was foreign to humans, even though we carry a very similar version of the same molecule - which may be one reason why animal-to-human organ and tissue transplants do not work well.
But in recent years, he has come to believe that the implications of this molecular difference are much wider. He has built up a range of evidence that potentially links Neu5Gc, a so-called sialic acid, to chronic disease.
This is because the animal version is absorbed by humans as a result of eating red meat and milk products, and there is evidence that the body views it as an invader.
Eating these foods could trigger inflammation and, over the long term, heart disease, certain cancers and auto-immune illnesses.
This sialic acid plays a number of roles: it helps us recognise cells and helps cells stick together (this stickiness is also exploited by microbes, which latch on to the sugary molecule to invade our cells). It also helps regulate our immune response, which may influence the progression of diseases and even play a part in human evolution.
The first evidence that this particular molecule is of unique importance to humans came a decade ago. Prof Varki's team, along with Prof Elaine Muchmore, also of the University of California, studied blood from chimps, bonobos, gorillas, orangutans and humans.
They found that we are the only primates whose bodies do not produce Neu5Gc - although further research established that our Neanderthal cousins were missing this version of the sugar acid, too.
Instead, human (and Neanderthal) cells bristle with a sugar called Neu5Ac. The two molecules are identical, apart from one little detail: the ape molecule has a single extra oxygen atom. Because of the many different jobs this sugar does throughout the body, this one atom was the first example found of a fundamental genetic and biochemical difference between humans and our closest relatives.
In recent studies, Prof Varki's team has found tantalising evidence that this mysterious molecule could be exerting a wider effect on our health, through the substances we eat.
After testing a range of foods, they found the highest levels of Neu5Gc in red meat: up to 11,600 micrograms could be absorbed from the recommended daily serving of beef, 5,100 from pork and 4,900 from lamb. The level in goat's cheese was 5,500, but fell to around 700 in milk and salmon. Cod, tuna, turkey and duck were in the twenties.
Given that food is broken down in the stomach, did eating animal tissue present the same dangers of provoking an immune attack as transplanting it? Following that great scientific tradition of self-experimentation, Profs Varki, Muchmore and Pascal Gagneux ate pure Neu5Gc to see what would happen.
Not only did the foreign sugar show up in the body soon after eating, but tests also revealed that many people carry antibodies that react to Neu5Gc - a protective immune response, but one which could trigger damaging inflammation.
Prof Varki's colleague - and wife - Prof Nissi Varki then found that small amounts of Neu5Gc were present in normal human tissue, probably as a result of long-term consumption. And as well as food, many biotherapeutic products made in animal cells and/or using animal materials were also contaminated with Neu5Gc.
This raised the fascinating possibility that anti-Neu5Gc antibodies are involved in auto-immunity. Auto-immune diseases, such as type-1 or juvenile diabetes and some types of arthritis, occur when the body mistakenly attacks healthy tissue.
Because the animal version of the sugar is so similar to the human one, the latter could be caught in the friendly fire directed by the immune system. Chronic inflammation is also linked with cancer; intriguingly, the team found that Neu5Gc was concentrated in tumours, particularly those that spread throughout the body. This could aid detection of such diseases, by getting scientists to look for the animal acid rather than the tumours themselves.
Some of this might sound familiar: several previous studies have linked ingestion of red meat to cancer and heart disease, and possibly to some other disorders involving inflammation, such as arthritis and lupus. But these focused mostly on the role of saturated fats, and on products that arise from cooking.
Prof Varki, however, believes that his little molecular difference could also be to blame: Neu5Gc elicits an immune reaction that might contribute to a whole spectrum of human-specific diseases. Although they have not proven this yet, the evidence is sufficiently compelling for his team to start work on ways to eliminate Neu5Gc from the body.