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Sleeping with no covers on

What’s the connection between a brain-parasite and marks on an ancient thigh-bone? It’s a strange and maybe even disturbing story. Let’s start with the marks on the ancient bone. Scientists at the National Museum of Natural History in France have recently published a paper describing their research into a skeleton found in a cave in Morocco, the desert kingdom in North Africa.

The skeleton is an incredible 500,000 years old, so it didn’t belong to a fully modern human. Instead, it’s from one of our recent ancestors or close relatives. To an untrained eye, there’s little difference and the skeleton can certainly tell us a lot about human history. What it says is grisly, because there are tooth-marks on one of its femurs, or thigh-bones. A large carnivore with powerful jaws dined on the thigh shortly after the death of the owner. It chewed off the flesh and crunched the softer parts of the bone.

That sounds like a hyaena, so perhaps it dined on the bone after the ancient human died naturally. Or perhaps not. The carnivore may have killed its food rather than scavenging it. Either way, the femur is more evidence that humans were regular food for wild animals in the past. Down the centuries, countless of us have ended up in a warm tomb: the belly of a hyaena, lion or leopard. That’s in Africa. Elsewhere, we’ve been eaten by tigers in Asia and jaguars or cougars in the Americas. The big cats are the most important predators, top of the meat-eating tree. That’s where the connection with a brain-parasite comes in.

The parasite in question is a microscopic organism called toxoplasma. It exists right around the world and is attracting more and more attention from scientists because of its unusual life-cycle. Although it breeds only in cats, big and small, it also infects prey-animals like rats. Remarkably, if toxoplasma finds itself in a rat, it begins to manipulate the rat’s brain. It produces powerful chemicals, altering the way the rat thinks and behaves. The rat stops being frightened of cats and becomes attracted to their odour. That increases the chance of the rat being eaten by a cat and toxoplasma being able to complete its life-cycle.

Scientists have been studying this brain-manipulation closely, because guess what? Toxoplasma also infects human beings – many millions of us right around the world. In most cases it doesn’t produce obvious symptoms, so the only way you can tell whether you’ve been infected is to have a blood-test. But an infection with toxoplasma does sometimes seem to affect personality and behaviour, making people more outgoing or reckless, as though it’s trying to manipulate our brains too. But what’s the point of that? When toxoplasma infects a human, it’s reached a dead end. Unlike rats, we aren’t eaten by cats, so the toxoplasma can never breed when it infects a human.

At least, we aren’t eaten by small cats. But lions, leopards and tigers still occasionally kill and eat people in countries like Kenya and India. In the past, as that ancient bone from Morocco proves, this must have happened much more often. This raises a disturbing possibility. If toxoplasma can trick rats into the jaws of cats, perhaps it once did the same to ancient humans. If an infection with toxoplasma made an ancient human more reckless, that would have increased the chance of them ending up as dinner for a big cat. That would have been a warm tomb for the human, but a breeding-ground for toxoplasma.

More study is needed, but even if turns out that toxoplasma didn’t manipulate human brains like that, it’s already certain that we haven’t always been the most powerful and dangerous creatures on earth. Once upon a time, we went in fear of big animals, having to hide from them for fear of being killed and eaten. But hiding didn’t always work: for thousands of years some of us have ended up in a carnivore’s stomach. It still happens occasionally today and the reaction of those left behind shows how people must have reacted in the past. Losing a loved one to a predator is a horrible thing, particularly if the body is partly or completely lost.

Unlike other animals, humans bury their dead, paying respect to the flesh and bones even when life has departed from them. Funerals are very important things and have been for a very long time. We are fascinated and disturbed by stories about animals that eat humans partly because they violate our sense of the way things should be. The dead should be laid to rest with respect and dignity, not have their flesh swallowed and bones crunched by a hungry hyaena.