Are humans actually parasitic organisms?

When the ethnologist Melanie Martin landed with her husband in the Amazon in Bolivia in 2012, she had two plans: to research the Tsimané, a people of hunters and gatherers, and to father a child. The latter succeeded surprisingly quickly. After only a few weeks, she and her husband were expecting a child, then in their mid-30s. "She joked that it could be because of all the parasites she was exposed to," says her colleague Aaron Blackwell of the University of California at Santa Barbara. "We then decided to actually look at the connection between parasites and fertility."

The result of the study was recently published in the science magazine Science published. Martin himself had no parasites, but the researchers found an astonishing effect on the Tsimané: women who were infected with roundworms became pregnant more often. A lifelong infected woman would give birth to an average of two more children than an uninfected woman, the researchers extrapolated. For the Tsimané that would mean eleven children instead of the usual nine.

More than half a billion people worldwide are infected with roundworms. The pests could well have an impact on the world's population. "Some of the demographic changes we see when people get medical care and clean water could also be related to the decline in these infections," says Blackwell. However, roundworms are not the only parasites that infest the tsimané. Others, such as roundworms, appear to decrease fertility. The new study shows once again how much parasites shape life on earth, including that of humans. "Parasites probably also have other effects that we haven't seen before," says Aaron Blackwell.

They exert an enormous force on life. They are the dark matter of evolution

For a long time, parasites were considered the lowest level of evolution, as degenerate living beings that depend on other, higher living beings for survival and reproduction. In reality, the parasites have developed an evolutionary model of success. They even made it that all other living beings have to adapt to them. The much maligned parasites have massively influenced the course of evolution.

"Take the zebra," says the ecologist Paul Schmid-Hempel from ETH Zurich, who has been researching parasites for 30 years. "The challenge for the zebra is not actually to escape the lion. It always manages to do that as long as it stays healthy. The zebra's job is to avoid or get rid of the parasites." The competition for food, the influences of climate and environment and the flight from predators are perhaps more obvious, but parasites play an at least as important a role, says Schmid-Hempel. In fact, parasites are more of a challenge for most animals because the inconspicuous companions can change much faster. With them, one generation follows the next so rapidly that the host has to make an effort not to lose the arms race. In 2014, researchers reviewed the numerous theories as to why zebras have stripes. Their result: Apparently the fur pattern confuses the visual system of the tsetse flies, they find it difficult to land. This reduces the risk of being infected with trypanosomes, protozoa that afflict many mammals and cause sleeping sickness in humans. Parasites may be invisible, but they exert a tremendous force on life. They are something like the dark matter of evolution.

The zoologist Hinrich Schulenburg is one of the researchers trying to make the influence of this dark matter visible. At the University of Kiel he is experimenting with one of the best-studied laboratory animals of all: Caenorhabditis elegans. The tiny worm is barely visible to the naked eye, not much more than a lint in a Petri dish. Numerous Nobel Prizes have already been awarded for research on the worm. But in the laboratory, the animal is usually viewed in isolation. Parasites are undesirable there.

It's different in Schulenburg's laboratory. Here the scientists infect the animal with other organisms, for example with the bacterium Bacillus thuringiensis. Generation after generation of worms they expose to the parasite, then they explore how the animals have adapted. As expected, the researchers found the strongest changes in the immune system. The constant arms race with the pathogen left its mark on the genome. But the researchers also found differences in behavior. The worms became more cautious over generations and learned better to differentiate between dangerous and harmless bacteria.

Evolution may just invented sex to keep parasites at bay

Evolution did not stop at the worm's sex life either. For six hours, the scientists checked every 15 minutes how many animals were mating. The result: the descendants of worms exposed to the parasite mated more frequently. The strategy will probably prevail because many different partners lead to many different descendants, says Schulenburg. And the more the offspring differ, the more likely it is that some of them will be able to cope with the pathogen.

Evolutionary biologists and ecologists have long neglected the role of parasites, says Robert Poulin from the University of Otago. Biologists could easily overlook the crucial role parasites play in the evolution of birds or mammals because the parasites are so tiny. "Darwin didn't care too much about parasites either," says Schmid-Hempel from Zurich. "He hadn't realized that these were the secret rulers."

In fact, researchers find parasites wherever they look. Well over half of all species on earth are believed to be parasitic. In the wild, animals are usually attacked by several parasites at the same time. In a study of perch in a lake in Finland, researchers found an average of 100 parasites per fish: worms, snails, leeches or protozoa.

The constant threat from parasites probably also explains one of the greatest puzzles in evolutionary biology: Why does sex even exist? From a biological point of view, sex is extremely ineffective. Since only females produce offspring, a species that reproduces asexually should be able to reproduce at least twice as fast as a species that relies on mating. But as quickly as cells divide, the descendants are so much alike. The idea: Only by mixing the genetic make-up during sex can host animals change sufficiently over the generations to offer resistance to parasites.

To test the theory, US researchers modified Caenorhabditis elegans so that it could only reproduce sexually or only asexually. (In nature the worm uses both routes.) Then they confronted the worms with the bacterium Serratia marcescens, which is dangerous for them, and let evolution take its course. The worms that had sex with each other managed to adapt sufficiently. The worms that reproduced asexually died out after 20 generations.

Like the fur of the zebra, the blaze of color in many birds is probably due to parasites. According to many scientists, the plumage serves primarily as a signal for the bird's health. If the bird is infected with parasites, it lacks the energy to produce the magnificent feathers and it becomes less attractive to potential mates.

Parasites also seem to have had a massive impact on humans. In a study published in 2011 in the specialist magazine Plos Genetics published, researchers compared genetic data from 55 groups of people from around the world. Neither the climate nor the local diet seemed to have changed the genome very much. Most of the genetic adaptations were due to the different parasites that people were confronted with in their location. "We can hardly understand people without the parasites and diseases that have made them what they are," says Frédéric Thomas from the University of Montpellier.

The human psyche is also likely to have been shaped by the constant threat from parasites. Disgust is probably a feeling that was created to protect people from parasites. The British researcher Valerie Curtis used detailed questionnaires to find out what people are particularly disgusted with: they find infected wounds more repulsive than dry ones, they find a full underground more uncomfortable than an empty one, and they tend to avoid a man with feverish symptoms rather than the same Man without these signs.