Parasites grow with humans

  Nowadays, many people like to keep pets, and they can see the living conditions of their owners from pets, so can we also see some silhouettes of their lives from the “pets” of ancient humans-parasites? Let’s take a look at what stories parasites tell about the basic necessities of ancient humans.
Eating meat and using fire:
Parasites and human beings

  In order to maintain life, parasites usually change several hosts during their lifetime. Scientists have discovered that the parasites living in the human body do not start from humans, so how do they meet humans?
  Scientists can study this by comparing the genome differences between parasites in humans and parasites in other species. The more similar a parasite in a species is to humans, the more likely it is the source of the human parasite. Eric Hoberg, an American parasite expert, chose the tapeworm as his research object. Tapeworm is the most common parasite in the human digestive tract, and it has a relatively single type in the human body, mainly Taenia solium and Taenia saginata. Using tapeworms as a standard to explore the diet of ancient humans is simple and the results are relatively accurate.
  Hoberg drew a genetic family tree of tapeworms and found that the most closely related to human tapeworms is a tapeworm that parasitizes lions and coyotes in Africa. The “divergence” of the two was about 2 million years ago. Lions and coyotes feed on antelopes, and the juvenile period of tapeworms is spent in antelopes, so we can think that 2 million years ago, human ancestors had already started hunting antelopes and eating meat, and the parasites were at this time Entering the human body, it began to adapt to the human environment and parted ways with its ancestors. What is unexpected is that although human tapeworms are named after “pig” and “cow”, the tapeworms in pigs and cattle are even less ancient than human tapeworms, which means that the tapeworms in pigs and cattle are infected by humans Therefore, the time when the two types of tapeworms “separated” may imply the time when people domesticated pigs and cattle.
  The tapeworm entered the human body by hiding in the antelope, and the unsuspecting people lost their armor and armor under the attack of the tapeworm, but soon, humans found a weapon to deal with the tapeworm, that is flame. People would use fire to cook food before eating. The fragile young tapeworms could not resist the high temperature and fell on the way to invade the human body, while the heat-resistant tapeworms survived and “accompanied” humans to evolve into what they are today. In this case, can we see when humans started to use fire from the changes in tapeworms?
  Heat shock protein is a kind of heat stress protein that widely exists in organisms. When organisms are exposed to high temperature, the body will synthesize this protein to protect itself. Genome analysis of the tapeworm found that Taenia solium has more heat shock protein genes than other tapeworms, making it more resistant to heat. This change in T. solium occurred about 160,000 years ago, when T. solium and T. bovis began to diverge into two subspecies. This time is roughly close to the time when other archaeological evidence shows that humans made fire. Can we assume that after people learned to make tinder to make fires, heating meat put a lot of pressure on the tapeworms, forcing them to become more heat-tolerant?
Hair removal and dressing:
Inseparable relationship between parasites and humans

  We know that human beings evolved step by step from ancient apes, and the most obvious difference in appearance between ancient apes and humans is the thick fur. Compared with the ancient apes, the body appearance of Homo sapiens is almost “naked”. Since when did we start shedding this fur?
  It is difficult to find answers from fossils, because protein, the main component of fur, has long been decomposed in the long river of history. But the little life hidden in the fur – parasites can answer this question. If the human body is covered with thick hair, it is easy to breed various parasites, and those individuals with less body hair will have fewer parasites. With fewer parasites, there will be fewer diseases caused by them. From this perspective, individuals with less hair have a higher probability of surviving and reproducing, and the trait of less hair will be inherited from generation to generation until Homo sapiens becomes what it is today.
  Body hair will not disappear completely at once, but will decrease little by little, that is to say, the living home of parasites on the human body surface will also become smaller little by little. When ancient humans grew hair all over their bodies, lice, a common parasite on the body surface, spread all over the body. As the body hair gradually fades away, people’s shoulders, chest and back become hairless, and the “home” of lice becomes smaller and smaller. Later, there are only two densely haired places left in the home of lice on the body surface , that is the hair and the crotch. These two parts are far apart, and the lice have been “separated” for a long time in the two places, without reproductive communication, and gradually divided into two subspecies, which we call head lice and pubic lice. When head lice and pubic lice begin to separate, it is time for humans to shed their hair.
  Scientists have used genetic methods to determine when head and pubic lice “divided” about 1.2 million years ago. This reminds us that it was 1.2 million years ago that human body hair began to fall off on a large scale. As a result, the shelter of the lice parasitic on the human body became smaller, and the two places had to be separated, and finally differentiated into two sub-regions. kind.
  Removing body hair reduces the number of parasites on the human body surface, but at the same time it brings another problem. In the freezing winter, how do people keep out the cold? You know the answer by now, and that is to get dressed. So, when did clothes enter the human stage?
  The earliest warm clothes were furs of ferocious beasts, but these furs can hardly be preserved as fossils to this day. How can we determine when people put on clothes? This question can also be answered by lice.
  After ancient humans shed body hair, most of the body became smooth, and lice could not survive in hairless places. However, people have discovered that besides head lice and pubic lice, there is a new subspecies of lice that live on the surface of the body. There are hooks on the front paws of this lice, which are not the same as its ancestors and close relatives. Scientists believe that this kind of lice, known as body lice, lived in the clothes of ancient humans. It has hooks on its front paws to hook the fibers on the clothes and hide in the clothes.
  So we know that body lice differentiated after having clothes. As long as we calculate the differentiation time of body lice, we can know when ancient humans began to wear clothes. The final determination was that body lice diverged from head lice 170,000 years ago. This also means that about 170,000 years ago, Homo sapiens began to wear clothes.


Taenia solium
Migration and integration:
The reunion of parasites and humans

  Parasites not only have stories of “sadness and joy” in individual human beings, but also repeated separation and recombination events in human groups, and the causes of these stories are related to human migration and integration.
  Neanderthals are close relatives of Homo sapiens, but their ancestors separated from the ancestors of Homo sapiens very early. The ancestors of Homo sapiens stayed in Africa, and the ancestors of Neanderthals went to Europe. After a long period of geographical isolation, the genes of the two showed obvious differences. The genetic map shows that the separation time was about 700,000 years ago. Scientists have also found the same signs in the genes of lice: there are obvious differences in the DNA of the lice that parasitize on the surface of modern humans and the lice found in the fossils of Neanderthals, and their separation time is also several years old. One hundred thousand years.
  Despite the long separation time, there is no reproductive isolation between Homo sapiens and Neanderthals, and there are still some Neanderthal genes in the DNA of modern Homo sapiens, and the same is true for lice! Scientists have discovered that modern lice also carry the DNA of Neanderthal lice, and that the two genes fused about 60,000 years ago, which is very close to the time when the genes of Homo sapiens and Neanderthals fused . In other words, about 60,000 years ago, when Homo sapiens and Neanderthals met again in Europe, the lice on the two races also reunited during their time together, and they may have exchanged genes through mating, It is also possible that some lice switched hosts.
  Mites, another parasite on humans, also hint at the story of human migration. According to the results of gene sequencing, people divide the mites on the human face into four subspecies. Studies have found that all four types of mites exist on the faces of African-Americans, three types of mites A, B, and D exist on the faces of Asians, but lack C, while almost all mites in Europeans come from type D.
  In this regard, the researchers speculate that when ancient humans set off from Africa, these four types of mites followed humans out of Africa. During the long human migration process, certain types of mites could not adapt to the changes in the environment and eventually became extinct. Therefore, people in Africa have the most “complete” mites on their faces, and people in other regions have different types of mites on their faces.
  The history of parasites and humans is longer than we imagined. They have experienced many “ups and downs” with humans. The traces of humans that cannot be found in other objects and places may be reproduced in parasites. Those shadows of history.