I don't know if you have noticed that although chimpanzees are close relatives of human beings, they don't need a haircut all their lives because their hair doesn't grow continuously at all. The same is true of many hairy animals in nature, because their hair stops growing when it grows to a suitable length to maintain body temperature. Most animals have hair, but in the process of evolution, we humans gradually lost our thick hair. A recent study by researchers at Washington University School of Medicine in St. Louis found that human hair is actually not the same as animal hair, because hair stops growing when it reaches a certain length, while our hair can keep growing. This is the real reason why people need a haircut and chimpanzees don't. In order to make people understand their point of view, they asked in a paper recently published in Evolutionary Anthropology: "Has anyone ever seen chimpanzees cut their hair?" On one occasion, Dr. Arthur Newfield, a professor of ophthalmology and visual science at Washington University School of Medicine, had dinner with his good friend and colleague, Dr. Glenn Conroy, a professor of anatomy and anthropology. Nuefeld told Conroy for the first time that human hair and animal hair are different things. Conroy replied that so far, he didn't know who was studying the difference between hair and hair in anthropology. Nuefeld recalled: "So we discussed this view and raised some questions, for example, why does human hair continue to grow when it is ignored?" How do hair and hair change during evolution? The more you pay attention to this problem, the more you find that hair and hair are really different. "Hair can only grow on the head. Under the microscope, the hair follicles of the hair pulled out of the thigh look the same as those of the hair. Anatomically speaking, the hair in our armpits, legs and other parts of the body is the same as hair. However, the experiment of hair transplantation shows that the growth mode of hair is determined by its location. When people transplant hair, they often take a hair from one part of the head and transplant it to another part of the head, because if they transplant hair from other parts of the body, it will not be successful. Similarly, if they transplant their hair to other parts of the body, their hair will not grow smoothly. Dr nuefeld said: "When researchers transplant hair from head to leg, it won't be as long as it is on the head, but the hair transplanted to the leg will be longer than the hair on the typical leg. Hair and body hair grow periodically. In the active stage called "growth period", the hair follicle grows a lock of hair, and then the growth rate slows down, and the hair follicle will "rest" for a while. This stage is called the "rest period" stage. When the hair falls off, it will reach the "exogenous" stage, and when the "growth period" begins again, the hair follicle will grow a new lock of hair. Hair on legs usually grows for 19 to 26 weeks and then falls off, while hair can grow for 2 to 6 years. Nuefeld and Conroy hypothesized that although hair follicles are the same as those on the body, the control of hair follicle growth cycle is different from other parts of the body. Conroy said: "Interestingly, anthropologists have been thinking about some questions for years, for example, why do humans no longer have thick hair? Why has our hair faded? How do they fade? This is a big problem and there are many different theories to explain it. However, people like Arthur, who are outside the field of anthropological research, have raised a question, why hair in different parts of the human body has evolved differently. "People who may never find the answer have slightly different keratin content in their hair. Keratin is a kind of fibrous protein, which constitutes the chemical basis of hair, nails, rhinoceros horn and many external features of animals. The keratin content in human hair is different from that of chimpanzees and gorillas. Although chimpanzees and gorillas are closest to us in biological relationship, there are still differences in keratin content. One possible reason for this difference is that there is human DNA called "pseudogene" in human body, while the true gene called φφhHaA in chimpanzees and gorillas produces keratin. Although the same DNA sequence is also stored in human body, human cells do not use this DNA to produce protein, which is why scientists call this gene in human body "pseudogene". Nuefeld and Conroy have already had some ideas about how to distinguish hair from body hair. They said that research in this area will be difficult because there are no animal prototypes for experiments. But they think this kind of research can be carried out, such as taking a hair follicle from a human head and a hair follicle from a leg, and conducting gene slicing experiments to study the differences of genes controlling the activities of these hair follicles. Similarly, study the differences between human hair follicles and chimpanzee hair follicles. However, even if it is possible to determine the factors that make hair different from body hair, scientists may never know why human hair is so different from that of recent animal species, but at least, nuefeld and Conroy raised this question.

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