【Popular Science Garden】
Science and Technology ** Beijing, Feb. 20 (Reporter Zhang Jiaxin) About 37.5 billion years ago, the comical-looking Titarik fish ventured ashore in a novel way of adapting to its environment: its lobed fins propelled it to "walk" on land, and the air sacs in its throat allowed it to "breathe" in the air. Titarik fish actually have gills and are thought to be the common ancestor of the earliest known tetrapods. Over the course of hundreds of millions of years, tetrapods have spawned countless species, including Homo sapiens. So if humans evolved from fish, why don't they have gills? According to the American Fun Science Network 18**, the reason why humans use their lungs instead of gills to breathe is related to natural selection in the process of evolution.
One of the explanations is that gills need to be kept moist to work effectively, and therefore, it is only suitable for aquatic animals. The gills have a large surface area and thousands of tiny blood vessels, allowing oxygen to easily enter the bloodstream. Chris Orgen, an evolutionary biologist at Montana State University in the United States, said that when water passes over the gills, oxygen is absorbed and carbon dioxide is diffused out. If a land animal has gills, these gills dry up quickly, which is not an efficient way to breathe.
Another explanation is that the lungs have significant advantages for life on land. When the fish ancestors were still living in an aquatic environment, they already had lungs in addition to their gills. Because only fish with lungs can go ashore to reach land and survive on land. The available evidence suggests that early lungs evolved into simple lungs like lizards and then into subdivided lungs specific to mammals. Notably, mammals evolved the diaphragm, which may date back 300 million years.
On the contrary, useless structures tend to disappear. Over time, the gills gradually degenerate, to about 3During the Carboniferous period, 1.5 billion years ago, gills finally disappeared completely from land-dwelling animals. Around that time, the ancestors of the first reptiles, as well as the first birds and mammals, began to evolve.
It's amazing that primitive fish had lungs. While gills are good at extracting oxygen from water, they provide less oxygen and are not suitable for larger animals that need more oxygen. In addition, seasonal changes can also affect the amount of oxygen in the water. Thus, the fish air sacs (primitive lungs) allow them to inhale air above the water surface to replenish oxygen intake.
Actually, humans did not evolve to completely lose their early gills. Human embryos have a physiological feature: tiny gills-like folds called pharyngeal arches. Although they are not true gills, they are definitely "remnants" of early gills. Throughout embryonic development, the pharyngeal arch becomes part of the jaw, throat, and ears. Without the pharyngeal arch, the animal's head cannot be formed. Aquatic, gills-breathing species also have this structure in embryonic development. The difference is whether this structure evolved into true gills in different animals. (Zhang Jiaxin).
*:Technology**.