Is evolution a circle or a line

  Is there a direction for evolution? Is it a reciprocating circle, or a straight line that never looks back? It is difficult to have a definite answer to this question. If we consider the evolution of different species over different lengths of time, perhaps we will get different answers.
Evolution is a reincarnation?

  Based on the fossil evidence found, bioarchaeologists speculate that there have been five mass extinctions on the earth so far. If we regard each mass extinction as the end of the cycle and the recovery of species as the starting point, are the evolutionary processes of species similar circles?
  The third mass extinction that occurred at the end of the Permian Period 251 million years ago is recognized as the most serious extinction. That extinction caused the disappearance of 98% of marine life and 96% of terrestrial life, and there are almost no signs of life in the world. The recovery process of life is also extremely long and repetitive. From the reappearance of life to the restoration of the prosperity and prosperity before extinction, it lasted for about 10 million years.
  The first to recover was algae, but because of the high ocean temperature and lack of nutrients, it took millions of years to recover the algae. The emergence of algae provided food for the mollusk, ammonites, nautilus, and ammonites became the ancestors of shrimp and crabs and the food of early marine benthic reptiles. Until the primitive crocodiles on the land and the pterosaurs in the sky ruled the earth, the prosperity before the extinction basically recovered.
  But the good times did not last long. At the end of the Triassic period, 195 million years ago, the earth ushered in another mass extinction. The fastest recovery this time is the marine creatures, and the dinosaurs that dominate in the end. The most recent mass extinction in modern times was the mass extinction of dinosaurs at the end of the Cretaceous period 65 million years ago. This time, the ammonites in the ocean did not escape death, while the seed plants and insects on the land became a revival. From the beginning, Homo sapiens finally ruled the earth.
  Starting from simple low-level creatures, to complex higher-level creatures occupying more survival resources, are the evolutionary recovery processes of these lives similar cycles? Is the evolution of the same species a cycle? The evolution of elephants seems to have this trend.
  The ancestor of the elephant The ancestor of the elephant is flat-looking and short in stature, more like a modern Asian tapir, with a long snout, and has not yet grown a long nose and ivory.
  Long teeth are more conducive to the survival of elephants than long noses. Tooth is not only a tool for predation, but also helps courtship. The larger the male elephant’s tusks, the more favored by the female elephant, so that the gene for tusking has a better chance of being passed on. Slowly, the elephant’s teeth grew longer and longer, and many types of tusks appeared, such as horror elephants whose teeth were longer than the chin, shovel-tooth elephants whose jaws and teeth were specialized as shovel, and those whose teeth were longer than their height. Mutual prism and so on.
  After millions of years of evolution, the developed ivory and the long trunk became the two major symbols of elephants, until the emergence of mankind changed everything. When the human purpose of hunting elephants changed from predation to obtaining ivory, the longer the ivory, the faster the individual would die. The longer the ivory, the less adapted to the “environment”, and the tusking gene could not be passed on. . In the Addo National Elephant Park in South Africa, 98% of female elephants have no tusks at all.
  Elephants have spent millions of years adapting to the environment. Their ivory grows bit by bit, but when they encounter a new environment and a more powerful “screening” power than natural selection, it takes hundreds of years. Retreat quickly. When this “screening” power disappears, will ivory grow back? Is the evolution of the elephant a circle?

Has evolution gone or not?

  Although the evolution of some species seems to have the phenomenon of “return to ancestors,” the evolution of many species now seems to be a straight line that never looks back.
  From single-celled algae that were invisible to the naked eye 1.9 billion years ago, to macroalgae that were visible to the naked eye 1.6 billion years ago, to the algae that flourished 540 million years ago, the aquatic algae grew bigger and bigger. Bryophytes are the transitional type of plants from aquatic to terrestrial evolution. Ferns and gymnosperms once dominated, but now the most diverse species on earth are still angiosperms with flowers and fruits. Growth, terrestrial growth, and seed reproduction seem to be the direction of plant evolution. The later the population appears, the more it is, and there are few “turning back”.
  Can I go ashore again when I enter the ocean from land? The evolution of whales is also a “way of no return.”
  About 48 million years ago, on the west coast of Pakistan today, a small cloven-toed herbivore that originally lived on land plunged into shallow water to avoid predators. Later, it never came up again, not dead, but found a more suitable place to live underwater-it was the ancestor of the whale. Scientists discovered its fossil in 1979, because it has ear bones and auditory systems similar to modern cetaceans, thus establishing the status of the ancestor of the whale, and was named “bucky ancient whale”.
  After the bucky ancient whale came to the ocean from land, the original limbs slowly turned into flippers, the nose turned into drainage holes, and the lungs and trachea were more suitable for breathing the oxygen in the water. They gradually adapt to and love the life on the bottom of the sea, where the space is vast, there is more food, and there are fewer natural enemies. They have become huge overlords of the sea, but at the cost of losing their legs, they can no longer go ashore like their ancestors.
  At present, it seems that even if whales are chased by humans in the ocean, it is difficult to grow a pair of long legs and get a chance to go ashore again. Is its life a straight line that cannot be turned back?
Humans have “regret medicine”?

  If we measure it over ten thousand years, perhaps the evolution of most species is an upward spiral. But if you only look at the life of a single population, the evolution of a species is usually “one way to the dark”, death or extinction is the end, and there is no chance to repent. But maybe humans can bring them back to the beginning.
  Many species became extinct in ancient times, and many species became extinct under the influence of human beings. How to resurrect them? Now we have at least three methods. The first is to find existing species that are similar to extinct species, and to breed the closest version to the extinct animal, such as the “resurrection” of the European bison. The second is cloning, which can only be used for newly extinct species. Complete cells are extracted from extinct animals for cloning. Scientists in Spain and France obtained the cells of the last bucardo wild goat in the world in 1999 and successfully cloned an extinct wild goat in 2003. However, due to lung problems, the young sheep only survived less than 10 minute.
  The final trick is genetic engineering, to find the “direct descendants” of extinct animals, use gene editing technology to modify the genes of the offspring animals into the genes of extinct animals, and create new extinct animals. Scientists are applying this method to try to resurrect dinosaurs. , Mammoths and passenger pigeons.
  We know that dinosaurs are the ancestors of modern birds, so can we modify the genes of birds to make them become dinosaurs again? Chilean scientists did such an experiment. The thigh bone of a dinosaur is different from that of a bird. It has a long fibula directly connected to the ankle, while the leg bone of a bird is shorter and does not connect to the ankle. Scientists from the University of Chile discovered that the cells that control the growth of the fibula of birds stop dividing before they grow to the ankle. So the scientists injected a gene that delayed the growth period of the fibula cells of chicken embryos, and finally made the birds grow and grow. The fibula that grows to the ankle the same as the dinosaur.
  George Church, a geneticist at Harvard University in the United States, is working on transforming modern elephants into mammoths. Previously, a research team had discovered a fully preserved mammoth corpse in the permafrost of Siberia, Russia. It even had liquid blood, and the DNA map of the mammoth could be obtained from the blood. After understanding the mammoth’s genes, George Church needed to modify the genome of modern elephants and input the mammoth’s feature codes. He changed 15 genes, including genes that control cold-resistant hemoglobin, subcutaneous fat, and sweat glands.
  After the gene editing is completed, Church will cultivate these edited elephant stem cells in the laboratory to see if they can grow tissues and organs with the desired functions. If they succeed, they can conduct in vivo experiments. Similar to Church’s method of resurrecting mammoths, “Resurrection”, a non-profit organization in the United States that aims to protect genetic diversity, is also using gene editing to resurrect passenger pigeons.
  Under the power of natural selection, species survive and evolve at their own pace. Even after experiencing mass extinction, they will be resurrected in the long years. But human intervention disrupted their rhythm, and their extinction time was advanced, which also made the time of resurrection distant indefinitely. In order to make up for these mistakes, can human beings resurrect them early? The survival problem of cloned individuals needs to be solved urgently. The method of gene editing is still in its infancy. If the resurrection is really successful, can the evolutionary trajectory continue?