In the evolution of animals, the gradual increase in body size is a natural trend. As early as 2018, scientists discovered a phenomenon by analyzing 17,208 genera of marine animals in the ocean: from the Cambrian to the present, the size of marine animals has increased 150 times. This conclusion provides scientific support for a hypothesis of evolutionary biology - Cope's law.
It can be seen from this that the size of animals tends to eat bigger and bigger, and bigger is more beneficial. After all, a larger size can keep herbivores away from predators, and larger predators can hunt larger prey, which is beneficial to survival.
However, if we look at several major extinction events on the earth, we can find a problem: the larger the animal, the easier it is to become extinct. Let's give two examples. The Carboniferous period is called the Age of Giant Insects. During this period, the size of arthropods reached the largest average size so far. There were not only dragonflies with a wingspan of 1 meter, but also giant millipedes with a body length of 3 meters. However, they did not even survive the third mass extinction event before they became extinct.
Beginning at the end of the Triassic period, the earth officially entered the age of dinosaurs. This period was the period when the overall size of reptiles on the earth was the largest, and giant dinosaurs with a body length of tens of meters appeared. However, dinosaurs only ruled the earth for about 160 million years and were taken away during the mass extinction event at the end of the Cretaceous. Only some of the remaining small theropod dinosaurs evolved into birds to survive.
However, during the mass extinction of the dinosaurs, small animals survived, such as primitive mammals. This gave mammals the opportunity to make their debut. So, since is advantageous in being large, why is it easy to become extinct? Let’s find out the truth about together.
Animals that are too large have a greater demand for food
Although animals that are too large will put themselves in an "invincible" state, its drawbacks are also very obvious, that is, they are too dependent on food. Let’s take dinosaurs as an example and see how herbivorous dinosaurs became extinct?
Regarding the extinction of dinosaurs, the current relatively unified theory is the planetary impact theory, but it is not a planet, because at least three large impact craters have been discovered that are consistent with the event of the extinction of dinosaurs: the Chicxulub crater in Mexico, the Potes crater in Ukraine, and the Silver Crater in the UK.
Chicxulub crater
The huge meteorite impact caused a large amount of dust and impact objects to enter the atmosphere, blocking the sky and the sun. The violent impact also made earthquakes, tsunamis and volcanic eruptions more and more frequent. Large and large-scale volcanic eruptions caused a large amount of sulfide to enter the atmosphere, forming a long period of acid rain. Without the dual blessing of sunlight and acid rain, a large amount of vegetation died.
At the end of the Cretaceous, herbivorous dinosaurs were generally very large in size. They needed a lot of food every day, but the significant reduction in vegetation left them without a stable food source, and then the herbivorous dinosaurs died on a large scale.
Obviously, not all the plants at that time died, only a small part was left, but it was not enough to support the survival of huge dinosaurs, so only those small animals survived. This was the result of large animals' excessive demand for food.
After talking about herbivores, let's look at carnivores. It is also disadvantageous for carnivores to be too large, because carnivores rely on speed and flexibility to hunt. Once they are too big, not only will their flexibility be limited, but they will also only be able to choose prey. It could be some large, slow-moving herbivores . We also take the carnivores of the dinosaur era as an example. Large carnivorous dinosaurs can only hunt large herbivorous dinosaurs. When herbivorous dinosaurs become extinct on a large scale, carnivorous dinosaurs will become extinct faster, because carnivorous dinosaurs need a large group of herbivorous dinosaurs to support them.
Animals that are too large have poor ability to respond to changes in the environment.
Animals that are too large will have poor adaptability to the environment. We can find the answer to this from extinct animals and existing animals. Once upon a time, mammoths such as mammoths appeared under the family Elephantidae. They were a product of the Ice Age. They had thick long hair and a thick layer of fat. This form was necessary for the Ice Age.
However, once the world starts to warm, it will be difficult for them to adapt. The extremely poor heat dissipation ability will cause the mammoths to overheat. At this time, they can only migrate to cold places, but there is little vegetation in cold places. As a large herbivore, there is not enough food in the polar regions to support their survival, and they will eventually become extinct.
Today's large animals are usually concentrated in the tropics, such as the world's largest anaconda, Burmese python, reticulated python. They all live in the tropics. The largest mammals on land live in the tropics. If these super large animals are not in the ocean, their ability to cope with environmental changes is very poor.
Because, in the ocean, they can migrate without hindrance and always find a place suitable for their survival. For example, blue whales migrate almost all over the world. But for these large animals on land, it will be extremely difficult for them to survive once the environment changes, such as entering an ice age and the tropical environment is no longer there.
The balancing effect of the earth
In fact, all creatures living on the earth are dependent on the earth and are "constrained" by the earth. However, usually this constraint is invisible. Let's take an example. The resources on the earth are limited, and the amount of time these resources can support animals is also limited. When the size of species tends to get larger and larger, there will be no change in a short period of time, but in the long run, the resources on the earth will be over-consumed.
At this time, the balancing effect of the earth appears. Excessive demand for resources will lead to extreme climate and environmental changes, which will cause most animals to be eliminated, and then the remaining animals will slowly evolve again.
Among the animals that are eliminated, large animals are often the first to be affected. This is caused by poor adaptability to the environment. However, the evolution of animals cannot escape the "curse" of getting larger and larger, because in animal competition, it is always the individuals with larger body sizes who can win and leave offspring. In other words, natural selection may eliminate most genetic mutations, but genetic mutations with larger body sizes will definitely be retained.
Therefore, the evolution history of life on earth is actually an evolution history of extinction and rebirth.
written at the end
In any geological period, large animals are always more likely to become extinct than similar smaller species. The essence is that although large size is advantageous in species competition, it is disadvantageous when the environment changes.
In fact, the balancing effect of the earth can also be used today. Today's humans are constantly demanding and destroying nature, which is intensifying the depletion of resources. Therefore, if the environment is not protected, humans will inevitably be taken away by the balance of the earth in the future.
