Why are various animals getting smaller and smaller compared to their ancient counterparts?

According to most of the scientists, this is actually related to the oxygen content in the air, which is currently 21%. In ancient times, the vegetation was lush, so in which time no one could destroy it, the oxygen content in the air was 20% higher than now. Therefore, at that time centipedes were 1 meter long, dragonflies had wings 1 meter wide and dinosaurs could grow up to 25 meters. In the case of the giant dinosaurs, they lived in a relatively stable and long period of time.

Over a long period of time, the Earth's land and sea plants were quite prosperous, enabling the Earth's creatures to produce large amounts of organic matter and fix large amounts of sunlight energy, the Earth's creatures prospered so that the creatures were not short of food, and the number of species allowed for the production of more mutant genes, relatively speaking. survival is easier for larger species that are more adapted to their environment. The fierce competition for survival made choices for organisms, which became larger in shape as a result.

At the same time, the law of the jungle, small size means a greater risk of survival, easy to become food for other predators, so natural selection, the size of the animal is getting bigger and bigger, which is actually equivalent to the current development of the business from rough to intensive, they no longer need to be so large to maintain the need for survival. Instead, the larger and heavier ones were not adapted to the current environment and most of them were eliminated, in ancient times creatures needed a greater physical advantage for predation and other activities, which was an advantage over smaller creatures.

Then, the Earth's oxygen content was several times higher than it is today, and the oxygen uptake of living organisms increased.The most dangerous things in the Carboniferous period were not reptiles, but giant insects, of which giant dragonflies were as big as eagles, because they did not use blood to transport oxygen, but through hollow tubes, where the air went directly to all parts of the body.There was a lot of dense and tall vegetation during the Carboniferous period, and the oxygen content was dozens of times greater, and the tubes throughout the insect's body became thicker and thicker as they breathed in more and more oxygen, and their heads became larger and larger.

That's all we have to say about why animals got smaller and smaller in size compared to their ancient counterparts.