The symmetry of living things is divided into bilateral symmetry and radiative symmetry, this symmetry is very common in nature, and the body structure of most animals, including humans, exhibits mirror symmetry, that is, bilateral symmetry.
Symmetrical animals on both sides are easier to understand, that is, the midline of the body is the axis of symmetry, and both sides of the body are the same. Most of our common animals are like this.
For example, carp, crayfish, frogs, lizards, butterflies and even humans are such symmetrical animals. Another type of radiation symmetry animals is also found in the animal kingdom, and their bodies are radially symmetrical at a central point. For example: jellyfish, polyps, starfish and other animals.
Therefore, many scientists believe that this is not an evolutionary coincidence.
According to fossil evidence, bilateral symmetry has been formed in organisms as early as 500 million years ago. Therefore, we believe that there must be some intrinsic reason for the bilateral symmetry of living things. Over the years, biologists have come up with many hypotheses about this problem.
Scientists have made many assumptions about the advantages of bilateral symmetry, such as this symmetry that allows animals to move effectively in purposeful and intentional directions, toward food sources or favorable environments, away from danger, and bilateral cooperation can promote improved sensory organs, such as eyes and ears, which help symmetrically the body to be able to perform focused movements.
I have seen relevant information on the Caiye Network before, from the perspective of cytology: we all know that animals are developed from fertilized eggs. The initial fertilized egg has only one cell, and then it splits into two, two to four, four to eight, and so on, and so on, and the cells are always even. In this way, each division, each of the same kind of cells is in the same position, and no factor appears to cause them to be asymmetrical, so the final result of development is symmetry.
Another popular hypothesis is that the evolution of symmetry has contributed to mate selection, and experiments on birds and insects have shown that females prefer to mate with males with the most symmetrical decorations. For example, female peacocks prefer male peacocks with long and symmetrical tails, while domestic swallows prefer male domestic swallows with long and symmetrical tails.
From the evolutionary history of animals, the earliest occurrence of radial symmetry animals. For example, polyps, their lifestyle is fixed, and the radiation symmetry allows them to perceive information from all directions, which is very beneficial to their survival.
Since the Ordovicians 550 million years ago, animals have evolved symmetrical body shapes on both sides, out of the need for survival, the nervous system and sensory organs will evolve with higher requirements, and the development of the head will appear in the direction of the trunk' progress, because this is the most convenient and fast source of perceived danger!
Of course, this is still early in time for the emergence of the earliest mammals in humans, but it is clear that mammals have inherited these symmetrical structures and the evolution of the brain from 5.5 years ago!
We have found the source of the left-right symmetrical structure of human beings, but the brain is actually divided into left and right brains, which are mainly due to the evolution and development of the contralateral distribution requirements of human sensory and motor organs, and gradually tend to refine the division of labor is also a trend!
Are there any asymmetrical animals? Yes, there is. Let's give a few examples!
Mollusks such as snails and conchs are asymmetrical animals. We look at their shells and we know that these animals are asymmetrical.
Then there's the flounder, which is a typical asymmetrical animal in vertebrates. Flounder's eyes are on the same side of the body, and they swim on one side of the body when swimming. When the flounder is small, the body is also symmetrical on the left and right sides. Flounders have grown up like this because of their habit of living on the ocean floor.