The earth is an ordinary planet in the solar system. From the table 1. 1, we can see that among the nine planets in the solar system, the mass, volume, average density and revolution and rotation of the earth have their own characteristics, but they are not special.
However, the earth is a planet suitable for living things. Although we believe that there are planets in the universe that can give birth to life, so far, we have not found them. Why do creatures appear on the earth? This is closely related to the cosmic environment in which the earth is located and the conditions of the earth itself.
There are primitive traces of life from the birth of the solar system to the earth, which has gone through a long period. now
During this period, the sun has not changed significantly, the lighting conditions of the earth have remained relatively stable, and the evolution of life from low to high has not been interrupted.
In the interplanetary space near the Earth, the orbits of large and small planets around the sun are in the same direction, and the orbits around the sun are almost on the same plane (Figure 1.3). The large and small planets go their own ways and do not interfere with each other, making the earth in a relatively safe cosmic environment.
The moderate distance between the earth and the sun makes the average temperature of the earth's surface 65438 05℃, which is beneficial to the occurrence and development of life processes. If the earth is too close to the sun and the temperature is too high, because of strong thermal disturbance, atoms can't combine at all, so it is impossible to form molecules, let alone complex life. If the earth is too far away from the sun and the temperature is too low, molecules will gather together firmly and can only exist in solid and crystal state, and organisms cannot survive.
The earth's volume and mass are moderate, and its gravity can make a large number of gases gather around the earth to form an atmosphere around the earth. But at this time, there is no oxygen in the atmosphere, which is mainly composed of carbon dioxide, carbon monoxide, methane and ammonia. After a long process of atmospheric evolution, an atmosphere dominated by nitrogen and oxygen has gradually formed, which is suitable for biological respiration.
The decay of radioactive elements in the earth causes the heat and gravity contraction of the original earth, which raises the internal temperature of the earth and evaporates the crystal water. The material movement inside the earth, such as volcanic eruption, accelerates the process of water vapor escaping from the earth. With the gradual decline of surface temperature, water vapor condenses into rain and falls to low-lying places on the ground, forming a primitive ocean. The first single-celled life on earth appeared in the ocean.
As can be seen from the above, the earth is in a relatively stable and safe cosmic environment, and it has the necessary conditions such as temperature, atmosphere and water for the survival of living things, so it is not surprising that the appearance and evolution of living things.