At present, science cannot give the final answer to this question, because they are related to another common and equally unsolved problem of the origin of the solar system.

Current research proves that some large and small nebulae separated from the solar nebula about 5 billion years ago.

They revolve around the sun while rotating.

In the process of movement, they collided with each other, and some lumps combined with each other, from small to large, and gradually became the original earth.

During the collision of nebula clusters, they shrink sharply under the action of gravity, and the radioactive elements in them degenerate, which makes the primitive earth heat up continuously. When the internal temperature reaches high enough, underground substances, including iron and nickel, begin to melt.

Under the action of gravity, the weight sinks and tends to concentrate in the center of the earth, forming the core; Lighter ones float to form the crust and mantle.

At high temperature, the water inside evaporates, rushes out with the gas and rises into the air.

But because of gravity, they will not run away, but will only turn around the earth and become a circle of air and water.

In the process of cooling and condensation, the crust on the earth's surface is constantly impacted and squeezed by the violent movement inside the earth, so it becomes uneven and sometimes crushed, forming earthquakes and volcanic eruptions, spewing magma and hot gas.

At first, this kind of situation often happens, and then it gradually decreases and tends to be stable.

This division of light and heavy substances led to great turmoil and reorganization, which was completed about 4.5 billion years ago.

After the crust is cooled and shaped, the earth is like an apple that has been dried for a long time, and its surface is wrinkled and uneven.

There are mountains, plains, river beds and basins.

For a long time, the water vapor in the sky coexisted with the atmosphere; Thick clouds are gathering.

It's dark. With the gradual cooling of the earth's crust, the temperature of the atmosphere is also slowly decreasing. Water vapor takes dust and volcanic ash as condensation nuclei and becomes water droplets, which accumulate more and more.

Due to uneven cooling and strong air convection, thunderstorms and turbidity currents were formed, and it rained harder and harder for a long time.

Surging floods, through thousands of rivers and valleys, converge into a huge water body, which is the primitive ocean.

In the primitive ocean, seawater is not salty, but acidic and anoxic.

Water is constantly evaporating, and clouds repeatedly cause rainfall, and then fall back to the ground, dissolving the salt in the rocks on the land and under the sea, and continuously collecting it into the seawater.

After hundreds of millions of years of accumulation and integration, it has become a generally uniform salt water.

At the same time, because there was no oxygen and ozone layer in the atmosphere at that time, ultraviolet rays could reach the ground directly. With the protection of seawater, life was first born in the ocean.

About 3.8 billion years ago, organic matter was produced in the ocean, and lower single-celled organisms first appeared.

In the Paleozoic era 600 million years ago, there were algae that photosynthesized in the sun to produce oxygen, which gradually accumulated and formed the ozone layer.

At this time, creatures began to land.

In a word, after the gradual increase of water quantity and salinity and the vicissitudes of geological history, the primitive ocean gradually evolved into today's ocean.