A study of some of the oldest rocks in the world reveals a different answer. The isotope ratio there is not the same as that of the asteroids, but similar to the ice in the early nebula. These rocks have more normal hydrogen and less deuterium. However, the current ratio of water in the sea is similar to that of asteroids. This seems to have changed something some billion years ago.
If the Earth’s oceans did not come from asteroids but existed with Earth, then some problems would have been solved. The first problem is why there is so much water on Earth, and the second is why living things that need water start to form as soon as the earth’s surface solidifies. The latest theory is that the oceans have always been in the interior of the earth since the beginning of the world. Earth’s crust causes the water inside it to rise to the surface, forming oceans. In this way, the waters of the earth’s crust and the oceans on the surface of the earth are in rotation.