Tiny stones just a millimetre across were the original building blocks of the Earth, research suggests.
Computer simulations show that early in the life of the solar system they gave rise to asteroids and "protoplanets" which then collided to create the Earth.
All traces of the sand-like "chondrules" vanished from our planet long ago, but they can still be found in asteroid fragments which land on the Earth as meteorites.
Previous research has shown that the Earth was formed through collisions between protoplanets the size of Mars over a period of 100 million years.
However, how the protoplanets were created has been a mystery.
Dr Martin Bizzaro, one of the authors of the new research from Copenhagen University in Denmark, said: "Our study shows that protoplanets may have formed very quickly from asteroids, by capturing chondrules in the same way as the asteroids did."
The computer simulation revealed how asteroids started off much smaller than they are today as a result of gravity acting on a "cosmic ocean" of chondrules.
They then grew quickly to become objects up to 1,000km across, the same size as those found today in the asteroid belt between Mars and Jupiter.
The largest asteroids went on growing to produce Mars-sized protoplanets with 10% of the mass of the Earth.
"We suddenly realised that this rapid process could say something about the formation of the Earth as well," said co-author Dr Anders Johansen, from Lund University in Sweden.
The research, published in the journal Science Advances, is supported by studies of meteorites originating from material blasted off the surface of Mars by impacts.
They have shown that Mars was formed over a period of just one to three million years, which is within the time span shown in the computer simulation.
Dr Johansen said: "Traces of this process remain in asteroids that still contain intact chondrules. The terrestrial planets, however, have all melted after their birth and therefore do not show any direct traces of their original building blocks."
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