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LONDON (PTI): Changes in the asteroid belt between Mars and Jupiter some three to four billion years ago hurled a devastating barrage of space rocks at our planet that probably delivered the ingredient crucial to the beginning of life, scientists have claimed.

Analysing tiny "spherules" embedded in layers of rock, a team from the Purdue University in the US said they found the "first real solid evidence" of asteroid bombardment.

According to the team, the spherules were created when asteroids crashed into the Earth, vapourising rock that expanded into space as a giant vapour plume. Small droplets of molten and vapourised rock in the plume then condensed and solidified, falling back to Earth as a thin layer.

These particles were preserved in layers of rock, and the scientists analysed them to record precise information about ancient asteroid impacts on Earth, 3.5 billion to 35 million years ago.

"What we have done is provide the foundation for understanding how to interpret the layers in terms of the size and velocity of the asteroid that made them. This is the first real solid evidence that this asteroid bombardment actually happened," study author Prof Jay Melosh was quoted as saying by the Daily Mail.

The period of heavy asteroid bombardment between 4.2 billion and 3.5 billion years ago is thought to have been influenced by changes in the early Solar System that altered the trajectory of objects in an asteroid belt between Mars and Jupiter, Melosh said.

Some of the asteroids were bigger than the nine-mile rock that wiped out the dinosaurs, but the 700-million-year barrage may have delivered chemicals crucial to the beginnings of life on our planet, the team said.

When the team looked at the number of impactors as a function of size, they got a curve that showed a lot more small objects than large ones, a pattern that matches exactly the distribution of sizes in the asteroid belt.

"For the first time we have a direct connection between the crater size distribution on the ancient Earth and the sizes of asteroids out in space," Melosh said.