Between 2.5 and 4 billion years ago it was not uncommon for asteroids or comets to hit the Earth. In fact, the largest ones, more than 9 kilometres wide, altered the chemistry of the planet’s earliest atmosphere.

Now, a study by U.S. researchers analysed remnants of ancient asteroids and modelled the effects of their collisions to show that the strikes took place more often than previously thought. These strikes may have delayed when oxygen started accumulating on Earth, says a Harvard University press release. The new models can help scientists understand more precisely when the planet started its path toward becoming the Earth we know today.

The researchers found existing planetary bombardment models underestimate how frequently asteroids and comets would hit Earth. The new, higher collision rate suggest impactors hit the planet roughly every 15 million years, about 10 times higher than current models.

The scientists realised this after analysing records of what appear to be ordinary bits of rock. They are actually ancient evidence, known as impact spherules, that formed in the fiery collisions each time large asteroids or comets struck the planet.

Researchers modelled how all these impacts would have influenced the atmosphere. They essentially found that the accumulated effects of meteorite impacts by objects larger than 9 kilometres probably created an oxygen sink that sucked most of the oxygen out of the atmosphere.

The findings align with the geological record which shows that oxygen levels in the atmosphere varied but stayed relatively low until around 2.4 billion years ago. Then the bombardment slowed down and the Earth then went through a major shift in surface chemistry triggered by the rise of oxygen levels known as the Great Oxidation Event.