Peering back through the millennia isn't easy, and scientists working in Ireland think they've come up with a better hypothesis for how the oldest crystals on Earth originally formed. Rather than being created by plate tectonics, as was previously believed, new research suggests they came from craters left by violent asteroid impacts on the surface of a very young Earth.

The ancient rocks in question are called zircon crystals, and they're important in giving geologists clues about the origins of life, where water on our planet first came from, and how our climate has varied over time. So that means any insight we can glean about the formation of zircon crystals helps to inform research in these other areas too.

To test the hypothesis that these crystals came from asteroid craters, researchers from Trinity College in Dublin examined a relatively new impact crater to see if the same rocks were found. Thousands of zircons were picked up from the Sudbury impact crater in Ontario, Canada, believed to be almost 2 billion years old.

"What we found was quite surprising," said lead researcher Gavin Kenny. "Many people thought the very ancient zircon crystals couldn't have formed in impact craters, but we now know they could have."

The 2-billion-year-old crystals closed matched the composition of crystals that are much more ancient, thought to be over 4 billion years old, and that's very significant: it shows that the crater hypothesis may well be right.

"There's a lot we still don't fully understand about these little guys but it looks like we may now be able to form a more coherent story of Earth's early years," says Kenny. "One which fits with the idea that our planet suffered far more frequent bombardment from asteroids early on than it has in relatively recent times."

The idea of plate tectonic movements as the source of these crystals has itself only been around for 10 years or so, but recently scientists have come to the conclusion that tectonic movements weren't happening in the very early stages of Earth's life - at least not in the same way as we understand them today.

That left the zircon crystals without a plausible origin story, though it looks as though this team might have cracked it. It's a sign of just how difficult it is to trace Earth's history back through billions of years.

The final report on the study has been published in the journal Geology.