The search for the origins of life on Earth has always pushed us to look back in time, looking for traces and clues in the most ancient layers of the Earth's crust. So far, the oldest evidence of living conditions dates back to around 3,5 billion years ago. A new study, conducted by analyzing very ancient zircon crystals from Australia, could rewrite history, anticipating the appearance of two key elements for life by as much as 500 million years: fresh water and land.
A discovery, published in Nature Geoscience (I link it to you here), which could have profound implications on our understanding of how life originated and evolved on our planet.
A very long journey through time
Pretend you're Marty McFly. Get into a DeLorean, the flying version, and set your destination to 4 billion years ago. As the dials spin wildly, you prepare to see an alien, hostile and inhospitable planet. And in fact, when you land, you find yourself surrounded by a vast ocean, not blue as we are used to, but brown, rich in iron and other dissolved minerals.
You look up at the sky and see it an eerie orange color, with a pall of carbon dioxide and the intermittent glow of incoming meteorites. Not exactly the ideal place for a picnic.
You are about to lose all hope of finding the conditions for life, when something appears on the horizon: land! You get closer and focus better: it is a volcanic island, with lava flowing along its sides. But you also see something unexpected: raindrops on the windshield. At the foot of the volcano, even pools of water. Fresh water. Now, consider that the time machine really exists, but it's not a DeLorean: it's a group of rocks. How do you mean?
Crystals tell an ancient story
The answer is contained in the rocks and the chemical signals they preserve. Just like a time capsule. The Earth is just over 4,5 billion years old, and the oldest rocks scientists have found date back just over 4 billion years. To truly understand what our planet was like in its first 500 million years, we must rely on crystals from even older rocks.
This is the case of zircons Jack Hills, in Australia. A team of researchers analyzed over a thousand of these crystals, known for their extraordinary resistance to alteration. And the results were surprising: about 10% of them were more than 4 billion years old. Most importantly, some of these very ancient zircons showed a very light oxygen isotopic signature, which could only have formed from the interaction between fresh water and rocks.
The conditions for life, sooner than expected
Fresh water and dry land: two fundamental ingredients for the origin of life. And now we know that they were present on Earth at least 4 billion years ago, 500 million years earlier than we thought. This doesn't necessarily mean that life began so early, but it opens up fascinating scenarios. Perhaps, while we were busy looking for conditions and traces of life in "young" rocks of 3,5 billion years old, the true first forms of life were laughing under their mustaches (or what they had instead of moustaches) in some remote pool of fresh water, half a billion years ago.
Of course, proving the existence of such ancient life will be an even greater challenge. But this discovery reminds us that, when it comes to unlocking the secrets of origins, Earth still has many surprises in store for us. And who knows, maybe one day we will actually find a time capsule from 4 billion years ago, with a message from our single-celled ancestors: “Hey, it took you a while to find us! Welcome to the club of life, we had fun waiting for you.” It will be our turn to do the same for future generations of explorers, on this and other worlds. The search continues.
