Fresh water and key conditions for life appeared on Earth half a billion years earlier than thought, study finds

In order for life on the planet to begin, we need two ingredients: dry land and (fresh) water. Strictly speaking, water does not have to be fresh, but fresh water can only occur on dry land.

Only when these two conditions are met can you transform the building blocks of life, amino acids and nucleic acids, into tangible bacterial life that heralds the beginning of the evolutionary cycle.

The oldest life on Earth left in our fragmented rock record is 3.5 billion years old, with some chemical data suggesting it may be as old as 3.8 billion years. Scientists have hypothesized that life may be even older, but we have no record of it being so.

Our new study published in Nature Geoscience provides the first evidence of fresh water and dry land on Earth 4 billion years ago. Knowing when the cradle of life first appeared on Earth—water and land—ultimately gives us clues to how we came to be.

Water and Earth: The Essence of Life

Imagine stepping into a time machine and going back 4 billion years. When the dials come to a screeching halt, you look out and see the vast ocean all around you. Not blue as you know it, but brown with iron and other dissolved minerals. You look up at the sky and it’s deep orange, with carbon dioxide smog and periodic flashes of incoming meteors. Inhospitable to life.

According to scientists, this is what Earth looked like 4 billion years ago. But did he do it?

Just as you abandon all hope of life, you see it on the horizon: earth. When you kick the time machine into travel mode, you fly over to this expanse of outcropping rock and land.

You quickly realize that you have entered a volcanic island with lava spewing from its sides. But you can also feel the raindrops on your nose and see the water collect in small pools at the base of the volcano. You carefully take your hands and taste… it’s fresh. The first evidence that there was fresh water on Earth, at least 4 billion years ago.

Fresh water and upland go hand in hand. If the whole earth is under water, then you can only have salty, ocean water. This is because the salt water wants to seep under the land, a phenomenon known as seawater intrusion.

So if you can find fresh water, you must have dry land – and a reasonably large area of ​​it.

How do we know that there was fresh water and land on the early Earth?

Fresh water is very different from sea water. You might say, but how do you know if one or both were present on Earth when you can’t actually go back in a time machine?

The answer is in the rock record and chemical signals preserved in that time capsule. The Earth is just over 4.5 billion years old, and the oldest rocks scientists have found are only a little over 4 billion years old.

To really understand our planet during its first 500 million years, we need to turn to crystals that once came from older rocks and ended up embedded in younger rocks.

Unlike rocks, the oldest extant crystals are up to 4.4 billion years old. And most of these superstar crystals come from one place on Earth: the Jack Hills in the Midwest of Western Australia.

That’s exactly where we went. We have dated more than a thousand crystals of a mineral called zircon, which is known for its extreme resistance to weathering and alteration.

This is quite important because over billions of years, many later processes can erase the primary chemical signal when the crystals first formed. Most other types of minerals can be altered much more easily, a process that would erase their original chemistry and give us no clues to Earth’s deep past.

Truly ancient grains

Our work shows that about 10% of all the crystals we analyzed were older than 4 billion years. That may not seem like much, but compared to other places around the world, that’s a huge amount of superstar grains.

To determine whether these grains hold the freshwater record, we used tiny beams of ions on these dated zircon grains to measure the ratio of heavier to lighter oxygen. This ratio, known as the oxygen isotope ratio, is thought to be nearly constant over time for seawater, but much lighter for fresh water.

Strikingly, a small fraction of zircon crystals from 4 billion years ago had a very light signature that could only have been formed by the interaction of fresh water and rocks.

Zircon is extremely resistant to change. For the Jack Hills zircon to acquire this light oxygen signature, the freshwater-altered rock must have melted and then resolidified to impart the light oxygen isotopic signature to our zircon.

So fresh water must have been on Earth 4 billion years ago.

Whether life also began so early in Earth’s history is a question we can’t yet be entirely sure about. But we have at least found evidence for the cradle of life on Earth sometime 4 billion years ago – very early in our planet’s 4.5 billion year history.