The study finds that fresh water and the basic conditions for life appeared on Earth half a billion years earlier than thought

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Credit: Zircon crystal under a microscope. Hugo Ollerock/Curtin University

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Credit: Zircon crystal under a microscope. Hugo Ollerock/Curtin University

We need two ingredients to start life on the planet: dry land and (fresh) water. Strictly speaking, the water does not have to be fresh, but fresh water can only exist on dry land.

Only with these two conditions met can the building blocks of life, amino acids and nucleic acids, be transformed into tangible bacterial life that heralds the beginning of the evolutionary cycle.

The oldest life on Earth remains in our fragmentary rock record It is 3.5 billion years oldwith some chemical data showing that it might be so It is up to 3.8 billion years old. Scientists have hypothesized that life might be older, but we have no records to prove that this is true.

Our new study Published in Natural earth sciences It provides the first evidence of fresh water and dry land on Earth 4 billion years ago. Knowing when the cradle of life – water and land – first appeared on Earth ultimately provides clues about how we came to be.

Water and Earth: the essence of life

Imagine that you entered a time machine and went back 4 billion years ago. When the discs stop, you look out and see a vast ocean around you. It is not blue as you know it, but brown with iron and other dissolved minerals. You look up at the sky and it’s dark orange, with smog of carbon dioxide and regular flashes of incoming meteors. Inhospitable to life.

This is what scientists think the Earth looked like 4 billion years ago. But did you do it?

Just as you abandon all hope in life, you spot it on the horizon: Earth. By pushing the time machine into travel mode, you will fly across this expanse of jutting rocks and land.

You’ll quickly realize you’ve entered a volcanic island, with lava flowing down its sides. But you also feel raindrops on your nose, and notice water collecting in small pools at the base of the volcano. Carefully you put your hands in and taste it…it’s fresh. The first evidence of fresh water on Earth was at least 4 billion years ago.

Fresh water and emerging land go hand in hand. If all the Earth were under water, all you would have was salty ocean water. This is because salt water wants to go underground, a phenomenon known as seawater seepage.

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

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

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

The answer is found 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 discovered are just over 4 billion years old.

To truly understand our planet in its first 500 million years, we have to turn to crystals that once came from ancient rocks and ended up deposited in younger rocks.

Unlike rocks, the oldest preserved crystals go back much further As much as 4.4 billion years ago. The bulk of these ultra-old crystals come from one place on Earth: the Jack Hills in the midwest of Western Australia.

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

This is very important, because over billions of years, many subsequent processes can erase the initial chemical signal when crystals first formed. Most other types of minerals are easy to alter, a process that would erase their original chemistry and provide us with no clues about Earth’s deep past.

Really ancient grains

Our work shows that about 10% of all the crystals we analyzed were more than 4 billion years old. That may seem small, but it’s a huge amount of very ancient grains compared to other places around the world.

To find out if these grains contained a freshwater record, we used small ion beams on dated zircon grains to measure the ratio of heavier to lighter oxygen. This ratio, known as the oxygen isotope ratio, is thought to be roughly constant over time for seawater, but much lighter for freshwater.

Clearly, a small fraction of the zircon crystals from 4 billion years ago had a very light imprint that could only have formed from the interaction of fresh water and rock.

Zircon is very resistant to change. For the zircon at Jack Hills to have this light oxygen signature, the rocks that had been altered by fresh water had to melt and then re-solidify to transfer the light oxygen isotopic signature to our zircon.

Thus, there must have been fresh water on Earth 4 billion years ago.

Whether life also began very early in Earth’s history is a question we cannot yet be sure. But we have at least found evidence of the cradle of life on Earth sometime 4 billion years ago, which is very early in our planet’s 4.5 billion year history.

more information:
Hamid Gamal El-Din and others, the beginning of the Earth’s hydrological cycle four billion years ago or earlier, Natural earth sciences (2024). doi: 10.1038/s41561-024-01450-0

Magazine information:
Natural earth sciences


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