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

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Credit: A zircon crystal under the microscope. Hugo Olyruk/Curtin University

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Credit: A zircon crystal under the microscope. Hugo Olyruk/Curtin University

Life on a planet needs two ingredients: dry land and (fresh) water. Strictly speaking, water is not necessarily fresh, but fresh water can only occur on dry land.

Only when these two conditions are met can the building blocks of life, amino acids and nucleic acids, be transformed into definitive bacterial life, marking the beginning of the evolutionary cycle.

Our fragmentary rock record contains the oldest life on Earth 3.5 billion years oldSome chemical data may also show 3.8 billion years old. Scientists have hypothesized that life could be even older, but we have no record of it being so.

Our new study Published in Natural Earth Sciences Provides the first source of fresh water and dry land on Earth 4 billion years ago. Knowing that the cradle of life — water and land — first appeared on Earth ultimately provides clues to how we came to be.

Water and land: the essence of life

Imagine you stepped into a time machine and went back 4 billion years. As the dials stop, you look out and see a vast ocean around you. Not blue as you know, but brown with iron and other dissolved minerals. You see the sky, which is dark orange, with carbon dioxide and the usual flashes from incoming meteors. Life is inhospitable.

What scientists think Earth looked like 4 billion years ago. But did he?

Just as you give up all hope for life, you see it on the horizon: land. Kick the time machine into travel mode and fly to the exposed rock expanse and touch down.

You’ll soon realize that you’ve stepped onto the side of a volcanic island with lava seeping out. But you feel the raindrops in your nose, and you can see the water collecting in small pools at the base of the volcano. Carefully cup your hands and taste…it’s fresh. The first evidence of fresh water on Earth is at least 4 billion years ago.

Freshwater and exposed land go hand in hand. If all the land is underwater, you can only get salty, sea water. This is because salt water tends to seep under the ground, which is called seawater intrusion.

So, if you find fresh water, you must have dry land – a reasonably large area.

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

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

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

To really understand our planet in its first 500 million years, we need to go back to the crystals that once came from older rocks and deposited on younger rocks.

Unlike rocks, primitive preserved crystals go back Up to 4.4 billion years. Most of these super-old crystals come from one place on Earth: the Jack Hills in Western Australia’s midwest.

This is where we went. We dated more than a thousand crystals of a mineral called zircon, which is renowned for its extreme resistance to weathering and alteration.

This is important because, over billions of years, many later processes can destroy the primary chemical signal when the crystals first formed. Other types of minerals are much easier to alter, which destroys their original chemistry and leaves us with no clues to Earth’s deep past.

A truly ancient grain

Our work shows that about 10% of all the crystals we analyzed are older than 4 billion years. It may seem small, but compared to other places around the world, it is a very large old grain.

To determine whether these grains contain a record of fresh water, we used beams of tiny ions to measure the ratio of heavy to light oxygen in these dated zircon grains. This ratio, called the oxygen isotopic ratio, is considered nearly constant over time for seawater, but much lighter for fresh water.

Notably, a small fraction of zircon crystals from 4 billion years ago had a very faint signature, formed only by contact with fresh water and rocks.

Zircon is very resistant to alteration. For Jake Hills’ zircon to have this light oxygen signature, the rock replaced by fresh water must have melted and then resolidified, injecting the light oxygen isotopic signature into our zircon.

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

Whether life began so early in Earth’s history is still an undecided question. But at least we’ve found evidence of life on Earth as early as 4 billion years ago — very early in our planet’s 4.5 billion-year history.

More information:
Hamed Gamaleldien et al, the beginning of Earth’s hydrologic cycle four billion years ago or earlier, Natural Earth Sciences (2024) DOI: 10.1038/s41561-024-01450-0

Press Information:
Natural Earth Sciences


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