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

With just these two conditions, you can turn 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, and some chemical evidence suggests it may be as old as 3.8 billion years. Scientists have suggested that life could be even older, but we have no evidence that this is the case.

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—water and land—first appeared on Earth ultimately provides 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. As the dials stop, you look out and see a 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 dark orange, with smog from carbon dioxide and regular flashes from incoming meteors. Inhospitable to life.

This is what scientists think Earth looked like 4 billion years ago. But did he succeed?

Just when you give up all hope of life, you notice it on the horizon: land. Putting the time machine into travel mode, you fly to this space of outcropping rock and tap.

You quickly realize that you have stepped onto a volcanic island whose hills are spewing lava. But you also feel raindrops on your nose and notice water collecting in small pools at the base of the volcano. You gently squeeze your hands and taste it… it’s fresh. The first evidence that there was fresh water on Earth at least 4 billion years ago.

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

So if you find fresh water, you must have dry land – and a relatively large space 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. Obviously, you might say, but how do you know if either or both were present on Earth if you can’t actually go back in time with a time machine?

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

To really understand our planet during the first 500 million years, we need to look to crystals that once came from older rocks and were deposited in younger rocks.

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

This is where we went. We dated over a thousand crystals of a mineral called zircon, known for its exceptional resistance to weathering and alteration.

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

Truly ancient beans

Our work shows that about 10% of all the crystals we analyzed are older than 4 billion years. This may seem small, but it is a huge amount of super old grains compared to other places in the world.

To find out if these grains contain a record amount of fresh water, we used small ion beams 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 almost constant over time for seawater, but much less so for fresh water.

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

Zircon is extremely resistant to changes. For the Jack Hills zircon to have this light oxygen signature, the fresh water-altered rock had to melt and then resolidify to give our zircon the light oxygen isotopic signature.

Therefore, fresh water should have been present on Earth 4 billion years ago.

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