Research shows that fresh water and the most important living conditions appeared on Earth half a billion years earlier than thought

For life to emerge on a planet, we need two ingredients: dry land and (fresh) water. Strictly speaking, the 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 convert the building blocks of life, amino acids and nucleic acids, into tangible bacterial life that marks the beginning of the evolutionary cycle.

The oldest life on Earth still present in our fragmented rocks is 3.5 billion years old, and some chemical data show it may be as old as 3.8 billion years. Scientists have hypothesized that life might be even older, but we have no data showing that this is the case.

Our new study published in Natural Geosciences 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 will ultimately provide clues to how we came to be.

Water and land: the essence of life

Imagine if you stepped into a time machine and went back 4 billion years ago. As the dials come to a 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 is dark orange, with a smog of carbon dioxide and frequent flashes of incoming meteors. Inhospitable to life.

This is what scientists think the Earth looked like 4 billion years ago. But did that work?

Just as you give up all hope of living, you see it on the horizon: land. If you put the time machine in travel mode, you will fly to this vast rock and land.

You soon realize that you have stepped onto a volcanic island, with lava spewing over its sides. But you also feel raindrops on your nose and see water collecting in small pools at the foot of the volcano. Carefully cup 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 emerging land go hand in hand. If all the land is underwater, all you can have is salty ocean water. This is because salt water wants to penetrate under land, a phenomenon known as seawater intrusion.

So if you find fresh water, you must have dry land – and a fairly large expanse of it.

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

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

The answer lies in the rock 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 just over 4 billion years old.

To truly understand our planet in its first 500 million years, we must turn to crystals that once emerged from older rocks and were eventually deposited in younger rocks.

Unlike rocks, the oldest preserved crystals date back as far as 4.4 billion years. And the bulk of these super-old crystals come from one place on Earth: the Jack Hills in the mid-west of Western Australia.

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

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

Truly ancient grains

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

To find out whether these grains contained 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 nearly constant over time for seawater, but much lighter for freshwater.

Strikingly, a small portion of the 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 rock.

Zircon is extremely resistant to change. In order for Jack Hills’ zircon to acquire this light oxygen signature, the freshwater-altered rock had to melt and then re-solidify to give the isotopic light oxygen signature to our zircon.

So fresh water had to have been present on Earth before 4 billion years ago.

Whether life also began so early in Earth’s history is a question we cannot yet be completely sure about. But at least we have found evidence for the cradle of life on Earth more than four billion years ago – extremely early in our planet’s 4.5 billion year history.

This article is republished from The Conversation under a Creative Commons license. Read the original article.