Could Earth's fresh water—and life itself—have appeared earlier than scientists once believed?
Earth formed about 4.5 billion years ago and has undergone many transformations before becoming the planet we know today. It took hundreds of millions of years for the oceans to form, with the first continents emerging even later. Until recently, scientists believed fresh water occurred later still, based largely on fossil evidence of single-celled organisms dating to around 3.5 billion years ago. But could fresh water have existed even earlier? A new study suggests it may have been at least 500 million years earlier.
To explore questions about Earth’s earliest days, researchers study exceptionally ancient and stable materials—those that have remained largely unchanged since their formation. One of the oldest known rock deposits lies in the Jack Hills of Western Australia, home to crystals of the mineral zircon.
Zircon, a mineral that forms in cooling magma flows - such as those of early Earth - is extremely durable, making it an invaluable source of information for studying the planet’s ancient geology. One of its greatest advantages is that it can be precisely dated: zircon contains uranium, a chemical element that undergoes radioactive decay over time at a predictable rate, allowing geologists to determine the crystal’s age. The oldest known zircon crystal, also discovered in the Jack Hills, dates back between 4.4 and 4 billion years—making it the oldest surviving fragment of our planet ever discovered.
But how can zircon shed light on Earth’s ancient water cycle? When water interacts with zircon, it leaves behind a chemical trace in the form of oxygen atoms. These atoms come in different forms, or isotopes, which have the same number of protons but different numbers of neutrons—and therefore different weights. Contact with fresh water tends to leave behind lighter oxygen isotopes than contact with saltwater. By analyzing the ratio of oxygen isotopes in zircon crystals, the research team uncovered evidence of fresh water dating back at least 4 billion years—500 million years earlier than previously believed.
One of the oldest known rock deposits lies in the Jack Hills of Western Australia, containing crystals of the mineral zircon. A researcher holds a rock sample from the Jack Hills | Curtin University
Land and Water
The presence of fresh water suggests that dry land existed much earlier than previously believed—a finding with far-reaching implications for our understanding of early Earth. The connection between fresh water and land is rooted in the water cycle: when water evaporates, it leaves dissolved salts behind, so the resulting vapor becomes fresh water. This vapor condenses into clouds and falls as rain. If the rain falls over the ocean, it simply mixes back into the saltwater. But if it falls on land, it can create freshwater sources—such as rivers or lakes—before eventually flowing back to the sea. From there, the water can evaporate again, continuing the cycle.
This continuous process is known as Earth’s water cycle. Before continents existed, evaporated water would have fallen directly back into the ocean, remaining part of the saltwater system. The presence of land is therefore a necessary condition for fresh water to form and accumulate in nature. According to the study, Earth’s continents may have emerged much earlier than previously thought.
The study also raises questions about the emergence of the first life on Earth—particularly life on land. The oldest direct evidence of life has been found in a freshwater environment and dates back to around 3.5 billion years ago, though there are contested claims of even older evidence originating from seawater. This new research suggests that suitable conditions for life on land may have existed as early as 4 billion years ago, opening the possibility that life began even earlier than currently identified.