According to new research, ancient Earth was a water world with little to no land. And this could have far-reaching consequences for the origin and evolution of life.
While the modern Earth’s surface is about 70% water, new research indicates that our planet was once a true ocean world 3 billion years ago. At this point, only a few archipelagos have breached the briny surface of our global ocean. That is if any land existed at all.
The scientists’ findings were based on unique rock samples discovered in Western Australia’s Panorama district. Because rocks carry imprints of the environments in which they formed, the researchers determined the rocks formed around 3.24 billion years ago in a hydrothermal vent system on the sea floor. Over the eons, the rocks were exposed and turned on their sides, allowing scientists to investigate Earth’s watery past from the comfort of dry land. This led them to believe that ancient Earth was a waterlogged planet with little landmass.
“An early Earth without emergent continents may have resembled a ‘water world,’ providing an important environmental constraint on the origin and evolution of life on Earth, as well as its possible existence elsewhere,” wrote the authors of the new study, which was published in Nature Geoscience.
Despite the abundance of today’s oceans, many questions about their origins remain. Was water always present on Earth, or did it arrive later? If later, how much later? And were the water’s origins comets, asteroids, or something else?
Scientists are still debating these and other questions. This is because evidence — such as ancient minerals called zircons that appear to have formed in a watery environment — clearly indicates that Earth had water around 4.4 billion years ago, just after our planet formed. That’s a long time in ocean history.
However, it is unclear how much water existed on early Earth. And the researchers were able to answer that question by studying their piece of the ancient seafloor.
The oxygen network
When rocks form in water, that water imprints its story in stone. Water, also known as H2O, is always composed of hydrogen and oxygen. However, the isotope, or type of oxygen, in the water reveals information about the environment in which the water formed. For example, how warm it was, or how the water cycled over time between land, sea, and air.
There are two types of oxygen isotopes. Oxygen-16 (O16) is a lighter version with eight protons and eight neutrons. And oxygen-18 (O18), a heavier cousin with eight protons and ten neutrons. Because those two extra neutrons give O18 extra weight, water molecules containing O16 evaporate faster than heavier O18 versions. Furthermore, rocks and dry land are more likely to absorb and capture O18, removing it from the sea’s stores.
When the authors of the new study examined their piece of ancient seafloor, they discovered a lot of O18 — more than is found in our modern oceans on average. And, because dry land is a massive reservoir of heavy oxygen, the presence of O18 in Earth’s early days suggests that such a reservoir did not exist. The researchers determined that the excess of heavy oxygen in their sample was most likely caused by the fact that dry land had not yet emerged from the ancient ocean.
Implications for life
Scientists often debate the origins of Earth’s first single-celled organisms. Did life first appear near hydrothermal vents in the ocean, where there was both heat and mineral-rich water? Or did life begin on land, possibly near Darwin’s proposed warm little pond? There are numerous theories, and scientists do not know for certain.
However, if further research confirms that the early Earth was entirely covered in water, this information could help researchers refine their theories about how life came into existence.
“The history of life on Earth tracks available niches,” Boswell Wing, a geology professor at the University of Colorado Boulder, said in a statement. “If you’ve got a water world, a world covered by ocean, then dry niches are just not going to be available.”
In other words, if the Earth was completely covered in water when life first began, life could not have formed on land at all. If this is the case, it suggests that exoplanets completely covered in water could be ideal places to look for extraterrestrial life. But let’s not get too far ahead of ourselves.
Despite the fact that this Australian seafloor sample represents only a single point in time, it covers a large and well-preserved area. As a result, the researchers hope to conduct similar research on rock samples spanning Earth’s history to track the emergence of the continents. These samples, which stretch a few billion years of Earth’s history, are waiting in Africa, Canada, New Mexico, and Arizona. Together, they’ll tell the story of when Earth stopped being an aquatic world and started offering up the dry land we inhabit today.
READ MORE: Scientists announce a breakthrough in determining life’s origin on Earth—and maybe Mars