When did the ocean stop being too acidic for life to emerge? Scientists have uncovered this mystery.

The history of Earth spans over 4.5 billion years, during which our planet has undergone an incredible number of changes. For instance, it is believed that the early oceans of Earth were too acidic to support life. In a new study, scientists focused on assessing the timelines of this process, as reported by IFLScience.

During the research, a team from Yale University evaluated the timelines of the processes that led to these changes. The results indicate that the global ocean has sufficiently de-acidified and reached the necessary pH level to host the first organisms around 500 million years ago.

Today’s oceans are slightly alkaline, with a pH of about 8.1. The addition of carbon dioxide to the atmosphere alters this process—a phenomenon known as ocean acidification—but it is expected that the pH will remain above the neutral point of 7, which is also positive. Unfortunately, even a slightly more acidic ocean is bad news for some shell-forming organisms, while a truly acidic ocean, even to a small extent, is considered incompatible with life, as it hinders the formation of organic molecules.

Nevertheless, scientists assert that conditions on the early planet favored the formation of extremely acidic oceans until processes driven by plate tectonics changed ocean chemistry. According to the study's lead author, Dr. Man Gao, who previously worked at Yale University, understanding when life first emerged on Earth is incredibly important for comprehending how and when a more neutral pH ocean appeared.

In the study, Gao and Professor Jun Korenaga calculated how long these processes would take to reach a point compatible with life. If the scientists are correct, then during the first 500 million years in Earth's history, the oceans of our planet were unsuitable for the formation of any life forms. Areas with lower pH may have existed in semi-isolated bays with little exchange with the global ocean, but not much farther.

The new finding essentially creates a paradox, as the age of Earth is over 4.5 billion years, while molecular clocks date the first life forms to around 4.2 billion years ago—before scientists believe the oceans became habitable.

The authors of the study note that there are several possible explanations for this paradox. Firstly, the scientists' new estimate may simply be incorrect; secondly, the validity of molecular clocks may also be questioned; thirdly, it could answer the question of whether life began in hydrothermal vents on the ocean floor or in island ponds.

Scientists agree that modeling the long-term evolution of ocean acidity is a complex issue, as it involves nearly all components of the Earth system: the atmosphere, oceans, crust, and mantle.

By modeling the rate of weathering of seafloor silicates and the first proto-continents, as well as the ocean's absorption of carbon dioxide from the atmosphere, Gao and Korenaga estimate the initial pH to be around 5, reaching neutrality 4 billion years ago. The authors believe that their work will also be important for the search for life on other worlds.