Scientists know this through the discovery of ancient zircon crystals that were dated around this time. According to the most recent scientific studies, an ancient ocean likely covered the entire planet 150 million years after the formation of Earth, about 4.4 billion years ago. This has happened five times and could happen again.Д«etween asteroid and comet bombardments, scientists believe enough time passed for vaporized water to condense and settle on the earthвЂ™s surface.

Evolution takes time, and when the ocean changed too rapidly for species to respond, mass extinctions occurred across the globe. Some species existed for a geological moment before they went extinct, while others slowly adapted to changing seas. Even creatures more familiar to us, like sharks, whales, and octopuses have long and storied pasts with ancestors very different than the creatures now roaming the seas. Later, bizarre and alien-like creatures reigned supreme. The first ocean lifeforms were microscopic, so small they would be invisible to the naked eye. Indeed, life had been evolving and changing for more than 3 billion years-the majority of the planet's existence-before the first creatures made their way out of the water. As terrestrial creatures, humans are largely unaware that much of life's history has taken place in the ocean. The ocean may seem like a vast and unchanging landscape, but the reality is that the world beneath the waves has continuously evolved over time.

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