Earth 3.5 billion years ago was a violent, water-covered world of microbial life

Nature · Published 3 October 2026 Swipe more like this →

In short: The Paleoarchean era, spanning 3,600 to 3,200 million years ago, offers our earliest glimpse into life on Earth. Amidst a landscape dominated by underwater crust and massive asteroid impacts, ancient microbes thrived. This era challenges our understanding of how continents formed and how life survived in a volatile, alien environment.

The Paleoarchean is defined chronometrically, marking a period when Earth was fundamentally different from the planet we inhabit today. Because the mantle was significantly hotter, modern plate tectonics did not exist. Instead, researchers propose a model of flake tectonics, where silicified upper oceanic crust delaminated rather than subducting. Consequently, most of the era's geological record has been erased by metamorphism and deformation, leaving only two primary windows into this time: the Pilbara Craton in Western Australia and the Kaapvaal Craton in Southern Africa.

These two regions provide the oldest confirmed evidence of life, primarily through microbial mats and structures like stromatolites. In the Pilbara’s Dresser Formation, dated to 3.48 billion years ago, scientists have identified fossilized bacteria. Similar evidence appears in the Barberton Greenstone Belt, where the Kromberg Formation preserves signs of microbial reproduction via binary and multiple fission. However, interpreting these ancient rocks remains difficult; some features, such as those in the Strelley Pool Chert, might be abiogenic, potentially formed by chemical precipitation on the seafloor rather than biological activity.

The era was also defined by extreme celestial bombardment. The North Pole Dome in the Pilbara represents the oldest surviving impact crater remnant at 3.47 billion years old. Even more catastrophic was the S2 impact 3.26 billion years ago. This collision, involving an asteroid up to 58 kilometers wide, likely triggered global megatsunamis, massive earthquakes, and the temporary boiling of ocean surfaces. Such events would have plunged the planet into years of darkness, creating a harsh filter for the earliest life forms.

Whether any land existed above the waves remains a subject of intense debate. While some formations suggest surface exposure, it is probable that most continental crust remained submerged, with hotspot islands serving as the only dry land. These cratons may have been connected as part of Vaalbara, one of the earliest supercontinents, though the timing of its assembly remains contested. Ultimately, the Paleoarchean serves as a testament to the resilience of life in a world defined by geological instability and cosmic violence.

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