What life on Earth was like for one billion boring years | Science

If we were to compress Earth’s 4.54 billion years of history into a single year, primitive life would have emerged early, before February ended. However, organisms with nucleated cells like ours would not have appeared until August. From then until November, hardly anything new would have happened. These three months, in the context of the analogy, represent a billion years — an unimaginably vast span of time. Now, a new study has helped illuminate this gap by detailing just how slowly life evolved during that period.

Roughly 2.5 billion years ago, Earth entered the Proterozoic Eon, the third of its four major geological eras, which lasted until 538 million years ago. By that time, life had already existed for at least 3.8 billion years but remained limited to simple, bacteria-like organisms known as prokaryotes. During the 1.5 billion years of the Proterozoic, a critical evolutionary milestone occurred: approximately 1.8 billion years ago, cells with nuclei — eukaryotes — emerged. These cells would eventually form the foundation of all multicellular life, from bees and sponges to oak trees and any other multicellular being.

However, after this revolutionary step, life seemed to stagnate. For about a billion years, evolutionary progress slowed to a crawl, a phenomenon paleontologist Martin Brasier famously labeled the “Boring Billion.” “Our analysis shows that during the so-called Boring Billion, eukaryotic diversity was very stable, and species turnover rates was quite low,” explains paleobiologist and geobiologist Shuhai Xiao of Virginia Tech, co-director of the new study published in Science, which compiled and analyzed the Proterozoic fossil record to investigate the life that populated the seas in this period. “In this sense, the Boring Billion was truly boring,” Xiao confirms.

Boring, but only in appearance

During this so-called Dark Age of Earth, as it is sometimes called, a hypothetical observer might have grown tired of the endless waiting and concluded that nothing more was going to happen, that the planet and its organisms had exhausted their potential. The climate was stable, free of dramatic ice ages or massive tectonic disruptions. The landmass consisted of a single supercontinent, Columbia, which eventually gave way to Rodinia. Bacteria dominated the biosphere, while eukaryotes, were present, sophisticated species were rare; life forms were largely microscopic, punctuated by a few multicellular organisms resembling algae and sponges. The appearance of something as complex as a trilobite seemed like a distant fantasy, and a creature as advanced as a dinosaur was utterly inconceivable.

But that’s only part of the story. According to Xiao, “Diversity is just one aspect of evolution; from another perspective, the Boring Billion may actually have been very exciting.” During this seemingly uneventful period, eukaryotic cells underwent profound internal transformations. They developed tiny internal structures known as organelles, likely by incorporating simpler prokaryotic cells and repurposing them for their own benefit. A novel method of packaging DNA emerged, enabling the accommodation of larger genomes. Sexual reproduction made its first appearance in a red alga known as Bangiomorpha. And eukaryotes began to differentiate into the major groups we recognize today — plants, animals, fungi, and others. “The few new species births represented groups of eukaryotes that would later be ecologically successful,” Xiao says.

The picture Xiao and her colleagues paint, distilled from extensive data and advanced algorithmic analysis, portrays the Boring Billion as a preparatory phase — a period where life was quietly laying the groundwork for future complexity. In the words of Indrani Mukherjee, a paleogeologist at the University of New South Wales, this era was nothing less than “the slingshot of complex life on Earth.”

The monotony of this epoch came to an end roughly 720 million years ago, with the onset of a drastic climate shift that transformed Earth into a giant snowball. This glacial period, which lasted until about 635 million years ago, marked a turning point. When these deep ice ages finally relented at the close of the Proterozoic, the metaphorical slingshot was released, and life on Earth began to thrive.

From 0 to 100 in a few million years

The results of the study confirm that, compared with the previous period, what followed was an explosion of life. But what exactly caused this prolonged pause, and how did the glaciation contribute to the subsequent surge in biological diversity?

“There are a few hypotheses,” explains Xiao. Data from the period suggest that oxygen and nutrients in the oceans were scarce, while compounds toxic to eukaryotes — such as hydrogen sulfide, notorious for its rotten egg smell — were abundant, sustaining certain types of bacteria.

Simon Poulton, a biogeochemist from the University of Leeds who specializes in the chemistry of the Proterozoic ocean, paints a stark picture of this time: “If you went on a safari back in time, you would find completely sterile landscapes.”

Everything changed after the Great Ice Age. According to Shuzhong Shen, study co-leader from Nanjing University in China, “the breakup of the Rodinia supercontinent may have played a significant role in triggering the rapid diversification of eukaryotes.” Shen elaborates that throughout Earth’s history, periods of intense geological activity, often linked to volcanism, have driven environmental shifts that have led to major transformations in life on Earth.

And once the Boring Billion was over, the Earth became a stage of constant surprises, with unpredictable plot twists and an astonishing finale. In the context of the year analogy, humanity made its grand entrance just 20 minutes before New Year’s Eve. All of human history since the beginning of our era occupies only the final 14 seconds of this cosmic calendar.

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