Starting roughly 1.8 billion years ago and continuing for a "Boring Billion" years hence, life on Earth was slimy, and evolution stagnated. When the first eukaryotic organisms evolved many millennia before, things looked like they were going to get exciting! What followed was a mega letdown. Life consisted predominantly of microbes and algae. Our planet, so resplendent and colorful today, amounted to little more than a watery, brown-green mess.

Scientists don't have time machines, so how do they know this? The best ancient snapshots come from fossilized structures called stromatolites (pictured above). When scientists examine these primordial records of life -- specifically the ones that date back approximately 800 million to 1.8 billion years -- they see bacteria, single-celled organisms called Archaea, some eukaryotic algae, and little else.

While geologists generally agree on the monotony of the period, they offer many competing explanations for precisely why it was so dull. Through the first half of the year alone, three new studies joined the theoretical fray.

In a study published in April, geologists at the University of St. Andrews blamed the gradual cooling of Earth's interior, which may have caused Earth's surface to stabilize. Thus, with almost no tectonic activity to shake things up, life remained contentedly simplistic.

Ross Large, a distinguished professor of geology at the University of Tasmania, offered a different theory in March. By analyzing seafloor sediments from all across the world, Large and his team found that trace metals like cobalt, selenium, copper, zinc, molybdenum, vanadium, and cadmium seemed to decline in the oceans during the "boring billion." These elements, he says, are critical to life and evolution, and without them, life wouldn't have been able to diversify and advance.

Another recent hypothesis points the finger squarely at oxygen, or rather the lack thereof. Around 2.3 billion years ago, cyanobacteria sent atmospheric oxygen levels soaring 1000 times over, which in turn sent an untold number of oxygen-hating anaerobic bacteria to their graves. But according to a team of American researchers, after the brief spike -- which could very well have catalyzed the evolution of eukaryotes -- oxygen levels seem to have dipped back down. And so, eukaryotes persisted in oxygenated parts of the ocean, but evolution as a whole stalled.

While scientists research and debate why the "Boring Billion" was so boring, a few holdouts, like Harvard University's Andrew Knoll, proffer that the period wasn't as banal as its name suggests.

"There’s reason to believe that all of the properties of cell biology that made complex life possible in the next geologic era were put in place here: cytoskeletons that allow eukaryotic cells to change shape, and cell polarity that allows cells to send a molecular message to one side of the cell but not the other, and to interact with nearby cells. The molecular circuitry and cross talk that allow complex organisms like us to exist today all took root in the so-called Boring Billion."

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