Exactly How Much Life is on Earth? – The New York Times

Source: Exactly How Much Life is on Earth? – The New York Times

By Dennis Overbye

Dec. 1, 2023

What’s in a number?

According to a recent calculation by a team of biologists and geologists, there are a more living cells on Earth — a million trillion trillion, or 10^30 in math notation, a 1 followed by 30 zeros — than there are stars in the universe or grains of sand on our planet.

Which makes a certain amount of sense. The overwhelming majority of these cells are microbes, too small to see with the unaided eye; a great many are cyanobacteria, the tiny bubbles of energy and chemistry that churn away in plants and in the seas assembling life as we know it and mining sunlight to manufacture the oxygen we need to breathe.

Still, it boggled my mind that such a calculation could even be performed. I’ve been pestering astrobiologists lately about what it means. Could Earth harbor even more life? Could it have less? How much life is too much?

“The big take-home is this really sets up Earth as a benchmark for comparative planetology,” Peter Crockford, a geobiologist at Carleton University in Ottawa and the lead author of the report, which was published last month in the journal Current Biology, said in an email. The finding “allows us to more quantitatively ask questions about alternative trajectories life could have taken on Earth and how much life could be possible on our planet.”

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The seeds for animal life were sown sometime in the dim past when some bacterium learned to use sunlight to split water molecules and produce oxygen and sugar. By 2.4 billion years ago, with photosynthesis well-established, the amount of oxygen in the atmosphere began to rise dramatically. The Great Oxidation Event “was clearly the biggest event in the history of the biosphere,” said Peter Ward, a paleontologist from the University of Washington.

Without photosynthesis, the rest of creation would have little to eat. But it is just one strand in a web of geological feedback loops by which weather, oceans, microbes and volcanoes conspire to keep the globe basically stable and warm and allow life to grow.

The carbonate silicate cycle, for example, regulates the amount of carbon dioxide in the atmosphere; the gas traps heat and keeps the planet temperate and mostly stable. Rain washes carbon dioxide from the air and into the ocean; volcanoes disgorge it again from the underworld. As a result, Dr. Crockford and his colleagues estimate, a trillion gigatons of carbon have been cycled from gas to life and back again over the millenniums. That’s about 100 times as much carbon as exists on Earth, which suggests that, in principle, every atom of carbon has been recycled 100 times.

The rise of cyanobacteria set off what is known as the Cambrian Explosion about 550 million years ago, when multicellular creatures — animals — appeared in sudden splendiferous profusion in the fossil record. We were off to the Darwinian races.