Exactly how much life is on Earth?

NEW YORK: 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.”

For example, he said, what if photosynthesis — that miraculous transformation of sunlight into food and oxygen— had never evolved? The question highlights the long, underrated relationship between geophysics and biology.

As Michael Kipp of Duke University, who was not part of the study, wrote in Current Biology Dispatches: “In the vast cosmic arena, there are perhaps planets that live fast and die young, while others are slow and steady. Where does Earth sit on this spectrum?” Caleb Scharf, an astrobiologist at NASA’s Ames Research Center in Mountain View, Calif., echoed Dr. Crockford.

“There have been a number of interesting works in the last year or two where people have taken a step back to really think about the ways that life imprints itself on a planet,” he wrote in an email.

He called Dr. Crockford’s paper “a sort of neo-Gaian way of looking at things,” referring to the hypothesis, proposed in the 1970s by James Lovelock, that life and the environment work together to maintain a habitable planet.

According to the fossil record, geology and evolution have been engaged in a dance for 3.8 billion years, since our planet was only 700 million years old. It was then that the first single-celled creatures appeared, perhaps in undersea volcanic vents, feasting on the chemical energy around them.

The population of cells has been growing exponentially ever since, even through geological disasters and extinction events, which opened up new avenues of evolution.

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.