Exactly how much life is there on Earth? – 12/10/2023 – Science

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

Which makes some sense. The overwhelming majority of these cells are microbes, too small to be seen with the naked eye; many of them are cyanobacteria, little bubbles of energy and chemistry that work in plants and the seas, assembling life as we know it and harnessing sunlight to produce the oxygen we need to breathe.

Still, I was perplexed that such a calculation could be performed. I’ve been bugging astrobiologists lately about what this means. Could Earth support even more life? Could there be less? How much life is too much?

“The big takeaway is that this really establishes Earth as a reference point for comparative planetology,” said Peter Crockford, a geobiologist at Carleton University in Ottawa, Ontario, and lead author of a report published in October of this year in an email. Current Biology magazine. The discovery “allows us to ask more quantitative questions about the alternative trajectories life could have taken on Earth and how much life might 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 and underappreciated relationship between geophysics and biology.

As Michael Kipp of Duke University, who was not part of the study, wrote in a Current Biology Dispatches article: “In the vast cosmic landscape, perhaps there are planets that live fast and die young, while others are slow and stable. Where Earth Does it fit into that spectrum?”

Caleb Scharf, an astrobiologist at NASA’s Ames Research Center in Mountain View, California, echoed Crockford. “There has been a lot of interesting work in recent years where people have taken a step back to really think about the ways in which life imprints itself on a planet,” he wrote in an email.

He called Crockford’s paper “a kind of neo-Gaian way of looking at things”, referring to the Gaia 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 just 700 million years old. It was then that the first single-celled beings appeared, perhaps in underwater hydrothermal vents, feeding on the chemical energy around them.

The cell population has grown exponentially since then, even through geological disasters and extinction events, which have opened new pathways for evolution.

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

Without photosynthesis, the rest of creation would have little to eat. But it is just one thread in a web of geological feedbacks by which climate, 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 retains heat and keeps the planet temperate and mostly stable. Rain washes carbon dioxide from the air and carries it to the ocean; volcanoes release him again from the underworld. As a result, Crockford and his colleagues estimate that a trillion gigatons of carbon have been recycled from gas to life and back again over the millennia. That’s about a hundred times more carbon than exists on Earth, which suggests that, in principle, each carbon atom has been recycled a hundred times.

The emergence of cyanobacteria triggered what is known as the Cambrian Explosion, about 550 million years ago, when multicellular creatures — animals — appeared in sudden, splendid profusion in the fossil record. We were ready for the Darwinian races.

Crockford and his colleagues realized they could track cell population growth over time by measuring mineral isotopes and the amount of oxygen in ancient rocks. As a result, they were able to estimate the total life that Earth has produced since its inception — about 10^40 cells, approximately 10 billion times more than currently exists.

While this number seems enormous, it represents just 10% of all the cells that will emerge by the time the curtain falls on life on Earth a billion years from now. As the Sun ages, it will become brighter, astronomers say, amplifying the erosion and washing away of carbon dioxide. At the same time, as the Earth’s interior gradually cools, volcanic activity will decrease, halting the replenishment of greenhouse gas.

As a result, Crockford said, “Earth’s biosphere is unlikely to grow beyond a total of ∼10^41 cells over the entire habitable lifetime of the planet.”

But for now, Crockford and his colleagues wrote in their paper, “Extending today’s relatively high rates of primary productivity will likely compress more life into less time.” The more cells there are, the more times they replicate, producing more mutations, Crockford explained. We, inhabitants of Earth’s biosphere, have billions of years of surprises ahead of us.

When it comes to other planets, he added, we still only have basic information about their sizes and habitability and our imaginations. Some of the most likely candidates for harboring extraterrestrial life are ice-covered oceanic worlds that are the moons of Saturn and Jupiter — like Europa, soon to be visited by a new robotic explorer, the Europa Clipper.

If there is life in these oceans, it is likely to be primitive, Crockford said, as these cold environments do not have enough energy to drive evolution.

“However,” he said, “it becomes extremely interesting to think about how the biosphere of such icy moons will change as the sun gets brighter.”