According to a recent calculation by biologists and geologists, there are more living cells on Earth—a million billion billion, a 1 followed by 30 zeros—than there are stars in the universe or grains of sand on our planet.
Most cells are microbes; many are cyanobacteria, the tiny bubbles of energy and chemistry that move in plants and in the seas, assembling life and extracting sunlight to produce oxygen.
“The big takeaway is that this really puts Earth as a benchmark for comparative planetology,” said Peter Crockford, a geobiologist at Carleton University in Ottawa and lead author of the report, published in the journal Current Biology.
The finding “allows us to raise questions in a more quantitative way about alternative trajectories that life could have taken on Earth.”
Geology and evolution have been in a dance for 3.8 billion years, since our planet was 700 million years old. That's when the first single-celled creatures appeared, perhaps in volcanic vents under the sea.
The seeds of animal life were sown when some bacteria learned to use sunlight to split water molecules and produce oxygen and sugar. 2.4 billion years ago, when photosynthesis was well established, the amount of oxygen in the atmosphere began to increase markedly.
However, photosynthesis is just one strand in a web of geological feedback loops through which climate, oceans, microbes and volcanoes keep the planet 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 warm and largely stable. Rain carries carbon dioxide from the air into the ocean; the volcanoes expel it again.
As a result, Crockford and his colleagues estimate, one trillion gigatons of carbon have cycled from gas to life and back again over thousands of years.
The rise of cyanobacteria triggered the Cambrian Explosion about 550 million years ago, when multicellular creatures appeared in sudden profusion in the fossil record.
The researchers tracked cell population growth over time by measuring mineral isotopes and the amount of oxygen in old rocks. They estimated the total life the Earth has produced since its inception — about 10^40 cells, about 10 billion times more than exist today.
This number represents only 10 percent of all cells that will form by the time the curtain falls on life on Earth a billion years from now. As the Sun ages, it will shine brighter, amplifying erosion and removing carbon dioxide, astronomers explain. At the same time, as the Earth's interior cools, volcanic activity will decrease, disrupting the replenishment of greenhouse gases.
As for other planets, we only have basic information about their habitability. Some of the most likely candidates for life are the moons of Saturn and Jupiter, ice-covered ocean worlds, Crockford said.
By: DENNIS OVERBYE
THE NEW YORK TIMES
BBC-NEWS-SRC: http://www.nytsyn.com/subscribed/stories/7044711, IMPORTING DATE: 2023-12-26 22:15:05
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