Scientists have identified the single chance encounter about 1.9 billion years ago to which almost all life on Earth owes its existence.

It saw an amoeba-like organism engulf a bacterium that had developed the power to use sunlight to break down water and liberate oxygen.

The bacterium was probably intended as prey but instead it became incorporated into its attacker’s body – turning it into the ancestor of every tree, flowering plant and seaweed on Earth. The encounter meant life on the planet could evolve from bacterial slime into the more complex forms we see today. “That single event transformed the evolution of life on Earth,” said Paul Falkowski, professor of biogeochemistry and bio-physics at Rutgers University in New Jersey. “The descendants of that tiny organism transformed our atmosphere, filling it with the oxygen needed for animals and, eventually, humans to evolve.”

It had been thought such organisms emerged many times over on the early Earth, but the unique nature of the event has become clear from studies of chloroplasts, the bodies in plant cells that absorb sunlight and use its energy to generate nutrients and oxygen.

They show that the genes within chloroplasts, and the proteins they produce, are so similar in all plants, ranging from tiny algae to oak trees, that they must all be the direct descendants of a single cell.

“It is an astonishing thought that a single random encounter between two tiny cells so long ago could have had such huge consequences,” said Falkowski, who will describe the latest research to next month’s meeting of the American Geophysical Union.

Falkowski’s group at Rutgers is one of several around the world using powerful scientific tools such as DNA analysis to work out how life evolved after Earth was formed about 4.5 billion years ago.

They have also refined methods for dating ancient rocks using radioactive isotopes. This method can show as well how much oxygen was in the atmosphere when the rocks were formed. Such evidence suggests primitive life emerged up to 3.5 billion years ago – but only as bacterial-type organisms. Then, more than 2.2 billion years ago, one group, the cyanobacteria, evolved the ability to use sunlight to break down water, making nutrients and liberating oxygen.

This event was a breakthrough, but cyanobacteria were inefficient, so oxygen levels in the air remained minimal. It took several hundred million more years before the chance encounter that would lead to flowering plants took place – a hiatus showing how unlikely it was to happen at all.

Nick Lane, a researcher at University College London and author of Oxygen: The Molecule that Made the World, said a picture of life evolving through a series of unique chance events was emerging. “Oxygen energises all life, and makes it big,” he said. “Nothing else can provide the energy needed to fuel the demands of multicellular organisms. True photosynthesis evolved only once, and the chance encounter that gave rise to plants also happened just once. These were two freak accidents in the 3.5 billion-year history of life on Earth.”

“As oxygen accumulated,” he added, “plants could grow ever larger. Animals evolved as these new food sources became available.”

One puzzle has been why oxygen accumulated in the air at all – because plants are consumed by organisms that use oxygen to break them down. This should mean oxygen is used up as fast as it is generated.

Falkowski suggests that, in reality, many plants never are consumed by other organisms and are instead permanently incorporated into rocks through geological processes.

In the sea, dead marine phytoplankton sink to the bottom and become incorporated into sediments that turn into rocks such as chalk. The white cliffs of Dover are made of the compressed remains of trillions of such organisms.

Falkowski calculates that in the past 2 billion years, about 15 billion billion tons of carbon has been removed from the atmosphere and locked into the Earth’s crust by such means. He said: “The burial of large amounts of organic carbon by plants, especially over the past 750m years, caused a sharp rise in atmospheric oxygen, which almost certainly triggered the explosion of animal life seen since then.”

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