Their courtship, which came about 1.9 billion years ago, was very brief, and it wasn't exactly love at first sight:

It came when an amoeba-like organism engulfed 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 of today.

"That single event transformed the evolution of life on Earth," said Paul Falkowski, professor of biogeochemistry and biophysics 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."

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Professor 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 4.5billion years ago.

They have refined methods for dating ancient rocks using radioactive isotopes. This method can also show 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 usesunlight 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.

I know, I know.  To some people, this sounds farfetched.  Unlike the other theory, which suggests a bearded magic man blinked his eyes and suddenly all life was created.