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

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."

(Via SciTechDaily.)