Really. New studies point to mats of microbes, or scum like you see on a stagnant lake, as the source of oxygen that made other forms of life possible.
Our scummy ancestors had the planet to themselves up to about 2.2 billion years ago because these single-celled organisms were the only things that could survive in a world without oxygen.
But then things changed. Some process suddenly pumped Earth's atmosphere full of oxygen. Life was kick-started and about 2.1 billion years ago the first multi-celled organisms appeared. These eventually led to all forms of life that now live, or ever lived, on Earth.
Researchers have puzzled for decades over what caused the breath of fresh air. "Bacteria in the early oceans were able to separate water into hydrogen and oxygen," said David Catling, a NASA Ames Research Center scientist who led one of the studies. The primitive microbes also produced methane gas, in which hydrogen atoms became trapped. Being light, the methane, with its trapped hydrogen, was lost to space. Excess oxygen, left behind because it's too heavy to escape the Earth's gravity, built up in the atmosphere making "life as we know it" possible.
So, next time you're tempted to call someone "pond scum," remember, that isn't necessarily a bad thing.