Nearly 4 billion years ago, cyanobacteria (otherwise known as Blue Green Algae) figured out how to get energy from light, and the world has never been the same since.
The photosynthesis reaction takes in water, sunlight and carbon dioxide, and outputs oxygen and (more importantly) carbohydrates which store energy needed by the bacteria. Plants rely on photosynthesis – and so do all animals, since they either eat plants, or eat animals that eat plants.
Back when photosynthesis first started, our planet’s atmosphere was made up of four gases: nitrogen, carbon dioxide, methane, and steam. There was no oxygen, and the carbon dioxide trapped heat via the greenhouse effect. But the bacteria started consuming the carbon dioxide, and a couple of billion years later, it had mostly disappeared.
As CO2 levels dropped, so did the earth’s temperature. Ice began to form around the poles, and the more ice formed, the more sunlight got reflected back into space, until the Earth became covered in a kilometre deep layer of ice about 2 billion years ago. The temperature from pole to equator was a chilly -40 degrees.
Cyanobacteria are very hardy, and when frozen, they just go into hibernation until a thaw happens. There are rivers in Antarctica that have frozen and then dried out, and hibernating bacteria on the river beds have been thawed out and immediately come back to life. So when the Earth became a giant snowball, they just waited for something to happen.
The first deep ice age lasted about ten million years. Eventually, volcanic eruptions belched enough CO2 into the atmosphere to restart the greenhouse effect, and the first thaw happened. The cyanobacteria then woke up and started the process all over again. There were dozens of ice ages, each about ten million years and with thaws of about a million years in between.
Finally, about 600 million years ago, during a thaw, more complex life evolved during the cambrian explosion, which restored the CO2 balance and stopped the process.
Could this happen again? Possibly, but not likely. There are only two plausible scenarios: first, wait a few million years until the continents have moved around: if Antarctica travelled more towards the equator, it would be possible for a large area of ice to deflect enough sunlight to reduce surface temperature and kick off the process again. Alternatively, a large asteroid strike could possibly block out sunlight long enough for ocean surfaces to freeze (which would have happened 65 million years ago except that the water temperature was higher then).
Something to think about on a cold day like today.