Approximately 2.3 billion years ago, bacterial life had generated enough oxygen to make the air breathable for larger creatures. Unfortunately, this was also a stage when our planet went through one of its "Snowball Earth" phases, when the entire planet was encased in kilometer-thick snow and ice. Scientists have found evidence that multi-celled eukaryotes - our distant ancestors - were thriving on Earth, before and after the big chill. While most of the planet was covered, there were probably pockets that were ice free, where islands of life could survive.

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