Perhaps the answer is now clearer. From the beginning of life on Earth, maybe 3.5 billion years ago, until about 700 mya, bacteria were the dominant organisms. Then over a relatively sort period in the Earth’s history, the planet first became ‘Snowball Earth’, probably completely covered in ice and glaciers, before entering a rapid global warming phase, after which eukaryotic organisms became dominant, and evolved onwards to life as we now know it. Why did those eukaryotic organisms, some of which were possibly present before 700 mya, suddenly flourish? The answer may have a lot to do with the great melt of ‘Snowball Earth’ and the subsequent deposit of mineral nutrients in the oceans, which allowed photosynthetic algae to flourish and oxygen levels to rise in the atmosphere.
This article below is a media summary, from the on-line Independent Newspaper, 16th August 2017. If you want to go further, the link to the original article in Nature is active, though much of the article is very, very difficult to follow and focusses upon biochemical, steroid markers. Good for the biochemists! Below the Independent article is a link to the same story from BBC. Same story but a different take.