744-704 million years ago
Tweeting was sparse for the past week, because for a billion years Earth was fairly stable. Any biological evolution towards greater complexity that was going on left little fossil evidence.
Then things changed dramatically. Before 720 million years ago, we find thick limestone deposits left by decaying algae. These were sequestering carbon, taking carbon dioxide out of the atmosphere, and cooling the Earth. At some point a positive feedback cycle kicked in, as polar seas froze and reflected more sunlight, cooling the planet further. The result was a succession of extreme Ice Ages. The Ice Age of the last few million years, which merely covered high latitudes with glaciers, off and on, were nothing compared to the Snowball Earth of the Cryogenian: at a minimum, polar seas were frozen, and tropical seas were slushy with icebergs. It’s possible that things were even more extreme: the entire sea may have been covered by a thick layer of ice, with a few photosynthetic algae surviving in the ice, and other organisms hanging on around deep sea hot water vents. For a hundred million years, climate oscillated abruptly between two steady states, frozen and warm.
It’s only in the last two decades we’ve begun to figure out this amazing story. If there’s a lesson here, it’s that Earth over the long run is far from a stable system. We will see again and again that the history of life, like human history, has been punctuated by catastrophes.