Portal to the past: Geologist identifies metamorphic rock as a crucial feature of the ancient Earth's carbon cycle

If Earth's history were a calendar year, humans would not appear until the last few minutes before midnight on Dec. 31. During the Proterozoic Eon—2.5 billion years to 543 million years ago—the sun was still a young star, much dimmer than today, and Earth required a stronger greenhouse effect to compensate and maintain habitable temperatures for the planet's earliest lifeforms.