Final answer:
Life began on Earth around 4.1 billion years ago, corresponding to the evening of December 31 on the "cosmic calendar." The presence of life on Earth can be traced back to this time, although the exact origins of life are still uncertain.
Step-by-step explanation:
On the "cosmic calendar," where the time since the Big Bang is compressed into 1 year, life began on Earth in the evening of December 31. This corresponds to around 4.1 billion years ago according to geological and geochemical evidence. The presence of life on Earth can be traced back to this time, although the exact origins of life remain a subject of ongoing speculation.