Final answer:
The earliest definite evidence for life on Earth is from the Archaean Eon, with actual cells dating back to 3.5 to 3.95 billion years ago.
Step-by-step explanation:
The earliest definite evidence for life on Earth exists in the Archaean Eon. Solid geological evidence places the appearance of actual cells between 3.5 to 3.95 billion years ago and with softer evidence of microbial life dating even further back to about 4.1 billion years ago at the end of the Hadean Eon, while indirect geochemical evidence of life, though highly controversial, dates from more than 3.83 billion years ago. The Hadean Eon, which spanned from 4.0 to 4.6 billion years ago, had conditions that were thought to be unfavorable for life, but softer evidence suggests that microbial life could have existed near the end of this eon, possibly as early as 4.1 billion years ago.