I would think it would be capitalized as "life on Earth", treating it as the name of the planet, like "life on Mars." I may be too tired to be making much sense, though. "Life on earth began millions of years ago" looks normal to me without capitalization.
I am still confused because there is a grammar rule: Definite — the
Uses :Before nouns of which there is only one.
"The earth is round." In: After visiting several galaxies, the aliens found themselves captivated by life on earth. I think the earth is only one within galaxies, so we should use the. Am i wrong?