For this question, we'll assume that life on Earth was brought by aliens. They didn't do anything specific to our planet: they saw that it was compatible with the lifeform they wanted to seed, so they just got down and put life on Earth.
What did this form of life look like?
I want to assume that they dropped the most evolved lifeform possible, without denying any of today's science. Fossils and other proofs of the earliest stages of life on Earth must be taken into account in your question. But see that in this question, everything that we have no scientific proof was physically on earth at some point needn't have been.
Also these aliens dropped one and only one form of life, many specimens if need be. If the alternative is fun, you can include in your response a scenario where they dropped more than one lifeform, but this is not the main purpose of this question.
Bonus: how long ago was this?