Question

What modern organisms are thought to be most like the first life-forms on Earth?

Answer 1

Probably either cyanobacteria or archaea, both of which flourish today in all kinds of wet environments.

Explanation:

There is a presumption in the question that the earliest life-forms on Earth were what we would call organisms today. Depending on your definition of "life-form", pre-cellular arrangements of molecules may qualify as life. Different authorities use different definitions.

The earliest unicellular forms of life of which I am aware are still living today, namely cyanobacteria and archaea. The classification of archaea into phyla seems to be in a state of flux - or at least contention.

It is difficult to distinguish whether the earliest evidence of life is from cyanobacteria or from archaea, but evidence has been found dating from between `3.48` and `4.1` billion years old.

Prior to cellular life-forms may have been a primitive form of life based on self reproducing RNA strands. This is essentially the "RNA world" hypothesis. Prior to that, there may have been even more primitive self reproducing molecules.