If you want a one-word definition, the best equivalent is "only." But that doesn't really have the same level of formality. You could also define it as "no more than" without changing the tone.

Here, "but" = "nothing other than" I agree with J.K. that it means "only", but I think it is stronger than "only". It adds a feeling of loneliness when you say "we are each but a tiny ...". "Only" doesn't give that same feeling - it sounds more factual.