"...wind"

An alien geological expedition from the stars on Earth 4 billion years from now. These beings will find a thin black line. This through the deep strata. Our Isotope traces will guide them. That line will be us.

First Contact.
Humanity all we were and did. From when we walked out of the seas at our beginning to when we raised its level at our end. We'll be catalogued and made a footnote among sentient species.