[onedoug]

These microorganisms would have to constantly renew their populations while presumably older ones could not be any older than 400my as seafloor is continually subducted beneath continental arcs.

Thus it would seem they originate in seawater, filter down, and become incorporated into then relatively new sediments.

[null and void]

while presumably older ones could not be any older than 400my as seafloor is continually subducted beneath continental arcs.

Second paragraph, first line:

Samples drilled from 3.5-million-year-old seafloor rocks...