From the latest Nature magazine:

(Nature 15 April 2010 Volume 464 Number 7291 pp957-1094)

Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean p1029


It has been thought that ocean temperatures during the early Palaeoarchaean era (around 3.5 billion years ago) were 55–85 °C. But a recent study indicated that the temperatures might be no higher than 40 °C. Here, studies are reported of the oxygen isotope compositions of phosphates in sediments from the 3.2–3.5-billion-year-old Barberton Greenstone Belt in South Africa. The findings indicate a well-developed phosphorus cycle and evolved biological activity in an Archaean ocean with temperatures of 26–35 °C.

Ruth E. Blake, Sae Jung Chang and Aivo Lepland

Amazing how they can determine all of these things. (Just mentally replace "evolved" with "developed" if you have a problem with that. Thanks.)