The gathering was supported by NASA and its Astrobiology Program, which has numerous reasons to be especially interested in the question of the provenance of the Isua stromatolites. Certainly the agency wants to know what they might or might not say about Earth’s earliest life, but also they wanted to know what the site could tell them about the even thornier question of how to identify biosignatures on Mars and beyond.

That, in fact, was an engine driving the expedition. At this point in space science and deep time science, the issue of biosignatures is both central and confounding. They are the pathway to learning if life has existed beyond Earth but, as has become increasingly clear, they are very difficult to find and even harder to interpret.

Allwood described the kind of biosignatures now being pursued as follows: While insects encased in amber can be seen and immediately identified, biosignatures are more like the “footprints and trapped breath of insects. Textual and chemical ghosts.”

Dawn Sumner, one of the geologists present at Isua and someone with long experience looking for microbial influences in old rocks on Earth and on Mars (as a leader of the Curiosity Mars rover team ), put it another way. The dynamic playing out, she said, is “a kind of detective game, but one with very few good clues.” Even when microbes influence rocks, the clues often disappear when those rocks are heated and deformed.

No clear signs of life have been found in rocks as old as Isua’s, which are accepted to be in the range of 3.7 to 3.8 billion years old. Any clues that might indicate life was present have generally been obliterated by the churning of the Earth’s crust, or are more subtle than the ability of today’s science to detect them. Which is too bad, because the very old rocks hold the information needed to understand the origins of life on Earth and early evolution, as well as whether life might exist beyond our planet.

So it was quite an important and dramatic moment when an Air Greenland helicopter landed on a rocky piece of Isua land, and the final group of scientists jumped out and descended to what is known as Site A. ( A previous group of eight had already been in the field for two days.)

The scientists clambered down a rocky slope to the contested site, eager to see and touch and study the ancient and controversial outcrop, and to hear what it might have to tell.