Suppose you are trekking through remote Greenland on a day off from your summer job at a s...

Suppose you are trekking through remote Greenland on a day off from your summer job at a scientific camp, and you find an unusual layer of sedimentary rock that is not mapped on your geological charts. You suspect this rock might be even older than the 3.7-billion-year-old rocks from Isua (Figure). What would you do to determine whether these rocks have any evidence of ancient life? What results would show that life was indeed present before 3.7 billion years ago?

Figure billionyear-old evidence suggesting life (a) This sedimentary rock is 3.7 billion years old. Note geologist’s hammer for scale. (b) The rock contains microscopic graphite particles, which appear as black dots. The particles contain ratios of carbon isotopes that suggest they are derived from living cells. From Rosing (1999).