Palaeobiology: Dating earliest life:
To my regret, the ancient Greenland rocks have not yet produced any compelling evidence for the existence of life by 3.8 billion years ago. The reader is reminded that another debate on early life is currently in progress on 3.5-billion-year-old rocks in Western Australia, where chains of cell-like structures, long identified as genuine fossils, have recently been downgraded by some workers to the status of artefacts produced by entirely non-biological processes. To have a chance of success, it seems that the search for remnants of earliest life must be carried out on sedimentary rocks that are as old, unmetamorphosed, unmetasomatized and undeformed as possible. That remains easier said than done. For the time being, the many claims for life in the first 2.0–2.5 billion years of Earth's history are once again being vigorously debated: true consensus for life's existence seems to be reached only with the bacterial fossils of the 1.9-billion-year-old Gunflint Formation of Ontario.
Rocks thought to contain the earliest evidence of life may not. Other possible early evidence of life is still being examined. This is how science works. We gather evidence, generate hypotheses, test them, generate new hypotheses, and move forward. Evolutionary biology inspired this research, and all evidence indicates that life existed at least 2 billion years ago. Did it exist 4 billion years ago? Maybe, but we'll only find out with more research.