Earth is about 4.6 billion years old, but few sedimentary rocks have survived destruction by geologic processes and remain to document our planet’s earliest history. Sedimentary rocks deposited nearly 3.5 billion years ago in Western Australia and South Africa provide some of our earliest records of Earth’s history. Remarkably, they preserve a scarce but discernible signature of life. Tiny organic structures preserved in these rocks have been interpreted as microfossils, although this conclusion remains controversial.

More pervasive (and persuasive) evidence of life in early oceans is provided by stromatolites. Stromatolites in ancient rocks are strikingly similar to modern structures formed by microbial communities, and so provide evidence of life that goes back about 3.5 billion years (Fig. 26.21). Also, because purely physical processes cannot explain the isotopic composition of carbon in very old limestones and organic matter, we can conclude that a biological carbon cycle existed 3.5 billion years ago (and, from the evidence of 3.8-billion-year-old rocks in Greenland, possibly earlier).

The chemistry of Earth’s oldest sedimentary rocks also shows that the early atmosphere and ocean contained little or no free oxygen. What would the carbon cycle look like on an oxygen-free planet? We know the answer from our discussion of prokaryotic metabolism. Photosynthesis can proceed in oxygen-poor waters, using H₂S, H₂, or Fe²⁺ as electron donors, and fermenters and anaerobic respirers can recycle carbon using electron acceptors such as SO₄^2– and Fe³⁺. Geologic evidence suggests that iron played a particularly important role in cycling carbon for the first billion years of evolutionary history.

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The evolution of cyanobacteria, with their capacity for oxygenic photosynthesis, made possible the accumulation of oxygen in the atmosphere and oceans. As discussed in Chapter 25, oxygen began to accumulate in the atmosphere about 2.4 billion years ago, making possible aerobic respiration and, eventually, our present-day carbon cycle. Both microfossils and molecular fossils indicate that cyanobacteria and other photosynthetic bacteria continued to dominate primary production until about 800 million years ago. Thus, our modern carbon cycle, powered overwhelmingly by oxygenic photosynthesis and with major participation by eukaryotic organisms, may have existed for only the last 20% of Earth’s history.