Most of what we think of as fossils—old bones, hardened bits of plants, impressions of leaves—go back no more than 600 million years. Yet life on the planet is about 3.8 billion years old, more than six time further into the past. How do we know that? What evidence do we have of life on earth so much earlier than the oldest fossilized bones?
One might think that there are older fossils that haven’t been found yet. But in fact there were no animals and plants at all before 600 mya. For about the first 3 billion years—much of the entire course of life on the planet—almost all life was tiny, even microscopic.
(One exception is fossilized stromatolites, hardened layers of bacterial cells piled in paddy-shaped colonies, seen today in their living versions at Shark Bay, Australia. Stromatolites thrived globally around 1.25 billion years ago but date back 2 billion years before that.)
So instead of digging through dirt, today’s paleontologists start by searching for the oldest rocks. Samples of rocks that formed up to four billion years ago from Australia, Greenland, South Africa and elsewhere are sliced, studied under a microscope, and tested with chemicals. Scientists find microfossils, tiny creatures’ cell walls that have mineralized into tough material. Or they find chemical smears of carbon or the products of the earliest photosynthesis. A recurring challenge is to figure out whether such traces are signs of early organisms or only part of the rock itself.
A less direct but more common sign of early life is oxygen, especially as it appears in bands of rust in rocks. The same bacteria that built the stromatolites gave off oxygen as a waste product, most of which was absorbed by iron in the oceans. The result was masses of rust that eventually formed in bands in rocks, most abundantly around 2.4 bya. It wasn’t until 2 billion years ago that enough iron had turned to rust so that bacterial oxygen was no longer absorbed by the metal and instead accumulated in the atmosphere.
Who does this kind of ancient detective work? We’ve come a long way from Indiana Jones. The field today consists of hybrids of the disciplines that I’ve always been familiar with. For example, the curriculum in Geobiology at the California Institute of Technology includes the following course titles: Earth’s Biogeochemical Cycles, Isotopic Biogeochemistry, Microbial Metabolic Diversity, Paleooceanography, and Geobiological Constraints on Earth History.
As these titles suggest, the study of the history of early life parallels how we view ecology today: the state of living things is inseparable from the state of the planet; a change in one always means a change in the other, back and forth, continually.