The story of our world is written in the rocks. Turning over the stony pages, a trained geologist can read about mountains that rose and fell in the distant past, oceans that dried out in the sun, and continents that came together and drifted apart. The ground beneath us was shaped by processes like these over millions of years, and perhaps it is unsurprising that these epic shifts in the landscape should have left traces behind. But here and there, to our immense good fortune, the rocks yield something else: remains or traces of ancient life. If a good rock can be read like a good book, then a well-preserved fossil is like a finely wrought illustration, a vivid depiction of a fleeting moment in the life of the world. Fossils, whether on the scale of dinosaurs or individual molecules, provide our most decisive evidence for testing hypotheses about the abundance, diversity and evolution of life on Earth over the past three-and-a-half billion years. They are also our best hope for answering one of the most compelling questions in science: was there ever life on Mars?

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