Prehistoric creatures left seemingly endless proof of their existence fossilized in age-old rocks. Monstrous thighbones of lumbering dinosaurs that drowned in flooded rivers now lie encased in sandy mudstone. Jagged fronds of tropical ferns once growing in muddy swamps are pressed between jet-black layers of coal. Squiggly worm burrows, excavated within a slimy seabed, lace steel-gray limestone. These signs of life are unmistakably distinct from their stony tombs. But the more ancient the creature, the more obscure its grave.

Before life began walking, slithering or putting down roots, nothing much more than solitary microscopic cells populated the globe. Virtually all traces of those that lived before about 2.5 billion years ago--a period known as the Archean eon--have since become nearly indistinguishable from the rocks that entomb them. Millions of brutal years of burial and resurfacing, akin to repeated pressure cooking, permitted very few fossilized cells to survive in rocks accessible at the earth's surface today. Often geologists must instead rely on other signs of life, or biosignatures--including rather subtle ones, such as smudges of carbon with skewed chemical compositions unique to biology.

This article was originally published with the title Questioning the Oldest Signs of Life.

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