LIFE
The Oldest Traces of Life
~3.7 billion years ago (claims 3.95–3.5 Ga)
The oldest traces claimed for life, about 3.7 billion years old, are stromatolites, layered structures built by mats of microbes, and carbon with the chemical signature of living cells, and some claims go back 3.95 billion years.
In the oldest rocks, life leaves only two kinds of trace. The first is the stromatolite, a layered structure built by mats of microbes. The second is carbon that has passed through a living cell and kept a chemical signature of it. Carbon has a lighter isotope, carbon-12, and a heavier one, carbon-13. Isotopes are atoms of the same element with different weights. The enzymes of metabolism, the proteins that carry out the chemical reactions of a cell, take up carbon-12 in preference to carbon-13. Organic carbon, the carbon of living matter, therefore comes out measurably lighter than carbon that never entered a cell. Researchers have claimed both traces in rocks from Greenland and Labrador at least 3.7 billion years old. The earliest case that most researchers accept is younger. It comes from Australian rocks 3.5 billion years old, whose microbial structures still contain organic matter sealed inside pyrite, an iron sulfide mineral.
Why it matters
Life appeared within the planet's first billion years, and possibly as soon as the oceans were established. If the 3.7-billion-year-old traces are confirmed, simple life is the ordinary outcome on a rocky planet that has water.
Dating & uncertaintyDisputed
The oldest claims are contested because both kinds of trace, layered structures and light carbon, can be produced without life. Circulating hot water alone stacks minerals into layers. Certain inorganic reactions, in which no living thing takes part, sort carbon isotopes as cells do. The doubt is smaller for younger rocks, and most specialists accept the traces in rocks 3.5 billion years old.
Earth history as one day
04:26:2519 h 34 min before midnight
if Earth’s 4.54 billion years were compressed into a single day