c. 3.8 Billion BCE

Life on Earth

No one knows exactly how, when, or why life first appeared on planet Earth, but we know that almost as soon as it could, it did. The oldest signs of life on Earth are chemical, not fossil, and are inferred as evidence because all known life on this planet is based on a common chemical architecture. Specifically, certain biogeochemical processes and reactions that are common to all life on Earth—reactions involving certain amino acids commonly associated with DNA or RNA, for example—create recognizable patterns in the isotopes of carbon and some other elements. Life prefers to use (and create) certain materials, in essence, and anomalous chemistries, like the occurrence of extra carbon-12 (¹²C) compared to carbon-13 (¹³C) in some 3.8-billion-year-old rocks from Greenland, provide circumstantial but controversial chemofossil evidence for life very early in our planet’s history.

The oldest known fossil evidence of microbial life on our planet is dated at around 3.5 billion years old and is preserved in the layers of ancient stromatolites, which are rock and mineral structures built up by colonies of simple organisms such as blue-green algae. Stromatolites still form in places such as Shark Bay in Western Australia, making them among the oldest life forms on our planet.

Recent studies of the very earliest period of Earth’s history, the Hadean (4.5–3.8 billion years ago), provide evidence that oceans and continents may have formed much earlier than previously thought, and that conditions may have been suitable for life just a few hundred million years after our planet formed. The Late Heavy Bombardment of 3.8 to 4.1 billion years ago may have killed off earlier life forms, or perhaps just frustrated their attempts to flourish. Whatever the case may be, soon after Earth’s crust cooled, the oceans formed, the late heavy bombardment ended, and Earth became stable enough to support life. The fact that it thrived and began to evolve into so many niches is remarkable. Now astronomers, planetary scientists, and astrobiologists are searching for evidence of life on other Earthlike worlds.

SEE ALSO Earth (c. 4.5 Billion BCE), Late Heavy Bombardment (c. 4.1 Billion BCE).

Cross-sectional view of a stromatolite fossil; the reddish layers are thought to be fossilized remains of the ancient blue-green algae that are some of the oldest preserved evidence for life on Earth. This particular piece, from the Ord Range of Western Australia, is about 2.4 inches (6 centimeters) tall.