Life made its appearance relatively early in Earth's 4.5-billion-year history.
Researchers from the University of Tokyo, Yuji Sano and Tsuyoshi Komiya, have found evidence of life existing on Earth as far back as 3.95 billion years ago. Their discovery, published in the journal Nature, was made in Northern Labrador, Canada, where rock from Earth’s early history is known to have survived. It was there that a deposit of mineral graphite was found.
Altas Obscura explains the significance of the find:
Graphite can form when decaying organic material is heated to hundreds of degrees, and mineral formed in this way has a distinctive chemical clue that it was formed from once-living material. Graphite is made of carbon, and if the isotope carbon-12 is present in particularly high concentrations, compared to carbon-13, it’s a sign of organic material.
After examination, the researchers found that this blend of carbon isotopes found in crystals of the mineral graphite suggests that microbes were living and using carbon dioxide to build their cells. If they are correct then this Canadian graphite may be the oldest evidence of life on Earth.
The Atlantic uses the calendar as an excellent analogy to explain how old this graphite is:
The Earth was formed around 4.54 billion years ago. If you condense that huge swath of prehistory into a single calendar year, then the 3.95-billion-year-old graphite that the Tokyo team analyzed was created in the third week of February. By contrast, the earliest fossils ever found are 3.7 billion years old; they were created in the second week of March.