History of Life on Earth

The US Geological Survey gives Earth's age as 4.54 billion years, with an uncertainty of less than 1%. The oldest known rock, the Acasta Gneiss of Canada, is about 4.03 billion years old, and single zircon grains reach 4.3 to 4.4 billion years (see USGS).

Geologists split this span into eons, eras and periods, set out in the International Chronostratigraphic Chart (see ICS chart). Numbers this big are hard to picture, so the tool below shrinks all of Earth's history into one day or one year.

How species change over time is covered in Evolution. This lesson is about the timeline itself.

No one knows yet. Scientists test several hypotheses, and none is proven.

A soup of simple chemicals. In 1953 Stanley Miller sent electric sparks through methane, ammonia, water and hydrogen for a week, and amino acids formed (see Miller). A later reanalysis of his samples found 22 amino acids (see Scripps). Making the building blocks of proteins is not the same as making life.

Hydrothermal vents. At some deep-sea vents, warm fluids rich in hydrogen meet carbon dioxide. Martin and others argue that these vents, with their natural supply of chemical energy, are a likely place for life to have begun (see Martin and others).

The RNA world. RNA can carry information, and in 1982 RNA molecules were found that also act as catalysts (see Alberts and others). In 1986 Walter Gilbert described an early stage when RNA molecules catalysed their own copying, before proteins and then DNA took over; this is the “RNA world” hypothesis (see Gilbert).

For most of Earth's history, life was microbial. Microbes still matter today, as Microbiology and Immunity shows.

Stromatolites are layered structures built by communities of microbes. The 3.48-billion-year-old Dresser Formation of Australia holds the oldest generally accepted evidence of life (see Nutman and others). In 2016 Nutman and others reported 3.7-billion-year-old stromatolites from Isua, Greenland (see Nutman and others), but in 2018 Allwood and others argued the Isua structures formed without life (see Allwood and others). The oldest date is still debated.

Cyanobacteria make oxygen by photosynthesis, and they are responsible for oxygenating the air and oceans since the Great Oxidation Event around 2.4 billion years ago (see Demoulin and others). Rocks from South Africa show oxygen first rose about 2.43 billion years ago, then swung up and down for about 200 million years, still swinging after 2.32 billion years ago, and only stayed for good from about 2.22 billion years ago (see Poulton and others).

Cells with a nucleus are called eukaryotes. In 1967 Lynn Margulis, then publishing as Lynn Sagan, proposed that mitochondria, plastids and basal bodies were once free-living cells that came to live inside other cells (see Sagan). The paper was turned down by at least 15 journals before it was accepted (see Embryo Project).

Genetic tests later backed her up for mitochondria and chloroplasts: chloroplast DNA turned out to be cyanobacterial DNA, and mitochondrial DNA resembles that of a group of bacteria that includes the cause of typhus (see Understanding Evolution). How living things are sorted into domains and kingdoms is in Classification.

Large, many-celled life shows up late. The Ediacaran Period began about 635 million years ago (see ICS chart). Its rocks hold the first large fossils of soft-bodied organisms, called the Ediacara biota. How they relate to living groups is still unsettled, though trace fossils suggest wormlike animals were moving about (see UCMP).

The Cambrian Period began 538.8 ± 0.6 million years ago (see ICS chart). It is when most major groups of animals first appear in the fossil record, often called the Cambrian explosion. The fastest burst of new groups took about 13 million years (see UCMP).

Darwin was troubled that he knew of no fossil-rich rocks below the Cambrian (see UCMP). The Ediacaran fossils later filled part of that gap.

The best-known Cambrian site is the Burgess Shale in British Columbia, Canada (see UCMP). Charles Walcott, then Secretary of the Smithsonian, discovered it in 1909, and the Smithsonian now holds over 65,000 specimens from it (see Smithsonian).

The earliest spores of land plants date from about 475 million years ago, in the mid-Ordovician, and the oldest plant fossils large enough to see by eye from about 425 million years ago, in the Late Silurian (see Wellman and others).

Animals with backbones followed. Tiktaalik roseae, described in 2006, is a Late Devonian fish from Arctic Canada with features between fish and four-legged animals (see Daeschler and others). The Evolution lesson looks at it as evidence.

In 1982 David Raup and Jack Sepkoski counted fossil families of sea animals and found mass extinctions in the Late Ordovician, Late Devonian, Late Permian, Late Triassic and Late Cretaceous (see Raup and Sepkoski). These are often called the Big Five. The periods they close end at these ages (see ICS chart):

• Ordovician: 443.1 million years ago
• Devonian: the Late Devonian runs from 382.31 to 358.86 million years ago
• Permian: 251.902 million years ago
• Triassic: 201.4 million years ago
• Cretaceous: 66.0 million years ago

The end-Permian was the most severe loss of life in the last 542 million years. High-precision dating puts it between 251.941 and 251.880 million years ago, so it lasted about 60,000 years (see Burgess and others).

The end-Cretaceous. In 1980 Luis and Walter Alvarez and colleagues found far more iridium than usual in clay at the end of the Cretaceous, and proposed that an asteroid had struck (see Alvarez and others). In 1991 the Chicxulub crater in Mexico was put forward as the impact site (see Hildebrand and others). A 2010 review concluded that the Chicxulub impact triggered the mass extinction, while noting that volcanic eruptions in India were happening at the same time (see Schulte and others).

Mammals bounced back fast once the dinosaurs other than birds were gone. Fossils from Colorado show that within about 100,000 years of the end-Cretaceous extinction, the number of mammal kinds had doubled. By about 700,000 years after it, larger mammals had appeared, along with the first plants of the bean family (see Lyson and others).

Our own species is very young. Fossils from Jebel Irhoud in Morocco, dated to about 315,000 years ago, are early Homo sapiens (see Hublin and others). On the one-day clock that is 6 seconds before midnight.

Relative dating puts rocks in order without numbers. Steno's principle of superposition says that in undisturbed layers, the oldest are at the bottom. He also saw that layers are laid down flat, so tilted layers were moved later (see UCMP).

Radiometric dating gives ages in years. Some atoms decay at a steady rate, and the half-life is the time for half of them to decay (see USGS). After n half-lives the fraction of the parent left is (½)ⁿ, and the age is n × the half-life.

• Carbon-14: half-life 5,730 years, useful to about 50,000 years (see USGS)
• Potassium-40: half-life 1.25 billion years, decaying to argon-40 (see USGS)
• Uranium-238: half-life 4.47 billion years, decaying to lead-206 (see USGS)

In 1913 Arthur Holmes published the first geologic time scale (see Walker and others).

The story was pieced together from rock layers, fossils, chemistry and physics.