When did life first begin on Earth?

No one knows the precise time that life began on Earth. One reason is that early life consisted of single-celled organisms. Because the soft parts of an organism are the first to decay and disappear after death, it is almost impossible to find the remains.

In addition, because the organisms were so small, they are now difficult to detect in ancient rocks. Some modern viruses are only about 18 nanometers (18 billionths of a meter) across and modern bacteria typically measure 1,000 nanometers across, which is much larger than the early organisms.

In addition, because scientists have found so little fossil evidence, it is difficult to know all the true shapes of the earliest life. Scientists believe that early life was composed of primitive single-cells and started in the oceans. The reason is simple: life needed a filter to protect it from the incoming ultraviolet energy from the Sun and the ocean waters gave life that protection.

Despite such gaps in knowledge, scientists estimate that the first life began about four billion years ago. These organisms did not survive on oxygen, but carbon dioxide.

What were the conditions on the early Earth that scientists believe may have led to life?

Two major theories explain how life could have grown on early Earth. The first theory states that life grew from a primordial soup, a thick stew of biomolecules and water. Chemical reactions were then triggered by the Suns ultraviolet rays, lightning, or perhaps even the shockwaves from violent meteor strikes that were more common at the time. These reactions produced various carbon compounds, including amino acids, which make up the proteins found in all living organisms. This theory was postulated after a famous experiment performed at the University of Chicago in 1954 by then-graduate student Stanley Miller (1930-2007), and his advisor, chemist Harold Urey (1893-1981). They showed that the amino acids could be formed from chemicals thought to exist in the early Earth atmosphere when they were combined with water and zapped by lightning.

One of the earliest forms of life to appear on Earth was cyanobacteria (inset), which have left behind unusual fossil rocks called stromatolites. (iStock).

The second theory of life centers around a discovery made within the last half century: hydrothermal vents, which are cracks caused by volcanic magma seeping through the deep ocean floor. There were probably many more hydrothermal vents during the early history of Earth, as the crust was newer, and thus thinner, than todays cooled, thicker crust. The organisms around these vents did not need to rely on photosynthesis for energy. Scientists know that todays volcanic vent organisms live off the bacteria around the vents, which in turn extract energy from the hot, hydrogen sulfide-rich water found around the sunless cracks in the ocean floor. Early organisms could have survived in much the same way.

In actuality, the conditions described by both theories could have existed simultaneously to produce the planets early life.

What are the oldest-known fossils found in rock on Earth?

The oldest-known fossils in rock have been found in Australia. One set of fossils found in Western Australia is dated between 3.45 and 3.55 billion years old. They show evidence of layered mounds of limestone sediment called stromatolites, which were formed by primitive microorganisms similar to blue-green algae called cyanobacteria.

Scientists know that stromatolites exist today. The fossils look amazingly like the stromatolites from the shallow waters off the coast of modern Australia.

There are other contenders for the oldest-known fossils. Tiny, simple cells have also been found in ancient cherts (crystalline-rich sedimentary rocks) from Australia, and there are similar ones in Africa. These cells are preserved by the silica from the chert, and appear to show a cell wall of some kind.

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