The first life on Earth

The story

How to search for the oldest life

Imagine looking for the footprint of an animal that walked through mud billions of years ago. The mud has become rock, and the rock has been squashed, heated, folded and worn away. That is what scientists face when they look for the first forms of life. Those first living things were very simple, very tiny cells, with no skeleton or shell, and they left no obvious fossils. That is why this subject is full of disputed evidence, and scholars often disagree.

One thing is clear, though: life appeared very early, and for nearly three billion years, from its beginning until about 600 million years ago, almost all living things were microscopic. Animals, plants and fungi arrived very, very late.

The rocks of the Pilbara

One of the pieces of evidence considered most solid today comes from Western Australia, from the Pilbara region, from a place called Strelley Pool (similar structures, about 3.48 billion years old, are also found at North Pole, again in the Pilbara). At Strelley Pool, in rocks about 3.43–3.45 billion years old, there are stromatolites, that is, layered structures, sometimes dome-shaped or cone-shaped. Stromatolites are built by colonies of microbes that form mats on the shore: the microbes trap grains of sediment or produce minerals, and layer by layer the colony grows. They are like small reefs made by living things.

How do we know that those of Strelley Pool were made by living things and not by chemical forces alone? In 2006 the geologist Abigail Allwood and her colleagues studied the shape of the structures, their position along an ancient coast and the make-up of the rocks, and concluded that a biological origin was the best explanation. Living stromatolites still exist today, for example in Shark Bay, also in Western Australia, and they allow comparison.

The oldest and most disputed evidence

Further back there are clues that divide scientists. In Greenland, in the Isua region, in 2016 a team led by Allen Nutman described cone-shaped structures in rocks about 3.7 billion years old, which they interpreted as stromatolites. Other researchers, a few years later, argued that they could have been formed by the deformation of the rocks. The debate is not closed.

Other clues are chemical. When microbes use carbon, they prefer the lighter kind, and they leave a signature in the rocks. Isua rocks about 3.8 billion years old have this signature, but it too can have non-biological causes. In 2015 a grain of graphite inside a 4.1-billion-year-old zircon, with the same signature, was announced. These traces are suggestive, but nobody considers them definitive proof.

In 2024 a team of researchers tried to reconstruct the last common ancestor of all living things, called LUCA, by comparing today's genes. According to their estimate it lived about 4.2 billion years ago and already had a complex system. The result is based on calculations and on the way of estimating how fast genes change, and it is not accepted by everyone.

How did it begin?

On this question science today knows less. We do not know how non-living matter became living. In 1953 the American chemist Stanley Miller, in the laboratory of his professor Harold Urey, carried out a famous experiment: in a glass container he put water and gases, which he thought were similar to those of the early atmosphere, and passed electric sparks through them, like lightning. After a few days some amino acids had formed, the building blocks of proteins. Today it is thought that the atmosphere was different, but the experiment showed that the building blocks of life can form through simple processes.

Several hypotheses are being studied today: hot springs on the seabed, pools of hot water on land, a world of RNA molecules able to copy themselves, materials brought by meteorites. Probably the truth will be a combination. Meanwhile we can say that life did not wait long: as soon as the Earth had cooled and become covered with oceans, and perhaps even earlier, the first microbes appeared.