Nature

How Life Began
Geological evidence suggests that life has existed on Earth for about 4 billion years of its 4.6-billion- year history - but how it began is a subject of great speculation. Most scientists believe the first life forms were probably self-replicating molecules, similar to DNA, that formed by chance early in the planet's history.

The Primitive Earth
Rocks from the Moon brought back for analysis by the Apollo astronauts, as well as observations of the cratered surfaces of other planets, show that the Solar System underwent an intense bombardment by meteorites between 4.1 and 3.9 billion years ago. This would have melted the Earth's surface, making it impossible for life to exist. But when the bombardment ceased it did not take long for life to gain a foothold.
Paleontologists have discovered accumulations of single-celled organisms, called cyanobacteria, in rocks formed only 200-400 million years after the bombardment era, and there is even evidence of biological molecules in the oldest known sedimentary rocks on the planet.

The primitive Earth was an inhospitable place. Its atmosphere consisted chiefly of hydrogen and other gases that today we regard as poisonous. There was widespread volcanic and earthquake activity, as well as gigantic electrical storms. The Sun was brighter than it is today, and its harsh ultraviolet radiation reached the planet's surface, which is now protected by ozone in the upper atmosphere. There were no oceans - only small pools of murky rainwater, cloudy with volcanic
ash.

The First Replicators
Most scientists believe that life began among the chemical reactions taking place on the young planet. Life probably arose from the increasingly complex chemicals that formed as simple compounds reacted together. The key event was the appearance of a molecule that could make copies of itself or self-replicate. In doing so it demonstrated one of the fundamental properties of life: the ability to reproduce.

In the Earth's early days, lightning and ultraviolet light from the Sun broke apart the simple, hydrogen-rich molecules of the primitive atmosphere. When this happened, the fragments recombined spontaneously to make larger and more complicated molecules. This process is thought to have produced a soup of increasingly complex molecules in lakes and oceans until, quite by accident one molecule appeared that was able to replicate, using the other molecules in the soup as its building blocks. This molecule was the earliest ancestor of DNA. All life on Earth today depends on the ability of DNA (or the related molecule RNA) to store information and replicate. The DNA and RNA in every living creature, from a virus to a whale, is thought to be descended from one of the first self-replicating molecules that appeared on the planet.

Scientists believe the first replicator was probably not a DNA molecule, because DNA cannot copy itself without using proteins, and proteins cannot
be produced without using DNA. This chicken- and-egg situation has led some to suggest that the first replicator was an RNA molecule, which can replicate like DNA, but which can also act as an enzyme, like proteins. It is also possible that the first replicator was something quite different from DNA and RNA that no longer exists.

The emergence of a replicating molecule represented the beginning of biological evolution. As the population of replicators grew, some were damaged by chemicals or by radiation from the Sun. In most cases these would have "died" but some were subtly changed, resulting in new types of replicators. Some of the variants would have been better at copying themselves, and would have become more common. Others learned to cooperate with one another, forming complex
colonies of molecules. Over time, these colonies are thought to have evolved into the first microorganisms.

The Stanley Miller Experiment
In 1953 the US chemist Stanley Miller (born 1930) performed a historic experiment to demonstrate how organic chemicals could have appeared spontaneously on Earth. In a glass jar he mixed the chemicals thought to have been in the Earth's atmosphere: hydrogen, methane, ammonia, water vapor, and liquid water. He then simulated lightning by generating an electrical discharge in the jar. After several days the mixture changed color. Upon analysis it was shown to contain some of the amino acids that are common in proteins today. Later experiments involving a slightly different mixture of chemicals produced some of the building blocks of DNA and RNA.

While these experiments show how simple organic compounds could have originated, the huge step to the appearance of a replicator remains a mystery to science.

Where Did Life Begin?
Research carried out since the classic Miller experiment suggests that large, complex organic molecules would be unlikely to form in a soup. A more likely site might have been the surface of rocks. There is some evidence that amino acids splashed onto clay surfaces from pools can combine to form long chain molecules. Scientists have called this the "primordial pizza' model for the origin of life, as it would have taken place in many small areas scattered over various types of rocks.

It is unlikely that life began on land, where conditions would have been harsh. Intense radiation from the Sun, for example, would have destroyed complex molecules as quickly as they formed. Some scientists believe that life began in vents of hot water on the ocean floor, where the key chemicals and energy would have been present and where the complex organic compounds would have been protected from the Sun.

Cells and Beyond
The first replicators were not nearly as complex as modern single-celled organisms, which are highly sophisticated forms of life. The smallest living things known today are minute viruses made of less than 10,000 atoms. These are so simple that some scientists do not classify them as living. They are thought to have evolved from fragments of more complex organisms, rather than from simpler organisms.

Scientists believe that the first cells may have been fat droplets in which organic compounds became concentrated, and in which the first replicators may have formed. Another theory is that tiny spheres formed from proteins in the organic soup, and replicators formed inside these. Other scientists believe that the first replicators were free-floating "naked genes" and that cells evolved later.

The first life forms were heterotrophs - organisms that obtain food from their surrounding. As the population of heterotrophs grew in number, they would have exhausted the supply of organic materials in their environment. Some variants might have overcome this by becoming predatory, evolving the ability to destroy and assimilate their rivals. Others evolved the ability to synthesize their own nutrients using energy (perhaps from the Sun) and inorganic chemicals in the environment. These were the first autotrophs, and they eventually evolved into the cyanobacteria - primitive bacteria that make food autotropically by photosynthesis. The cyanobacteria, which still exist today, made the oldest known fossils on Earth - 3.8-billion-year-old mineral deposits called stromatolites.

An important episode in the evolution of life was the evolution of sex, which seems to have emerged about two billion years ago. Before then, new types of microorganisms could only arise as a result of mutations, which meant that evolution was very slow. However, sex allowed organisms to exchange genes and combine them in new ways, which increased the rate of evolution dramatically.

All life on Earth can be traced back to the first self-replicating molecules - perhaps to a single molecule - assembled by chance when the planet was young. They arose with astonishing speed, perhaps only a few tens of millions of years after the surface of the Earth cooled down. Inside all our cells are their direct descendants.

The Theory of Evolution made easy

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