All life on Earth sprang from tiny single-cell organisms that emerged around 3.5 billion BC.
All life on Earth sprang from tiny single-cell organisms that emerged around 3.5 billion BC.
No one yet knows how they were created. Millions believe God created them.
Others believe life arrived in a cloud of dust from a passing comet, or even that it was planted on Earth by aliens.
Whether or not you believe in God, it is possible to arrive at a scientific explanation. Producing living cells from nothing would have needed a remarkable chain of chemical events brought about by the conditions on primitive Earth.
The planet was an extremely inhospitable place by human standards.
Volcanoes erupted all over the surface. It was constantly hit by lightning and battered by space debris.
Earth’s atmosphere had more hydrogen than oxygen. It had copious amounts of carbon dioxide, water vapour, methane and ammonia.
When the heat from the volcanoes and lightning mixed with these chemicals, it produced simple organic carbon-based chemicals essential for life.
Scientists have tested this by simulating Earth’s early atmosphere in laboratories and passing sparks through it, producing amino acids and the components of nucleic acids (DNA is a nucleic acid).
Billions of years ago these chemicals are thought to have become concentrated in the lakes and tidal pools which formed as water vapour from Earth’s volcanoes condensed.
The result was a rich primordial “soup”. It is believed that the effect of heat and the sun’s ultraviolet rays condensed these simple molecules into more complex ones resembling proteins and nucleic acids.
Some of these must have been capable of replicating themselves. From these, the first living cells arose.
From that point, Charles Darwin’s theory of natural selection kicks in.
Only those cells which could adapt best to the surroundings survived.
The first cells developed into bacteria and algae, a lower form of plant life.
Some of the world’s oldest fossils, found in Transvaal, South Africa, and dating back 3.1 billion years, have traces of both.
They were to prove crucial for the development of all other life. For they fed themselves by a process called photosynthesis, which uses the sun’s energy to produce carbohydrates from carbon dioxide and water — giving off oxygen.
Over millions of years these tiny organisms transformed Earth’s atmosphere into one based on nitrogen and oxygen.
A layer of ozone, a form of oxygen, built up around the planet and shielded it from the sun’s ultraviolet rays.
Around 1.4 billion years ago, more complex cells evolved and led to more complex creatures.
Some 600 million years ago the only life on Earth was still in the oceans.
But the oxygen level gradually grew and sea creatures evolved that were capable of breathing oxygen in the water.
By 400 million years ago there was enough oxygen for animals to venture on to land and breathe the air.