It's hard to imagine the world not full of life—everywhere we look there are signs of it, from busy colonies of ants to streams of stressed commuters. But how did life on Earth first begin?
If you could travel back 4 billion years in time, you would find Earth a very different place. You would not be able to breathe because the air would be filled with poisonous gases, and you would not be able to eat because there would be no plants or animals. Most of the ground would be bare rock. But out in the oceans, the first signs of life would be stirring…
How did life get started?
Life probably began as a series of chemical reactions between substances in the sea and atmosphere. Scientists have tested this idea by re-creating the conditions of early Earth.
In one experiment, they put water in a glass container to imitate the sea and replaced the air above it with the gases they think were in the atmosphere 4 billion years ago. Then they passed electric sparks through the gases, to imitate lightning.
Afterwards, the water was found to contain some of the complex chemicals that make up living things.
Where did the gases on Earth come from?
Most of the gases, including methane, hydrogen and ammonia, came from inside Earth and were released through volcanoes.
Four billion years ago, there were many more volcanoes on Earth than there are now, and they were erupting all the time because the centre of the planet was intensely hot.
What were the first life forms?
The first life forms almost certainly appeared in the oceans and were a type of microscopic bacteria, each consisting of just a single cell (your body, on the other hand, is made up of trillions of cells).
Critically, these bacteria were able to reproduce themselves.
What is the earliest evidence of life on Earth?
The earliest evidence of life—traces of bacteria in ancient rocks found in Western Australia—dates from about 3.5 billion years ago, about 800,000 years after Earth formed.
However, some scientists think life may have emerged even earlier, perhaps around 4.3 billion years ago, soon after Earth’s atmosphere began to form.
How did the first life forms develop?
The first bacteria grew by using chemicals in the sea. As those chemicals ran out, it became harder for them to survive. But another kind of bacteria then appeared, which learned to develop in a different way, by photosynthesis. This involves using the energy in sunlight to make food, and it produces oxygen as a waste product.
So, as this new gas began to build up, Earth’s atmosphere changed, forming the kind of air we breathe today and creating opportunities for other life forms to develop.
Why didn't life form this way on other planets?
It may have done, but so far we don’t know of such a planet. In our Solar System, Earth is the only planet suited to life. The main thing that makes it so is water. No other planet we know of has the huge oceans we have, nor the many lakes and rivers.
Water is a very important ingredient of all living things because it has many properties that no other substance has. All the important reactions that occur inside our cells require water, and all the organs in our bodies, such as the heart and lungs and liver, rely on being surrounded by water.
Without water, life as we know it could not go on. It’s possible that there could be a type of life form that doesn’t depend on water, but so far scientists have not worked out what that could be.
When did the first fish appear?
Evidence for the first fish-like creatures dates back to about 470 million years ago. These were also the first creatures with a backbone—they were the first vertebrates, in other words.
The earliest fish were jawless forms, such as the eel-like conodonts and the armoured ostracoderm. Appearing about 420 million years ago, the earliest known jawed fishes were the placoderms, another kind of armoured fish.
Which were the first animals to live on land?
Among the first animals to appear on land, about 450 million years ago, were primitive insects, spiders, centipedes and millipedes.
About 370 million years ago, some fish developed legs and began to live partly on land and partly in the sea—they were amphibious, in other words.
The earliest creatures of this type, such as Hylonomus, probably resembled modern salamanders.
Why did some animals and plants disappear?
Every so often in the fossil record, large numbers of plants and animals disappear abruptly. Such mass extinctions were usually the result of massive volcanic eruptions, ice ages, asteroid impacts or other sudden environmental changes.
Major extinctions have occurred several times in Earth’s history. About 445–440 million years ago, for example, about three-quarters of all species on Earth vanished, probably as a result of falling temperatures and sea levels associated with the onset of an ice age.
Similar events took place about 365 million years ago; about 245 million years ago, just before the first dinosaurs; about 208 million years ago, wiping out many of the dinosaurs’ rivals; and 66 million years ago, killing off the dinosaurs themselves.
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