An animal is a living being that can react to the surrounding world with its senses, typically seeing with eyes, hearing with ears, and feeling with nerves. All animals experience emotions as well.

For billions of years, life on Earth consisted of tiny unicellular organisms. The first cells – the smallest units of life – came into being in the hot oceans of early Earth and hung on in freezing waters when our planet became entirely covered by kilometres of thick ice and snow (Snowball Earth 635 MYA).

When glaciers eventually melted, life began to grow and diversify. Cells grouped together like never before, and the first animals came into being some 600 million years ago (plants and fungi probably earlier). Some animals remained small, while others increased in size. They varied in shape more and more. A few animals developed a backbone (vertebrates¹), yet most survived without one (invertebrates²). Some became predators, others prey. But for a long, Earth remained an underwater world. Its rocky surface was still too toxic for life.

As the ozone layer³ began to form and protect organisms from the lethal radiation of our Sun, life started to occupy the barren ground. At first, pioneering bacteria, lichens, and fungi took over the surface, creating fertile soils for plants to live upon around half a billion years ago. Grim land began soon to turn green – habitable for animals, too.

More than 400 million years ago, whether or not looking for fresh food, invertebrates eventually laid their many tiny feet on the shores and began exploring the new habitat. Meanwhile, in water, some fishes turned fins into arms and legs, growing fingers and toes, soon enabling the first vertebrates to crawl onto dry land as well.

Within the following hundred million years, marine life became plentiful, and the first sharks began to prey underwater. Plants grew into trees, forming the first extensive forests where salamander-like animals took initial steps on four limbs before evolving into amphibians, reptiles, and mammals. Ancestors of spiders, millipedes and scorpions roamed in vegetation alongside giant insects, the first ones to spread their wings up into the air more than 300 million years ago, long before birds and bats.

¹ Vertebrates are animals that have a backbone or a spinal column. Vertebrates include mammals, birds, reptiles, amphibians, and fish.

² Invertebrates are animals that do not have a backbone or a spinal column. They make up the majority of animal species on Earth, with an estimated 90% of all animal life being invertebrates. Some examples of invertebrates include insects, spiders, scorpions, worms, jellyfish, millipedes, and snails.

³ The ozone layer is a layer of ozone molecules in the Earth's atmosphere at an altitude of 15 kilometres (in the stratosphere). It protects life on Earth by absorbing most of the sun's ultraviolet (UV) radiation. This prevents harmful UV rays from reaching the surface of the Earth and causing damage to living organisms.

Bibliography

Panciroli, E. 2022. The Earth a Biography of Life The Story of Life On Our Planet Through 47 Incredible Organisms. London, United Kingdom: An Hachette UK company. 255 p. ISBN 9781529413984. Pages 28-30, 35, 38-39, 46, 60-67, 72-81, 86-107.

Brusatte, S. 2019. The Rise and Fall of the Dinosaurs. London, United Kingdom: Picador. 404 p. ISBN 9781509830091. Pages 20-21.

Brusatte, S. 2022. The Rise and Reign of the Mammals. London, United Kingdom: Picador. 500 p. ISBN 9781529034233. Pages 9, 42-43.

Frankopan, P. 2023. The Earth Transformed an Untold History. London, United Kingdom: Bloomsbury Publishing. 695 p. ISBN 9781526622563. Pages 27-28.

Dartnell, L. 2019. Origins How the Earth Shaped Human History. London, United Kingdom: Penguin Random House. 346 p. ISBN 9781784705435. Pages 173, 261.

National Geographic Education. Ozone Layer. Available at: https://education.nationalgeographic.org/resource/ozone-layer/ (Accessed: 2 October 2023).

Next Story

200 MYA
From one continent to many

Our planet is restless, constantly changing its face. The crust of the Earth is cracked into large rocky pieces – tectonic plates – floating around on a slowly flowing but solid rock beneath them. Continents laying on tectonic plates get together only to be ripped apart, while new oceans open before shrinking and disappearing all alone.

Previous Story

635 MYA
Snowball Earth

Ice on our planet is more or less uncommon, but once in a while, temperatures on Earth fall low enough, and kilometres of thick glaciers begin to form on its poles. Sometimes, they spread even further and cover more and more of the surface. These periods are called ice ages. The first ice age appeared on Earth more than two billion years ago, but the most severe one occurred hundreds of millions of years later, an episode threatening the birth of all complex life forms.