Creatures from the Past

Written by Anne Rooney Illustrated by Kunal Kundu

Contents How did we get here?

Digging down Animals our ancestors saw Grasslands Mammals make their move Early birds The golden age of the dinosaurs Flying and swimming reptiles Shambling and sails Out of the water The start of four-legged animals Life in the sea The Cambrian explosion Getting started Still going Glossary Index Timeline

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How did we get here?

Living things come in all shapes and sizes, from giant whales to creatures too small to see. Each one has developed over time to be just right for how and where it lives. This process is called evolution. But the world changes too, and to stay just right, organisms have to keep adapting. Evolution is still happening – plants and animals continue to change.

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Starting out Everything living on Earth today has evolved from the tiny, simple organisms that started life 3,500Â million years ago.

Over time, organisms became different from each other.

Some lived in the sea and others moved to the land, eventually becoming the trees, plants and animals we see around us now.

Polar bears from brown bears Most bears are dark, but polar bears are white.

They evolved from brown bears at least 350,000 years ago, changing to live in cold, snowy conditions.

They grew a thick layer of fat and thicker fur to keep them warm.

Their fur became white, helping them creep up unseen in the snow, to capture the animals they eat.

Climate change puts polar bears at risk. They’re not suited to living in warmer temperatures.

They hunt animals that live in cold sea water. Will they evolve or die out?

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Charles Darwin’s theory of evolution In 1831, the naturalist Charles Darwin set off on a sea journey around the world. His job was to collect, study and draw the wildlife he saw. He developed a theory about how organisms change, which he called the theory of evolution. Darwin suggested that organisms change in small steps.

If a change makes it better suited to its environment, the organism will survive and breed, passing on the change.

Darwin’s finches Darwin visited a group of islands called the Galapagos.

He found a type of bird called finches that looked similar, but had differently-shaped beaks on each island. He decided that the finches had originally been the same, but slowly adapted to have beaks suited to the types of food available on each island.

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All change Changes build up over time, or sometimes happen quickly, so that eventually organisms look completely different.

Biologists study similarities and differences between organisms to understand how they evolved.

Today, we can use evidence from DNA to see how organisms are related to one another. Scientists draw branched diagrams, called cladograms, to show how different organisms are related through evolution.

No organism is better, “higher” or “more evolved” than any

house cat

leopard

wolf

horse

turtle

other, they’re all well suited to being themselves.

can purr has retractable claws has teeth for eating meat has hair has a backbone

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Digging down We know about animals and plants from the past because some have been preserved as fossils.

Bones and tails Fossils can form in different ways.

Body fossils form when an animal’s body is slowly turned to stone or replaced by stone through chemical changes.

Usually, only the hard parts of an animal survive as fossils. fossil of an archaeopteryx, an early bird-like dinosaur

feathers

beak

wings feet

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fossilised footprints of a dinosaur

Another type of fossil can form when an organism has pressed against a soft surface, such as mud, and made an imprint or trace. If the surface hardens over time, a fossil forms.

There are trace fossils of dinosaur footprints, and drag marks made by tails.

Sometimes, soft body parts or pieces of plant dissolve, leaving a hole in the surrounding sand or mud. If this fills with minerals that harden, it forms a natural cast. It can preserve the imprint of feathers, the texture of skin, or the shape of a plant’s leaves.

Mind the gaps Very few of the organisms that have ever lived have become fossils. There will have been many types of plants and animals that have never been fossilised, so we know nothing about them. But every year, more fossils are found and identified. Around 10 –15 new types of dinosaur are found each year, for example.

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Deeper and deeper If an organism dies in mud or sediment, such as the sludge of a river bed, it can be buried before it decays and then it might be fossilised. As more sediment collects above it, the fossil is buried deeper and deeper. The sediment hardens into rock under pressure. Because rock is laid down from above, the older fossils are buried more deeply than recent fossils.

Geologists study the rocks that make up the Earth’s crust.

The rocks form in layers, called strata. From the position of a fossil in the rock strata, experts can work out how long ago the organism lived.

Mountain-top fish Fossils of fish have been found in mountain ranges such as the Himalayas. The rock that the fossils formed in was once at the bottom of the sea.

All the Earth’s land and oceans are held on a thin crust of rock. It’s divided into plates, which move very slowly over the Earth’s surface.

In some places, they crunch together and push upwards.

Over millions of years, rock from the sea bed can be forced upwards to become mountains, carrying fish fossils with it.

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monkeys and apes

Cenozoic era: 66 million years ago to present day

recent mammals

reptiles amphibians jawed fish jawless fish

invertebrates

Paleozoic era: 542 million years ago to 251 million years ago

dinosaurs

Mesozoic era: 251 million years ago to 66 million years ago

early mammals

The geological column works as a timeline, showing fossils in layers of rock going back millions of years.

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