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3.5 billion years ago (bya)

Earliest life-forms—bacteria

3.4 bya

First stromatolites

565 million years ago (mya)

Ediacarans—first complex animals

545 mya

Mollusks evolve protective shells

520 mya

Explosion of ocean lite, including sponges, worms, and arthropods

480 mya

First vertebrate—a jawless fish

460 mya

First land plants

428 mya

First land animals—arthropods

420 mya

First fish with jaws

400 mya

First insects

375 mya

Tiktaalik marks transition between fish and tetrapods (four-legged creatures)

360 mya

First land vertebrates

315 mya

First reptiles, such as Hylonomus

295 mya

Sail-backed reptiles, such as Dimetrodon

245 mya

Marinę reptiles, such as nothosaurs and ichthyosaurs

228 mya

First dinosaurs, such as Eoraptor

200 mya

First, shrewlike mammals

151 mya

First known birds

130 mya

First flowering plants

122 mya

First feathered dinosaurs, such as Microraptor

65 mya

Mass extinction of many animal groups, including the dinosaurs

62 mya

Explosion of new mammal species

60 mya

First grasses

7 mya

Earliest hominins (humans), such as Sahelanthropus

1.8 mya

Homo habilis, a possible ancestor of modern humans

What is evolution?

Evolution is the gradual change over time that produces new species. Life on Earth began billions of years ago. Some organisms, because of their genetic make-up, have always been morę likely to survive and breed than others. This natural selection is the driving force of evolution —and it has led to today’s amazing diversity of species.

Fantastic fossil evidence

The coiled shells of sea ►creatures called ammonites evolved from ancestors with straight and curved shells.

Cynodonts were the first ►reptiles to have mammal features, such as hair and warm-blooded bodies, but they still laid eggs.

Charles Darwin found

►    the remains of an extinct mammal, Glyptodon, and realized that it was a relative of modern armadillos living in South America.

A Sahelanthropus skuli

►    known as "Toumai” belongs to an apelike human who lived around seven million years ago—about the time that chimps and humans parted company from a common ancestor.

Stromatolites - structures

►    madę up of bacteria and sediment - are the oldest-known fossils. Living stromatolites can be seen in the warm, shallow waters off the coast of Western Australia.

Ho w to:

evolve (over many generations)

02 ■ Grow slightly longer legs - thatll make you faster. Certain genes in your body will make this possible.

Get better at escaping predators. You need to be able to outrun animals that prey on primitive rabbits iike yourself.

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Growing apart

Asiatic and American black bears shared a common ancestor around four million years ago. They still look alike because of their similar habitat and diet.

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In North America, polar bears evolved from brown bears around 100,000 years ago. Brown bears eat plants as well as meat; polar bears are meateaters.

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