How Life Began

Earth first formed about 4.6 billion years ago. When the planet was very young, life would have been impossible—the ground was blisteringly hot and there was no water in sight. So how did life begin?


A sea of molten rock covered the newly formed Earth. In time, this cooled to solid rock, but volcanoes continued to spew out floods of lava. The volcanoes also released gases from deep inside the planet, forming Earth’s atmosphere, though the air at first was poisonous.



For millions of years, Earth’s surface was bombarded by comets, asteroids, and even small planets. The collisions tore open the planet’s newly formed crust, releasing more floods of lava. But they also delivered water.

Oceans form

Life cannot exist without liquid water. Today water covers 71% of the Earth’s surface

As the young Earth slowly cooled, so did its atmosphere. Scalding steam released by volcanoes condensed to form liquid water that fell as rain, producing a downpour that lasted as long as a million years. Comets and
asteroids brought yet more water. All the water pooled on the surface to form vast oceans.

A watery beginning

Many scientists think life began about 3.8 billion years ago in the deep sea, which was safer than Earth’s deadly surface. The first life-forms might have lived around hot volcanic vents, feeding off energy-rich chemicals dissolved in the boiling water. Special kinds of
bacteria thrive in these scalding habitats even today.

Life in hot water


At Grand Prismatic Spring in Yellowstone National Park, bacteria thrive in water too hot for any other organism to bear.

Copycat molecules

The first life-form was not a whole organism or even a cell—it was just a

Model of a DNA molecule

molecule that could make copies of itself. This is what DNA does today. DNA can’t copy itself outside cells, so the first living molecule must have been something different. Later on, it evolved into DNA.

The age of bacteria

Bacteria are single-celled organisms that are too small to see. Millions live on your skin and inside your body

Soon after life began, the self-copying molecules built cells around themselves and became bacteria. Bacteria were the only forms of life on Earth for the next 3 billion years, a vast span of time.

A true survivor – Stromatolites

Living stromatolites can still be found today – Shark Bay, Western Australia

Some of the oldest evidence of life on Earth comes from stromatolites. These are rocklike mounds formed by colonies of bacteria. Fossil stromatolites date back to 3.5 billion years ago. The bacteria in stromatolites live like plants, using the Sun’s energy to make food and in doing so releasing oxygen. Billions of years ago, they made enough oxygen to transform Earth’s air, paving the way for air-breathing animals to evolve.


Fossils of prehistoric animals show us that life on Earth is always changing. Over time, old species disappear and new ones develop from them, like new relatives appearing in a family tree. These new species appear thanks to a process of gradual change we call evolution.


Breeding frogs lay many hundreds of eggs, but only a tiny number will survive to become adults themselves.

Evolution is driven by a process called natural selection. Animals and plants produce more offspring than survive to adulthood, all of them slightly different. Nature selects those with the best characteristics, which then pass on these characteristics to the next generation.

The giraffe’s neck

Giraffes feeding from a tall tree


The giraffe’s long neck evolved because natural selection weeded out individuals that couldn’t reach food high in the trees. With each generation, the tallest giraffes got the most food and had the most babies. Over time, the species changed as its neck grew longer.


This picture is a result of Darwin’s researches

The most famous person to collect evidence for the idea of evolution was the English naturalist Charles Darwin. He visited the Galápagos Islands in the 1830s, where he found a range of similar finch species, each with a beak suited to its particular diet. He realized they’d all evolved from a common ancestor that had settled on the islands long ago.

An unpopular theory

Charles Darwin’s satirical portrait from 1871

People made fun of Darwin for his ideas; he was drawn with the body of a chimpanzee in 1871 when he proposed that humans were related to apes.


 Archaeopteryx had feathers but also teeth, claws and a tail like those of dinosaurs (above); Fossil remains of Archaeopteryx (below);
Archaeopteryx had feathers but also teeth, claws and a tail like those of dinosaurs (above); Fossil remains of Archaeopteryx (below);

One reason Darwin was ridiculed is that the fossil record is much too sparse to show a process of gradual change. However, some key fossils show clear links between related animal groups. One example is Archaeopteryx—a missing link between dinosaurs and birds.



In a few rare examples, we can see gradual evolution in fossils. The elephant belongs to a group of animals called proboscideans. Over time, proboscideans became larger and developed larger tusks and trunks. But the ancient animals shown here may not be direct ancestors of the elephant—they are merely glimpses of parts of the elephant’s large and hidden family tree.

Artificial selection

All domestic dogs today around the world have a common ancestor in the wolf

Darwin realized that animal breeders change their breeds using a process very similar to natural selection. Instead of letting nature choose which animals will breed, breeders make the choice themselves.

Darwin called this artificial selection. All dog breeds were created this way from their wild ancestor, the wolf.


Timeline of Life

Earth’s history stretches back 4.6 billion years to our planet’s birth. Scientists divide this vast span of time into different periods, such as the Jurassic Period, when many of the dinosaurs lived. Here you can see all the periods on a timeline showing the history of life.


The past leaves clues buried in the rock below our feet. Certain types of rock build up in layers (strata) over millions of years. Different layers correspond to different periods in Earth’s history.


The Grand Canyon – Arizona, U.S.A.

The different periods in Earth’s history are named after the layers of rock in which fossils are found. At the Grand Canyon, you can see these ancient rock layers, which get older toward the bottom.


It’s divided into very long stretches of time called eras. These are further divided into shorter stretches called periods, such as the Jurassic and Triassic:


Paleozoic Era

CAMBRIAN: 542–488 million years ago

Trilobites scuttled around on the seafloor

ORDOVICIAN: 488–444 million years ago

Starfish (sea stars) became common in the sea

SILURIAN: 444–416 million years ago

Pseudocrinites anchored itself to the seabed in the late Silurian.

DEVONIAN: 416–358 million years ago

Dunkleosteus, a giant predator, terrorized the seas

CARBONIFEROUS: 358–299 million years ago

Dragonflies and other insects buzzed through the air

PERMIAN: 299–251 million years ago

Dimetrodon was the most fearsome predator of its time


Mesozoic Era

TRIASSIC: 251–200 million years ago

The first dinosaurs appeared, one of the earliest known being Herrerasaurus.

JURASSIC: 200–145 million years ago

The earlies known bird, Archaeopteryx, appeared

CRETACEOUS: 145–65 million years ago

The first mammals were small, mouselike animals


Cenozoic Era

PALEOGENE: 65–23 million years ago

One of the earliest known primates, Eosimias, appeared in this period

NEOGENE: 23 million years ago

Our apelike ancestors began walking

All About Fossils

Almost everything we know about prehistoric animals comes from fossils. A fossil is the preserved remains or trace of an ancient animal or plant. The word “fossil” comes from the Latin word fossilis. That means “dug up,” and that’s how some fossils are discovered, although most are exposed by erosion. Most fossilized animals have lain buried for millions of years.


Complete fossilized skeletons are rare, but when found they provide a huge amount of information for fossil hunters (paleontologists).


Fossils can be sorted, or classified, into different types, depending on how they formed. All take millions of years to form—fossilization is not quick. Total preservation: If an insect or spider was caught in the sticky sap released by a tree such as a pine, it may be preserved complete. Creatures that are millions of years old have been preserved in fossilized tree resin (known as amber) in this way.

A spider caught in amber


Dinosaur mineralized fossils

Dinosaurs, like us, had hard bones, and sometimes just these parts of an animal are preserved—though not as bone, which is replaced over time with minerals to form rock. Rock has to be carefully removed to expose the fossils.


Tree trunks, just like bones, can be turned to rock by mineralization over millions of years. Petrified trees still look like logs. Petrification means “change to stone.”

External mold

Sometimes the original organism dissolves completely, but leaves an impression of itself in the rock. This impression is called a mold. Natural cast This forms just like an external mold, but the hole then fills in as minerals from water slowly crystallize inside it, forming a rock such as flint.

Trace fossil

Occasionally an animal will leave a hint of its presence: a trace. This may be a footprint, a nest, tooth marks, or even droppings. These are called trace fossils.

What makes a fossil?

All kinds of living things have been discovered in fossil form. We have unearthed fossilized animal skeletons, skin impressions, footprints, teeth, animal droppings, insects, and plants. The hard parts of an animal, such as the bones, are the parts that fossilize best.



A fossil can only form if an animal’s body is buried quickly after death, so fossilized animals are animals that have died in a river and have sunk into mud, for example, or those that have died in a sandstorm and been buried in sand. These five diagrams show one way a dinosaur’s bones—in this case a Baryonyx—may be fossilized and found millions of years later.


A paleontologist analyizing analyizing a dinosaur’s fossil

People who study fossils are called paleontologists. Paleontologists may work in the field, digging up new fossils, or in labs or museums. They work like detectives, carefully gathering as many clues as they can to find out what happened in the past and to figure out where each new discovery fits in the tree of life.


■ Fossils are usually found in rock but may also be found in mud or gravel.

■ The parts of an animal most likely to fossilize are the hard parts: the bones or teeth or a creature’s shell.

■ Teeth are among the most commonly found fossils.

■ The oldest fossils are stromatolites (mounds of rock made by sea-dwelling bacteria). These have been dated to 3.5 billion years ago.

Changing Planet

Planet Earth is always changing. Areas of land (continents) move slowly around on Earth’s surface, changing the map of the world. The climate swings from warm to cold, and the plants and animals change from one era to the next, sometimes dramatically. Scientists divide the age of the dinosaurs into three periods, all of which were very different from today’s world.

Triassic Life

251–200 million years ago

In the Triassic period, Earth’s land formed a single continent called Pangaea. The coast and river valleys were green, but much of the interior was desert. There were no flowering plants; instead, tough leaved plants such as cycads (a palmlike tree), ginkgos, horsetails, and conifers flourished (all of which are still with us). Early dinosaurs included Herrerasaurus, Plateosaurus, Chindesaurus, Coelophysis, and Eoraptor.

The Triassic saw the planet’s first dinosaurs, all fairly small, like this Coelophysis. They lived in a hot, largely barren world


Pangaea began to break up during the Triassic, with the Tethys Sea pushing between its two halves.

Jurassic Life

200–145 million years ago

Pangaea broke into two continents around 200 million years ago, with oceans spreading over what had been land to create enormous shallow seas. The Jurassic saw the emergence of giant, plant-eating sauropods (such as Brachiosaurus and Diplodocus) and large predators (such as Allosaurus). Lush forests spread across the land and the deserts shrank. Common plants included conifers, monkey puzzle trees, and ferns.

The Jurassic enjoyed a slightly milder climate than the Triassic. Dinosaurs thrived and reached enormous sizes


Pangaea split into Laurasia in the north and Gondwana in the south, with shallow seas between.

Cretaceus Life

145–65 million years ago The continents continued to break up during the Cretaceous. As a result, dinosaurs on different continents evolved in different ways, giving rise to many new species. Tyrannosaurus emerged, as did Triceratops and Iguanodon. Flowering plants appeared; early species included magnolias and passion flowers. Dense forests contained trees we know today, such as oak, maple, walnut, and beech.

The Cretaceus was cooler still, although it was warmer than today’s world. Dinosaurs ruled the land but pterosaurs and insects ruled the air (Ankylosaurus)


The continents began to resemble those we recognize today during the Cretaceous period.



Today Earth’s land is divided into seven major areas that we call continents: Europe, Africa, Asia, North America, South America, Antarctica, and Australasia. All the continents are still moving, but very slowly—at about the speed your fingernails grow.

Fossil Hunting

You may have seen a fossil hunt on television, or you may have visited a fossil site. Perhaps you have been lucky enough to find your own fossil. What happens on an organized fossil dig?


An Ouranosaurus exposed at Royal Ontario Museum – Canada

Every dinosaur dig is different. Some fossils are found embedded in solid rock that needs to be chipped away bit by bit. Others fall out of soft, crumbly cliffs and can be very fragile, falling apart easily. The dinosaur above was found buried in the desert sand and was easy to dig out by hand.

A fossil hunter’s toolbox


Scientists who study fossils are called paleontologists. Paleontologists use basic digging tools to remove fossils from the ground, such as hammers, chisels, and trowels. Brushes help sweep away dust.


Paleontologists classify the dinosaur fossils they uncover in one of four ways.
Articulated skeleton. This is a skeleton that is still joined together. It may be complete, but pieces are usually missing.
Associated skeleton. This means the bones have broken up and spread out, but they can be identified as belonging to the same dinosaur.
Isolated bone. This is a bone that has been separated from its skeleton, and fossilized alone. It may be a leg bone such as a femur (thigh bone), which is a large fossil.
Float. These are scraps of fossilized bone—the fossil has shattered, and the scraps are usually too small to be useful.



Once the paleontologists have carefully removed all dirt from around each of the fossilized bones, the position of each bone is carefully mapped on graph paper, with the help of a square grid called a quadrat.



The excavation of two dinosaur fossils, Afrovenator (a theropod) and Jobaria (a sauropod) has started in Africa. The bones were first discovered by local tribesmen, who found them jutting out of desert rock. It can take many months to excavate a complete dinosaur find, and this dig was no exception.

MAKING A START. Painstaking work over a number of weeks to remove rock finally revealed each fossil. A large team of people worked on this dig.

ON SHOW. As more soil is removed, the fossils become clear. The team was dealing with a theropod that could reach 30 ft (9 m) and a sauropod that could reach 60 ft (18 m) in length, so the bones were large.

SITE MAP. One paleontologist made a final, detailed drawing of the bones in position. This showed clearly how some bones had separated from the animal over the millions of years it had lain encased in rock.

WRAP IT UP! Once the bones were ready to be removed, they were covered with bandages soaked in a plaster solution. When the plaster sets hard, this protects the fossil, ready for its removal to a museum laboratory for further study.

So many bones…

Dinosaurs bones founded at Dinosaur National Monument – Utah-Colorado border, U.S.A

One quarry has yielded far more dinosaur bones than any other. From 1909 to 1924, 385 tons (350 metric tons) of dinosaur fossils were removed from the Dinosaur National Monument on the Utah-Colorado border.
That’s a lot of bones!

A Look at Dinosaur Sizes

From dinosaurs the size of chickens to lumbering sauropods, the animals that have walked and swum on Earth have varied enormously in size and shape and length. Let’s take a look at a few examples:

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* The sizes shown in the gallery are the maximum these animals are known to have reached. The artwork is not perfectly to scale, but it gives an idea of what these animals may have looked like if they could have been brought together.

Biggest killer on land?


The dinosaur Spinosaurus is the largest known land dwelling meat eater. It was 50 ft (16 m) long and weighed 13½ tons (12 metric tons).

Largest flying animal?


Hatzegopteryx was a pterosaur—a kind of flying reptile. Its wingspan was about 36 ft (11 m), making it as big as a small aeroplane. For comparison, the bird with the largest wingspan today is the wandering albatross, with a span of 12 ft (3.6 m).


The blue whale

The blue whale is the world’s largest living animal. Its heart alone is the size of a small car.

Smallest dinosaur?


Pigeon-sized Anchiornis is the smallest known prehistoric dinosaur. The Cuban bee hummingbird is the smallest living dinosaur.

Largest land animal?

Amphicoelias (the largest dinosaur in the picture)

The largest animal ever to walk on Earth might have been a dinosaur called Amphicoelias (am-fee SEE-lee-ass). More than a century ago, a single backbone of Amphicoelias was found. It was drawn and described, but then mysteriously disappeared. The description suggests Amphicoelias was an incredible 130–196 ft (40–60 meters) in length and 135 tons (120 metric tons) in weight.


What Are Invertebrates?

From insects to mollusks, and from worms to jellyfish, invertebrates dominate our planet in terms of their numbers: they make up around 97 percent of the animal kingdom. What features do these animals share in common? Very few! However, they are animals that possess neither a backbone, nor a bony internal skeleton.
Invertebrates are divided into about 30 groups. They include:


Imperial scorpion

The group Arthropoda includes insects, arachnids (creatures such as spiders and scorpions), and crustaceans. Arthropods make up the largest group of invertebrates, and account for about 90 percent of known animal species.


A centipede has at least 15 pairs of legs. Centipedes are carnivorous, hunting prey such as insects and spiders.


There are more than 300,000 species of beetle, some very brightly colored.



From a small garden snail to a giant squid, the group that forms Molluska is incredibly varied. Most mollusks have a shell, or at least the remnants of one, but not all—octopuses have no shell, and neither do slugs.


These marine mollusks are often called “sea slugs.” The infant form (larva) has a shell.


These are large snails—they can reach 8 in (20 cm) in length.




Annelid worms have bodies that are divided into segments. Earthworms and bristleworms are types of annelid worm. Members of this group can be found living in seawater, in fresh water, and on land. Amazingly, there are more than 12,000 recognized species of annelid worm.


These creatures have a pair of swimming legs (called parapods) on each segment of their bodies.


Some leeches, like this one, will wait for a passing animal and feed off that animal, sucking its blood.


Sea anemone

This group includes sea jellies (also known as jellyfish), corals, and sea anemones. They have stinging cells called nematocysts. Some can swim, while others remain fixed to the seabed, waiting for food to drift past.


Many corals are named for their appearance, like this heavily wrinkled coral.


Jellyfish, such as these sea nettles, have bodies that are largely made of water. Take a jellyfish out of water, and the shape will collapse.



Sunflower sea star

Many echinoderms have very spiny bodies and nearly all live on the seafloor—none can survive in fresh water. They include starfish (sea stars), sea urchins, and sea cucumbers. Most of these animals can move around, and they have up to 20 legs—but no brain.


These echinoderms are found on seabeds all over the world.


This is the largest starfish and a voracious predator, feeding on corals. It has needle-sharp spines, each capable of injecting a nasty venom.



Porifera are also called sponges. They were mistaken for plants until the 1700s but, in fact, are very simple animals, with no arms, legs, heads, or sense organs. Sponges have simple baglike or tubelike bodies and live stuck to the seafloor, filtering food from the water.


There are thousands of sea sponges, some very colorful.


Some sponge species can grow quite large.




Most invertebrates leave the egg as a larva and undergo several developmental stages before reaching adult form. This is known as metamorphosis.


After hatching from an egg, a butterfly caterpillar proceeds to eat and eat and eat. Its job is to grow quickly.


A tough, leathery coat forms around the caterpillar, and it becomes a pupa. After some time, a butterfly will break free.


Finally, the butterfly emerges. It has to spread out its wings to dry before it can fly. A butterfly is the adult form.

The First Animals

Fossils tell us that animal life began about 600 million years ago. The first animals lived in darkness, rooted to the seabed, and had simple, soft bodies shaped like disks or leaves, with which they gathered nutritious chemicals or particles in the water. These strange beings seem to have had no legs, no heads, no mouths, no sense organs, and no internal organs.

First life

Living stromatolites in Shark Bay, Australia

For nearly nine-tenths of the Earth’s history, there were no animals or plants. During most of this early era, called the Precambrian period, the only life forms were microscopic single cells. Some grew in colonies on the sea floor, building up over time to form cushion-shaped mounds of rock – “stromatolites” – that still form today.


ANCHOR: Some fossil of Charnia have a stem with a disk at the base. These disks, buried in the sandy seabed, may have been anchors that held Charnia fixed in place while the feathery top waved about in the current.

Discovered by a schoolboy in 1957, Charnia caused a sensation because it came from rocks thought far too old to contain animal fossils. It had a feather-shaped body and lived rooted to the seafloor by a stem, perhaps feeding on microbes filtered out of the water. Its main body was made of rows of branches that gave it a striped, quilted appearance. Some experts think its body might have housed algae that made it green and allowed it to gather energy from sunlight (photosynthesis).

Fossil location: England, Australia, Canada, Russia

Habitat: Seafloor

Length: 6 in–6. ft (0.15–2 m)

Spriggina (sprig-EEN-a)

SEGMENTS: Fossils show that Spriggina’s body was made of segments. Most fossils are curved in different ways, suggesting it had a flexible body.

Spriggina may have been one of the very first animals with a front and back end. It may even have had a head with eyes and mouth, suggesting it was one of the first predators to exist. Some scientists think it may have been an early trilobite. Others liken it to worms.

When: 550 million years ago (Late Precambrian)

Fossil: location Australia, Russia

Habitat: Seafloor

Length: 1. in (3 cm)

Dickinsonia (dickin-SO-nee-a)

Dickinsonia fossils are usually oval, with what look like segments extending from a central groove. Hundreds of fossils have been found, with a huge variety of sizes.

One of most baffling Ediacaran fossils is Dickinsonia—a flat, round organism that appears to have had distinct front and back ends but no head, mouth, or gut. Studies suggest Dickinsonia lived fixed to the seafloor, perhaps absorbing food through its base.

When: 560–555 million years ago (Precambrian)

Fossil location: Australia, Russia

Habitat: Seafloor

Length: 3⁄8–39 in (1–100 cm)

Cyclomedusa (cy-clo-med-OO-sa)


Mysterious Cyclomedusa was originally mistaken for a jellyfish because of its circular shape, but neighboring fossils are often misshapen, as though growing around each other on the seafloor. Some scientists think Cyclomedusa was just a colony of microbes or the anchor for the stalk of a bigger creature.

When: 670 million years ago (Precambrian)

Fossil location: Australia, Russia, China, Mexico, Canada, British Isles, Norway

Habitat: Seafloor

Length: 1–12 in (2.5–30 cm) across

Parvancorina (PAR-van-coe-REE-na)



Parvancorina had a shield-shaped front end that may have been a head and that faced into the current when it was alive. It also had a central ridge flanked by what look like segments. Many fossils have a well-preserved shape, suggesting that its body had a hardened outer casing.

When: 558–555 million years ago (Precambrian)

Fossil location: Australia, Russia

Habitat: Seafloor

Length: 3⁄8–1 in (1–2.5 cm)


In 1946, a scientist named Reg Sprigg was eating a packed lunch in the Ediacara Hills of Australia when he spotted what looked like jellyfish fossils in the rocks. He’d discovered something amazing: the oldest animal fossils in the world. One was named Spriggina, after him, and all the fossils from the period are now called Ediacaran fossils.