History Of Animals On Earth Throughout Geological Ages
For most of Earth’s 4.6-billion-year history, the only living things on the planet were microscopic organisms such as bacteria. Then 600 million years ago a different form of life appeared—the first simple animals. They originated in the oceans because ocean water contains all the chemicals needed to make the complex substances vital to life. But some 430 million years ago the earliest land animals appeared, and since then animals have managed to colonize almost every habitat on Earth.
Scientists divide the history of Earth into eras, which are then further divided into smaller time spans called periods. This is the basis of the timeline shown here, measured in millions of years ago (MYA). During this immense span of time, the processes of evolution and extinction have created and destroyed an astonishing diversity of animal life.
PRECAMBRIAN (4.6 billion to 541 MYA)
Throughout much of this vast stretch of time, the only life forms were single-celled organisms. Eventually, these began forming colonies, and about 600 million years ago some colonies evolved into the first multicelled animals.
Water vapor erupting from ancient volcanoes created the oceans where life first evolved.
These early animals resemble jellyfish or worms, with no hard parts, which is why their fossils are so rare.
CAMBRIAN (541–485 MYA)
Near the beginning of this period, there was an increase in the diversity of animal life. Many had hard shells, so they were more likely to form fossils than the soft-bodied animals that existed before.
The spiny, hard-shelled Marrella lived on the seabed about 500 million years ago. It had jointed legs like a crab, and was less than 3/4 in (2 cm) long.
ORDOVICIAN (485–443 MYA)
Early fish—the first vertebrates appeared in the oceans during the Ordovician Period and lived alongside other animals, such as trilobites. However, many of these animals were wiped out by a mass extinction event at the end of this period.
The earliest fish lacked hinged, bony jaws, as do modern lampreys. They also had no pectoral or pelvic fins, so were probably not very strong swimmers.
SILURIAN (443–419 MYA)
Bony fish with movable jaws evolved during the Silurian. By this time life had spread from the seas on to land, in the form of the earliest green plants.
This was one of the first plants to have stems. Such plants provided foo for the earliest land animals—invertebrates that resembled scorpions and centipedes.
DEVONIAN (419–358 MYA)
Many new types of sea fish evolve during the Devonian, and some 375 MYA a group of animals with four bony limbs started living partly on land. These were the first amphibians—the ancestors of all land vertebrates.
This late Devonian fish used its broad jaws to crush shellfish.
One of the earliest four-legged animals, Ichthyostega retained some fishlike features such as its tail fin and small, fishlike scales.
CARBONIFEROUS (358–298 MYA)
During the Carboniferous, life on land expanded rapidly, with primitive treelike plants forming large areas of forest. Insects and spiders flourished, and were hunted by large amphibians.
This dragonfly-like insect had a 29-in (75-cm) wingspan.
Plants such as this grew to 30 m (100 ft) tall, providing food and habitats for animals.
PERMIAN (298–252 MYA)
By the Permian, the amphibians had given rise to scaly-skinned reptiles that could live in all warm land habitats.
This sail-backed animal was related to the ancestors of mammals.
TRIASSIC (252–201 MYA)
The Permian Period ended with catastrophic mass extinction. But by the end of the Triassic Period that followed, the first dinosaurs had evolved, along with the flying pterosaurs and the earliest true mammals.
A plant-eater that walked on all four feet, this dinosaur could also rear up to feed in tall trees.
This mouse-sized, furry insect-eater was typical of the first mammals, which appeared about 225 million years ago.
JURASSIC (201–145 MYA)
Dinosaurs dominated animal life in this period. They included huge plant-eaters and powerful predators, but also smaller, feathered types that gave rise to the birds.
Toward the end of the Jurassic Period, a group of feathered dinosaurs developed wings that enabled them to fly. This species is one of the earliest we know of.
CRETACEOUS (145–66 MYA)
The Cretaceous Period saw the evolution of some of the biggest, most spectacular dinosaurs. But it ended with a mass extinction that wiped out all the giant dinosaurs.
The most famous dinosaur, T. rex lived at the end of the Cretaceous.
Early flying dinosaurs had teeth and long, bony tails. But by the Cretaceous, the first true birds had evolved.
PALEOGENE (66–23 MYA)
Some small mammals survived the catastrophe that destroyed the giant dinosaurs. They gave rise to larger plant-eaters and hunters that took the place of the extinct giants.
A heavily built herbivore, Uintatherium was the size of a modern rhinoceros.
NEOGENE (23–2 MYA)
During the Neogene Period, many modern types of mammal appeared, including some fearsome carnivores adapted for hunting the big plant-eaters.
This saber-toothed hunter lived in South America some 3 million years ago.
QUATERNARY (2 MYA to the present)
This period has included long ice ages separated by warmer phases, such as the one we live in today. The dominance of humans has driven many types of animals to extinction.
Most closely related to the modern Asian elephant, this was adapted for life in icy climates. It died out 3,700 years ago.