Since life began, more than 3.5 billion years ago,
evolution has produced an enormous variety of living things. The earliest
living things were simple, microscopic forms, such as bacteria. They evolved
into increasingly complex creatures, eventually developing into the animals we
know today. However, some animals, such as DINOSAURS, have become
extinct.
Like many
insects, dragonflies have barely changed since they first evolved. Insects are
a remarkably successful group of animals that first appeared around 400 million
years ago. About 320 million years ago, some insects evolved wings, making them
the earliest animals to fly.
Between about 206 and 65 million years ago, long necked reptiles
called plesiosaurs lived in the oceans. Elasmosaurus was one of the largest. As
plesiosaurs have no living relatives, scientists can only guess how they lived
and moved. The huge flippers may have worked like the oars of a boat, pulling
the animal along, or they might have provided propulsion by moving up and
down.
More than 540 mya, dating back to when life began.
Beginning 540 mya, this era lasted for 290 million years. Fish
appeared in the seas and rivers around 500 mya. Over time, some of these
developed legs and lungs, giving rise to amphibians. Later, reptiles, such as
Dimetrodon, evolved from these air-breathing amphibians.
This era lasted from 250 to 65 mya. It was dominated by dinosaurs,
pterosaurs, and giant sea reptiles. Birds evolved about 150 mya. Placental
mammals evolved from more primitive mammals at the end of the era.
This era began 65 mya and continues today. The ancestors of most
modern mammal groups appeared. Our own ancestors, the first upright hominids,
evolved about 5 mya.
An extinct group of land-living reptiles, dinosaurs had
erect legs set under their bodies rather than out to the side. All dinosaurs
fall into one of two types, depending on the shape of their hip bones. Their
closest living relatives are birds, which evolved from meat-eating
dinosaurs.
The hips of all dinosaurs had three bones, the ilium, ischium, and
pubis. The so-called bird-hipped, or ornithischian, dinosaurs were all
plant-eaters.
Lizard-hipped, or saurischian, dinosaurs included meat-eaters such
as Velociraptor and Tyrannosaurus rex, as well as gigantic long-necked
herbivores, such as Diplodocus. Bird-hipped dinosaurs may have evolved from a
lizard-hipped ancestor.
