Life Before Dinosaurs:
As you already know, dinosaurs were not the first living things. Life began billions of years before them with the simplest living thing -- the single cell. Gradually animals with many cells, such as reptiles and mammals, evolved.
First Life -- 3 BYA (Billion Years Ago):
The first living things on Earth were very simple, single-celled forms of life. Fossils of cyanobacteria, commonly known as blue-green algae, have been found in rocks dating back to 3 billion years old. Hot springs in Yellowstone Park, North America contain bacteria and simple algae. Perhaps this scene is similar to 3 billion years ago when life began. Cyanobacteria are capable of photosynthesis - using the Sun's energy to make food, releasing oxygen as a by-product. Cyanobacteria transformed the planet as oxygen accumulated in the ocean waters and eventually created 'free oxygen' for the atmosphere.
Life in the Sea -- 600 to 530 MYA (Millions of Years Ago):
Before fish, the seas were home to other creatures. Many of them were like sea animals that live today. There were jellyfish, shellfish (brachiopods) and many sorts of sea worms. One type of animal that hasn't survived is the trilobite. Trilobites were sea creatures with hard bodies like armor, which were jointed so that they could move. They had legs like those of shrimps. To protect themselves some could curl up into a ball, rather like a woodlouse. Some trilobites may have been the first animals to obtain 'complex eyes' - an organ which may have helped accelerate evolution.
During the Silurian Period, the land surfaces of the Earth were mostly barren. The first plants, known as Cooksonia, were leafless and flowerless and no more than 4 or 5 cm tall. They lived on boggy ground, near the water's edge during this time. Through this miniature jungle, scorpions hunted millipedes that fed on the plants. Algae and fungi were also making their place on land. One particular plant, known as lichen, was responsible for breaking down rock from the barren land into smaller pieces, which later became soil.
First Teem in the Sea -- 390 MYA:
The first animals with backbones were fish. It is thought that sometime during this period Eusthenopteron, a fish that used its front fins to help it 'walk', crawled out of the water to live on land. It was the ancestor of land animals. Some scientists believe that the fish was strictly aquatic. The fish had special internal nostrils, which are exclusively found in land animals and tetrapods (those which walk or crawl on all four limbs).
Amphibians Rule -- 370 to 280 MYA:
Amphibians live on both land and in water, where they lay their eggs. Amphibians evolved from fish. At this time in history, amphibians were very successful. Some amphibians were quite large. Ichthyostega was an early tetrapod in the late Devonian Period. Scientists classify this unique creature based on its mixed characteristics of fish and amphibian. Its lungs and limbs were more amphibian-like, while its body had scales like that of a fish. They probably spent most of its time on land, especially the smaller juveniles.
Reptiles Rule -- 310 MYA:
There were more reptiles on land than amphibians. Scientists think that one group of reptiles were the ancestors of mammals. Dimetrodon belonged to this group. Dimetrodon was a big, mobile predator whose evolution heralded the beginning of the age of large reptiles. Unlike amphibians, reptiles had waterproofed skin that could hold the moisture within their bodies. This was a big advantage for those animals living in an arid climate during the Permian Period. The most obvious feature of this reptile was the large sail on its back. This sail was not just for display, but rather was for regulating body temperature. Thus, it was cold-blooded.
Flying Insects and Towering Forests -- 300 MYA:
Through huge forests flew the first flying insects such as dragonflies. Some of these, such as Meganeura, had vast wingspans up to 2.5 feet across. They were the largest flying insects ever to exist and probably the most deadly predators of their time. It has been suggested that these eagle-sized dragonflies grew to enormous size due to the increased levels of oxygen that existed in the Carboniferous atmosphere. Meganeura probably spent most of its time in the air, flapping or gliding its way through the forest in search of food. It hunted on the wing, plucking other insects in the air, but could also take small, ground-dwelling animals by swooping downwards.
Dinosaur Ancestor -- 245 MYA:
Archosaur reptiles, some of which were rather like modern crocodiles, were among the animals on Earth. Some, such as Euparkeria, may have have been the ancestors of the dinosaurs. This creature was a small, nimble reptile that lived in open woodland areas. It had a light, lean body, long tail and a small skull with tiny needle-like teeth. It fed on insects and any other small animals that it could find on the forest floor. It was one of the first to be able to run on two legs.