The Earth is roughly 4.5 billion years old[]
How do you explain in a few paragraphs over 4 billion years of Earth's history? Well, we really can't without a very long book, so we will touch on a few highlights and skip a lot of the really technical stuff. To begin at the beginning, the earth was formed from a ball of dust and hot gases about 4.5 billion (4,500,000,000) years ago.
The first life forms appeared 3.5 billion years ago[]
Approximately a billion years later, some very simple life forms appeared. These were single-cell creatures, neither plant nor animal, that lived in the soupy mixture that would eventually become the world's oceans. No one really understands how these first life forms started, but they did. It took another 2.5 billion years for them to get together and form a sort of a city of single-cells that all worked together, but at different jobs - some were in control of eating, some controlled movement, some reproduction. This was the first complex life form. From that point on, about 600 million years ago, there was an explosion of life. In the Timeline you can find more detailed descriptions of the actual events that occurred before dinosaurs came onto the scene in the Paleozoic Era. Up until about 517 million years ago everything that could have been considered an animal was kind of squishy and slimy, like big slugs or jellyfish. Then a very important creature appeared in the seas. This small animal was called a Pikaia (pie-KI-ya) and it was only about two inches long, but it had something no other animal had ever had before - a spine. This would make it the ancestor of every fish, bird, reptile, and mammal (including humans).
The first fish didn't have movable jaws[]
It wasn't long after Pikaia appeared that many different types of fish began to evolve. The first fish looked a lot different than any that exist today. For one thing, they didn't have jaws. Their mouths were mostly just an open hole through which they would filter water and collect small plants and animals that were floating in the water. It would be another 42 million years (475 million years ago) before movable jaws and teeth would appear, but when they did, life in the sea became much more competitive. The ability of predators to attack and inflict serious damage increased dramatically as they could now feed with greater ease and therefore grew in size. This situation lasted quite a while until a few simple creatures began to leave the water for the safety of the land. These pioneers became insects, and along with plants, would be the first to colonize the land.
Insects were the first creatures to begin living on land[]
Insects were the first creatures on Earth to leave the sea, but fish weren't far behind. Some fish learned how to gulp air and started to develop lungs. Eventually, these simple lungfish began to pull themselves out of the water using their fins and gradually these fins, through the process of evolution, developed into simple legs. At this time, life on land was much easier as food would have been plentiful and there wasn't much competition so these early visitors were able to prosper relatively quickly. They soon developed more advanced legs and feet and evolved into the first amphibians. Shortly thereafter these amphibians had established a life cycle on land similar to that of the sea - predator and prey.
Life on land became highly diversified about 270 million years ago[]
By about 270 million years ago life on land was easily as complex as that in the sea. There were lizards, amphibians, insects, lots of plants and a new kind of creature - reptiles. The most interesting types of these creatures were the Mammal-Like Reptiles. There isn't a lot known about these amazing animals, but scientists do know that they were much bigger than the first dinosaurs - some grew to almost 20 feet in length. They were divided into herbivores (plant-eaters) and carnivores (meat-eaters) just like the dinosaurs. Some had huge teeth, which were very advanced for so long ago. In fact, their teeth were more advanced than dinosaur teeth. They ruled the land until the middle of the Triassic period when dinosaurs first appeared on the scene.
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