The First Organisms

Close to 2.5 billion years ago, the earth's surface and atmosphere were stable enough to support primitive life. Single-cell organisms began to develop in the seas that covered the planet. Most of them were very simple single-cell bacteria that fed on chemicals in the ocean's waters.

A simple organism known as blue-green algae appeared and spread across the seas. Blue-green algae are still alive today. It was very important to the future of the planet because blue-green algae used sunlight and water to make food, and in the process, created oxygen. As the blue-green algae grew in the earth's seas, they began to fill the atmosphere with oxygen.

The oxygen blue-green algae produced made it possible for other types of organisms to develop. These organisms needed oxygen to carry out their life processes of growth, feeding, responding and reproducing. Unlike the blue-green algae, these organisms could not produce their own food. They needed oxygen to perform their life processes of growth, feeding, responding, and reproducing. In return, they produced CO2, which the algae needed to perform its life processes. A precise balance between plants and animals was established.

Classification: In order to study nature, scientists have classified the life forms in nature, or put them into groups. Organisms are classified according to how closely they are related. Large groups are broken down into smaller and smaller groups. A kingdom is the largest unit of classification. There are five kingdoms in nature. One of those kingdoms is Animalia, or the animal kingdom.

There are two main groups of animals: vertebrates and invertebrates. Vertebrates are a subgroup of the Phylum Cordata, or animals that have a spinal chord. These include fish, amphibians, reptiles, birds and mammals. You might not think it, but invertebrates, or animals without a spinal chord, make up most of the animal kingdom. These include sponges, jellyfish, worms, arthropods (insects, shrimp, spiders), mollusks (snails, clams, octopuses), and echinoderms (sea urchins, sea stars).

Organisms in the animal kingdom consist of many different species. Some animals most familiar to us are mammals, birds, fish and insects.

Animal cells don't have the rigid cell walls that plant cells have. Most animal bodies are made up of organized cells that are specialized to perform a specific task. Other cells are organized into even more specialized organs. Most animals are capable of moving relatively fast, unlike plants. Most animals reproduce sexually.