Life has evolved into an incredible variety of forms, with almost 1.8 million known species and many more yet to be discovered. Scientists classify them in six kingdoms. Each kingdom is divided into groups of related living things called phyla, classes, orders, and families.
These organisms have evolved different ways of surviving in their particular habitats, but many find it harder and harder to live in a changing world.
There is life almost everywhere on Earth, but some regions have many more species than others. Such areas are known as biodiversity hotspots. The warm tropics are the richest - especially tropical rainforests and tropical coral reefs. These habitats offer many ways in which organisms can survive, encouraging the evolution of different types of life.
Every species of living thing is related to others that evolved from the same ancestors. They form groups of species that are related to other groups in the same way, like a giant family tree. This tree has six main stems - the six kingdoms of life. Three of the kingdoms are made up of organisms that are mostly too small to see without a microscope, but the others consist of the animals, plants, and fungi.
The first life forms to evolve on Earth were the microscopic organisms called archaea. They have a very simple structure - just a single cell enclosing a tiny drop of watery fluid containing the molecules vital to life. Some archaea live in hostile places such as hot, acidic springs.
These are very like archaea, with the same structure. But their chemistry is different, showing that they evolved separately. Some bacteria cause disease, while others are essential to our survival. One type, cyanobacteria, produced nearly all the oxygen in the atmosphere.
Also known as algae and protozoans, these are mainly microscopic, single-celled organisms. Unlike bacteria or archaea, each cell has a nucleus that contains the main life-giving molecules. There are also other structures within the cell that make food or turn it into energy.
Although they seem to grow like plants, fungi feed on other living or dead organisms. Some fungi are single-celled microbes, but most fungi form spreading, multi-celled root networks. These produce mushrooms and molds like those that grow on decaying fruit.
Green plants are multi-celled living things that use the energy of sunlight to make food. In the process, they release oxygen into the air, which is vital to animal life. They mostly live on land or in fresh water, and range from low-growing mosses to magnificent trees.
Like plants, all animals are multi-celled organisms. But unlike them, they cannot make their own food, and must eat other organisms instead. Most do so by moving around, using their senses to find the food they need, and this has led to the evolution of intelligence.
Each of the six kingdoms of life is divided into several phyla of similar organisms. These are split into classes, orders, and families. Within each family there are usually groups of closely related organisms, each called a genus. This normally contains several individual species.
The scientific name of an organism is made up of its genus and species. The tiger is in the genus Panthera, so its scientific name is Panthera tigris.
All living things need energy. Plants and some other organisms gather energy from sunlight and use it to make sugar. Animals eat the plants and turn the sugar back into energy and body tissues. Other animals eat the plant-eaters, so the energy - along with vital nutrients - passes down a food chain. But eventually, the energy and nutrients are turned into a form that can be recycled by plants.