Named after a monk's room
In 1665 the English scientist Robert Hooke looked at a thin slice of cork through his microscope and saw that it was made of tiny boxes. They reminded him of the small rooms monks lived in, called cells, so that is what he called them. Over the next two centuries scientists showed that every living ORGANISM is made of one CELL or many, and that every cell comes from another cell.
Most cells are far too small to see. A cell is wrapped in a thin MEMBRANE, and in plant and animal cells the NUCLEUS holds the instructions for running it. Your body replaces a vast number of cells every day, hundreds of billions of them, mostly blood and gut cells, by dividing old ones in two in a process called MITOSIS.
Two metres of instructions in every cell
The instructions are written in DNA, a long, thin molecule coiled up tightly on each CHROMOSOME. Stretched out, the DNA in a single human cell would be about two metres long, packed into a nucleus far smaller than a full stop. Its double-helix shape was worked out in 1953, a story with a famous twist told on our scientists page.
The complete set of instructions for a living thing is its GENOME. The Human Genome Project, which read the whole human set for the first time, was finished in 2003. It found that people have only around 20,000 genes that make PROTEIN, far fewer than expected. GENETICS, the study of how these instructions are passed on, began with a monk breeding pea plants in the 1850s and 1860s.
A letter from the other side of the world
Charles Darwin spent about twenty years quietly gathering evidence for his theory of EVOLUTION by natural selection. In 1858 he received a letter from a young naturalist, Alfred Russel Wallace, who was collecting insects and birds in what is now Indonesia and had come up with almost exactly the same idea on his own. Their papers were read together at a meeting in London that summer, and Darwin rushed to finish his book, On the Origin of Species, which came out the following year.
How many kinds of life?
Scientists have named about 1.2 million SPECIES, but a 2011 study estimated there might be around 8.7 million on Earth, most of them still undiscovered. Animals with backbones, called VERTEBRATE animals, are only a small share of the total; insects and other small creatures make up most of it.
Some of the most important life is tiny. The PLANKTON drifting in the sea, especially the tiny plant-like ones, produce around half of the oxygen we breathe. Your body carries roughly as many BACTERIA cells as human cells, most of them helpful ones in your gut. A FUNGUS, surprisingly, is more closely related to animals than to plants. The AMOEBA, the MICROBE, the ENZYME that speeds up the chemistry of life, TISSUE and the ORGAN, the HABITAT and ECOLOGY all belong to the study of life, and our ecosystems page shows how living things depend on each other.