Biology

Biology is the science of living things: how they grow, how they work, how they make more of themselves, and how they fit into the rest of the world. Every blade of grass, every spider, every dolphin and every human being is a biological machine of astonishing complexity. Biology covers everything from the tiniest microbe you cannot see without a microscope up to the giant blue whale, the biggest animal ever to have lived. Life on Earth has existed for at least 3.7 billion years, and biologists are still discovering new species every year.

• Age of life on Earthapprox. 3.7 billion yearsThe first single-celled life
• Known speciesapprox. 2 millionAnd up to 10 million still undiscovered
• Largest living thingPando aspen groveA single tree in Utah, 43 hectares, 80,000 tonnes
• SmallestMycoplasma bacteriaapprox. 0.2 micrometres across, simpler than viruses
• Six kingdomsAnimals, plants, fungi, protists, bacteria, archaeaPlus viruses (not officially "alive")
• Largest animal everBlue whaleUp to 30 m, 200 tonnes

The big branches of biology

Biology is huge, so biologists usually specialise in one part of it. Some major branches include:

• Zoology: the study of animals (everything from worms to whales).
• Botany: the study of plants.
• Microbiology: the study of microscopic life such as bacteria, viruses and protists.
• Genetics: how living things pass on their features through DNA.
• Ecology: how living things interact with each other and their environment.
• Evolution: how living things change over many generations.
• Anatomy and physiology: how the bodies of living things are built and how they work.

What makes something alive?

Biologists usually agree on a checklist of seven things that every living thing does. Together they are sometimes remembered as MRS GREN:

• Movement (moving at least part of themselves)
• Respiration (releasing energy from food)
• Sensitivity (responding to their surroundings)
• Growth (getting bigger or more complex)
• Reproduction (making more of themselves)
• Excretion (getting rid of waste)
• Nutrition (taking in food or producing it from sunlight)

Anything that does all seven is alive. (Viruses are a famous edge case: they reproduce only by hijacking other cells, and some scientists do not count them as truly alive at all.)

Fact Every living thing on Earth, from a bacterium to a blue whale to a banana, is made of cells and uses the same chemical code (DNA) to pass on instructions to the next generation. That is why biologists are confident every living thing on Earth shares a single common ancestor that lived around 4 billion years ago, nicknamed the Last Universal Common Ancestor or LUCA.

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