Classification and extinction · GCSE Biology

The Linnaean system

Carl Linnaeus classified living things into groups based on their structure and characteristics: kingdom, phylum, class, order, family, genus and species.
Organisms are named by the binomial system: the genus followed by the species, for example Homo sapiens.

As microscopes improved and scientists understood more biochemistry, new models of classification were proposed.
Carl Woese proposed the three-domain system, using chemical analysis.
Archaea: primitive bacteria, usually living in extreme environments. Bacteria: true bacteria. Eukaryota: protists, fungi, plants and animals.

Evolutionary trees show how scientists believe organisms are related.
They use current classification data for living organisms and fossil data for extinct ones. Organisms that share a more recent common ancestor are more closely related.

Fossils are the remains of organisms from millions of years ago, found in rocks.
They form from parts that did not decay because some conditions for decay were absent, from parts replaced by minerals as they decayed, or as preserved traces such as footprints, burrows and root traces.
Many early life forms were soft-bodied, so they left few traces, and many traces were destroyed by geological activity. This is why scientists cannot be certain how life began.
Fossils show how much or how little organisms have changed as life developed.

A species becomes extinct when there are no individuals left.
Causes include new diseases, new predators, new competitors, changes to the environment over geological time such as climate change, and single catastrophic events such as huge volcanic eruptions or asteroid collisions.

Classification: Sorting living things into groups based on their characteristics.
Binomial system: Naming an organism by its genus and species, for example Homo sapiens.
Genus: A group of closely related species.
Species: The smallest group in classification; organisms that can interbreed to produce fertile offspring.
Three-domain system: Classification into Archaea, Bacteria and Eukaryota, proposed by Carl Woese.
Archaea: Primitive bacteria, usually found in extreme environments.
Evolutionary tree: A diagram showing how organisms are thought to be related.
Fossil: The remains or traces of an organism from millions of years ago, found in rock.
Extinction: When there are no individuals of a species left.