Evolution is one of the best-supported theories in all of biology. The evidence comes from many independent sources, all of which point in the same direction.
Fossils
A fossil is the preserved remains, or impression, of an organism that lived in the past. Fossils form when:
- An organism dies and is buried quickly enough to avoid being eaten or decomposing fully.
- The soft parts rot away, leaving hard parts (bones, shells, teeth, wood).
- Over thousands of years, minerals seep into the hard parts and replace them, leaving a stone copy in the rock.
- Geological processes eventually expose the rock at the surface, where palaeontologists can dig up the fossils.
Rocks are arranged in layers (strata), with the older rocks at the bottom and younger ones on top. Fossils from each layer can be dated by the rock they are found in.
What fossils tell us
- The age of life on Earth. The oldest fossils (of bacteria-like cells) are about 3.5 billion years old. Life on Earth is therefore extremely ancient.
- The order in which different groups appeared. Fossils show that life began with single-celled organisms, then evolved through simple multicellular organisms, fish, amphibians, reptiles, dinosaurs, mammals, and birds. The order matches what evolution predicts: complex organisms developed gradually from simpler ones, not all at once.
- Transitional forms. Fossils show animals with features that are part-way between two groups. For example, Archaeopteryx (a famous fossil from 150 million years ago) has feathers and a wishbone (like a bird) but also teeth and a long bony tail (like a reptile). It is a transitional form between reptiles and birds.
- Extinction. Many fossil species no longer exist today. Mass extinctions (such as the one that wiped out the dinosaurs 66 million years ago) are visible as sudden changes in the kinds of fossils found in successive rock layers.
Why the fossil record is incomplete
Fossilisation is rare. Most dead organisms decompose completely or are eaten before they can fossilise. Soft-bodied organisms (like worms or jellyfish) almost never leave fossils. Whole groups of organisms have therefore left no record at all. The fossil record we have is therefore a small, patchy sample of the history of life, but the patterns it shows are still strong enough to support evolution.
Other evidence for evolution
Although the syllabus focuses on fossils, evolution is supported by several other independent lines of evidence:
- Anatomy: many distantly-related species share the same underlying body plan. Human arms, bat wings, whale flippers, and horse legs all have the same arrangement of bones (a single upper bone, two lower bones, wrist bones, and five digits). These homologous structures make sense if all four species evolved from a common ancestor.
- DNA sequencing: the more closely related two species are, the more similar their DNA sequences. Comparing genomes lets biologists draw evolutionary trees that match the trees previously drawn from anatomy.
- Direct observation: evolution has been seen happening within human lifetimes in fast-reproducing organisms, including antibiotic resistance in bacteria, pesticide resistance in insects, and beak changes in Galápagos finches in response to droughts.