Theories of the origin of life
Several ideas have been proposed. Special creation says that life was created by a divine being. Spontaneous generation said that living things arose from non-living matter, such as maggots from meat; it was disproved by Redi and Pasteur, who showed that life comes only from life (biogenesis). The chemical evolution theory of Oparin and Haldane says that, in the early Earth, simple molecules such as water, methane, ammonia and hydrogen formed organic molecules, then the first simple cells. Miller and Urey supported this in 1953 by passing electric sparks through such a mixture and obtaining amino acids. Another idea, panspermia, says that life came from space.
| Theory | Main idea | Evidence or problem |
|---|---|---|
| Special creation | Life created by a divine being | Based on belief; not testable by science |
| Spontaneous generation | Life arises from non-living matter now | Disproved by Redi and Pasteur |
| Chemical evolution | Organic molecules formed in early Earth | Miller–Urey experiment |
| Panspermia | Life came from space | Organic molecules found in meteorites |
Organic evolution
Organic evolution is the change of species over time. Lamarck suggested that features acquired during life by use and disuse are inherited (for example, a giraffe stretching its neck), but acquired features are not inherited. Darwin and Wallace proposed natural selection. Modern theory adds genetics: variation comes from mutation and meiosis, and selection changes the allele frequencies in a population. When populations become separated and change, new species form (speciation).
Evidence
| Evidence | Example |
|---|---|
| Fossil record | Archaeopteryx links reptiles and birds; horse fossils show gradual change |
| Comparative anatomy | Homologous limbs (pentadactyl limb) in vertebrates; vestigial organs such as the appendix |
| Biochemistry | Cytochrome c and DNA are more alike in close relatives |
| Embryology | Gill slits in the embryos of all vertebrates |
| Direct observation | Antibiotic resistance in bacteria; insecticide-resistant mosquitoes |
Start with a small share of resistant bacteria. Choose the advantage of resistance when the drug is used and the number of generations.
Key terms
- Biogenesis
- The idea that life comes only from life.
- Speciation
- The formation of a new species.
- Mutation
- A random change in DNA.
- Vestigial organ
- A reduced organ with little or no function.
Summary
- Theories of life’s origin include special creation, chemical evolution and panspermia.
- Pasteur disproved spontaneous generation.
- Natural selection acting on variation from mutation changes allele frequencies.
- Evidence: fossils, anatomy, biochemistry, embryology and observed change.