The origin of life
The universe is about 13.8 billion years old, according to current measurements, and is thought to have begun with the Big Bang. The Earth formed about 4.5 billion years ago. Its early atmosphere had water vapour, methane, carbon dioxide and ammonia, but no free oxygen.
Theories:
Spontaneous generation claimed that life arose from non-living matter (decaying matter, mud). Louis Pasteur disproved it: in pre-sterilised flasks, no life appeared from killed yeast, while in flasks open to air, new organisms arose.
Panspermia suggested life came from space as spores.
Oparin and Haldane proposed that the first life arose from non-living organic molecules (such as RNA and proteins) through chemical evolution, under the hot, reducing conditions of the early Earth.
Miller-Urey experiment (1953). Stanley Miller created electric sparks in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800 °C. He observed the formation of amino acids. Others later found sugars, nitrogen bases, pigments and fats in similar experiments. This supported chemical evolution.
The first non-cellular forms of life may have arisen about 3 billion years ago. The first cellular forms did not appear until about 2000 million years ago, probably as single cells in water.
Evidence for evolution
Fossils. Fossils are the remains of hard parts of ancient organisms preserved in rocks. Different-aged rock layers (sediments) hold different life forms, which shows that life forms have changed over time. Their age is determined by radioactive dating.
Comparative anatomy:
Homologous organs | Analogous organs | |
|---|---|---|
Structure | same basic structure and origin | different structure and origin |
Function | different | similar |
Examples | forelimbs of whales, bats, cheetahs and humans; vertebrate hearts and brains; thorn of Bougainvillea and tendril of Cucurbita | wings of a butterfly and a bird; eyes of an octopus and a mammal; flippers of penguins and dolphins; sweet potato (root) and potato (stem) |
Shows | divergent evolution (common ancestry) | convergent evolution (similar habitats lead to similar solutions) |
Molecular evidence. Similarities in proteins and genes between organisms point to common ancestry.
Natural selection in action:
Industrial melanism: in England before industrialisation, white-winged moths outnumbered dark ones on lichen-covered trees. After industrial smoke darkened tree trunks and killed the lichens, dark-winged moths became more common, because predators could no longer easily spot them.
Excessive use of herbicides, pesticides and antibiotics has selected for resistant varieties in a short time.
Adaptive radiation
Evolution of different species from a common ancestor in a geographical area, spreading into different habitats, is adaptive radiation.
Darwin's finches on the Galápagos Islands evolved varied beaks (for seeds, insects and more) from an original seed-eating ancestor.
Australian marsupials radiated from an ancestral stock into many forms, such as the kangaroo, koala, wombat and marsupial mole. When adaptive radiation happens in more than one isolated area, it gives convergent forms: Australian marsupials resemble placental mammals elsewhere (marsupial mole and placental mole, Tasmanian wolf and wolf).
Theories of evolution
Lamarck proposed that organisms evolve through the use and disuse of organs, and that acquired characters are inherited. He argued that the giraffe's neck lengthened because generations stretched to reach high leaves. This idea of inheritance of acquired characters is not supported by evidence.
Darwin's theory of natural selection (from his voyage on HMS Beagle and his book On the Origin of Species, 1859), with Alfred Wallace reaching similar conclusions:
Populations tend to produce more offspring than can survive.
Individuals vary in their characteristics, and much of this variation is inherited.
Resources are limited, so there is a struggle for existence.
Individuals with variations better suited to the environment survive and reproduce more: survival of the fittest. Fitness ultimately means reproductive fitness: leaving more offspring.
Over many generations, these favourable variations accumulate, giving rise to new species.
Hugo de Vries, working on the evening primrose, proposed mutation theory: large, sudden variations (saltation) cause speciation. Darwin's variations were small and directional; mutations are random.
The Hardy-Weinberg principle
In a population, the frequencies of alleles stay constant from generation to generation unless something disturbs them. This genetic equilibrium is expressed as
p² + 2pq + q² = 1, where p and q are the frequencies of two alleles (p + q = 1).
Five factors disturb the equilibrium and cause evolution:
gene migration (gene flow),
genetic drift: chance changes in allele frequency, strongest in small populations (a sharp change in a small founding group is the founder effect),
mutation,
genetic recombination,
natural selection.
Natural selection can be stabilising (favouring the average), directional (favouring one extreme) or disruptive (favouring both extremes).
A brief history of life
Plants: from algae to bryophytes, pteridophytes, gymnosperms and finally angiosperms.
Animals: about 350 million years ago, jawless fish gave rise to lobefins, which led to the first amphibians (the coelacanth, a lobefin thought extinct, was found alive in 1938). Amphibians gave rise to reptiles, which dominated as dinosaurs until about 65 million years ago. Mammals and birds evolved from reptiles; the first mammals were shrew-like.
Human evolution
Ancestor | Approximate time | Features |
|---|---|---|
Dryopithecus, Ramapithecus | about 15 million years ago | hairy, walked like gorillas and chimpanzees; Ramapithecus more man-like |
Australopithecus | about 2 million years ago | lived in East African grasslands; hunted with stone weapons; ate fruit |
Homo habilis | the first human-like being; brain about 650–800 cc; probably did not eat meat | |
Homo erectus | about 1.5 million years ago | brain about 900 cc; probably ate meat |
Neanderthal man | 1,00,000–40,000 years ago | brain about 1400 cc; lived in the Near East and Central Asia; used hides to protect their bodies and buried their dead |
Homo sapiens | arose in Africa | moved across continents; developed into distinct races |
During the ice age (75,000–10,000 years ago), modern Homo sapiens arose. Prehistoric cave art developed about 18,000 years ago. Agriculture began about 10,000 years ago, leading to human settlements.
Summary
Chemical evolution (Oparin-Haldane) was supported by the Miller-Urey experiment.
Fossils, homologous organs (divergence) and analogous organs (convergence) are evidence for evolution.
Darwin: variation, struggle for existence, survival of the fittest; de Vries: mutation.
Hardy-Weinberg equilibrium (p² + 2pq + q² = 1) is disturbed by gene flow, drift, mutation, recombination and selection.
Human brain capacity grew from about 650 cc (H. habilis) to about 1400 cc (Neanderthals).