Life has existed on Earth for approximately 3.5 billion years and has changed and diversified over time. The life forms we see on Earth today are incredibly varied, and many of these diverse life forms seem to assist various species in survival in their unique environmental conditions. The diversity has come about through many changes; changes that occur in a population of living things accumulate over generations as they are inheritable and transform a population's gene pool. This type of change is known as evolution, the process of cumulative, inheritable change in a population over many generations.

The Theory of Evolution by Natural Selection

In the 1850s, two naturalists by the name of Alfred Russel Wallace and Charles Darwin both arrived at the same idea of how species 'came to be', terming the idea 'natural selection'. The theory of evolutionary change through natural selection links all species to a common ancestor. An ancestor is a species from which other species have evolved, and a common ancestor refers to an ancestor that is shared by different species. This is supported by molecular evidence, through the fact that there is a common genetic code in the form of DNA and RNA. The basis of Darwin's theory of evolution was that individuals within a population showed a range of variation in their characteristics, and those most suited to their environment would have an advantage and be more likely to pass these traits to the next generation. Phylogeny is the reconstruction of the evolutionary history of any group of organisms, studying the similarities and differences between them.

Life Forms and Times of the Earth

Scientists use a geological timescale within which they can place the major changes that occurred in the history of the Earth. Time has been divided into segments covering millions, sometimes billions of years. Geologic time is divided into eons, which are split into eras, eras into periods, and periods into epochs. Geological time is usually expressed as mya (millions of years ago). Conditions on Earth have significantly changed over time, and populations have either adapted or become extinct.

Continential Drift

The major plates of Earth's crust (tectonic plates) float on a fluid called the mantle that lies over the Earth, causing continental drift. These plates are in constant movement and may result in earthquakes, volcanic activity and tsunamis. Fossil and geological evidence show that 200 mya, a supercontinent called Pangaea existed. Pangaea broke into two landmasses, the northern continent Laurasia and the southern continent Gondwana. Over the next 100 mya, further separation led to the formation of the continents we know today.

Magnetic Evidence

The Earth's magnetic field changes frequently, alternating between both normal polarity seen today and reversed polarity where magnetic poles are reversed. Iron-rich minerals in rocks are magnetised frozen compasses; these record orientations are relative to the position of the magnetic poles at the time of formation, which provides evidence for evolution.

Temperature, Climate and Atmosphere

Changes to Earth's climate have been drastic at times, for example, ice core samples can display this. Generally, the Earth has been warmer in the past, however, periods of warming and cooling have been regular. Temperature differences between polar and the equator have not been as big in the past. A cold, dry Earth was seen at 570, 280 and 23mya. In between, much warmer, humid and wet periods have existed where vegetation diversified and increased animal life (diversity). The first atmosphere of Earth likely had very little oxygen, and evidence suggests that oxygen started to increase around 2,500mya. At 1,500mya, oxygen levels were at 1% of current levels, and 600mya, oxygen levels were at 5% of current levels due to photosynthetic bacteria (cyanobacteria). More complex, multicellular life then evolved in response to the increasing availability of oxygen.

Biogeography and Evidence for Evolution

Biogeography is the study of the distribution of organisms and ecosystems across time. Some fossils of animals and plants (250mya) have similarities in Australia, South America, India and Antarctica due to the existence of Gondwanaland. The Wallace Line is explored in biogeography in great detail, and it observed a distinct change in flora and fauna across the Indonesia achepello, with the dividing line passing between Bali and Lombok. BIrd, mammal and fish species differed markedly on either side of this 'imaginary line'. The mystery of the line was solved with an understanding of plate tectonics, and the separation of the South Asian and Australasian plates.