2Earth formed about 4. 6 billion years ago Earth formed about 4.6 billion years ago. By about 4 billion years ago, less dense compounds had cooled to form a solid crust, water vapour had condensed, and ocean basins had filled.
3Primordial EarthMiller-Urey experiment showed the spontaneous formation of macromolecules was possible with the conditions in the earth’s early atmosphere.
4Chemical EvolutionAlong with amino acids and proteins, experiments have shown the spontaneous development of other macromolecules such as lipids and nucleic acids (such as RNA). As soon as a molecule formed which could self- replicate, the possibility then exists for errors in those copies. Errors in replication produces variation. And natural selection acts on variation. Any molecule which could copy itself more efficiently or faster, could be selected for in an environment.
5Formation of Protocells The next important step in formation of cells is to develop a membrane to protect those self-replicating molecules.Phospholipid molecules naturally arrange themselves into spherical shapes called liposomes.
6Prokaryotic cellsThe oldest known fossils on Earth are dated to billion years old – from Western Australia lsyered in formations called stromatolitesThese microscopic fossils resemble present-day cyanobacteriaAlthough the oldest fossil bacteria resemble photosynthetic cyanobacteria, which use oxygen, the very first prokaryotic cells would certainly have been anaerobic, as the atmosphere would then have contained little or no free oxygen.These first prokaryotic organisms would likely have relied on abiotic sources of organic compounds. They would have been chemoautotrophic, using compounds like H2S
7Changing the atmosphere Although the first photosynthetic organisms may have also used hydrogen sulfide as a source of hydrogen, those that used water would have had a virtually unlimited supply.As they removed hydrogen from water, they would have released free oxygen gas into the atmosphere—a process that would have had a dramatic effect.The accumulation of oxygen gas, which is very reactive, would have been toxic to many of the anaerobic organisms on Earth.While these photosynthetic cells prospered, others would have had to adapt to the steadily increasing levels of atmospheric oxygen or perish.Some of the oxygen gas reaching the upper atmosphere would have reacted to form a layer of ozone gas, having the potential to dramatically reduce the amount of damaging ultraviolet radiation reaching Earth.At the same time, the very success of the photosynthetic cells would have favoured the evolution of many heterotrophic organisms.
8Evolution of the three domains Comparisons of present-day prokaryotic and eukaryotic DNA, however, suggest that the earliest prokaryotic cells probably gave rise to eubacteria and archaebacteria.The eukaryote lineage and archaebacteria lineage are thought to have separated about 3.4 billion years ago.
10Eukaryotic cellsThe distinguishing feature of eukaryotic cells is the presence of membrane-bound organelles, such as the nucleus and vacuoles. A nuclear membrane and the endoplasmic reticulum may have evolved from infolding of the outer cell membrane.Initially, such folding may have been an adaptation that permitted more efficient exchange of materials between the cell and its surroundings by increasing surface area, and it may also have provided more intimate chemical communication between the genetic material and the environment.
12Mitochondria and chloroplasts Early eukaryotic cells engulfed aerobic bacteria in a process similar to phagocytosis in amoebaHaving been surrounded by a plasma membrane, the bacteria were not digested but, instead, entered into a symbiotic relationship with the host cell. The bacteria would have continued to perform aerobic respiration, providing excess ATP to the host eukaryotic cell,which would have continued to seek out and acquire energy-rich molecules from its surroundings.Endosymbiotic bacteria, benefiting from this chemical-rich environment, would have begun to reproduce independently within this larger cell.This process is referred to as endosymbiosis
13EndosymbiosisEvidence which supports this theory is that both mitochondria and chloroplasts:have their own DNAundergo division independently of their own cell’s divisioncontain two sets of membranes (outer and inner membranes)
15MulticellularityFor the first 3 billion years of life on Earth, all organisms were unicellular.Eubacteria gave rise to aerobic and photosynthetic lineages, while archaebacteria evolved into three main groups: methanogens, extreme halophiles, and extreme thermophiles.Once eukaryotic organisms evolved complex structures and processes, including mitosis and sexual reproduction, they would have had the benefit of much more extensive genetic recombination than would have been possible among prokaryotic cells.Photosynthesis continued to increase the oxygen concentration in the atmosphere to the benefit of aerobic organisms.Multicellular organisms, including plants, fungi, and animals, are thought to have evolved less than 750 million years ago.
16DiversificationThe oldest fossils of multicellular animals date from about 640 million years ago.However, during a 40-million-year period beginning about 565 million years ago, a massive increase in animal diversity occurred, referred to as the Cambrian explosion.Fossil evidence dating from this period shows the appearance of early arthropods, such as trilobites, as well as echinoderms and molluscs; primitive chordates – which were precursors to the vertebrates—also appeared.Animals representing all present-day major phyla, as well as many that are now extinct, first appeared during this period, a time span that represents less than 1% of Earth’s history.
18Rate of EvolutionWhen Darwin proposed the theory of natural selection, he predicted that species would change gradually over time, following the pace of geologic change.The theory of gradualism contends that when new species first evolve, they appear very similar to the originator species and only gradually become more distinctive, as natural selection and genetic drift act independently on both species.One would expect to find, according to this theory, as a result of slow incremental changes, numerous fossil species representing transitional forms.
19Rate of EvolutionNiles Eldredge of the American Museum of Natural History and Stephen Jay Gould of Harvard University rejected this explanation and, in 1972, proposed an alternative theory called the theory of punctuated equilibrium. It consists of three main assertions:Species evolve very rapidly in evolutionary time.Speciation usually occurs in small isolated populations and thus intermediate fossils are very rare.After the initial burst of evolution, species do not change significantly over long periods of time.