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The History of Life on Earth (Key events in life’s history include the…
The History of Life on Earth
Conditions on early Earth made the origin of life possible
Scientists have proposed a four-stage hypothesis for the origin of life on early Earth
The abiotic synthesis of small organic molecules
The joining of these small molecules into macro molecules
The packaging of these molecules into protocells
The origin of self-replicating molecules
Experiments simulating possible early atmospheres have produced organic molecules from inorganic precursors
The first genetic material may have self-replicating, catalytic RNA
The fossil record documents the history of life
The fossil record, based largely on fossils found in sedimentary rocks, documents the rise and fall of different groups of organisms over time
Sedimentary strata reveal the relative ages of fossils. The absolute ages of fossils can be estimated by radiometric dating and other methods
The fossil record shows how new groups of organisms can arise via the gradual modification of preexisting organisms
Key events in life’s history include the origins of unicellular and
multicellular organisms and the colonization of land
By studying rocks and fossils at many different sites, geologists have established a geologic
record of the history of life on Earth, which is divided into three eons
The first two eons—the Archaean and the Proterozoic—together lasted approximately 4
billion years
The Phanerozoic eon covers the last half billion years and encompasses much of the time that
animals have existed on Earth
The oldest known fossils are 3.5-billion-year-old stromatolites, rocks composed of layers of
cyanobacteria and sediment
The endosymbiont theory suggests that mitochondria and plastids were formerly small
prokaryotes that began living within larger cells
The hypothesis of serial endosymbiosis supposes that mitochondria evolved before plastids
Many phyla of animals appear suddenly in the fossil record in a phenomenon known as the
Cambrian explosion
The rise and fall of organisms reflect differences in speciation and
extinction rates
According to the theory of plate tectonics, Earth’s continents drift across the planet’s surface
on great plates of crust that float on the hot, underlying mantle
About 250 million years ago, all the continental landmasses came together into a
supercontinent called Pangaea
The result of these environmental changes is a mass extinction, in which large numbers of
species become extinct
Adaptive radiations occur when a few organisms make their way into new areas, when
novel adaptations arise, or when extinction opens up ecological niches for surviving species
Major changes in body form can result from changes in the sequence
and regulation of developmental genes
Heterochrony, an evolutionary change in the rate or timing of developmental events, has led
to many striking evolutionary transformations
If the rate of reproductive development accelerates compared to the development of other
organs, then a sexually mature stage can retain juvenile structures a process called
paedomorphosis
Homeotic genes control the spatial arrangement of body parts
Evolution is not goal oriented
Novel and complex biological structures can evolve through a series of incremental modifications, each of which benefits the organism that possess it
Evolutionary trends can be caused by factors such as natural selection in a changing environment or species selection
Like all aspects of evolution evolutionary trends result from interactions between organisms and their current environments