18-2: ORIGIN & HISTORY OF LIFE
HISTORY OF LIFE
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Earliest were prokaryotes (fossil record supports this)
o Prokaryotes appeared at least 1.4 billion years before the oldest eukaryotes
o Bacteria and Archaea diverged early, between 2-3 billion years ago.
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Photosynthetic bacteria started the production of O2 about 2.5 billion years ago
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Eukaryotes emerged some 2.1 billion years ago
o Symbiotic community of prokaryotes
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Plants evolved from green algae.
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Fungi and animals arose from different groups of heterotrophic unicells.
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Fungi are more closely related to animals than they are to plants.
Transition from the aquatic environment to land was a pivotal point in the history of life.
Fossils are used to explore the history of life on Earth.
Determining the Age of Fossils
1) The Law of Superposition = younger strata are found on top of older strata
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Organisms found in the same stratum layer are believed to be about the same age 
“Relative Dating”
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Relies on Index Fossils = have a known age that can be used to identify the approximate
age of the layer
2) Absolute Dating = using radiometric techniques (radioactive isotopes and their half-life) to
determine the age of a fossil
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Radiocarbon Dating = uses the ½-life of 14C  5730 years
o Only good for a fossil that is <100,000 years old
o Ex. A sample of a fossil is found to have ¼ of the original amount of carbon-14.
How old is it?
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Older fossils require the use of K-Ar (potassium-argon) or 238U dating
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Can be used to date fossils that are 100,000-4.5 billion years old
GEOLOGIC TIMESCALE
Divides Earth’s history into eras  periods  epochs
Precambrian Time
o 3.5 bya  1st prokaryotes appear
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Archaea  tolerate harsh environments well (hot springs, salt marshes, anaerobic)
o Stomatolites contain evidence of cyanobacteria from 3.4 bya
o Added free O2 to the atmosphere, so that by 2.0 bya cyanobacteria and aerobic bacteria began
to proliferate
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Ozone developed and UV light began to be filtered
o 2.1 bya  Eukaryotic Cells Arise
The Endosymbiotic Theory
Many cellular components had to arise between the prokaryotes and the development of the eukaryotes:
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A membrane-bound nucleus
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Mitochondria, chloroplasts, and the endomembrane system
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A cytoskeleton
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Multiple chromosomes of linear DNA molecules associated with proteins
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Life cycles that involve mitosis, meiosis, and sex
Two Processes Involved:
1) Specialization of Plasma Membrane Invaginations
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Led to the nuclear envelope, endoplasmic reticulum, Golgi Apparatus, and other
endomembrane components
2) Serial Endosymbiosis Events
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Theory developed by Lynn Margulis, which states that certain prokaryotic species (called
endosymbionts) lived within larger prokaryotes and over time the arrangement became
permanent.
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Chloroplasts are descended from endosymbiotic photosynthesizing prokaryotes, such as
cyanobacteria, living in larger cells
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Mitochondria are postulated to be descendants of prokaryotic aerobic heterotrophs
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May have been parasites or undigested prey of larger prokaryotes
o Progressed from parasitism or predation to mutualism
o Became more interdependent with time until the integrated into a single organism
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Evidence lies in the similarities between these organelles and prokaryotes:
o Similar SIZE to bacteria
o Have inner membranes containing several enzyme and transport systems similar to
those on prokaryotic plasma membranes
o Replicate by splitting processes similar to binary fission
o Have their own DNA that is circular and not associated with histones or other
proteins
o Contain their own tRNA, ribosomes, and other components for DNA transcription
and translation into proteins
o Ribosomes are more similar to prokaryotic ribosomes
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Led to development of multicellular Organisms about 1.4 bya
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1st animals (soft-bodied invertebrates) appeared about 630-545 mya
The Paleozoic Era
o The Cambrian Explosion = when invertebrate life in the oceans became very abundant
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About 542 mya
o Made possible due to the development of the exoskeleton
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Adaptive Zone = when the development of a new trait makes adaptive radiation more
likely
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Often created by the occurrence of Mass Extinctions = the disappearance of a large
number of species in a few million years
o Molecular Clocks = determining age by looking at the number of mutations that have
occurred in the genome
o Invasion of Land
o The ability to move to land created a HUGE adaptive zone
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Plants: Nonvascular 1st (like mosses) and then seedless vascular (like ferns)
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Invertebrates: Arthropods (things with exoskeletons) first
o Evolution of Flight and wings (on insects)
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Vertebrates evolved: Jawless fishes  Jawed fishes  amphibians
The Mesozoic Era
o Seed Plants evolved: Gymnosperms (like evergreens) and then Angiosperms (flowering
plants)
o Animals: Amphibians diversified, reptiles appeared, and then birds & mammals
The Cenozoic Era
o Flowering plants diversify
o Mammals Diversify
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Primates appear
Climate cooled  many ice ages
GEOLOGICAL FACTORS IN EVOLUTION
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Continental drift results from the movement of great plates of crust and upper mantle that float
on the Earth’s molten core.
o Ex. North America and Europe are drifting apart at a rate of 2 cm per year.
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250 million years ago, plate movements brought all land masses together into a super-continent
called Pangaea.
o Species evolving in isolation where brought together and competition increased.
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180 million years ago Pangaea began to break up
o Marsupials probably evolved on the portion of Pangaea that is now North America and
migrated into the area that would become Australia.
The History of Life is Punctuated by Mass Extinctions Followed by Major Adaptive Radiations
Examples of Major Adaptive Radiations
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The evolution of some novel characteristics opened the way to new adaptive zones allowing
many taxa to diversify greatly during their early history.
o Ex. Insects and wings
o Ex. The Precambrian Era burst in diversity of sea animals, due to the origin of shells and
skeletons (700 million years ago).
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An empty adaptive zone can be exploited only if the right evolutionary novelties arise.
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An evolutionary novelty cannot enable organisms to exploit adaptive zones that are occupied or
that do not exist.
Examples of Mass Extinctions
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May be caused by habitat destruction or by unfavorable environmental changes
o Ex. The Permian Extinctions eliminated over 96% of the marine animals and 70% of
terrestrial organisms about 250 million years ago (during the formation of Pangaea)
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Period of extreme vulcanism  extreme global warming?
o Ex. The Cretaceous Extinction – 65 million years ago
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50% of the marine species and many terrestrial plants and animals (including
dinosaurs) were eliminated.
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climate was cooling (Ice age) and shallow seas receded from continental lowlands
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Evidence also indicates than an asteroid or comet (a bolide) struck the Earth
(Impact Hypothesis) while the Cretaceous extinctions were in progress.
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May or may not have been the cause