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Outline
• 18.1 Origin of Life• 18.2 History of Life• 18.3 Geological Factors That Influence
Evolution
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18.1 Origin of Life • The last universal common ancestor, (LUCA), is
common to all organisms that live, and have lived, on Earth since life began. Today, we say that “life only comes from life.” The molecules of living organisms, called biomolecules,
are organic molecules.• However, the first cells had to arise from nonliving chemicals,
inorganic substances.
Studies in chemistry, evolutionary biology, paleontology, microbiology, and other branches of science help scientists develop hypotheses about life’s origins.
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Origin of Life
• The earth came into being about 4.6 BYA. • The earth’s mass provides a gravitational field strong
enough to hold an atmosphere.• Primitive Earth atmosphere:
Most likely consisted of:• Water vapor• Nitrogen• Carbon dioxide• Small amounts of hydrogen, methane, ammonia, hydrogen sulfide,
and carbon monoxide• Little free oxygen
Originally too hot for liquid water to form
• As the earth cooled, water vapor condensed to liquid water.
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Origin of Life
• Four stages in life’s origins Stage 1: Organic monomers evolved from inorganic compounds. Stage 2: Organic polymers were joined to form organic
polymers. Stage 3: Organic polymers became enclosed in membranes to
form protocells or protobionts. State 4: Protobionts acquired the ability to self-replicate.
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Stages of the Origin of Life
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early Earth
Inorganic chemicals
Stage 1
Stage 2
Stage 3
Stage 4
Stage 4
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Origin of Life:first self-replicating cell
extinct lineages
Common ancestorof all life on Earth orLUCA (last universalcommon ancestor)
Nutrition• The process would have had to carry on nutrition in order
to grow.• If organic molecules formed in the atmosphere and were
carried into the ocean by rain, simple organic molecules could have served as food.
– According to this hypothesis, the protocell was a heterotroph, an organism that consumes preformed organic molecules.
• If the protocell evolved at hydrothermal events, it could have carried out chemosynthesis.
– Chemosynthesis is the synthesis of organic molecules by the oxidation of inorganic compounds.
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Origin of Life
• Evolution of the protocell• Nutrition (cont’d)
Natural selection would have favored cells that could extract energy from carbohydrates to produce ATP.
• Oxygen was not available.• The protocell may have carried on a form of fermentation.• Scientists speculate that it took millions of years for glycolysis,
a metabolic pathway that transforms high-energy chemical bonds into energy for a cell to do work, to evolve completely.
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Origin of Life
• A Self-Replication System RNA-first hypothesis
• The first cell would have had an RNA gene that directed protein synthesis.
• Reverse transcription could have led to DNA genes.• RNA was responsible for both DNA and protein formation.• Eventually, protein synthesis would have been carried out according
to the central dogma, with information flowing from DNA to RNA to protein.
Protein-first hypothesis• The protocell would have developed a plasma membrane and
enzymes.• Then, DNA and RNA synthesis would have been possible.• After DNA genes evolved, protein synthesis would have been carried
out according to the central dogma. After DNA formed, the genetic code had to evolve.
18.2 History of Life
• Fossils are the remains and traces of past life.• Paleontology is the study of the fossil record.• Most fossils are traces of organisms embedded in
sediment. Sediment is produced by weathering and erosion of rocks. Sediment becomes a recognizable stratum in a
stratigraphic sequence. Strata of the same age tend to contain the similar fossil
assemblages (index fossils) that can be used for relative dating.
This helps geologists determine relative dates of embedded fossils despite upheavals (relative dating).
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The History of Life
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Fossils
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History of Life• Absolute dating assigns an actual date to a fossil.• One absolute dating method relies on radiometric (radioactive)
dating techniques using, for example 14C, which decays to 14N.• Half-life:
The length of time required for half the atoms to change into another stable element
Unaffected by temperature, light, pressure, etc. All radioactive isotopes have a dependable half-life.
• Occurs at a constant rate• Some only fractions of a second• Some billions of years• Most in-between
• Many isotopes are used in absolute dating, and their combined half-lives make them useful over all periods of interest.
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History of Life• The geologic timescale
Divides the history of the earth into• Eras
• Periods
• Epochs
Derives from accumulation of data from the age of fossils in strata all over the world
Life arose during the Precambrian
Geologic Timescale: Cenozoic and Mesozoic Eras
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Geologic Timescale: Paleozoic and Precambrian Eras
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History of Life
• The Precambrian includes about 87% of the geological timescale. Little or no atmospheric oxygen in the early atmosphere
Lack of ozone shield allowed UV radiation to bombard Earth
• The first cells came into existence in aquatic environments. Prokaryotes appeared about 3.5 BYA.
Cyanobacteria fossils have been found in ancient stromatolites.
Photosynthetic cyanobacteria added oxygen to the atmosphere.
Aerobic bacteria proliferated in the oxygen-rich atmosphere.
New metabolic pathways evolved.
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The Tree of Life
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ARCHAEA
BACTERIA
first cells
photosynthetic bacteria (produce oxygen)
aerobic bacteria
other photosynthetic bacteria (do not produce oxygen)
• Contain a nucleus as well as other membranous organelles
Endosymbiotic Theory• Mitochondria were probably once free-living aerobic
prokaryotes.• Chloroplasts were probably once free-living photosynthetic
prokaryotes.• A nucleated cell probably engulfed these prokaryotes that
became various organelles. Cilia and flagella may have originated from slender
undulating prokaryotes that attached to the host cell.
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History of Life• Support for the Endosymbiotic Theory
Mitochondria and chloroplasts are similar in size to bacteria.
Mitochondria and chloroplasts have their own DNA and make some of their own proteins.
Mitochondria and chloroplasts divide by binary fission.
The outer membranes of mitochondria and chloroplasts differ.
• The outer membrane resembles a eukaryotic membrane.
• The inner membrane resembles a prokaryotic membrane.
History of Life
• Multicellularity Arises
About 1.4 BYA Separating germ cells from somatic cells may have
contributed to the diversity of organisms. Early multicellular organisms lacked internal organs
and could have absorbed nutrients from the sea. It’s possible that they practiced sexual
reproduction.• Among today’s protists are colonial forms in which some
cells are specialized to produce gametes needed for sexual reproduction.
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Ediacaran Fossils
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History of Life
• The Paleozoic Era It begins with the Cambrian period. It lasted over 300 million years. It includes three major mass extinction events. An extinction is the total disappearance of all
members of a species or higher taxonomic group. Mass extinction:
• Disappearance of a large number of taxa• Occurred within a relatively short time interval, a few million
years (compared to geological time scale)
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History of Life
• The abundance of fossils of animals of the Cambrian period may be due to the evolution of outer skeletons.
• The ancestry of all modern animals can be traced to the Cambrian period based on molecular clock data.
• Molecular Clock: Based on hypothesis that
• Changes in base-pair sequences of certain DNA segments occur at a fixed rate.
• The rate is not affected by natural selection or other external factors. When these base-pair sequences are compared between two
species:• Count the number of base-pair differences.• Count tells how long two species have been evolving separately.
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Sea Life of the Cambrian Period
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History of Life
• Invasion of land Began around 500 MYA Plants
• Seedless vascular plants date back to the Silurian period.• They later flourished in Carboniferous period.
Invertebrates• Arthropods were the first animals on land.• Outer skeleton and jointed appendages pre-adapted them to live
on land. Vertebrates
• Fishes first appeared in the Ordovician period.• Amphibians first appeared in the Devonian period and diversified
during the Carboniferous period. A mass extinction occurred at the end of the Permian
period.
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Swamp Forests of the Carboniferous Period
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History of Life
• The Mesozoic Era Triassic Period
• Nonflowering seed plants became dominant. Jurassic Period
• Dinosaurs achieved enormous size.• Mammals remained small and insignificant.
Cretaceous Period• Dinosaurs declined at the end of the Cretaceous period due
to a mass extinction.• Mammals:
– Began an adaptive radiation – Moved into habitats left vacated by dinosaurs
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Dinosaurs of the Late Cretaceous Period
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History of Life• The Cenozoic Era
It is divided into Tertiary and Quaternary periods.
Mammals continued adaptive radiation.
• Several species took to the air.
Flowering plants were already diverse and plentiful. Primate evolution began.
• Some primates adapted to living in trees for protection from predators and to obtain food in the form of fruit.
• Ancestral apes appeared during the Oligocene epoch.
• Megafauna during Pleistocene epoch– Human hunting may have been responsible for the extinction of mammalian megafauna.