Transcript Origin of Life on Earth

Origin of Life on Earth
Primary Abiogenesis
 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.
Primordial Earth
Miller-Urey experiment showed the spontaneous formation of macromolecules
was possible with the conditions in the earth’s early atmosphere.
Chemical Evolution
Along 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 selfreplicate, 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.
Formation 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.
Prokaryotic cells
 The oldest known fossils on Earth are dated to 3.465 billion
years old – from Western Australia lsyered in formations
called stromatolites
 These microscopic fossils resemble present-day
cyanobacteria
 Although 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
Changing 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.
Evolution 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.
Evolution of the three domains
Eukaryotic cells
 The 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.
Development of internal membranes
Mitochondria and chloroplasts
 Early eukaryotic cells engulfed aerobic bacteria in a process
similar to phagocytosis in amoeba
 Having 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
Endosymbiosis
Evidence which supports this theory is that both mitochondria and chloroplasts:
• have their own DNA
• undergo division independently of their own cell’s division
• contain two sets of membranes (outer and inner membranes)
Multicellularity
 For 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.
Diversification
 The 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.
Diversification and extinction
Rate of Evolution
 When 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.
Rate of Evolution
 Niles 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.
Gradualism vs Punk Eek