Download Chapter 18 Origin and History of Life

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Chapter 18
Origin and
History of Life
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Origin of Life
The Early Earth
• The early atmosphere most likely consisted mainly of
these inorganic chemicals:
–
–
–
–
water vapor,
nitrogen,
carbon dioxide,
small amounts of hydrogen, methane, ammonia, hydrogen
sulfide, and carbon monoxide.
• The early atmosphere contained little free oxygen (O2)
and was probably a reducing atmosphere with little free
oxygen; a reducing atmosphere lacks free O2 and
allows formation of complex organic molecules.
• The early Earth was so hot that H2O only existed as a
vapor in dense, thick clouds.
• As the Earth cooled, H2O vapor condensed to form
liquid H2O, and rain collected in oceans.
Monomer Evolve
• There are three hypotheses that explain how organic
monomers could have evolved.
• Hypothesis one: Monomers came from outer space
– Comets and meteorites, perhaps carrying organic
chemicals, have pelted the Earth throughout history.
– A meteorite from Mars (ALH84001) that landed on Earth
13,000 years ago, may have fossilized bacteria.
• Hypothesis two: monomers came from reactions in the
atmosphere
– Oparin/Haldane independently suggested organic
molecules could be formed in the presence of outside
energy sources using atmospheric gases.
– Experiments performed by Miller and Urey (1953)
showed experimentally that these gases (methane,
ammonia, hydrogen, water) reacted with one another to
produce small organic molecules (amino acids, organic
acids).
– Lack of oxidation and decay allowed organic molecules to
form a thick, warm organic soup.
• Hypothesis three: monomers came from reactions at
hydrothermal vents
– Ammonia may have been scarce in the early atmosphere;
undersea thermal vents, which line ocean ridges, might
have been responsible for converting nitrogen to
ammonia.
– Under ventlike conditions, 70% of various nitrogen
sources were converted to ammonia within 15 minutes.
Polymers Evolve
• Gunter Wachterschaüser and Claudia Huber have shown that
organic molecules will react and amino acids will form peptides
in the presence of iron-nickel sulfides under ventlike
conditions.
• Protein-first Hypothesis
– Sidney Fox demonstrated amino acids polymerize abiotically if
exposed to dry heat.
– Amino acids collected in shallow puddles along the rocky shore;
heat of the sun caused them to form proteinoids (i.e., small
polypeptides that have some catalytic properties).
– When proteinoids are returned to water, they form cell-like
microspheres composed of protein.
– This assumes DNA genes came after protein enzymes; DNA
replication needs protein enzymes.
• The Clay Hypothesis
– Graham Cairns-Smith suggests that amino acids polymerize in clay,
with radioactivity providing energy.
– Clay attracts small organic molecules and contains iron and zinc
atoms serving as inorganic catalysts for polypeptide formation.
– If RNA nucleotides and amino acids became associated so
polypeptides were ordered by and helped synthesize RNA, then
polypeptides and RNA arose at the same time.
• RNA-first Hypothesis
– Only the macromolecule RNA was needed at the beginning to lead
to the first cell.
– Thomas Cech and Sidney Altman discovered that RNA can be both
a substrate and an enzyme.
– RNA would carry out processes of life associated with DNA (in
genes) and protein enzymes.
A Protocell Evolves
• Before the first true cell arose, there would have been a
protocell or protobiont.
• A protocell would have a lipid-protein membrane and
carry on energy metabolism.
• Sidney Fox showed that if lipids are made available to
microspheres, lipids become associated with
microspheres producing a lipid-protein membrane.
• Alec Bangham discovered that when he extracted lipids
from egg yolks and placed them in water, the lipids
would naturally organize themselves into doublelayered bubbles roughly the size of a cell. Bangham’s
bubbles soon became known as liposomes.
Fluorescent image of a liposome with a fluorophore-labeled
phospholipid incorporated in the bilayer.
• David Deamer and Bangham realized that liposomes might
have provided life’s first boundary. Perhaps liposomes with a
phospholipid membrane engulfed early molecules that had
enzymatic, even replicative abilities. These investigators called
this the “membrane-first” hypothesis.
A Self-Replication System Evolves
• Cairns-Smith suggests that polypeptides and RNA evolved
simultaneously.
– The first true cell would contain RNA genes that replicated because
of the presence of proteins; they become associated in clay in such a
way that the polypeptides catalyzed RNA formation.
• Once the protocell was capable of reproduction, it became a
true cell and biological evolution began.
A Recap of the Steps
• Most biologists suspect life evolved in basic steps.
– Abiotic synthesis of organic molecules such as amino acids
occurred in the atmosphere or at hydrothermal vents.
– Monomers joined together to form polymers at seaside
rocks or clay, or at vents; the first polymers could have
been proteins or RNA or both.
– Polymers aggregated inside a plasma membrane to make a
protocell that had limited ability to grow; if it developed in
the ocean it was a heterotroph, if at a hydrothermal vent, a
chemoautotroph.
– Once the protocell contained DNA genes or RNA
molecules, it was a true cell.
Fig. 18.4
History of Life
• Fossils Tell a Story
– A fossil is the remains or traces of past life, usually preserved in
sedimentary rock.
– Paleontology is the study of fossils and the history of life, ancient
climates, and environments.
– The great majority of fossils are found embedded in or recently
eroded from sedimentary rock.
– Sedimentation has been going on since the Earth was formed; it is
an accumulation of particles forming a stratum, a recognizable layer
in a stratigraphic sequence laid down on land or in water.
– The sequence indicates the age of fossils; a stratum is older than the
one above it and younger than the one below it.
• Relative Dating of Fossils
– However, geologists discovered that strata of the same age
contain the same fossils, termed index fossils.
– Therefore, fossils can be used for the relative dating of
strata.
– A particular species of fossil ammonite is found over a
wide range and for a limited time period; therefore, all
strata in the world that contain this ammonite are of the
same age.
• Absolute Dating of Fossils
– Absolute dating relies on radioactive dating to determine the actual
age of fossils.
– Radioactive isotopes have a half-life, the time it takes for half of a
radioactive isotope to change into a stable element.
– Carbon 14 (14C) is a radioactive isotope contained within organic
matter.
•Half of the carbon 14 (14C) will change to nitrogen 14 (14N)
every 5,730 years.
•Comparing 14C radioactivity of a fossil to modern organic matter
calculates the age of the fossil.
•After 50,000 years, the 14C radioactivity is so low it cannot be
used to measure age accurately.
– It is possible to determine the ratio of potassium 40 (40K) and argon
40 to date rocks and infer the age of a fossil.
The Precambrian Time
• As a result of their study of fossils in strata, geologists
have devised the geologic time scale, which divides the
history of the Earth into eras and then periods and
epochs.
• The first cells were probably prokaryotes.
• Prokaryotes, the archaea and bacteria, can live in the
most inhospitable of environments which may typify
habitats on early Earth.
Table 18.1
Eukaryotic Cells Arise
• The eukaryotic cell, which originated around 2.1 BYA,
is nearly always aerobic and contains a nucleus as well
as other membranous organelles.
• The nucleus may have developed by an invagination of
the plasma membrane.
• The mitochondria of the eukaryotic cells were once
free-living aerobic bacteria, and the chloroplasts were
free-living photosynthetic prokaryotes.
• The endosymbiotic theory states that a nucleated cell
engulfed these prokaryotes, which then became
organelles. The evidence for the theory is the
following:
– Present-day mitochondria and chloroplasts have a size that
lies within the range of that for bacteria.
– Mitochondria and chloroplasts have their own DNA and
make some of their own proteins.
– Mitochondria and chloroplasts divide by binary fission
similar to bacteria.
– The outer membrane of mitochondria and chloroplasts
differ.
Fig. 18.7