Chapter 26
EARLY EARTH AND THE ORIGIN OF LIFE
HISTORY OF LIFE
Earth Chronology:
4.5 billion years ago earth formed
4-3.9 bya life originated
3.5 bya oldest known fossils (prokaryotes)
2.7 bya photosynthesis, oxygen accumulated
2.1 bya first eukaryotes
1.2 mya soft-bodies multicellular life
540 mya hard-bodied multicellular life
"Arrhenius and graduate student Stephen Mojzsis ... recently discovered the oldest chemical
evidence of life in sedimentary rocks from Greenland. The rocks are estimated to be more than
3.85 billion years old. Carbon in these rocks had an isotope profile seen only in remains of
organisms (S.J. Mojzsis et al., Nature, 384:51-9, 1996). "The evidence of the carbon signatures
is crucial in arguing that life on Earth was present before 3.85 billion years ago," maintains
Mojzsis. "Furthermore, the carbonaceous matter was found in intimate association with the
phosphate mineral apatite, a common biologically formed substance." Phosphates exist in cell
membranes, enzymes, genetic material, and biological energy molecules." http://www.thescientist.com/yr1997/mar/research_970331.html; The Scientist, March 31, 1997.
Life on earth originated between 3.5 and 4.0 billion years ago. The oldest fossils are those of
prokaryotes 3.5 billion years old.
Prokaryotes dominated the earth from about 3.5 to 2.0 billion years ago.
The domains Prokaryota and Archaea evolved in hydrothermal vents and in shallow water
communities that left fossils called stromatolites.
3.5 billion-year-old cyanobacteria fossils have been found in Warrawoona, Australia.
Mats of microorganisms, precipitates and sediments that are cemented together to form
laminated knobs called stromatolites.
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Fossil stromatolites lived in shallow water and intermittently exposed environment about
3.5 billion years ago.
Probably made by communities of filamentous and unicellular microorganisms some of
which were aerobes, others anaerobes and autotrophs and heterotrophs.
Modern stromatolites live in high salinity and high temperature waters well protected
from herbivores.
Modern stromatolite communities are composed of bacteria, cyanobacteria and algae.
Photosynthesis probably evolved very early in prokaryotic history but in metabolic versions that
did not split water and liberate oxygen.
Cyanobacteria are photosynthetic prokaryotes that probably evolved before 2.7 million years
ago.
Most atmospheric oxygen is of biological origin, from the water-splitting step of photosynthesis,
photolysis.
About 2 billion years ago the concentration of oxygen in the atmosphere began to gradually
increase to its present level.
An oxidizing aerobic environment began to form as oxygen accumulated and posed a challenge
to life.
We don't know at what speed oxygen began to accumulate in the atmosphere.
The eukaryotic cell probably evolved from a prokaryotic cell that was the host of several
prokaryotes called endosymbionts.
There are fossils of multicellular algae dating back to 2.1 million years ago.
The oldest fossils of animals are about 600 million years old.
By the Cambrian (545 m.y.a.) the levels of O2 had increased enough to allow the rapid
evolution of aerobic organisms.
Most phyla of animals appear in the fossil record between 540 and 520 million years ago.
Plants, fungi and animals colonized the land about 500 million years ago.
Plants colonized the land in a symbiotic relationship with fungi.
THE ORIGIN OF LIFE
Biogenesis is the principle that states that life comes from life.
The theory of Spontaneous Generation stated that life came from inanimate matter, e. g.
dead leaves in water produced frogs.
Louis Pasteur disproved the theory of Spontaneous Generation.
The first cells may have arisen from inorganic molecules that became progressively more
"organized" into polymers and then into pre-cellular droplets, protobionts.
Protobionts are droplets surrounded by membranes capable of carrying on chemical reactions
different from those in their surroundings.
Four stage hypothesis of the Origin of Life.
1. The synthesis of small organic molecules, such amino acids and sugars, and then more
complex ones like nucleotides.
2. The joining of simple molecules, monomers, into complex ones, polymers.
3. The origin of self-replicating molecules that eventually made inheritance possible.
4. The organizing of these molecules into protobionts.
Abiotic synthesis of organic molecules.
Oparin and Haldane (1920s) proposed independently that the primitive atmosphere of the
Earth was reducing and that organic compounds formed in such an atmosphere might be similar
to those presently used by living organisms.
The early atmosphere contained very little oxygen and was rich in hydrogen (H2) and
compounds that can donate hydrogen atoms to other substances.
Organic compounds have been detected in interstellar clouds and fallen meteorites.
Many but not all amino acids found in meteorites are the same as those used in protein
formation.
Oparin's and Haldane's ideas inspired the famous Miller-Urey experiment, which in 1953 began
the era of experimental prebiotic chemistry.
In 1953, Stanley Miller demonstrated that amino acids and other organic compounds could be
synthesized spontaneously from hydrogen gas, ammonia, methane and other compounds
presumed to have been present in the second atmosphere of the Earth if energy is provided.
Miller found that as much as 10 percent of the carbon in the system was converted to a
relatively small number of identifiable organic compounds, and up to 2 % of the carbon went to
making amino acids of the kinds that serve as constituents of proteins.
Glycine was the most abundant amino acid, resulting from the combination of formaldehyde
(CH2O), ammonia and hydrogen cyanide. A surprising number of the standard 20 amino acids
were also made in lesser amounts.
Since then, workers have subjected many different mixtures of simple gases to various energy
sources. The results of these experiments can be summarized neatly:
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Under sufficiently reducing conditions, amino acids form easily.
Conversely, under oxidizing conditions, they do not arise at all or do so only in small
amounts.
Small organic molecules polymerize when they are concentrated on hot sand, rock or clay.
Thermal energy was probably the energy originally used by the early protocells.
Heat in unreliable and varied from place to place.
Chemical energy providers allowed a constant and regulated source of energy to be used in
controlled and localized reactions.
Organic catalysts had to evolve to restrict reactions to the right time and place.
The cell developed a system that would provide energy in a controlled fashion when it is
needed.
Proteins add speed to the process of catalysis.
The first genes may have been abiotically produced RNA, whose base sequences served as
templates for both alignment of amino acids in polypeptide synthesis and alignment of
complementary nucleotide bases in a primitive form of self-replication.
In 1982, Kruger and colleagues discovered a catalytic and self-splicing RNA.
RNA could subsequently have developed the ability to link amino acids together into proteins.
This scenario could have occurred if prebiotic RNA had two properties not evident today:
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a capacity to replicate without the help of proteins
ability to catalyze every step of protein synthesis.
A nucleotide polymer (RNA) could self-replicate and also make proteins through the interaction
with AA.
In 1983 Thomas R. Cech of the University of Colorado at Boulder and, independently, Sidney
Altman of Yale University discovered the first known ribozymes, enzymes made of RNA.
Ribozymes are RNA fragments that posses catalytic properties.
"One can contemplate an RNA world, containing only RNA molecules that serve to catalyze the
synthesis of themselves" (W. Gilbert, Nature, 319:618, 1986).
The question that remains is the making of nucleotides and their polymerization. Attempts to
synthesize nucleotides directly from their components under prebiotic conditions have met with
only modest success.
Natural selection in the RNA world.
In a particular environment, RNA molecules of certain base sequences are more stable and
replicate faster and with fewer errors than other sequences.
The sequence best suited to the temperature, salt concentration, and other features of the
surrounding solution and having the greatest autocatalytic activity will prevail.
A variety of RNA will be produced because of replicating errors.
Occasionally a molecule will be produced that folds into a more stable shape and replicates
better than the ancestral form.
Protobionts
Phospholipids have a hydrophobic and a hydrophilic end.
Mechanical wave action can form vesicles composed of a bilayer through wave action or
mechanical agitation.
Properties of the bilayered vesicles:
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Selective permeability.
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Selective permeability allows the concentrations of particular compounds inside the vesicle.
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Concentration of substances allows certain reactions to occur within the droplet.
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The small size of the droplet allows a chain reaction to occur; the products of one reaction
become the reactants of the next.
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Some basic proteins cause an increase in the influx of nucleic acids into the vesicle.
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Droplet membranes could incorporate amphipathic proteins (span both sides of the bilayer)
that could act as H+ pumps.
The protobionts most effective at using resources from the environment and at reproducing
would increase their proportions among a population of varying protobionts through a process of
natural selection.
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Protobionts containing hereditary information will be favored.
MAJOR LINEAGES OF LIFE
Originally there were two kingdoms: Animalia and Plantae.
Over the years other kingdoms were recognized: Monera (or Prokaryotae), Fungi, Protista (or
Protoctista).
Most biologists recognize six kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae
and Animalia.
Domain is a new taxon above the kingdom based on fundamental molecular differences, e.g.
gene sequencing.
There are three domains recognized by many biologists: Archaea, Eubacteria and Eukarya.
The Protista is a polyphyletic group that will probably be split into several kingdoms when more
evidence accumulate.