CHAPTER 26
EARLY EARTH AND THE ORIGIN OF
LIFE
Life on Earth originated between 3.5
and 4.0 billion years ago
• The Earth formed about 4.5 billion years ago
• No clear fossils have been found in the oldest surviving
Earth rocks, from 3.8 billion years ago.
• The oldest fossils that have been uncovered were
embedded in rocks from western Australia that are 3.5
billion years ago.
Prokaryotes dominated evolutionary
history from 3.5 to 2.0 billion years ago
• The earliest organisms were prokaryotes.
• Relatively early, prokaryotes diverged into two
main evolutionary branches, the bacteria and the
archaea.
Oxygen began accumulating in the
atmosphere about 2.7 billion years ago
• Photosynthesis probably evolved very early in
prokaryotic history.
• Cyanobacteria, photosynthetic organisms that
split water and produce O2 as a byproduct,
evolved over 2.7 billion years ago.
• This early oxygen initially reacted with dissolved iron
to form the precipitate iron oxide. This can be seen
today in banded iron formations.
Eukaryotic life began by 2.1 billion
years ago
• The first clear evidence of a eukaryote appeared
about 2.1 billion years ago.
• This is the same time as the oxygen revolution
that changed the Earth’s environment so
dramatically.
• Chloroplasts and mitochondrion evolved
Multicellular eukaryotes evolved by 1.2
billion years ago
• A great range of eukaryotic unicellular forms
evolved into the diversity of present-day
“protists.”
• Recent fossils finds from China have produced a
diversity of algae and animals from 570 million
years ago, including preserved embryos.
Animal diversity exploded during the
early Cambrian period
• Most of the major
groups of animals
make their first
fossil appearances
during the
relatively short
span of the
Cambrian
period’s first 20
million years.
Plants, fungi, and animals colonized the
land about 500 million years ago
• Most orders of modern mammals, including
primates, appeared 50-60 million years ago.
• Humans diverged from other primates only 5
million years ago
The Origin of Life
• Spontaneous
generation- life arises
from nonliving matter
• In 1862, Louis Pasteur
conducted broth
experiments that
rejected the idea of
spontaneous generation
even for microbes.
• Biogenesis- all life
today arises only by the
reproduction of
preexisting life.
• One hypothesis is that chemical and physical
processes in Earth’s primordial environment
eventually produced simple cells.
(1) the abiotic synthesis of small organic molecules
(2) joining these small molecules into polymers
(3) origin of self-replicating molecules
(4) packaging of these molecules into “protobionts”
1. Abiotic synthesis of organic
molecules
• 1920’s, A.I. Oparin and J.B.S. Haldane
• Conditions on the early Earth favored the synthesis of
organic compounds from inorganic precursors.
• The reducing environment in the early atmosphere
would have promoted the joining of simple molecules
to form more complex ones.
• The energy required could be provided by lightning
and the intense UV radiation that penetrated the
primitive atmosphere.
• 1953, Stanley Miller
and Harold Urey
• The atmosphere
model consisted of
H2O, H2, CH4, and
NH3.
• It produced a variety
of amino acids and
other organic
molecules.
2. Laboratory simulations of earlyEarth conditions have produced
organic polymers
• Monomers should link to form polymers without
enzymes and other cellular equipment.
• Researchers have produced polymers, including
polypeptides, after dripping solutions of
monomers onto hot sand, clay, or rock.
3. RNA may have been the first genetic
material
• Many researchers have proposed that the first
hereditary material was RNA, not DNA.
• RNA can also function as an enzyme
• Short polymers of ribonucleotides can be
synthesized abiotically in the laboratory.
• 1980’s Thomas Cech- discovered that RNA
molecules are important catalysts in modern cells.
• Ribozymes- RNA catalysts that remove introns
from RNA.
• Ribozymes also help catalyze the synthesis of
new RNA polymers.
4A. Protobionts can form by self-assembly
• Protobionts- maintain an internal chemical
environment from their surroundings and may
show some properties associated with life,
metabolism, and excitability.
• Liposomes- droplets of abiotically produced
organic compounds that form from lipids. Forms
a molecular bilayer like the lipid bilayer of a
membrane.
• If enzymes are added, they are incorporated into
the droplets.
• The protobionts are
then able to absorb
substrates from
their surroundings
and release the
products of the
reactions catalyzed
by the enzymes.
4B. Natural section could refine
protobionts containing hereditary
information
• Once primitive RNA genes and
their polypeptide products were
packaged within a membrane,
the protobionts could have
evolved as units.
• The most successful
protobionts would grow and
split, distributing copies of
their genes to offspring.
• Later, DNA would replace
RNA for storing genetic
information.
Debates about the origin of life abounds
• Laboratory simulations cannot prove that these
kinds of chemical processes actually created life
on the primitive Earth. They describe steps that
could have happened.
• Among the debates is whether organic monomers on
early Earth were synthesized in deep-sea vents or
reached Earth on comets and meteorites.
• The leap from an aggregate of molecules that
reproduces to even the simplest prokaryotic cell is
immense, and change must have occurred in many
smaller evolutionary steps.
The five-kingdom system reflected
increased knowledge of life’s diversity
• 1969, R.H Whittaker, A five-kingdom system:
Monera, Protista, Plantae, Fungi, and Animalia.
Arranging the diversity of life into the
highest taxa is a work in progress
• Three-domain system: Bacteria, Archaea, and
Eukarya, as superkingdoms.