Chapter 25

CHAPTER 26
Early Earth and the Origin of Life
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Section A: Introduction to the History of Life
1. Life on Earth originated between 3.5 and 4.0 billion years ago
2. Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years
ago
3. Oxygen began accumulating in the atmosphere about 2.7 billion years ago
4. Eukaryote life began by 2.1 billion years ago
5. Multicellular eukarotes evolved by 1.2 billion years ago
6. Animal diversity exploded during the early Cambrian period
7. Plants, fungi, and animals colonized the land about 500 million years ago

• Life is a continuum extending from the earliest
organisms through various phylogenetic branches
to the great variety of forms alive today.
• The diversification of life on Earth began over 3.8
billion ago.
Introduction

• Geologic events that alter environments have
changed the course of biological evolution.
• For example, the formation and subsequent breakup of
the supercontinent Pangea has a tremendous impact on
the diversity of life.
• Conversely, life has changed the planet it inhabits.
• The evolution of photosynthetic organisms that release
oxygen into the air had a dramatic impact on Earth’s
atmosphere.
• Much more recently, the emergence of Homo sapiens
has changed the land, water, and air on a scale and on
a rate unprecedented for a single species.

• Historical study of any sort is an inexact
discipline that depends on the preservation,
reliability, and interpretation of past records.
• The fossil record of past life is generally less and less
complete the farther into the past we delve.
• Fortunately, each organism alive today carries traces
of its evolutionary history in its molecules,
metabolism, and anatomy.
• Still, the evolutionary episodes of greatest antiquity
are the generally most obscure.

• One can view
the chronology
of the major
episodes that
shaped life as a
phylogenetic
tree.
Fig. 26.1

• Alternatively, we
can view these
episodes with a
clock analogy.
Fig. 26.2

• For the first three-quarters of evolutionary history,
Earth’s only organisms were microscopic and
mostly unicellular.
• The Earth formed about 4.5 billion years ago, but rock
bodies left over from the origin of the solar system
bombarded the surface for the first few hundred
million years, making it unlikely that life could
survive.
• No clear fossils have been found in the oldest
surviving Earth rocks, from 3.8 billion years ago.
1. Life on Earth originated between 3.5
and 4.0 billion years ago

• The oldest fossils that have been uncovered were
embedded in rocks from western Australia that
are 3.5 billion years ago.
• The presence of these fossils, resembling bacteria,
would imply that life originated much earlier.
• This may have been as early as 3.9 billion years ago,
when Earth began
to cool to a temper-
ature at which
liquid water
could exist.
Fig. 26.3a

• Prokaryotes dominated evolutionary history from
about 3.5 to 2.0 billion years ago.
• The fossil record supports the hypothesis that the
earliest organisms were prokaryotes.
• Relatively early, prokaryotes diverged into two
main evolutionary branches, the bacteria and the
archaea.
• Representatives from both groups thrive in various
environments today.
2. Prokaryotes dominated evolutionary
history from 3.5 to 2.0 billion years ago

• Two rich sources for early prokaryote fossils are
stromatolites (fossilized layered microbial mats)
and sediments from ancient hydrothermal vent
habitats.
• This indicates that the metabolism of prokaryotes was
already diverse over 3 billion years ago.
Fig. 26.4

• Photosynthesis probably evolved very early in
prokaryotic history.
• The metabolism of early versions of photosynthesis
did not split water and liberate oxygen.
3. Oxygen began accumulating in the
atmosphere about 2.7 billion years ago

• 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.
• About 2.7 billion years ago oxygen began
accumulating in the atmosphere and terrestrial rocks
with iron began oxidizing.

• While oxygen accumulation was gradual between
2.7 and 2.2 billion years ago, it shot up to 10% of
current values shortly afterward.
• This “corrosive” O2 had an enormous impact on
life, dooming many prokaryote groups.
• Some species survived in habitats that remained
anaerobic.
• Other species evolved mechanisms to use O2 in
cellular respiration, which uses oxygen to help
harvest the energy stored in organic molecules.

• Eukaryotic cells are generally larger and more
complex than prokaryotic cells.
• In part, this is due to the apparent presence of the
descendents of “endosymbiotic prokaryotes” that
evolved into mitochondria and chloroplasts.
4. Eukaryotic life began by 2.1 billion
years ago

• While there is some evidence of earlier eukaryotic
fossils, the first clear eukaryote appeared about
2.1 billion years ago.
• Other evidence places the origin of eukaryotes to as
early as 2.7 billion years ago.
• This places the earliest eukaryotes at the same
time as the oxygen revolution that changed the
Earth’s environment so dramatically.
• The evolution of chloroplasts may be part of the
explanation for this temporal correlation.
• Another eukaryotic organelle, the mitochondrion,
turned the accumulating O2 to metabolic advantage
through cellular respiration.

• A great range of eukaryotic unicellular forms
evolved into the diversity of present-day
“protists.”
• Multicellular organisms,
differentiating from a
single-celled precursor,
appear 1.2 billion years
ago as fossils, or perhaps
as early as 1.5 billion
years ago from molecular
clock estimates.
5. Multicellular eukaryotes evolved by
1.2 billion years ago
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.6

• Recent fossil finds from China have produced a
diversity of algae and animals from 570 million
years ago, including beautifully preserved
embryos.
Fig. 26.7

• Geologic evidence for a severe ice age (“snowball
Earth” hypothesis) from 750 to 570 million years
ago may be responsible for the limited diversity
and distribution of multicellular eukaryotes until
the very late Precambrian.
• During this period, most life would have been
confined to deep-sea vents and hot springs or those
few locations where enough ice melted for sunlight to
penetrate the surface waters of the sea.
• The first major diversification of multicellular
eukaryotic organisms corresponds to the time of the
thawing of snowball Earth.

• A second radiation of eukaryotic forms produced
most of the major groups of animals during the
early Cambrian period.
• Cnidarians (the phylum that includes jellies) and
poriferans (sponges) were already present in the
late Precambrian.
6. Animal diversity exploded during the
early Cambrian period

• However, most of
the major groups
(phyla) of animals
make their first
fossil appearances
during the relatively
short span of the
Cambrian period’s
first 20 million years.
Fig. 26.8

• The colonization of land was one of the pivotal
milestones in the history of life.
• There is fossil evidence that cyanobacteria and other
photosynthetic prokaryotes coated damp terrestrial
surfaces well over a billion years ago.
• However, macroscopic life in the form of plants, fungi,
and animals did not colonize land until about 500
million years ago, during the early Paleozoic era.
7. Plants, fungi, and animals colonized
the land about 500 million years ago

• The gradual evolution from aquatic to terrestrial
habitats required adaptations to prevent
dehydration and to reproduce on land.
• For example, plants evolved a waterproof coating of
wax on the leaves to slow the loss of water.
• Plants colonized land in association with fungi.
• Fungi aid the absorption of water and nutrients from
the soil.
• The fungi obtain organic nutrients from the plant.
• This ancient symbiotic association is evident in some
of the oldest fossilized roots.

• Plants created new opportunities for all life,
including herbivorous (plant-eating) animals and
their predators.
• The most widespread and diverse terrestrial animals
are certain arthropods (including insects and
spiders) and certain vertebrates (including
amphibians, reptiles, birds, and mammals).
• Most orders of modern mammals, including
primates, appeared 50-60 million years ago.
• Humans diverged from other primates only 5
million years ago

• The terrestrial vertebrates, called tetrapods because
of their four walking limbs, evolved from fishes,
based on an extensive fossil record.
• Reptiles evolved from amphibians; both birds and
mammals evolved from reptiles.
• Most orders of modern mammals, including
primates, appeared 50-60 million years ago.
• Humans diverged from other primates only 5
million years ago.

CHAPTER 26
Section B: The Origin of Life
1. The first cells may have originated by chemical evolution on a young Earth:
an overview
2. Abiotic synthesis of organic monomers is a testable hypothesis
3. Laboratory simulations of early-Earth conditions have produced organic
polymers
4. RNA may have been the first genetic material
5. Protobionts can form by self-assembly
6. Natural selection could refine protobionts containing hereditary
information
7. Debate about the origin of life abounds

• Sometime between about 4.0 billion years ago,
when the Earth’s crust began to solidify, and 3.5
billion years ago when stromatolites appear, the
first organisms came into being.
• We will never know for sure, of course, how life on
Earth began.
• But science seeks natural causes for natural
phenomena.
Introduction

• Most scientists favor the hypothesis that life on Earth
developed from nonliving materials that became ordered
into aggregates that were capable of self-replication and
metabolism.
• From the time of the Greeks until the 19th century, it was
common “knowledge” that life could arise from nonliving
matter, an idea called spontaneous generation.
• While this idea had been rejected by the late Renaissance
for macroscopic life, it persisted as an explanation for the
rapid growth of microorganisms in spoiled foods.
1. The first cells may have originated by
chemical evolution on a young Earth:
an overview

• In 1862, Louis
Pasteur conducted
broth experiments
that rejected the
idea of spontaneous
generation
even for microbes.
• A sterile broth
would “spoil” only
if microorganisms
could invade from
the environment.
Fig. 26.9

• All life today arises only by the reproduction of
preexisting life, the principle of biogenesis.
• Although there is no evidence that spontaneous
generation occurs today, conditions on the early
Earth were very different.
• There was very little atmospheric oxygen to attack
complex molecules.
• Energy sources, such as lightning, volcanic activity,
and ultraviolet sunlight, were more intense than what
we experience today.

• One credible hypothesis is that chemical and
physical processes in Earth’s primordial
environment eventually produced simple cells.
• Under one hypothetical scenario this occurred in
four stages:
(1) The abiotic synthesis of small organic molecules;
(2) The joining these small molecules into polymers:
(3) The origin of self-replicating molecules;
(4) The packaging of these molecules into “protobionts.”
• This hypothesis leads to predictions that can be
tested in the laboratory.

• In the 1920s, A.I. Oparin and J.B.S. Haldane
independently postulated that conditions on the
early Earth favored the synthesis of organic
compounds from inorganic precursors.
• They reasoned that this cannot happen today because
high levels of oxygen in the atmosphere attack
chemical bonds.
2. Abiotic synthesis of organic
monomers is a testable hypothesis

• The reducing environment in the early atmosphere
would have promoted the joining of simple
molecules to form more complex ones.
• The considerable energy required to make organic
molecules could be provided by lightning and the
intense UV radiation that penetrated the primitive
atmosphere.
• Young suns emit more UV radiation and the lack of an
ozone layer in the early atmosphere would have
allowed this radiation to reach the Earth.

• In 1953, Stanley Miller and Harold Urey tested
the Oparin-Haldane hypothesis by creating, in the
laboratory, the
conditions that
had been postulated
for early Earth.
• They discharged sparks
in an “atmosphere” of
gases and water vapor.
Fig. 26.10

• The Miller-Urey experiments produced a variety
of amino acids and other organic molecules.
• The atmosphere in the Miller-Urey model consisted of
H2O, H2, CH4, and NH3, probably a more strongly
reducing environment than is currently believed to
have existed on early Earth.
• Other attempts to reproduce the Miller-Urey
experiment with other gas mixtures also produced
organic molecules, although in smaller quantities.

• The Miller-Urey experiments still stimulate
debate on the origin of Earth’s early stockpile of
organic ingredients.
• Alternate sites proposed for the synthesis of organic
molecules include submerged volcanoes and deep-sea
vents where hot water and minerals gush into the deep
ocean.
• Another possible source for organic monomers on
Earth is from space, including via meteorites
containing organic molecules that crashed to Earth.

• The abiotic origin hypothesis predicts that
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.
• Similar conditions likely existed on the early Earth
when dilute solutions of monomers splashed onto fresh
lava or at deep-sea vents.
3. Laboratory simulations of early-
Earth conditions have produced
organic polymers

• Life is defined partly by inheritance.
• Today, cells store their genetic information as
DNA, transcribe select sections into RNA, and
translate the RNA messages into enzymes and
other proteins.
• Many researchers have proposed that the first
hereditary material was RNA, not DNA.
• Because RNA can also function as an enzyme, it helps
resolve the paradox of which came first, genes or
enzymes.
4. RNA may have been the first genetic
material

• Short polymers of ribonucleotides can be
synthesized abiotically in the laboratory.
• If these polymers are added to a solution of
ribonucleotide monomers, sequences up to 10 bases long
are copied from the template according to the base-
pairing rules.
• If zinc is added, the copied sequences may reach 40
nucleotides with less than 1% error.

• In the 1980s Thomas Cech discovered that RNA
molecules are important catalysts in modern cells.
• RNA catalysts, called ribozymes, remove introns
from RNA.
• Ribozymes also help catalyze the synthesis of
new RNA polymers.
• In the pre-biotic world, RNA molecules may have
been fully capable of ribozyme-catalyzed
replication.

• Laboratory experiments have demonstrated that
RNA sequences can evolve in abiotic conditions.
• RNA molecules have both a genotype (nucleotide
sequence) and a phenotype (three-dimensional
shape) that interacts with surrounding molecules.
• Under particular conditions, some RNA
sequences are more stable and replicate faster and
with fewer errors than other sequences.
• Occasional copying errors create mutations and
selection screens these mutations for the most stable or
the best at self-replication.

• RNA-directed protein synthesis may have begun
as weak binding of specific amino acids to bases
along RNA molecules, which functioned as
simple templates holding a few amino acids
together long enough for them to be linked.
• This is one function of rRNA today in ribosomes.
• If RNA synthesized a short polypeptide that
behaved as an enzyme helping RNA replication,
then early chemical dynamics would include
molecular cooperation as well as competition.

• Living cells may have been preceded by
protobionts, aggregates of abiotically produced
molecules.
• Protobionts do not reproduce precisely, but they
do maintain an internal chemical environment
different from their surroundings and may show
some properties associated with life, including
metabolism and excitability.
5. Protobionts can form by
self-assembly

• In the laboratory, droplets of abiotically produced
organic compounds, called liposomes, form when
lipids are included in the mix.
• The lipids form a molecular bilayer at the droplet
surface, much like the lipid bilayer of a
membrane.
• These droplets can undergo osmotic swelling or
shrinking in different salt concentrations.
• They also store energy as a membrane potential, a
voltage cross the surface.

• Liposomes behave dynamically, growing by
engulfing smaller liposomes or “giving birth” to
smaller liposomes.
Fig. 26.12a

• If enzymes are included among the ingredients,
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.
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Fig. 26.12b

• Unlike some laboratory models, protobionts that
formed in the ancient seas would not have
possessed refined enzymes, the products of
inherited instructions
• However, some molecules produced abiotically do
have weak catalytic capacities.
• There could well have been protobioints that had a
rudimentary metabolism that allowed them to modify
substances they took in across their membranes.

• Once primitive RNA genes
and their polypeptide
products were packaged
within a membrane, the
protobionts could have
evolved as units.
• Molecular cooperation could
be refined because favorable
components
were concentrated
together, rather than
spread throughout the
surroundings.
6. Natural selection could refine
protobionts containing hereditary
information

• As an example: suppose that an RNA molecule
ordered amino acids into a primitive enzyme that
extracted energy from inorganic sulfur
compounds taken up from the surroundings
• This energy could be used for other reactions within
the protobiont, including the replication of RNA.
• Natural selection would favor such a gene only if its
products were kept close by, rather than being shared
with competing RNA sequences in the environment.

• The most successful protobionts would grow and
split, distributing copies of their genes to
offspring.
• Even if only one such protobiont arose initially
by the abiotic processes that have been described,
its descendants would vary because of mutation,
errors in copying RNA.

• Evolution via differential reproductive success of
varied individuals presumably refined primitive
metabolism and inheritance.
• One refinement was the replacement of RNA as the
repository of genetic information by DNA, a more
stable molecule.
• Once DNA appeared, RNA molecules wold have
begun to take on their modern roles as intermediates in
translation of genetic programs.

• Laboratory simulations cannot prove that these
kinds of chemical processes actually created life
on the primitive Earth.
• They describe steps that could have happened.
• The origin of life is still subject to much
speculation and alternative views.
• Among the debates are whether organic monomers on
early Earth were synthesized there or reached Earth on
comets and meteorites.
7. Debates about the origin of life
abounds

• Major debates also concern where life evolved.
• The prevailing site until recently was in shallow
water or moist sediments.
• However, some scientists, including Günter
Wachtershäuser and colleagues, have proposed that
life originated in deep-sea vents.

• Modern phylogenetic analyses indicate that the
ancestors of modern prokaryotes thrived in very
hot conditions and may have lived on inorganic
sulfur compounds that are common in deep-sea
vent environments.
• These sites have energy sources that can be used by
modern prokaryotes, produce some organic
compounds, and have inorganic iron and nickel
sulfides that can catalyze some organic reactions.

• As understanding of our solar system has
improved, the hypothesis that life is not restricted
to Earth has received more attention.
• The presence of ice on Europa, a moon of Jupiter, has
led to hypotheses that liquid water lies beneath the
surface and may support life.
• While Mars is cold, dry, and lifeless today, it was
probably relatively warmer, wetter, and with a CO2-
rich atmosphere billions of years ago.
• Many scientists see Mars as an ideal place to test
hypotheses about Earth’s prebiotic chemistry.

• Debate about the origin of terrestrial and
extraterrestrial life abounds.
• 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 point at which we stop calling membrane-
enclosed compartments that metabolize and replicate
their genetic programs protobionts and begin calling
them living cells is as fuzzy as our definition of life.
• Prokaryotes were already flourishing at least 3.5
billion years ago and all the lineages of life arose from
those ancient prokaryotes.

CHAPTER 26
Section C: The Major Lineages of Life
1. The five kingdom system reflected increased knowledge of life’s
diversity
2. Arranging the diversity of life into the highest taxa is a work in
progress

• Traditionally, systematists have considered
kingdom as the highest taxonomic category.
• As a product of a long tradition, beginning with
Linnaeus organisms were divided into only two
kingdoms of life - animal or plant.
• Bacteria, with rigid cell walls, were placed with plants.
• Even fungi, not photosynthetic and sharing little with
green plants, were considered in the plant kingdom.
• Photosynthetic, mobile microbes were claimed by both
botanists and zoologists.
1. The five-kingdom system reflected
increased knowledge of life’s diversity

• In 1969, R.H Whittaker argued for a five-
kingdom system: Monera, Protista, Plantae,
Fungi, and Animalia.
Fig. 26.15

• The five-kingdom system recognizes that there
are two fundamentally different types of cells:
prokaryotic (the kingdom Monera) and eukaryotic
(the other four kingdoms).
• Three kingdoms of multicellular eukaryotes were
distinguished by nutrition, in part.
• Plant are autotrophic, making organic food by
photosynthesis.
• Most fungi are decomposers with extracellular
digestion.
• Most animals digest food within specialized cavities.

• In Whittaker’s system, the Protista consisted of all
eukaryotes that did not fit the definition of plants,
fungi, or animals.
• Most protists are unicellular.
• However, some multicellular organisms, such as
seaweeds, were included in the Protista because of
their relationships to specific unicellular protists.
• The five-kingdom system prevailed in biology for over
20 years.

• During the last three decades, systematists
applying cladistic analysis, including the
construction of cladograms based on molecular
data, have been identifying problems with the
five-kingdom system.
• One challenge has been evidence that there are two
distinct lineages of prokaryotes.
• These data led to the three-domain system: Bacteria,
Archaea, and Eukarya, as superkingdoms.
2. Arranging the diversity of life into
the highest taxa is a work in progress

• Many microbiologists have divided the two
prokaryotic domains into multiple kingdoms
based on cladistic analysis of molecular data.
Fig. 26.16

• A second challenge to the five kingdom system
comes from systematists who are sorting out the
phylogeny of the former members of the kingdom
Protista.
• Molecular systematics and cladistics have shown that
the Protista is not monophyletic.
• Some of these organisms have been split among five
or more new kingdoms.
• Others have been assigned to the Plantae, Fungi, or
Animalia.

• Clearly, taxonomy at the highest level is a work in
progress.
• It may seem ironic that systematists are generally more
confident in their groupings of species into lower tax
than they are about evolutionary relationships among
the major groups of organisms.
• Tracing phylogeny at the kingdom level takes us back
to the evolutionary branching that occurred in
Precambrian seas a billion or more years ago.

• There will be much more research before there is
anything close to a new consensus for how the
three domains of life are related and how many
kingdoms there are.
• New data will undoubtedly lead to further taxonomic
modeling.
• Keep in mind that phylogenetic trees and taxonomic
groupings are hypotheses that fit the best available
data.

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