The Origins of Life on
Earth
(or, a History of our planet
in a week or less)
Biology 2, College of the Atlantic
Spring 2002
• How old is this planet anyway?
• Theories of Origin
• Geological and Biological
timescales
• Phylogeny (and an awful lot of it)
How old is this planet anyway?
• The Universe is probably ~13 billion
years old (Big Bang Theory/Doppler
Shift)
• Earth is ~4.5 billion years old (begins
with cooling of crust/solidification)
• Earliest records of life ~3.5 billion years
ago
• First humans (Australopithecus), 0.005
billion years ago
• Discovery of Australopithecus fossils ,
The Fragility of Life - Coincidence
#1
• Life can only exist
within temperatures
corresponding to the
boiling and freezing
point of water
• This range is a
fraction of the range
between absolute
zero (-273°C) and
the temperature of
the sun (106°C)
How did life evolve?
• Three theories
– Creationism
– Extraterrestrial origin (Panspermia)
– Spontaneous Origin (Coincidence #2)
Physical conditions of early
Earth - Coincidence #3
• Temperatures in correct range (in
general, water in fluid state, carbon
compounds non-brittle)
• Size of planet retains an atmosphere
• Early atmosphere lacked oxygen,
therefore highly reductive
• High energy bombardment from sun
promotes generation of organics
Spontaneous origins of life - 4
steps
• Abiotic synthesis and accumulation of
organic compounds
• Polymerization
• Aggregation of polymers into nonliving
structures (Protobionts)
• Origin of heredity
Experimental evidence of
Spontaneous Origin
• Theories of Oparin and Haldane—tested by
Miller and Urey—demonstrate formation of
organics under conditions typical of early
Earth
• Polymerization can occur with appropriate
substrate
• Abiotically produced proteins (proteinoids)
self-assemble into Protobionts (selectively
permeable membrane)
The final key - Heredity
• First passage of genetic information
probably occurred through short strands
of RNA (also autocatalyst, e.g
ribozymes)
• Mutations cause variation
• “Natural selection” of molecular
combinations
• Origin of DNA
Biological time scales
• Biological timescales by necessity follow
geological timescales
• Often, geological events marked by key
biological events (mass
extinctions/diversifications)
• First fossil record of life 3.5 billion years
ago (prokaryote), in the Precambrian
• Earliest eukaryote ~1.5 billion years ago
(endosymbiotic theory)
Earth - The Early Years
• Late Precambrian saw the first
eukaryotic multicellular life
• Boundary between Precambrian and
Cambrian (580 mya) marked by a rapid
adaptive radiation/diversification of
marine life (Cambrian explosion)
• By the middle of the Cambrian, all of the
animal phyla existing today had evolved
The drive behind MacroEvolution
• Biological forces: natural selection
working in general, but particularly
effectively on genes controlling
– allometric growth
– paedomorphosis
• Physical forces
– Plate tectonics, leading to formation and
splitting of supercontinents
The study of evolutionary
history: Phylogeny
• Modern Darwinian synthesis suggests
adaptive radiation from a common ancestor
• Concept of phylogeny supported through
studies of homology
• Traditional classification systems (Linnaeus)
are monophyletic, based on homology 
parallel or divergent evolution
• Some groupings are polyphyletic, with
analogous structure  convergent evolution
The Kingdom System
• Scientists follow various taxonomic
systems: Campbell uses the 5 kingdom
classification scheme
– Monera
– Protista
– Plantae
– Fungi
– Animalia
Phylogeny recounts the
“natural selection” of species
(Earth: the Middle Years)
• First major extinction at end of the Paleozoic era
(the Permian Extinction), probably caused by
collision of tectonic plates to form the
supercontinent, Pangaea
• Pangaea marks the birth of a new era, the
Mesozoic (Triassic, Jurassic, Cretaceous)
• Mesozoic ends with second mass extinction—
the Cretaceous Extinction (impact hypothesis)
And now...
• Currently in the Recent epoch of the
Quarternary period of the Cenozoic era
• History may tell of a third mass
extinction?
• Radically changing planet will continue
to apply selective pressure to species
Monera :
the Pioneers of Life on Earth
• The most ‘successful’ group of
organisms on the planet
• 3.5 billion year history
• Although only 4000 species known, the
number of extant species is thought to
be ~4,000 – 4 x106
• Found in all ecological niches, including
some where other forms of life cannot
exist
The current importance of
Monera
• In some cases at base of food chain
• Vital roles in various elemental cycles
– Carbon cycle
– Nitrogen cycle
• Interactions with human life
– Symbiosis (E.coli)
– Pathogenic bacteria (physical,
exo/endotoxins)
– Commercial/Industrial/Scientific uses
The phylogeny of Prokaryotes
Early Prokaryote
Domain
Bacteria
(Eubacteria)
Domain
Archaea
(Archaebacteria)
Protista
Domain
Eukarya
(Eukaryotes)
Fungi
Plantae
Animalia
Archaebacteria
• Treated either as Domain, or subphylum
• Cell plan similar to most primitive
prokaryotic fossils
• Tend to exist in extreme environments
• Smaller group of species
– Methanogens (mmm-mmmm!)
– Extreme Halophiles
– Extreme Thermophiles (Sulfolobus)
Eubacteria
• More diverse group
– Spirochetes (Treponema)
– Clamydias (Clamydia trachomatis)
– Gram-positive eubacteria (Bacillus)
– Cyanobacteria (blue-green alga)
– Proteobacteria (E.coli, Salmonella)
Structure
• Small (1-5 µm)
• Of three general shapes
– Coccus (pl. cocci), e.g. Streptococcus
– Bacillus (pl. bacilli) e.g. Bacillus
– Spirillum (pl. spirilla) e.g. Treponema
• Cell wall made of peptidoglycan
– Leads to gram +/-ve distinction
• Some have a capsule, and/or pili,
and/or flagella
Physiology
• Various forms of nutrition
– Autotrophs (obtain carbon from inorganic
CO2
• Photoautotrophs (energy from sunlight)
• Chemoautotrophs (energy from inorganics)
– Heterotrophs (carbon from organics)
• Photoheterotrophs
• Chemoheterotrophs**
• Origins of glycolysis, chemiosmosis and
cellular respiration
Reproduction
•
•
•
•
Single strand of DNA
No mitosis/meiosis
Only Binary fission
Some sexual recombination through
– Transformation
– Conjugation
– Transduction
• Some form endospores
Kingdom Animalia
Invertebrata
•
•
•
•
What is an animal?
Anatomy, Embryology and Ontogeny
Parazoa
Eumetazoa
– Radiata/Bilateria
– Acoelomate/Pseudocoelomate/Eucoelomate
– Protostomes/Deuterostomes
What is an animal?
• Likely ancestor is a protist: a Precambrian
choanoflagellate
• Multicellular, heterotrophic eukaryotes
(usually exhibit ingestion)
• Storage of energy-rich reserves as glycogen
• Lack of cell walls. Unique cell junctions
• Unique tissues: muscle, nervous tissue
• Unique embryology
Embryology
• Diploid zygote divides by the mitotic
process of cleavage
• Formation of blastula followed by
gastrulation (creation of gastrula)
• Mode of embryological development
provides a taxonomic key to invertebrates
First taxonomic dichotomy
• Asymmetry versus Symmetry divides
Kingdom Animalia into sub-kingdoms:
– Parazoa (lacks tissues, asymmetrical, “like
animals”). Represented by only one
phylum: Porifera (sponges)
– Eumetazoa (“true animals”, exhibit
symmetry, represented by remainder of
Kingdom Animalia
Porifera (Sponges)
• General form: Two layered cup
(separated by mesohyl), with porocytes
entering into spongocoel, exiting
through osculum
• Outer layer (epidermis) reinforced by
spicules (Si)
• Filter feeding by Choanocytes that line
endodermis
• Transport of materials by Amoebocytes
Second taxonomic dichotomy
• Radial versus bilateral symmetry
– Super-phylum Radiata exhibits radial
symmetry. Two phyla include:
• Cnidaria (jellyfish, anemones, corals and
hydra)
• Ctenophora (combjellies)
– Super-phylum Bilateria exhibits bilateral
symmetry (rest of invertebrata)
Bilateral symmetry leads to...
Cephalization
• Bilateral symmetry implies a directionality
to the animal
• With movement in a specific direction
comes development of sensory
equipment at end that encounters
environment first
• Collection of sensory nervous tissue at
anterior end of animal = cephalization
• Type of symmetry also reflected by
embryonic germ layers seen in
blastula/gastrula:
Ectoderm
Mesoderm
Endoderm
Diploblastic
= Radiata
(endoderm/ectoderm)
Triploblastic
= Bilateria
(endoderm/mesoderm
/ectoderm)
Cnidaria (Jellyfish, etc.)
• Diploblastic, radially symmetrical (i.e. no
cephalization)
• Phylum characterized by cnidocytes
that eject nematocysts
• Gastrovascular cavity (GVC), with one
opening (mouth/anus simultaneously)
• Tentacles to pull prey into GVC
• Some species exhibit alternation of
sexual and asexual forms (e.g. Obelia)
• Classes within Cnidaria include:
– Hydrozoa (e.g. Obelia, Hydra)
– Scyphozoa (jellyfish, e.g. Sea wasp,
Lionsmane, Portuguese Man-o-war)
– Anthozoa: calcareous secretions build an
exoskeleton (e.g. coral, anenomes,
Metridia)
Third taxonomic dichotomy
• Design of body cavity (or lack thereof)
characterizes Bilateria
– Acoelomates lack a body cavity, e.g.
Platyhelminthes (flatworms)
– Body cavities
• Pseudocoelomates have a body cavity lined
by mesoderm-derived tissue on one side only,
e.g. Nematoda (roundworms), Rotifera
• Eucoelomates (coelomates) body cavity is
lined on both sides by mesodermally-derived
tissue (everything else upwards)
Tissue derived from:
Ectoderm
Mesoderm
Endoderm
GVC
Acoelomate
GVC = Gastrovascular cavity
DT = Digestive tract
PC = Pseudocoelom
euC = Coelom
DT
DT
PC
euC
Pseudocoelomate
Eucoelomate
note: true digestive tract first seen in pseudocoloemates
Platyhelminthes (flatworms)
• Triploblastic, bilateral, cephalized
acoelomates that have been flattened
dorso-ventrally
• No internal transport system
• (Use of GVC, with muscular pharynx)
• Some species exclusively parasitic
• Classes of Platyhelminthes include
– Turbellaria (planarians, e.g. Dugesia,
Planaria). Free-living, aquatic. Movement
by cilia, eased by secretion of mucus
– Trematoda (parasitic flukes, e.g.
Schistosoma)
– Monogenea (parasitic flukes)
– Cestoda (parasitic tapeworms)
n.b. Parasitic forms do not possess GVCs
Body cavities have various functions
• Cushions internal organs
• Independence of movement
• Primitive circulatory system
– Transport of nutrients and metabolic wastes,
gaseous exchange
• Basis of hydrostatic skeleton
• Helped development of a true digestive
tract (phylum Nemertea)
Nematoda (Roundworms)
• Cylindrical triploblastic pseudocoelomates
• Some are freeliving saprobes important role in decomposition of dead
organic matter
• Other are parasitic (e.g. hookworm,
Trichinella*)
*hmmm, pork chop
Fourth taxonomic dichotomy
Protostomes
• Spiral/Determinate
cleavage
• Blastoporemouth
• Schizocoelous
development
e.g. Mollusca thru’ to
Arthropods
Deuterostomes
• Radial/Indeterminat
e cleavage
• Blastoporeanus
• Enterocoelous
development
e.g. Echinodermata
and Chordata
Mollusca
• Triploblastic coelomates: body plan
divided into three (foot, visceral mass,
and mantle)
• Mantle may secrete calcareous shell for
protection against dessication and
predators
• Gaseous exchange by gills. In some
cases, gills are modified to filter feed
• Open circulatory system
• Classes of Mollusca include
– Polyplacophora (Chitons, herbivorous
grazers)
– Gastropoda (snails and slugs: mostly
grazers, but some predators—e.g. cone
shells)
– Bivalvia (Bivalves: clams, oysters,
mussels—filter feeders)
– Cephalopoda (shelled—nautilus, or
unshelled—squid, octopus)
Segmentation
• Defined as a system of similar units
• Allows specialization of regions along
body length
• Evolved separately in protostomes
(Annelida, Arthropoda) and
deuterostomes (all Phyla)
Annelida (segmented worms)
• Triploblastic segmented eucoelomates
• Specialization of body regions (e.g. see
digestive tract)
• Closed circulatory system
• Three classes include:
– Oligochaeta (earthworm—Lumbricus)
– Polychaeta (marine segmented worms)
– Hirudinea (leeches)
Arthropoda
• The most successful animal phylum ever
• Characterized by highly developed
cephalization, exoskeleton (made from
armor-tough chitin), division of body into
head, thorax and abdomen
• Open circulatory system, including
haemocoel as well as coelom
• Modified appendages per segment: first
evolutionary development of flight
• Classification of arthropods is complex,
but subphylums include:
– Trilobitomorpha (extinct trilobites)
– Cheliceriformes (spiders, mites and ticks,
scorpions)
– Uniramia (insects)
– Crustacea (crabs, lobsters, shrimp,
copepods)
Chelicerates includes the class Arachnida
• Simple eyes
• Modified appendages into
– walking legs (4 pairs)
– feeding mouthparts, including pedipalps and
fangs (chelicerae), not mandibles
– Spinnerets
Uniramians
• Insects (Insecta), Millipedes (Diplopoda) and
Centipedes (Chilopoda). Have compound
eyes, mandibles, and sensory antennae
• Most insects have developed flight and occupy
most ecological niches
– Gaseous exchange through spiracles and
tracheae
– Waxy cuticle to prevent dessication
– Other dessication defenses through reabsorption
of water from faeces
– Some species undergo metamorphosis
Crustacea
• Mainly marine
• Extensively specialized, jointed
appendages
• Classes include
– Decapoda (crabs, lobsters, shrimp, prawns)
– Copepoda (copepods)
– Amphipoda (amphipods)
– Isopoda (isopods)
Deuterostomes: Echinodermata
• Triploblastic coelomates
• Pentaradially symmetrical as adult, but
bilaterally symmetrical as larva
• Unique water vascular system
• Classes include:
– Asteroidea (sea stars)
– Echinoidea (sea urchins and sand dollars)
– Holothuroidea (sea cucumbers)
The invertebrate chordates
• Phylum Chordata traditionally thought
of as a vertebrate group, but two of the
three subphyla are invertebrate:
– Urochordata (sea squirts, tunicates)
– Cephalochordata (lancelets, e.g.
Amphioxus)
• All chordates (including vertebrates)
share the common features of
pharyngeal slits, muscular post-anal tail,
notochord and dorsal hollow nerve cord
Protistan Ancestor
(Choanoflagellate)
Asymmetrical
Parazoa
Porifera
Symmetrical
Eumetazoa
Radiata
Diploblastic
Cnidaria
Bilateria
Triploblastic
Acoelomate
Platyhelminthes
Coelomates
Pseudocoelomate
Nematoda
Eucoelomate
Protostome
Mollusca
Annelida
Arthropoda
Deuterostome
Echinodermata
Chordata