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Early Earth and The Origin of Life

Timeline of Life on Earth

Timeline of Life on Earth

• Life on Earth is said to have originated 3.5 – 4.0 billion years ago

• Bacteria (prokaryotes) were the first organisms to inhabit Earth

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Miller-Urey Experiment

• In 1953, Miller and Urey did an experiment that simulated lab conditions that were similar to those of the early Earth

• After one week, they found a variety of organic compounds (including amino acids) that had been produced from inorganic material

Miller-Urey Experiment

Kingdoms of Life• Arranging the diversity of life into kingdoms is a

work in progress

• Early classification systems had two kingdoms: plants and animals

• Robert Whittaker proposed five kingdoms: Monera, Protista, Plantae, Fungi, and Animalia

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New information has revised our understanding of the tree of life

• Molecular data have provided insights into the deepest branches of the tree of life

• Early classification systems had two kingdoms:

plants and animals

• Robert Whittaker proposed five kingdoms:

Monera, Protista, Plantae, Fungi, and Animalia

Kingdoms of Life (R. Whittaker’s Classification)

Kingdoms of Life-Molecular data have provided insights into the deepest branches of the tree of life

-The five kingdom system has been replaced by three

domains: Archaea, Bacteria, and Eukarya

- Each domain has been split into kingdoms

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Universal ancestor

Domain Bacteria

Domain Eukarya

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Investigating the Tree of Life• Ex: Legless lizards have evolved independently in several

different groups

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• Phylogeny is the evolutionary history of a species or group of related species

• The discipline of systematics classifies organisms and determines their evolutionary relationships

• Systematists use fossil, molecular, and genetic data to infer evolutionary relationships

• Taxonomy is the ordered division and naming of organisms

Binomial Nomenclature

• In the 18th century, Carolus Linnaeus published a system of taxonomy based on resemblances

• Linnaean system is useful today: two-part names for species and hierarchical classification

• The two-part scientific name of a species is binomial:

-first part of the name is the genus

-second part, called the specific epithet (unique for each species within the genus)

• The first letter of the genus is capitalized, and the entire species name is italicized

• Both parts together name the species-its SCIENTIFIC NAME (not the specific epithet alone)

Hierarchical Classification

• Linnaeus also introduced a system for grouping species in increasingly broad categories

• The taxonomic groups from broad to narrow are domain, kingdom, phylum, class, order, family, genus, and species

• A taxonomic unit at any level of hierarchy is called a taxon

• The broader taxa are not comparable between lineages• For example, an order of snails has less genetic

diversity than an order of mammals

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Figure 26.3

Species:

Panthera pardus

Genus:

Panthera

Family:

Felidae

Order:

Carnivora

Class:

Mammalia

Phylum:

Chordata

Domain:

Bacteria

Kingdom:

Animalia Domain:

ArchaeaDomain:

Eukarya

Linking Classification and Phylogeny

• Systematists depict evolutionary relationships in branching phylogenetic trees

Figure 26.4Order Family

Pantherapardus(leopard)

Genus Species

Canislatrans(coyote)

Taxideataxus(Americanbadger)

Lutra lutra(Europeanotter)

Canislupus(gray wolf)

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• Linnaean classification and phylogeny can differ from each other

• Systematists have proposed the PhyloCode, which recognizes only groups that include a common ancestor and all its descendents

• A phylogenetic tree represents a hypothesis about evolutionary relationships

• Each branch point represents the divergence of two species

• Sister taxa are groups that share an immediate common ancestor

• A rooted tree includes a branch to represent the last common ancestor of all taxa in the tree

• A basal taxon diverges early in the history of a group and originates near the common ancestor of the group

• A polytomy is a branch from which more than two groups emerge

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Figure 26.5

Branch point:where lineages diverge

ANCESTRALLINEAGE

This branch pointrepresents thecommon ancestor oftaxa A–G.

This branch point forms apolytomy: an unresolvedpattern of divergence.

Sistertaxa

Basaltaxon

Taxon A

Taxon B

Taxon C

Taxon D

Taxon E

Taxon F

Taxon G

What We Can and Cannot Learn from Phylogenetic Trees

• Phylogenetic trees show patterns of descent, not phenotypic similarity

• Phylogenetic trees do not indicate when species evolved or how much change occurred in a lineage

• It should not be assumed that a taxon evolved from the taxon next to it

Applying Phylogenies

• Phylogeny provides important information about similar characteristics in closely related species

• A phylogeny was used to identify the species of whale from which “whale meat” originated

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Minke (Southern Hemisphere)

Unknowns #1a, 2, 3, 4, 5, 6, 7, 8

Minke (North Atlantic)

Humpback (North Atlantic)

Humpback (North Pacific)

Gray

Blue

Unknowns #10, 11, 12

Unknown #13

Unknown #1b

Unknown #9

Fin (Mediterranean)

Fin (Iceland)

RESULTS

Phylogenies are inferred from morphological and molecular data

• To infer phylogenies, systematists gather information about morphologies, genes, and biochemistry of living organisms

EXAMPLE: New views of animal phylogeny are emerging from molecular data

• Zoologists recognize about three dozen animal phyla

• Phylogenies now combine morphological, molecular, and fossil data

• Current debate in animal systematics has led to the development of multiple hypotheses about the relationships among animal groups

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ANCESTRALCOLONIALFLAGELLATE D

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Brachiopoda

Echinodermata

Chordata

Platyhelminthes

Rotifera

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Annelida

Arthropoda

Nematoda

MODEL HYPOTHESIS 1. based

mainly on morphological and

developmental comparisons

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Annelida

Nematoda

Arthropoda

Figure 32.11

MODEL HYPOTHESIS 2.

based mainly on

molecular data

Points of Agreement

1. All animals share a common ancestor colonial flagellate

2. Sponges are basal animals

3. Eumetazoa is a clade of animals (eumetazoans) with true tissues

4. Most animal phyla belong to the clade Bilateria, and are called bilaterians

5. Chordates and some other phyla belong to the clade Deuterostomia

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Body Symmetry• Animals can be categorized according to the symmetry of their bodies, or

lack of it

• Some animals have radial symmetry, with no front and back, or left and right

• Two-sided symmetry is called bilateral symmetry

• Bilaterally symmetrical animals have

• A dorsal (top) side and a ventral (bottom) side

• A right and left side

• Anterior (head) and posterior (tail) ends

• Cephalization, the development of a head

RADIALLY SYMMETRICAL: often sessile ex:

Hydra, sea anemone, coral polyp or planktonic

(drifting or weakly swimming) ex: jellyfish, comb

jellies

Bilateral animals often move actively w/ a

central nervous system ex: insects, man

Tissues

• Animal body plans also vary according to the organization of the animal’s tissues

• Tissues are collections of specialized cells isolated from other tissues by membranous layers

- Sponges lack true tissues

• During development, three germ layers give rise to the tissues and organs of the animal embryo

• Ectoderm is the germ layer covering the embryo’s surface

• Endoderm is the innermost germ layer and lines the developing digestive tube, called the archenteron

• Mesoderm – intervening layer

• Diploblastic animals have ectoderm and endoderm• These include cnidarians and comb jellies

• Triploblastic animals also have an intervening mesoderm layer; these include all bilaterians

• These include flatworms, arthropods, vertebrates, and others

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Muscle Tissue

Skeletal muscle

Nuclei

Musclefiber

Sarcomere

100 m

Smooth muscle Cardiac muscle

Nucleus Muscle fibers 25 m Nucleus Intercalated disk 50 m

Body Cavities

• Most triploblastic animals possess a body cavity• A true body cavity is called a coelom and is derived from

mesoderm• Coelomates are animals that possess a true coelom• A pseudocoelom is a body cavity derived from the

mesoderm and endoderm• Triploblastic animals that possess a pseudocoelom are called

pseudocoelomates• Triploblastic animals that lack a body cavity are called

acoelomates

© 2011 Pearson Education, Inc.

(a) Coelomate

Coelom

Digestive tract(from endoderm)

Body covering(from ectoderm)

Tissue layerlining coelomand suspendinginternal organs(from mesoderm)

(b) Pseudocoelomate

Body covering(from ectoderm)

Pseudocoelom Muscle layer(frommesoderm)

Digestive tract(from endoderm)

(c) Acoelomate

Body covering(from ectoderm)

Wall of digestive cavity(from endoderm)

Tissue-filled region(frommesoderm)

Figure 32.8

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Protostome and Deuterostome Development

• Based on early development, many animals can be categorized as having protostome development or deuterostome development

Cleavage

• In protostome development, cleavage is spiral and determinate

• In deuterostome development, cleavage is radial and indeterminate

- indeterminate cleavage, each cell in the early stages of

cleavage retains the capacity to develop into a

complete embryo

- makes possible identical twins, and embryonic stem

cells

(a) Cleavage

(b) Coelom formation

(c) Fate of the blastopore

Key

Ectoderm

Mesoderm

Endoderm

Protostome development(examples: molluscs,

annelids)

Deuterostome development(examples: echinoderms,

chordates)

Eight-cell stage Eight-cell stage

Spiral and determinate Radial and indeterminate

Archenteron

Coelom

Coelom

Blastopore BlastoporeMesoderm Mesoderm

Folds of archenteronform coelom.

Solid masses of mesodermsplit and form coelom.

Anus

Anus

Mouth

Mouth

Digestive tube

Mouth develops from blastopore. Anus develops from blastopore.

Figure 32.9

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Progress in Resolving Bilaterian Relationships• The morphology-based tree divides bilaterians into two

clades: deuterostomes and protostomes

• In contrast, recent molecular studies indicate three bilaterian clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa

• Ecdysozoans shed their exoskeletons through a process called ecdysis

• Some lophotrochozoans have a feeding structure called a lophophore

• Others go through a distinct developmental stage called the trochophore larva

Future Directions in Animal Systematics

• Phylogenetic studies based on larger databases will likely provide further insights into animal evolutionary history