CHAPTER 15

TRACKING EVOLUTIONARY HISTORY

Early Earth and the Origin of Life

Young Earth-prokaryotes were main form of life.

Earth probably began ca. 4.6 billion years ago from a vast swirling cloud of dust

Conditions on Early Earth

Different from those of today such that the first atmosphere was probably thick with water vapor along with various compounds released by volcanic eruptions including nitrogen and its oxides, CO2, CH4, NH3, hydrogen and H2S.

Lightning, volcanic activity and ultraviolet radiation were much more intense

When did life begin?

The earliest evidence of life on Earth is a fossilized STROMATOLITE which are about 3.5 byo, thus supporting the hypothesis that life in a simpler form arose as early as 3.9 bya

A x-section of a fossilized stromatolite

How did life arise?

People believed for some time flies came from rotting meat and fish from ocean mud.

Experiments performed in the 1600s proved this belief to be untrue as such experiments showed that large organisms cannot arise from nonliving matter, rather only by the reproduction of pre-existing life.

Observations and experiments have led scientists to hypothesize that chemical and physical

processes on early Earth could have produced very simple cells through a sequence of four main stages:

1. The abiotic synthesis of small organic molecules, such as amino acids and nucleotides

2. The joining of these small molecules into macromolecules, including proteins and nucleic acids

3. The packaging of these molecules into “protobionts,” droplets with membranes that maintain an internal chemistry different from that of their surroundings

4. The origin of self-replicating molecules that eventually made inheritance possible

Miller-Urey Experiment

Stanley Miller-first to show amino acids and other organic molecules could have been generated on a lifeless Earth.

Before early prokaryotes added oxygen to air, earth had reducing atmosphere instead of oxidizing one.

Energy needed for this reaction came from ultraviolet radiation and lightening discharge

In 1953, Miller and Urey, Tested the Oparin-Haldane

hypothesis by creating conditions in which there was an:  

Atmosphere above warmed sea water that contained H2O, H2, CH4, and NH3, and Electrodes that simulated lightning.

From this setup, they obtained organic compounds such as amino acids that were collected in cooled water.

The Miller-Urey experiment showed that organic molecules could be created out of inorganic molecules

Formation of polymers, membranes and self-replicating molecules represent stages in the origin of the first cells

Abiotic Synthesis of Macromolecules

Polymers are synthesized by dehydration reactions that add monomers to a growing chain

Polymers of life are synthesized by dehydration reaction that releases a water molecule for each monomer added to chain-Polymerization.

The first polymers may have formed on hot rocks or clay

Formation of Protobionts

A PROTOBIONT is an aggregate of abiotically produced molecules

surrounded by a membrane or membrane-like structures

Self-replicating RNA

RNA may have been the first genetic material, replicating itself without the aid of proteins, which idea was supported by laboratory experiments

RNA replication may have been aided by RNA molecules acting as catalysts

Scientists discovered RIBOZYMES, an RNA that carries out a number of enzyme-like functions

Major Events in the History of Life

As material circulated through the apparatus, Miller and Urey periodically collected samples for analysis. They identified a variety of organic molecules, including amino acids such as alanine and glutamic acid that are common in the proteins of organisms. They also found many other amino acids and complex,oily hydrocarbons.

RESULTS

Miller and Urey set up a closed system in their laboratory to simulate conditions thought to have existed on early Earth. A warmed flask of water simulated the primeval sea. The strongly reducing “atmosphere” in the system consisted of H2, methane (CH4), ammonia (NH3), and water vapor. Sparks were discharged in the synthetic atmosphere to mimic lightning. A condenser cooled the atmosphere, raining water and any dissolved compounds into the miniature sea.

EXPERIMENT

CONCLUSION Organic molecules, a first step in the origin of

life, can form in a strongly reducing atmosphere.

Origin of Prokaryotes

Prokaryotes existed from at least 3.5 to about 2 billion years ago resulting to the appearance of atmospheric oxygen, which then began to increase rapidly.

This oxygen revolution had an enormous impact on life: Many prokaryotes probably became extinct, while other species surived in anaerobic habitats. The evolution of cellular respiration allowed other prokaryotes to flourish.

Origin of Single-celled Eukaryotes

The oldest widely accepted fossils of eukaryotes are about 2.1 years old.

After the first eukaryotes appeared, a great range of unicellular forms evolved, giving rise to the diversity of single-celled eukaryotes that continue to flourish today.

Origin of Multicellular Eukaryotes

Eukaryotes include a variety of algae, plants, fungi, and animals

Molecular comparisons suggest that the common ancestor of multicellular eukaryotes lived 1.5 billion years ago

The oldest known fossils of multicellular eukaryotes are of relatively small algae that lived about 1.2 billion years ago

Larger multicellular soft-bodied eukaryotes apeared about 600 million years ago

A great increase in the diversity of animal forms occurred about 535-525 million years ago in a period known as the CAMBRIAN explosion

Colonization of Land

Larger forms of life such as fungi, plants and animals did not begin to colonize land until about 500 million years ago.

Plants colonized land in the company of fungi. Even today, the roots of most plants are associated with fungi that aid in absorption and receive nutrients in return. Such mutually beneficial associations are evident in some of the oldest plant fossils.

Arthropods and Tetrapods are the most widespread and diverse land animals.

The human lineage diverged from hominids (apes) around 6 to 7 million years ago and our species originated about 195,000 years ago.

GEOLOGIC RECORD

A time scale established by geologists that divides Earth’s

history into time periods, grouped into three eons

1. Archaean 2. Proterozoic 3. Phanerozoic

Such eons are further subdivided into eras, periods and epochs

Mechanisms of Macroevolution

CONTINENTAL DRIFT is the slow movement of Earth’s crustal plates

Early 1900’s: Alfred Wegener wrote of a single supercontinent named Pangaea, meaning “all land.” He portrayed the breakup of Pangaea and the movement of continents to their present position

The mantle circulates constantly resulting to the slow but incessant movement of the continental plates on the underlying mantle.

In some cases, the plates are moving away from one another. For instance, North America and Europe are drifting apart at the rate of about 2 cm per year.

Throughout geologic time, continental movements have reshaped the physical features of the planet and altered the habitats in which organisms live.

Continental drift during the Phanerozoic eon

Evidence support Continental Drift Theory

1. “Puzzle Pieces”

Continents look likethey could be part of a

giant jigsaw puzzle

2. Distribution of Fossils

By about 10 million yearsago, Earth’s youngestmajor mountain range,the Himalayas, formedas a result of India’scollision with Eurasiaduring the Cenozoic.The continents continueto drift today.

By the end of theMesozoic, Laurasiaand Gondwanaseparated into thepresent-day continents.By the mid-MesozoicPangaea split intonorthern (Laurasia)and southern(Gondwana)landmasses.

At the end of thePaleozoic, all ofEarth’s landmasseswere joined in thesupercontinentPangaea.

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Eurasia

Africa

Madagascar

Antarctica

Laurasia

Plant and animal fossilsfound on the coastlines of

different continents

3. Sequence of Rocks

Same rock patterns found in South America, India, Africa, Antarctica and Australia

4. Glacial features of the same age restore to a tight polar distribution

Effects of continental drift may imperil human life

The SAN ANDREAS FAULT, a boundary between two crustal plates. Plate boundaries are hotspots of geologic activity.

California’s frequent earthquakes result from movement along the San Andreas Fault.

The San Francisco earthquake and resulting fire of April 18, 1906, took about 700 lives and caused millions of dollars worth of damage.

Mass Extinction destroy a large number of species

is the phenomenon in which a large number of species of life on Earth become extinct in a relatively short period of time.

5 mass extinctions have occurred over the past 500 years. Of all mass extinctions, Permian and Cretaceous periods received the most attention

Permian (Paleozoic and Mesozoic) mass extinction claimed 96% of marine animals and took a tremendous toll of terrestrial life

At the end of Cretaceous period, 50% of marine life was lost as well as many lineages of terrestrial plants and animals

Causes of Mass Extinctions:1. IMPACT EVENTS – asteroid2. CLIMATE CHANGE – rapid

transitions in climate may stress the environment to the point of extinction

3. VOLCANISM - The formation of large igneous provinces through the outflow of up to millions of cubic kilometers of

lava in a short duration is likely to poison the atmosphere and oceans in a way that may cause extinctions.

4. GAMMA RAY BURST – a nearby gamma ray burst (less than 6,000 light years distance) could destroy the ozone layer and sufficiently irradiate the surface of the Earth to kill organisms living there.

5. PLATE TECTONICS - The opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem.

Consequences of Mass Extinctions

They can permanently remove species with highly advantageous features and change the course of evolution forever.

How long does it take the diversity of life to recover after a mass extinction?

The fossil record shows that it typically takes 5 to 10 million years for species numbers to return to previous levels. In some cases, it has taken much longer, such as marine families took 100 million years to recover after the Permian mass extinction

ADAPTIVE RADIATIONS have increased the diversity of life

Adaptive radiations followed each mass extinction when survivors become adapted to the many vacant ecological niches.

For example, fossil evidence indicates that mammals underwent a dramatic adaptive radiation after the extinction of terrestrial dinosaurs 65 mya.

Genes that control development play a major role in evolution

“EVO-DEVO”

The research field that combines evolutionary biology with developmental

biology

Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult

Heterochrony is an evolutionary change in the rate or timing of developmental events which can have a significant impact on body shape

Different allometric patterns contribute to the contrasting shapes of human and chimpanzee skulls

(b) Comparison of chimpanzee and human skull growth. The fetal skulls of humans and chimpanzees are similar in shape. Allometric growth transforms the rounded skull and vertical face of a newborn chimpanzee into the elongated skull and sloping face characteristic of adult apes. The same allometric pattern of growth occurs in humans, but with a less accelerated elongation of the jaw relative to the rest of the skull.

Heterochrony has also played a part in the evolution of salamander feet

Chimpanzee fetus Chimpanzee adult

Human fetus Human adult

In paedomorphosis, the rate of reproductive development accelerates compared to somatic development

The sexually mature species may retain body features that were juvenile structures in an ancestral species

An AXOLOTL, a paedomorphic salamander

Changes in Spatial Pattern

Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts

Homeotic genes determine such basic features as where a pair of wings and a pair of legs will develop on a bird or how a flower’s parts are arranged

New forms can evolve by changes in the number, sequences, or regulation of developmental genes

Stickleback fish from ocean and lake stained to show bony plates and spines. Freshwater stickleback fish has

no pelvic spine

Evolutionary novelties may arise in several ways

Most novel biological structures evolve in many stages from previously existing structures

Some complex structures, such as the eye have had similar functions during all stages of their evolution

Evolutionary trends do not mean that evolution is goal directed

Ground-dwelling salamander. A longer timeperoid for foot growth results in longer digits andless webbing.

Tree-dwelling salamander. Foot growth endssooner. This evolutionary timing change accounts for the shorter digits and more extensive webbing, which help the salamander climb vertically on treebranches.

(a)

(b)

Pigmented cells(photoreceptors)

Epithelium

Nerve fibers

Pigmentedcells

Nerve fibersPatch of pigmented cells.The limpet Patella has a simplepatch of photoreceptors.

Eyecup. The slit shellmollusc Pleurotomariahas an eyecup.

Fluid-filled cavityEpithelium

Cellularfluid(lens)

Cornea

Optic nervePigmentedlayer (retina)

Opticnerve

Pinhole camera-type eye.The Nautilus eye functionslike a pinhole camera(an early type of cameralacking a lens).

Cornea

Lens

RetinaOptic nerveComplex camera-type eye. The squid Loligo has a complexeye whose features (cornea, lens, and retina), though similar to those of vertebrate eyes, evolved independently.

(a) (b)

(d)(c)

(e)

Eye with primitive lens. Themarine snail Murex has a primitive lens consisting of a mass of crystal-like cells. The cornea is a transparent region of epithelium (outer skin) that protects the eyeand helps focus light.

The fossil record often shows apparent trends in evolution that may arise because of adaptation to a changing environment

According to the species selection model trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics

The appearance of an evolutionary trend does not imply that there is some intrinsic drive toward a particular phenotype

PHYLOGENY AND THE TREE OF LIFE

PHYLOGENY is the evolutionary history of a species or group of related species.

Convergent evolution occurs when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary lineage

Convergent evolution of burrowing adaptations in Australian mole

(top) and North American mole (bottom)

Analogous structures or molecular sequences that evolved independently are called homoplasies

Systematics connects classification with evolutionary history

SYSTEMATICS is a discipline of biology that focuses on classifying organisms and determining their evolutionary relationships

Biologists also use systematics as an analytical approach to under- standing the diversity and relationships of organisms, both present-day and extinct

Currently, systematists use morphological, biochemical, and molecular comparisons to infer evolutionary relationships

Phylogenetic systematics connects classification with evolutionary history

Taxonomy is the ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences

Binomial nomenclature is the two-part format of the scientific name of an organism

Was developed by Carolus Linnaeus

Hierarchical Classification

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

Recent(11,500 ya)

Pleistocene(1.8 mya)

Pliocene(5.3 mya)

Miocene(23 mya)

Oligocene(33.9 mya)

Eocene(55.8 mya)

Equus Hippidion and other genera

NannippusPliohippus

NeohipparionHipparion

Sinohippus MegahippusCallippus

Archaeohippus

Merychippus

Parahippus

HypohippusAnchitherium

Miohippus

Mesohippus

Epihippus

Orohippus

Paleotherium

Propalaeotherium

Pachynolophus

GrazersBrowsers

Key

Hyracotherium

Pantherapardus

Panthera

Felidae

Carnivora

Mammalia

Chordata

Animalia

EukaryaDomain

Kingdom

Phylum

Class

Order

Family

Genus

Species

Linking Classification and Phylogeny

Systematists depict evolutionary relationships in branching phylogenetic trees

Phylogenetic systematics informs the construction of phylogenetic trees based on shared characteristics

A cladogram is a depiction of patterns of shared characteristics among taxa

A clade within a cladogram is defined as a group of species that includes an ancestral species and all its descendants

Cladistics is the study of resemblances among clades

Clades can be nested within larger clades, but not all groupings or organisms qualify as clades

In cladistic analysis, clades are defined by their evolutionary novelties

A shared primitive character is a homologous structure that predates the branching of a particular clade from other members of that clade

Is shared beyond the taxon we are trying to define

A shared derived character is an evolutionary novelty unique to a particular clade

Systematists use a method called outgroup comparison to differentiate between shared

derived and shared primitive characteristics

As a basis of comparison we need to designate an outgroup which is a species or group of species that is closely related to the ingroup, the various species we are studying

Outgroup comparison is based on the assumption that homologies present in both the outgroup and ingroup must be primitive characters that predate the divergence of both groups from a common ancestor

Molecular Clocks

The molecular clock is a yardstick for measuring the absolute time of evolutionary change based on the observation that some genes and other regions of genomes appear to evolve at constant rates

Neutral theory states that much evolutionary change in genes and proteins has no effect on fitness and therefore is not influenced by Darwinian selection and that the rate of molecular change in these genes and proteins should be regular like a clock

The molecular clock, however, does not run as smoothly as neutral theory predicts

The Universal Tree of Life

The tree of life is divided into three great clades called domains: Bacteria, Archaea, and Eukarya

The early history of these domains is not yet clear

Panthera pardus

(leopard)

Mephitis mephitis

(striped skunk)

Lutra lutra (European

otter)

Canis familiaris

(domestic dog)

Canislupus (wolf)

Panthera Mephitis Lutra Canis

Felidae Mustelidae Canidae

CarnivoraOrd er

Fam

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Gen

us

Spec

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Bacteria Eukarya Archaea 4 Symbiosis of chloroplast ancestor with ancestor of green plants

3 Symbiosis of mitochondrial ancestor with ancestor of eukaryotes

2 Possible fusion of bacterium and archaean, yielding ancestor of eukaryotic cells

1 Last common ancestor of all living things

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Origin of life