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Earth’s History

All cells come from preexisting cells. Where did the 1^st cells come from?
Models approximating early earth’s conditions reconstruct life originating processes.

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12-1: How Did Life Begin?

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The Age of Earth

Earth formed 4.5 billion years ago
Cooling of Earth allowed water vapor in the atmosphere to condense into oceans
Life most likely evolved in the oceans over a period of hundreds of millions of years
Earth’s age is estimated by measuring the age of rocks

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Measuring Earth’s Age

Radiometric dating: way to determine the age of something by estimating the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope

Isotopes = atoms of the same element with different atomic masses

C-12 = 6 protons + 6 neutrons; C-14 = 6 protons + 8 neutrons

Radioisotope: radioactive isotopes that are unstable and undergo radioactive decay, releasing particles and energy (radiation)
Half-life = length of time it takes for one-half of the mass of the parent isotope to decay into a daughter isotope

Use rate of decay to estimate age of fossils/rock material

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Formation of the Basic Chemicals of Life

Life on Earth developed through chemical and physical processes
Life probably arose from the chemical reaction of molecules of nonliving matter
These chemical reactions were energized by the sun and volcanic heat
The resulting more complex, organic molecules probably became the building blocks of the first cells
This hypothesis has been tested and supported by many laboratory experiments

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The Primordial Soup Model

Primordial soup: Earth’s oceans most likely were filled with various organic molecules
Oparin and Haldane each hypothesized that these molecules formed spontaneously in chemical reactions energized by solar radiation, volcanic eruptions, and lightning

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Oparin & Urey’s Hypothesis

Earth’s early atmosphere: ↓O₂; ↑N₂, NH₃, H₂, H₂O_(g), CH₄; ↑ Temp.

Energy from lightning caused these gases to form simple organic compounds

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Miller-Urey Experiment

1953 – experiment simulated early earth’s conditions based on Oparin & Urey’s hypothesis
Procedures: chamber containing same gases (N₂, CH₄, H₂, and NH₃, H₂O_(g)) ; electric sparks provided energy; ↓↓O₂ content; heated water (ocean)
Results: over a few weeks, amino acids, fatty acids, & other hydrocarbons were created without human intervention
Significance: amino acids make proteins; basic chemicals of life could have formed spontaneously under early Earth’s conditions

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Other experiments produced additional proteins, ATP, and nucleotides
Atmosphere different?

No ozone layer
Would have destroyed NH₃ and CH₄…don’t get the same molecules

Where could the gases come from?

Produced in ocean bubbles?
Arose from deep sea vents?
Still under investigation…

http://www.ucsd.tv/miller-urey/miller1.mpg

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The Bubble Model

Gases from underwater volcanoes trapped in bubbles (NH₃, CH₄, etc.)
Chemical reactions occurred inside bubbles where the gases were shielded from UV radiation

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Organic molecules released into air when bubbles burst upon surfacing
Simple organic molecules underwent further chemical reactions driven by UV radiation & lightning in atmosphere
Complex organic molecules formed from additional chemical reactions carried by precipitation into oceans

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Precursors of the First Cells

RNA (ribonucleic acid) have been formed in the lab

RNA assists in carrying out DNA’s instructions

Inorganic Molecules

RNA nucleotides

RNA macromolecules

RNA molecules catalyze protein synthesis

Proteins

Self-replication

Inorganic molecules can form RNA building blocks (NTs)
RNA stores information like DNA
RNA NTs combine to form larger RNA molecules
RNA helps to make proteins (acts as an enzyme – catalyzes chemical reactions)
Proteins carry out DNA’s cellular instructions
RNA has the ability to copy itself
RNA can mutate
RNA could have been the first hereditary molecule in cells

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Microspheres & Coacervates

Cell-like structures form spontaneously from simple organic chemicals in the lab

Microspheres = spherical structures made of amino acids (building blocks of protein) which form a membrane
Coacervates = clusters of droplets made of different types of molecules (e.g. a.a. & sugars)
Both have life-like properties: engulf substances and enlarge – parts of cellular life not dictated by genes

Microspheres could have been the first step toward cellular organization
Not true cells without heredity

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Origin of Heredity

Most scientists agree that RNA probably evolved before DNA

RNA can act as an enzyme

Probably catalyzed formation of first proteins

Perhaps some microspheres contained RNA

RNA could be transferred to “offspring”

Much, more to be learned…

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12-2: The Evolution of �Cellular Life

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The Evolution of Prokaryotes

Fossil evidence is used to determine when the first organisms formed.
Fossil = preserved/mineralized remains or imprint of a dead organism

Form when an organism is covered by sediment and its tissues are eventually replaced by hard minerals

Mold = type of fossil that is an imprint
Cast = rock-like model of an organism created by the filling of a mold
Oldest fossils are 2.5 billion years old

Prokaryotes, see next slide

Fossils show morphology, behavior, mode of locomotion

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Fossilization Process

Brachiopod dies and falls on the sea-bed. Organic remains: dispersed or eaten.
Inorganic part (shell) is preserved. Worn down by erosion from water and sand.
Sediments cover the shell. Process continues for millions of years. Sediments harden petrifying the shell.
Fossil rises, through uplift & erosion, now being exposed to sun, the wind and the frost.

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Prokaryotes, continued…

Prokaryotes:

Single-celled organisms
No membrane-bound organelles (e.g. mitochondria, chloroplasts)
No nucleus (DNA floating)

Cyanobacteria – some of the first prokaryotes seen in fossil record

Photosynthetic, marine
Added O₂ to the atmosphere over hundreds of millions of years…accumulated (21% today)

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Two Groups of Prokaryotes

Two groups of prokaryotes evolved:

Eubacteria (AKA “bacteria”)

Escherichia coli, Sulfolobus
Contain peptidoglycan in cell walls
Many cause disease & decay

Archaebacteria

Lack peptidoglycan in cell walls; have unique lipids in cell membranes
Live in extreme environments: hot springs, deep sea vents, caves, etc.

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The Evolution of Eukaryotes

First eukaryotes appeared in fossil record ~1.5 billion years ago
Larger than prokaryotes
Complex internal membrane system
DNA stored in a nucleus
Specialized organelles:

Mitochondria – organelle that carries out cellular respiration; make cellular energy
Chloroplasts – organelle that carries out photosynthesis; in photosynthetic organisms
Both are the size as prokaryotes
Both contain their own DNA

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The Origins of Mitochondria �and Chloroplasts

Mitochondria & chloroplasts originated by the process of endosymbiosis (Margulis)
Bacteria entered larger cells as undigested prey or parasites
Continued to carry out photosynthesis or cellular respiration
Host cell benefited from energy-producing mechanism; engulfed cell benefited from safe environment…mutualism!

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Support for theory of endosymbiosis…

Size & structure:

Mitochondria & eubacteria ≈ same size; chloroplasts & cyanobacteria ≈ same size
Both organelles surrounded by two membranes (from being engulfed)

Smooth outer membrane resembles host cell’s organelle’s membrane
Rough inner membrane resembles modern eubacteria
Chloroplasts & cyanobacteria contain the same photosynthetic structures

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Support for theory of endosymbiosis…

Genetic material:

Mitochondria & chloroplast chromosomes are made of circular DNA like bacteria
Genes on DNA of mitochondria & chloroplasts are different than those on DNA in nucleus

Ribosomes:

Similar between bacteria and mitochondria & chloroplasts

Reproduction:

Mitochondria & chloroplasts reproduce in the same way as bacteria

Binary fission (independent of host cell)

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Multicellularity

6 Kingdoms of life

2 Prokaryotes:

Eubacteria & Archaebacteria

4 Eukaryotes:

Protists, Fungi, Plants, Animals

Protists: large, diverse group of multicellular and unicellular organisms

Paramecia, algae (seaweed), many human parasites, Amoeba, slime molds…

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Giardia

Trypanosoma

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Trichomonas vaginalis

Dinoflagellate

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The Ciliophora (ciliates), a diverse protist group, is named for their use of cilia to move and feed.

Fig. 28.8

Stentor

Paramecium

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Plasmodium, the parasite that causes malaria, spends part of its life in mosquitoes and part in humans.

Fig. 28.7

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Euglena, a single celled mixotrophic protist, can use chloroplasts to undergo photosynthesis if light is available or live as a heterotroph by absorbing organic nutrients from the environment.

Fig. 28.3

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DINOFLAGELLATE

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Chrysophyta – golden algae

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Brown Algae

FUCUS

Kelp

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Volvox and Green Algae

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Red Algae

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Slime Molds

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Multicellularity, continued…

Over time, some organisms evolved the trait of being multicellular (made up of at least two cells)
Multicellularity allows an organism to have specific cells with specialized functions

Cells for movement
Cells for digestion
Cells for vision
Cells for disease prevention (immune)
This is an advantage over a single-celled organism, whose cell must carry out all functions

Oldest fossils of multicellular organisms are 700 million years old

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Origins of Modern Organisms

Most animal phyla evolved during the Precambrian era and Cambrian periods between 570-505 million years ago
Known as the “Cambrian explosion” because of the rapid diversification of animals

Animals that evolved during this time period most likely were the ancestors of all animals thereafter

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Mass Extinctions

~440 m.y.a. the first mass extinction occurred…large % of all organisms went extinct (marks end of Ordovician period)
Extinction: death of all members of a species
Mass extinction: episode during which large numbers of species become extinct

Mass extinctions are probably caused by changes in worldwide geologic and weather patterns
Five mass extinctions have occurred during Earth’s history

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Mass Extinctions

6^th mass extinction underway

Human activity is causing the destruction of fragile ecosystems

Especially tropical rain forests
~1/2 the world’s tropical rain forests have already been lost
Most the world’s biodiversity lives in tropical rain forests

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12-3: Life Invaded the Land

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The Ozone Layer

Unsafe levels of UV radiation prevented the first organisms on Earth from leaving the oceans
~2.5 b.y.a. O₂ levels in the atmosphere began increasing

Cyanobacteria were doing photosynthesis
O₂ formed O₃ (ozone)
Over millions of years, ozone was “thick” enough to protect organisms

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Plants and Fungi on Land

First land dwelling organisms:

Fungi + plants or algae together
Benefit one another:

Plants make sugars from sunlight
Fungi get minerals from rock

Mycorrhizae: symbiotic (mutualistic) association between fungi and plant roots
Mutualism: relationship between two species in which both benefit
Plants and fungi began a terrestrial mutualism about 430 million years ago

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Arthropods

The evolution of land plants provided a food source for land-dwelling animals
Arthropods were first animals to invade land (probably a scorpion-like animal)
Arthropod: animal with hard outer skeleton, segmented body, and paired, jointed limbs

Lobsters, crabs, insects, spiders

Insects are most plentiful & diverse group…probably because of wings

Search for food, mates, nesting sites
Pollinators (flowering plant fossils 127 m.y. old)

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Vertebrates

Vertebrate: animal with a backbone

Mammals, reptiles, fishes, birds, amphibians

Fishes

First vertebrates (on record) were small, jawless fishes, ocean-dwelling, 530 m.y.a.
Jawed fishes evolved ~430 m.y.a.

Bite & chew food…efficient predators

Fishes are most successful living vertebrates
Land-dwelling animals are descendants of fishes

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Amphibians

First vertebrates left the sea ~370 m.y.a.
Amphibians:

Smooth-skinned, four-legged animals

Modern Ex: frogs, salamanders, toads, newts

Moist breathing lungs could absorb O₂ from air (versus extracting it out of water)
Evolution of rigid skeleton allowed:

Walking
Support (base for limbs to work against)

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Reptiles

Reptiles evolved from amphibian ancestors about 340 m.y.a.

Modern Ex: snakes, lizards, turtles, crocodiles

Reptiles better suited for dry land than amphibians:

Watertight skin (prevents moisture loss)
Watertight egg (can lay egg on dry land)

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Mammals & Birds

Birds evolved from dinosaurs during/after the Jurassic
Mammals evolved from therapsids during the Triassic

Therapsids: reptiles w/ complex teeth & legs under their bodies

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Mammals & Birds, continued…

All dinosaurs but ancestors of birds went extinct 65 m.y.a.
Climate became more moist, allowing birds & mammals to dominate

Previously dry climate favored reptiles

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