Lecture 9

Life, Death and Evolution
The Blue Planet: Chapter 14

Outline
• What is Life?
• Life: a Planetary Perspective
• Evolution: the History of Life
• Extinction: the History of Death

What is Life?
• The necessities of life
– Metabolism: the set of chemical reactions
by which an organism derives energy
– Autotrophs
• Organisms that produce their own energy
– Photosynthesis: from light energy
– Chemosynthesis: from chemicals (black smokers)
– Heterotrophs
• Organisms that derive energy by feeding on
other organisms or compounds
– Oxygen intolerant organisms use fermentation
– Oxygen tolerant organisms use respiration

What is Life?
– Reproduction
• The genetic plan is encoded in DNA, decoded
and executed by RNA
• Single-celled organisms undergo cell division
• Multicelled organisms reproduce asexually or
sexually
– Sexual reproduction involves combining genetic
material from two different individuals, resulting in
genetic recombination

What is Life?
– Growth
• Involves the ordering and organizing of atoms
and small molecules to make larger molecules
– Polymerization or crystallization
• The ordered pattern of atoms or molecules is
replicated throughout the structure
• Growth in living matter occurs by polymerization
– Polymerization absorbs energy
• Growth requires energy; through metabolism

What is Life?
– Evolution
• The potential to develop into new species
– Living species are defined by the ability to breed and
produce fertile offspring
– Fossil species are defined by their morphology
• The process whereby the genetic makeup of
species change over time, by passing genetic
information from generation to the next, and
new species arise
• The unifying theory in biology

What is Life?
• The hierarchy of life
– The cell is the structural and functional unit
of all living organisms
– Prokaryotes are cells that do not have a
true nucleus
– Eukaryotes are cells that have a nucleus
– Cells combine to form organs, organs
combine to forms individual multicelled
organisms

What is Life?
• The hierarchy of life
– Populations of one species interact and are
interdependent with populations of other
species in ecological communities
• An ecosystem comprises all of the interactions of
an ecological community with the nonbiological
components of its local environment
– On a regional scale, all ecosystems of one
general type are referred to as a biome, and
all coexisting ecosystems as a bioregion or
biozone

What is Life?
• The kingdoms of life
– Species are organized into taxonomic
ranks of 3 domains comprising 6 kingdoms
• Archaea
• Bacteria
• Eukarya
– Animals, plants, fungi and protists
– Forms of life are organized into groupings
according to their genetic and evolutionary
relationships called a phylogenetic tree

Life: a Planetary Perspective
• The ecosphere and the life zone
– Earth’s 4 reservoirs interact in a relatively
narrow zone called the life zone
– A zone around a star, within which orbiting
planets and their moons would be just the
right distance to allow for the existence of
liquid water and thus, possibly, to support
life, is called an ecosphere

• Life on Earth originated as must as 3.9
billion years ago, the end of the Hadean
– Very hot
– No free oxygen
– No oceans, lakes or rivers
– High atmospheric pressure
• As the planet cooled, water vapor
condensed and collected in low-lying
areas

• The earliest fossil found is from the
Archean at 3.55 billion years old
– Chemical signatures of biologic processes
have been dated to older rocks
• There are numerous hypotheses for the
formation of organic molecules, and we
can specify three steps that must have
been accomplished

1. Chemosynthesis
– Development of small organic molecules, such
as amino acids
1. Biosynthesis
– Polymerization of small organic molecules to
form biopolymers
1. Development of machinery needed for
metabolism
– DNA, RNA

• The history of the biosphere is closely
intertwined with that of the atmosphere,
hydrosphere, and geosphere
• Oxygen is necessary for life today
– However it is toxic to most organisms without
specialized enzymes
– Generated as a byproduct of photosynthesis
– Early photosynthesizers likely needed to
escape from the oxygen they generated as
waste

• Oxygen buildup in enough quantity to
oxygenate the atmosphere took place
over a long period of time
– Roughly by the beginning of the Proterozoic
• With an oxygenated atmosphere came
ozone, which filtered out harmful UV
radiation, allowing organisms to survive
and flourish in shallow waters and lands
– This critical stage occurred around 600 Ma

• Oxygen levels have fluctuated throughout
Earth’s history, during the Cretaceous it
was 40% higher than today
• The emerging biosphere also had
profound effects on the carbon cycle
– The calcium carbonate shells of marine
organisms provide a storage reservoir for CO2,
and when they die, they form limestone

• The Gaia Hypothesis
– Proposes that life has altered the
environment at a global scale throughout
life’s history on Earth and continues to do
so; that these alterations contribute to
biogeochemical stability, expressed as a
condition of homeostasis on Earth; and
that the alterations benefit life by
increasing the probability of the
persistance of life

Evolution: the History of Life
• Evolution is achieved through the
processes of natural selection and
adaptation
– Natural selection occurs when individuals
of a population that are well-suited to an
environment survive and are reproductively
successful, and individuals that are poorly
adapted are reproductively unsuccessful
– Adaptation is the process of change in
response to environmental pressure

• Speciation occurs if
– The characteristics of a population, or part of
a population, change so dramatically over
time that the individuals can no longer breed
successfully with individuals from the original
population
– Allopatric speciation: results from geographic
isolation
– Sympatric speciation: results from
segregation and reproductive isolation

• In natural selection certain genetic
variants (alleles) become more or less
common as a result of the survival-
related usefulness of heritable traits
• Another mechanism for this is genetic
drift, resulting from random sampling of
genes from one generation to the next
• Mutation during the passage of genes
can also change the genetic sequence

• Some of the most ancient fossils found
are 3.55 billion year old microscopic
prokaryotes, others are layered
structures made of thin sheets of
calcium carbonate precipitated by
certain bacteria
– These are stromatolites

• Prokaryote world
– Early life was prokaryotic and anaerobic
• Generated energy through fermentation
• Produced alcohol and oxygen as waste
• For 2 billion years they dominated
– This puts limitations on the anaerobic cell
• A large surface-to-volume ratio is required for
diffusion of food in and waste out
• Cannot afford energy on specialized organelles
• Need free space around them, so cannot form
3 dimensional structures

• Emergence of eukaryotes
– Appeared about 1.4 billion years ago
• Aerobic - use oxygen for respiration
• More efficient than fermentation
• Can maintain a nucleus and organelles
• Not inhibited by crowding, and can form three
dimensional structures

• The Ediacaran fauna
– The earliest animal fossils found
– 600 million year old rocks
– Jelly-like animals without external armor
• Disclike
• Penlike
• Wormlike
– Represent a huge jump in complexity from
the first unicelled eukaryotes 800 million
years earlier

• Phanerozoic life - the Cambrian radiation
– Beginning 542 million years ago
• Introduction of internal and external skeletons
• A time of almost unbridled growth and diversity
in the marine environment
• The pace of evolution increased dramatically
• Most major modern groups of organisms
emerged and established body plans at this time

Evolution: the
History of Life

• Phanerozoic life - life on land
– Began about 450 million years ago
– In order to survive on land, organisms had
some basic requirements
• Structural support: skeleton or stem
• Internal aquatic environment with plumbing and
water conservation devices
• Mechanism for gas exchange: lungs or gills
• Moist environment for reproduction

– Early land plants likely evolved from algae
– Eventually vascular plants with woody stems
evolved
– In the Devonian, gymnosperms (naked seed
plants) appeared with male pollen and female
cell, and were freed from the swamps
– By the Cretaceous, angiosperms (flowering
plants) became dominant, and over time
developed specialized relationships with
insects, birds and mammals

– From early arthropods came insects, the
first to set foot on land
– By the Carboniferous, insects were
abundant and included dragonflies with a
wing span of 60 cm
– Have a primitive respiration system, which
limits their size

– Chordates are the ancient ancestors to
vertebrates, and have a cartilaginous rod
running along the back of the body
– Pikaia and Cambrian fish were jawless
– Jawed fish evolved afterwards, along with
a great burst of diversification
– About 400 million years ago, the first fish
ventured on land and gave rise to the
amphibians, but still breed in the water

– Reptiles freed themselves from the water
with water-tight skin and amniotic eggs
• Filled terrestrial niches that amphibians could
not
– By the Jurassic, reptile diversification filled
the land, air, and moved back into water
• This resulted in the two orders of dinosaurs,
and gave rise to mammals and birds

• The human family
– The emergence of humans during the
Cenozoic is one of the most complex and
controversial fields in paleontology
• The first bipedal hominid was Australopithecus
• Fossil range from 3.9 to 3.0 million years old
• Homo erectus dates back about 1.8 ma
• Homo habilis is older, but used stone tools
• 230,000 - 30,000 is Homo neanderthalensis
• Replaced by our own species, Homo sapiens

• The history of life’s biological
diversification has been driven by
– Availability of new environments
– Evolutionary innovations
• Organisms not only respond to
environmental change, but also create it

Extinction: the History of Death
• Organisms have disappeared by
extinction just as new types of
organisms have emerged by speciation
• The ongoing disappearance of life is the
background rate of extinction
– This rate varies among species
– On average this is 1 extinction per million
species/years

• Evidence from the fossil record points
to at least 5 mass extinction events
– Where many types of organisms die out in
short time periods
– Typically these are followed by radiations
– There is evidence for extraordinary factors
that contributed to these events
• Meteorite impact
• Massive outpourings of lava
• Supercontinent assembly

• The ultimate fate of all Earth’s species
is extinction
• However, humans have greatly
accelerated the rate of extinction since
the Industrial Revolution
• The sixth great extinction is ongoing, at
thousands of times the background rate

• Humans influence species’ abundance with
– Fire
– Hunting
– Overharvesting
– Deforestation
– Land-clearing
– Desertification
– Introduction of non-native species
– Pollution

Ecosystems, Biomes, and
Cycles of Life
The Blue Planet: Chapter 15

Outline
• Energy and Matter in Ecosystems
• Global Cycles of Life
• Biomes: Earth’s Major Ecosystems

Energy and Matter in
Ecosystems
• The two fundamental requirements for
any life-supporting system are
1. Flow of energy
– Functions of energy
– Pathways through which it flows
1. Continual recycling of chemical elements

Energy and Matter in Ecosystems
• The basic function of energy is to make
the production of organic matter
possible
• The total amount of organic matter in
any particular ecosystem is the biomass
• Biomass increases as a result of
biological production, the transformation
of energy into matter by biological
processes

• Photosynthesis
– In addition to oxygen, photosynthesis
produces carbohydrates, which build the
body mass of autotrophs
– Primary production
• Heterotrophs build body mass by eating
other organisms
– Secondary production

• Primary producers convert energy and
inorganic compounds into biomass
– 1st: autotrophic organism produces organic
matter in its body
– 2nd: uses this organic matter as fuel in
metabolism and respiration, releasing heat
– 3rd: stores some of the organic matter for
future use

• Energy is transferred along food chains
in which one organism eats another and
is, in turn, eaten by another organism
– A food chain is pathway by which energy
moves through an ecosystem
• Each group of species that is the same
number of steps away from the original
source of energy is a trophic level
– Food-energy levels together form a tropic
pyramid

• Food chains and trophic pyramids are
limited: at each level used energy, that
can never be recovered or recycled, is
lost
• The amount of biomass also decreases
from bottom to top

• Decomposers
– Organisms at each trophic level excrete
waste products and die
– Both of these processes leave behind
biomass as waste material
– This biomass is recycled by decomposers
• Saphotrophs
• Mainly bacteria and fungi

• When there are several interconnected
food chains in an ecosystem that
involves complex eating relationships
and interactions, it is called a food web
– A food web illustrates the movement of
both energy and matter through a complex
set of feeding interactions
– Energy moves through the food web in a
complicated, circuitous, but one-way flow

• Only 24 chemical elements are
essential nutrients
– Elements known to be required for life
– Micronutrients
• Elements required in small amounts by all life
or in moderate amount by some forms of life
– Macronutrients
• Elements required in large amounts by all life
• Carbon, hydrogen, nitrogen, oxygen,
phosphorus, and sulfur

• For any form of life to persist, the required
chemical elements must be available
– At the right times
– In the right amounts
– In the appropriate relative concentrations to each
other
• When this does not happen, an element can
become a limiting factor
– Preventing the growth of an individual, population,
or species, even causing its extinction

• Organisms are selective in their uptake
of chemical elements for the synthesis
of specific compounds in their cells
– As a result, chemical elements can
become much more concentrated in
organisms than in the local environment
– Concentration factor
– Plants that concentrate substances that
are toxic can be used to extract them from
contaminated soils
• Phytoremediation

• Mechanisms of bioconcentration
– Occurs when an organism takes in a
substance faster than it can process and
excrete it
– Bioaccumulation
• A substance is taken in without being excreted at a
comparable rate, it will become more concentrated
as the organism ages
– Biomagnification
• Food-chain concentration: the substance is passed
from consumer to consumer up to the next trophic
level

• Through uptake of chemicals from the
environment, living things change their
own chemistry and the chemistry of the
environment
• A system with a continually renewed
source of energy and a sink for heat
energy is the minimum required to
support and sustain life

Global Cycles of Life
• A biogeochemical cycle is the complete
pathway that a chemical element
follows through the Earth system
– This can be viewed as a set of open
systems linked by the transfer of materials
– These systems maintain a mass balance
since Earth functions as a closed system
– Crucial aspects are
• The set of processes that control the flux
• Residence time
• Bioavailability

• The calcium cycle
– Metallic element, does not form a gas
– Cycles between the geosphere, hydrosphere
and biosphere
– Constituent in igneous, metamorphic and
sedimentary rocks
– Released by weathering and carried to the
ocean attached to water molecules
– Can occur as dissolved ions or be precipitated
– Returns to land by plate tectonics

• The sulfur cycle
– Nonmetallic element, can form a wide variety
of reduced and oxidized compounds
– Mostly in long-term reservoirs in the geosphere
– Released to the atmosphere by volcanoes
– Absorbed directly by the biosphere or via water
– Because of gaseous forms, it cycles through
much more rapidlly than calcium

• The carbon cycle
– The building block of life, several gaseous forms
– Enters the atmosphere through respiration, fires,
volcanoes, decay, and ocean diffusion
– Removed by photosynthesis, can also dissolve in
water to make carbonic acid
– Converted to carbonate in the ocean, used by
organisms, buried in sediments and stored
– Transformed into fossil fuels, graphite and diamond
– Operates on several time scales, tens to millions of
years

• The nitrogen cycle
– The atmosphere’s main gift to life
– Unreactive gas that is a necessary component
of all proteins
– 90% of the conversion of free nitrogen into
biologically useful forms is done by bacteria
• Nitrogen fixation
– Upon conversion, it can be taken up by plants
and algae and consumed by animals
– Upon death, bacteria convert nitrogen back by
denitrification, and it returns to the atmosphere

• The phosphorus cycle
– Plays two important roles in the biosphere
• As sugar-phosphate units, forms the helical
framework of DNA
• Facilitates all of life’s energy transactions
– Occurs in its oxidized state as phosphate,
forming minerals found in soils and water
– Eroded from rocks, used by life on land,
washed to the ocean, temporarily available
to plankton, then deposited on the seafloor

• Biogeochemical links among the spheres
– Atmosphere
• Has the greatest influence on the biosphere and
has been most affected by it
• Photosynthesis and respiration
– Hydrosphere
• Water is indispensable for the biosphere
• The medium for aquatic life and gas exchange
• Transfer mechanism for nutrients

• Biogeochemical links among the spheres
– Geosphere
• Contributes all the elements necessary to
support life
• Volcanism is the source of CO2
• Source of chemical energy for chemoautotrophs
– Soil
• Weathered rock material that has been altered
by the presence of living and dying organisms
• Partly mineral, partly organic, solid, liquid and
gas, it is a link between the geosphere,
hydrosphere, atmosphere and biosphere

• Soil
– Contains a variety of materials
• Fragments of rocks and minerals
– Colloids, clay, silt, sand, and gravel
• Humus
– Partially decayed organic matter
• Small living organisms
– Worms, spiders, mites, bacteria, and fungi
– The relative proportions of these determine
soil texture and properties

• Soil properties
– Color: indicative of chemistry
– Structure: measure of its clumpiness
– Electrical resistivity
– Thermal properties
– Water content
– Shear strength

• Soil horizons and the soil profile
– Formed from decomposition of parent rock
by weathering, soil gradually develops
downward from the surface
• Accumulation of decaying organic matter
• Dark, humus-rich topsoil
• Zone of leaching
• Maximum accumulation of clay minerals
• Weathered parent rock

• Soil
– Soil serves as the base from which all
nutrition in all the terrestrial ecosystems is
derived
– Arable soils are those suited for growing
– Soil fertility is its ability to provide nutrients
• Soil moisture is a crucial component
• Fills pore spaces and carries cations important
to plant growth

• Impacts from the anthroposphere
– Humans remobilize carbon in the carbon
cycle by burning fossil fuels, which are
normally long-term storage reservoirs
– Deforestation releases carbon and
removes a sink for carbon dioxide
– Sulfur is mobilized by mining and refining,
and forms sulfuric acid in water

• Impacts from the anthroposphere
– Industrial combustion produces nitrogen
oxides, contributing to smog
– Sewage, raw or treated, contains high
concentrations of nitrogen and phosphorus

Biomes: Earth’s Major Ecosystems
• Biomes are the most important unit of
biogeography
– The geographic distribution of living
organisms and the characteristics of their
communities and ecosystems
• A biome is a large geographic area
defined by its environmental attributes
– Different biomes that coexist constitute a
bioregion or an ecozone

• There are two basic types of biomes
– Terrestrial
• Most important attributes are usually temperature and
precipitation (which influences soil type)
– Aquatic
• Temperature, salinity and water depth are the main
defining characteristics
– There are obviously differences in the types and
structures of organisms that live in each type of
biome, also in the resources available and the
basic types of food

• Terrestrial biomes
– Tundra: occurs at high latitudes
• Alpine tundra occurs at high altitudes closer to
the equator
• Characterized by permafrost
• Winters are long, cold and harsh, summers are
short and cool
– Boreal forest: to the south of tundra
• Cold winters, short growing seasons, and low
precipitation
• Coniferous trees dominate
• Also called taiga

– Temperate rain forest: coniferous forest
• Winters are milder than the north
• Precipitation is high, so forests are thick and tall
– Temperate deciduous forest: northeast US,
Europe and eastern China
• Characterized by seasonal changes
• Consist of broad-leaved deciduous trees
• Soils rich in organic material and well-suited to
agriculture

– Tropical rain forest: equatorial regions
• Host an enormous diversity of organisms
• Temperature and precipitation are high
• Growing season lasts all year
• Closed forests with an almost continuous
canopy
• Soils tend to be highly weathered and low in
organic matter
• Abundant decomposers

– Tropical deciduous forest: equatorial
• Also called tropical seasonal forests or monsoon
forests
• Main seasonal variation is precipitation
– Savanna: tropical and subtropical open forest
• Consist of broad, grassy plains and scattered trees
• Temperatures are high, rainfall is low
• Have a long dry season

– Chaparral: hot, dry summer, cool, wet winter
• Low, scrubby evergreen bushes and short trees
• Surrounds the Mediterranean Sea, SW US, parts
of Australia, Africa and South America
– Grassland: huge temperate prairies
• Grasses have interconnected root systems
• Well-suited to agriculture due to rich soils
– Temperate moist grasslands (tallgrass prairies)
– Shortgrass prairies, which are drier and more drought-
resistant

– Desert: low-precipitation extreme
• Subtropical: in the two subtropical dry belts
• Continental interior: far from any water source
• Rainshadow: where mountains create a barrier
• Coastal: western coasts
• Polar: low precipitation due to cold sinking air
– Plant cover is sparse in all five types, but
adapted to a lack of water

• Aquatic biomes
– Flowing-water environments: rivers
• Vary dramatically from source to mouth
• From small, cold and swift to wide, deep,
cloudy, and warmer
• Change seasonally
• Organisms are adapted to survive in strong
currents

• Aquatic biomes
– Standing water: lakes, ponds and wetlands
• Occupies a depression in the land, filled with
freshwater
• Lakes contain several zones defined by depth,
temperature, and distance from the shore
• Warmer zones tend to be more biologically
productive

• Aquatic biomes
– Transitional environments: coastal
• Transitional between fresh and saltwater
• Estuaries, salt marshes, mangrove forests
• Water level, salinity and temperature fluctuate
with tides
• Organisms are adapted to tolerated these
variations

• Aquatic biomes
– Marine biomes: oceans
• Divided by depth and penetration of light
• Also divided by proximity to shore
– Near shore: can be an intertidal zone
• Dynamic, high energy
• Organisms can attach themselves to rocky
surfaces or survive periodic drying out
• Beach organisms can burrow to escape
breaking waves

• Aquatic biomes
– Open water environments: most of the open ocean
• Floating or swimming organisms
• The photic zone is the most productive
• Deep ocean is cold and dark, but many organisms have
adapted
– Bottom environments: the benthic zone
• Can be deep: dominated by burrowing organisms and
bacteria
• Or shallow: highly productive with sea grasses, fish,
crustaceans, reptiles, detritivores, algae, sponges, and
coral reef communities

Lecture 9

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Lecture 9