The Origin and History of Life on Earth
• What is Life?
– Life is, to the best of our knowledge to date, unique to
our planet Earth.
– There is no simple definition of Life, except that life
forms are able to act on their own behalf to support
their own existence, and to reproduce themselves.
– The field of science known as Biology is dedicated to
the study of life, its component species, and the
variations within species of life forms.
The Origin and History of Life on Earth
• What Is a Species?
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Species Are Groups of Interbreeding Populations
Appearance Can Be Misleading
Allopatric and sympatric speciation
Geographic Separation of a Population Can Lead to
Allopatric Speciation
– Ecological Isolation of a Population Can Lead to
Sympatric Speciation
Earth’s Biosphere and Life Science
• Our Earth is, to the best of our current knowledge,
the only planet in our Solar System (or beyond)
which supports life of any kind.
• The time table and processes of the origin of life is
poorly known at present, but there is evidence that
the simplest forms of life came into being more than
3 billion years ago (out of the total of 4.6 billion
years our Earth has been in existence).
• Earth’s original atmosphere contained little or no
molecular oxygen, which is required by current
animal and advanced plant life forms.
Earth’s Biosphere and Life Science
• The oxygen currently in our atmosphere is produced
by green plant photosynthesis, which began with
reduction of carbon dioxide by anaerobic bacteria,
also known as cyanobacteria or “blue-green algae”,
inhabiting the oceans in Earth’s early history.
• Animal life, as we know it (which requires
atmospheric oxygen), did not come into existence
until about 600 million years ago.
• An important topic of the present day is that humans
are returning CO2 to the atmosphere at a faster rate
than plants can reduce it to form O2, which can
cause global warming.
Evolution of Oxygen Content of Earth’s Atmosphere
How Do New Species Form?
• Changes in Chromosome Number Can Lead to Sympatric Speciation
– Speciation by polyploidy
• Change Over Time Within a Species Can Cause Apparent “Speciation”
in the Fossil Record
• Under Some Conditions, Many New Species May Arise
Need Text Description
How Is Reproductive Isolation Between
Species Maintained?
• Premating Isolating Mechanisms Prevent Mating Between
Species
– Members of Different Species May Be Prevented from Meeting
– Different Species May Occupy Different Habitats (Ecological
isolation)
• Different Species May Breed at Different Times (Temporal
Isolation)
• Different Species May Have Different Courtship Rituals
– Species’ Differing Sexual Organs May Foil Mating Attempts
• Postmating Isolating Mechanisms Limit Hybrid Offspring
– One Species’ Sperm May Fail To Fertilize Another Species’ Eggs
– Hybrid Offspring May Survive Poorly
– Hybrid Offspring May Be Infertile
What Causes Extinction?
• Localized Distribution and Overspecialization make
Species Vulnerable in Changing Environments
• Very localized distribution can endanger a species
• Extreme specialization places species at risk
• Interactions with Other Organisms May Drive a Species
to Extinction
• Habitat Change and Destruction Are the Leading Causes
of Extinction
• Natural events, of geological or extraterrestrial origin,
can cause sudden, and extremely catastrophic,
extinctions on a large, or even global, basis.
How Did Life Begin?
• Experiments Refuted Spontaneous Generation
• Chemical Evolution Preceded and Gave Rise to Life
– Organic Molecules Can Form Spontaneously Under Prebiotic
Conditions
– Organic Molecules Probably Accumulated Under Prebiotic
Conditions
– Organic Molecules May Have Become Concentrated in Tidal
Pools
• RNA May Have Been the First Self-Reproducing
Molecule
• Membrane-Like Microspheres May Have Enclosed
Ribozymes
Broth Heated to
Boiling to Sterilize
No growth
Growth
(Broth cooled
without exposure
to room air)
(Broth cooled
and exposed to
room air)
Louis Pasteur's experiment disproving the spontaneous
generation of microorganisms in broth.
Experimental Research on the Origin of Life
• One of the greatest unsolved mysteries of science is,
“how did life evolve on Earth?”
• We have evidence that the oldest and simplest forms of
life, anaerobic bacteria, may have formed more than 3.6
billion years ago (only 1 billion years after Earth was
created).
• These life forms, also called cyanobacteria or “bluegreen algae”, did not require oxygen to breathe (as do
nearly all organisms inhabiting Earth at the present
time).
Experimental Research on the Origin of Life
• However, the capability of these anaerobic bacteria to
convert atmospheric carbon dioxide (CO2) into oxygen
(O2) and carbohydrates made possible the evolution of
more complex life forms, including all animal life, that
breathe oxygen and feed upon plant carbohydrates.
• Since we have very few geologic records of the earliest
periods of life on Earth, scientists have performed
laboratory experiments with electric discharges (simulating
lightning) in various mixtures of gases (simulating Earth’s
earliest atmosphere) to see if the chemical substances
required for life forms could have been produced by
lightning early in Earth’s history.
The Experimental Apparatus of Stanley Miller and
Harold Urey
• Life's earliest stages left no fossils, so evolutionary historians have pursued
a strategy of re-creating in the laboratory the conditions that may have
prevailed on early Earth.
• The mixture of gases in the spark chamber simulates lightning in Earth's
early atmosphere.
Did Microspheres Enclose the Earliest Cells?
• Cell-like microspheres can be formed by agitating
proteins and lipids in a liquid medium. Each microsphere
in this photo is about 5 micrometers (µm) in diameter.
What Were the Earliest Organisms Like?
• The First Organisms Were Anaerobic Prokaryotes
• Some Organisms Evolved the Ability to Capture the
Sun’s Energy
• Photosynthesis Increased the Amount of Oxygen in the
Atmosphere
• Aerobic Metabolism Arose in Response to the Oxygen
Crisis
• Some Organisms Acquired Membrane-Enclosed
Organelles
– Mitochondria and Chloroplasts May Have Arisen from Engulfed
Bacteria
– Evidence for the Endosymbiont Hypothesis Is Strong
The Relationship between Time and the Decay
of Radioactive 40K (yellow) to 40Ar (blue)
The Probable Origin of Mitochondria and
Chloroplasts in Eukaryotic Cells
Symbiosis Within a Modern Cell
• The ancestors of the chloroplasts in today's plant cells
may have resembled Chlorella, the green,
photosynthetic, single-celled algae living symbiotically
within the cytoplasm of the Paramecium pictured here.
What Were the Earliest Multicellular Organisms
Like?
• Some Algae Became Multicellular
– Higher organisms with differentiated cells evolved more than 1
billion years ago
• Animal Diversity Arose in the Precambrian Era
– Original animal forms were primarily aquatic shelled invertebrates
(ocean dwellers), dating back to the Proterozoic (pre-Cambrian) era,
more than 600 million years ago.
• Life Invaded the Land beginning in the Cambrian Era
– Land plants began in the Cambrian era, about 500 million years ago
– Oxygen content of the atmosphere reached near-current levels in
the Devonian era, beginning nearly 400 million years ago
– Land animals date back to the Devonian era, about 350 million
years ago.
Diversity of Ocean Life during the Silurian Period
• (a) Life characteristic of the oceans during the Silurian period, 440
million to 410 million years ago. Among the most common fossils from
that time are (b) trilobites and their predators, such as nautiloids and (c)
ammonites. This (d) living Nautilus is very similar in structure to the
Silurian nautiloids, showing that a successful body plan may exist
virtually unchanged for hundreds of millions of years.
How Did Life Invade the Land?
•Some Plants Became Adapted to Life on Dry Land
•Primitive Land Plants Retained Swimming Sperm and
Required Water to Reproduce
•Seed Plants Encased Sperm in Pollen Grains
•Flowering Plants Enticed Animals to Carry Pollen
•Some Animals Became Adapted to Life on Dry Land
•Amphibians Evolved from Lobefin Fishes
•Reptiles Evolved from Amphibians
•Reptiles Gave Rise to Both Birds and Mammals
A Fish that Walks on Land
• Some modern fishes, such as this mudskipper, walk on
land. Like the ancient lobefin fishes that gave rise to
amphibians, mudskippers use their strong pectoral fins
to move across dry areas in their swampy habitats.
What Role Has Extinction Played in the
History of Life?
• Evolutionary History Has Been Marked by Periodic Mass
Extinctions
• Climate Change Contributed to Mass Extinctions
• Catastrophic Events May Have Caused the Worst Mass
Extinctions
• The extinction of the Dinosaurs has recently been
determined to have been caused by an asteroid impact
with the Earth about 65 million years ago
• There is evidence of an even more catastrophic
extinction event further back in Earth’s history (about 300
million years ago)
Continental Drift from Plate Tectonics
• The continents are passengers on plates moving on Earth's
surface as a result of plate tectonics.
• (a) About 340 million years ago, much of what is now North
America was positioned at the equator.
• (b) All the plates eventually fused together into one gigantic
landmass, which geologists call Pangaea.
• (c) Gradually Pangaea broke up into Laurasia and
Gondwanaland, which itself eventually broke up into West
and East Gondwana.
• (d) Further plate motion eventually resulted in the modern
positions of the continents.
Evolution of the Earth with Time: Continental Drift
200 Million Years Ago
50 Million Years Ago
150 Million Years Ago
100 Million Years Ago
Present
How Did Humans Evolve?
• Some Early Primate Adaptations for Life in Trees Were
Inherited by Humans
• Binocular Vision Provided Early Primates with Accurate
Depth Perception
• Early Primates Had Grasping Hands
• A Large Brain Facilitated Hand–Eye Coordination and
Complex Social Interactions
• The Oldest Hominid Fossils Are from Africa
How Did Humans Evolve?
• The Earliest Australopithecines Could Stand and Walk
Upright
• Several Species of Australopithecus Emerged in Africa
• The Genus Homo Diverged from the Australopithecines
2.5 Million Years Ago
• The Evolution of Homo Was Accompanied by Advances
in Tool Technology
• Neanderthals Had Large Brains and Excellent Tools
How Did Humans Evolve?
• Modern Humans Emerged Only 150,000 Years Ago
• Cro-Magnons and Neanderthals Lived Side by Side
• Several Waves of Hominids Emigrated from AfricaThe
Evolutionary Origin of Large Brains May Be Related to
Meat Consumption
• The Evolutionary Origin of Human Behavior Is Highly
Speculative
• The Cultural Evolution of Humans Now Far Outpaces
Biological Evolution
Representative Primates
• The (a) tarsier, (b) lemur, and (c) lion-tail macaque
monkey all have relatively flat faces, with forward-looking
eyes providing binocular vision. All also have color vision
and grasping hands. These features, retained from the
earliest primates, are shared by humans.
The earliest hominid
• This nearly complete skull of Sahelanthropus tchadensis,
which is more than 6 million years old, is the oldest
hominid fossil yet found.
Representative hominid tools
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(a) Homo habilis produced only fairly crude chopping tools called hand
axes, usually unchipped on one end to hold in the hand. (b) Homo ergaster
manufactured tools that were typically sharp all the way around the stone; at
least some of these blades were probably tied to spears rather than held in
the hand. (c) Neanderthal tools were works of art, with extremely sharp
edges made by flaking off tiny bits of stone. In comparing these weapons,
note the progressive increase in the number of flakes taken off the blades
and the corresponding decrease in flake size. Smaller, more numerous
flakes produce a sharper blade and suggest either more insight into tool
making, more patience, finer control of hand movements, or perhaps all
three.
Paleolithic Burial
• This 24,000-year-old grave shows evidence that CroMagnon people
ritualistically buried their dead.
• The body was covered with a dye known as red ocher, then buried
wearing a headdress made of snail shells and with a flint tool in its hand.
Competing Hypotheses for the Evolution of Homo Sapiens
• The "multiregional" hypothesis suggests that populations of H. sapiens
evolved in many regions simultaneously from the already widespread
populations of H. erectus.