Chapter 15
The History of Life on Earth
How Did Life Begin?
Louis
Pasteurs Experiments Refuted Spontaneous Generation
How Did Life Begin?
But:
The First Living Things Arose from Nonliving Ones
Example of Chemical Evolution
Spontaneous
formation of porphyrin rings from formaldehyde
Components
of chlorophylls and cytochromes
How Did Life Begin?
Organic
Molecules Can Accumulate under Prebiotic Conditions
Not fast
But no life,
or oxygen
However: UV
radiation
Simple
molecules combine to form more complex ones-primordial soup?
Organic
Molecules May Have Become Concentrated in Tidal Pools
RNA World
DNA is genetic
material now
DNA-to-RNA-to-protein system is complicated
RNA may have been
first genetic material
RNA can assemble
spontaneously
Ribozymes
catalyze reactions
How switch from
RNA to DNA might have occurred is not known
Possible Sequence
What Were the Earliest Organisms Like?
The First
Organisms Were Anaerobic Prokaryotes
Organisms
Evolved the Ability to Capture the Suns Energy
Photosynthesis
Increased the Amount of Oxygen in the Atmosphere
Aerobic
Metabolism Arose in Response to the Oxygen Crisis
Proterozoic Eon
Origin
of photosynthetic eubacteria
Noncyclic
pathway first
Cyclic
pathway next
Oxygen
accumulates in atmosphere
Origin
of aerobic respiration
The First Cells
Originated
in Archeon Eon
Were
prokaryotic heterotrophs
Secured
energy through anaerobic pathways
No
oxygen present
Relied
on glycolysis and fermentation
Theory of Endosymbiosis
Lynn
Margulis
Mitochondria
and chloroplasts are the descendents of free-living prokaryotic organisms
Prokaryotes
were engulfed by early eukaryotes and became permanent internal symbionts
Some Organisms
Acquired Membrane-Enclosed Organelles
Mitochondria and Chloroplasts May Have Arisen from
Engulfed Bacteria
Evolutionary Tree
Evolutionary Tree
Possible Origin of ER
Advantages of Organelles
Nuclear
envelope may have helped to protect genes from competition with foreign DNA
ER
channels may have similarly protected vital proteins
What Were the Earliest Organisms
Like?
Evidence for the Endosymbiont Hypothesis Is Strong
Symbiosis within a modern cell
Separate DNA
What Were the Earliest Multicellular
Organisms Like?
Some Algae Became
Multicellular
Animal Diversity
Arose in the Precambrian Era
Diversity of ocean life during the Silurian period
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
How Did Life Invade the Land?
Seed Plants Encased Sperm in Pollen
Grains
Flowering Plants Enticed Animals to Carry Pollen
Rise of Flowering Plants
How Did Life Invade the Land?
Some
Animals Became Adapted to Life on Dry Land
Amphibians Evolved from Lobefin Fishes
Reptiles Evolved from Amphibians
How Did Life Invade the Land?
Reptiles
Gave Rise to Both Birds and Mammals
What Role Has Extinction Played in the History of Life?
Climate Change
Contributed to Mass Extinctions
Continental drift from plate tectonics
K-T Asteroid Impact
Theory
is that an asteroid impact led to mass extinction
Iridium
Earliest Primates
Primates
evolved more than 60 million years ago during the Paleocene
First
primates resemble tree shrews
Long
snouts
Poor
daytime vision
Adaptations to an
Arboreal Lifestyle
During
the Eocene, certain primates became adapted to life in trees
Better
daytime vision
Shorter
snout
Larger
brain
Forward-directed
eyes
Capacity
for grasping motions
How Did Humans Evolve?
Humans
Inherited Some Early Primate Adaptations for Life in Trees
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
From Primates to Humans
Uniquely human traits evolved
through modification of traits that evolved earlier, in ancestral forms
Hominoids
Apes,
humans, and extinct species of their lineages
In
biochemistry and body form, humans are closer to apes than to monkeys
Hominids
Subgroup
that includes humans and extinct humanlike species
The Oldest Hominid Fossils Are from Africa
Earliest Fossils Are African
Africa
appears to be the cradle of human evolution
No
human fossils older than 1.8 million years exist anywhere but Africa
Homo
erectus left Africa in waves from 2 million to 500,000 years ago
Trends in Lineage
Leading to Humans
Less reliance on
smell, more on vision
Skeletal changes
to allow bipedalism
Modifications of
hand to allow refined hand movements
Bow-shaped jaw
and smaller teeth
Longer lifespan
and longer period of dependency
First Hominids
Earliest
known is Ardipithecus ramidus
Lived
4.4 million years ago in Africa
More
apelike than humanlike
Numerous
australopiths evolved during the next 2 million years
Large
face, protruding jaw, small skull
Walked
upright
Australopiths
Earliest known is A. anamensis
A.
afarensis and A. africanus arose next
All
three were slightly built (gracile)
Species
that arose later, A. boisei and A. robustus, had heavier builds
Exact
family tree is not known
Humans Arise
First member of
the genus Homo is H. habilis
Lived in
woodlands during late Miocene
How Did Humans Evolve?
The Earliest
Hominids 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
Homo erectus
Evolved
in Africa
Migrated
into Europe and Asia about 1.5 million - 2 million years ago
Had
a larger brain than H. habilis
Was
a creative toolmaker
Built
fires and used furs for clothing
How Did Humans Evolve?
Neanderthals
Had Large Brains and Excellent Tools
Modern
Humans Emerged Only 150,000 Years Ago
Paleolithic
burial
The
sophistication of Cro-Magnon people
Cro-Magnons
and Neanderthals Lived Side by Side
Homo sapiens
Modern
man evolved by 100,000 years ago
Had
smaller teeth and jaws than H. erectus
Facial
bones were smaller, skull was larger
Neanderthals
Early
humans that lived in Europe and Near East
Massively
built, with large brains
Disappeared
when H. sapiens appeared
DNA
evidence suggests that they did not contribute to modern European populations
Where Did H. sapiens Arise?
Two
hypotheses:
Multiregional model
African emergence model
Both
attempt to address both biochemical and fossil evidence
Multiregional Model
Argues
that H. erectus migrated to many locations by about 1 million years ago
Geographically
separated populations gave rise to phenotypically different races of H.
sapiens in different locations
Gene
flow prevented races from becoming species
African Emergence Model
Argues
that H. sapiens arose in sub-Saharan Africa
H.
sapiens migrated out of Africa and into regions where H. erectus had
preceded them
Only
after leaving Africa did phenotypic differences between races arise
Genetic Distance Data
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