Chapter 25: The History of Life on Earth
25.1: Conditions on early Earth made the origin of life possible
direct evidence of life on early earth from fossils of microorganisms that lived 3.5 billion years ago
hypothesis: chemical and physical processes on early earth, aided by emerging force of natural selection, produced very simple cells throguh sequence of 4 main stages
1) abioting (nonliving) synthesis of small organisc molecules, such as amino acids and nitrogenous bases
2) joining of these small molecules into macromolecules, such as proteins and nucleic acids
3) packaging of these molecules into protocells (abiotic precursor of a living cell that had a membrane like structure and maintained an internal chemistry different from that of its surroundings)
4) origin of self replicating molecules that eventually made inheritance possible
Synthesis of Organic Compounds on Early Earth
planet formed 4.6 billion years ago
condensed from vast cloud of dust and rocks that surrounded the sun
for the first few hundred million years, earth was bombarded by huge chucks of rock and ice left over from the formation of the solar system
collisions generated so much heat that all available water was vaporized, seas and lakes could not form
massive bombardment ended 4 billion years ago
first atmosphere had little oxygen
thick with water vapour
also thick with compounds released by volcanic eruptions
nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen
as earth cooled, water vapour condensed into oceans
hydrogen escaped into space
hypothesis that this atmosphere was a reducing (electron adding) environment
organic compounds could have formed from simpler molecules
energy for organic synthesis coming from lightning and intense UV radiation
early oceans were a solution of organic molecules (primitive soup)
some evidence suggests early atmosphere made up primarily of nitrogen and carbon dioxide
neither reducing nore oxidizing
small pockets of the early atmosphere, ex. near volcano openings, may have been reducing
first organic compounds may have formed near volcanoes
organic compounds may have first been produced in deep sea hydrothermal vents
= areas on the sea floor where heated water and minerals gush from earths interior into the ocean
alkaline vents release water with a high pH and is warm, which could be suitable for life
abiotic synthesis of organic molecules is possible under various conditions
source of organic molecules may have been meteorites
ex. murchison meteorite contains more than 80 amino acids
not contaminants from eath because has an equal mix of d and l isomers, organisms make and use only l isomers
also contains other key organic molecules (lipids, simple sugars, nitrogenous bases)
Abiotic Synthesis of Macromolecules
abiotic synthesis of adenine and guanine and cytosine and uracil bases can occur spontaneously from simple precursor molecules
but presence of small organic molecules is not sufficient for emergence of life
many types of macro molecules needed for self replication
proteins and nucleic acids could have formed on early earth
amino acid polymers could have acted as weak catalysts for a variety of chemical reactions on early earth
Protocells
all organisms must be able to carry out reproduction and metabolism
DNA molecules carry genetic information
incl. instructions needed to replicate accurately in reproduction
DNA replication requires enzymes and nucleotide building blocks provided by cells metabolism
self replicating molecules and metabolic source of building blocks may have appeared together in early protocells
may have met in vesicles (fluid filled compartments enclosed by a membrane-like structure)
abiotically produced vesicles can exhibit simple reproduction and metabolism, and maintain internal chemical environment different than surroundings
ex. can spontaneously form when lipipds are added to ware
molecules w hydrophobic and hydrophilic region organize into bilayer
can “reproduce” and “grow” on their own
some have selectively permeable bilayer and perform metabolic reactions using external source of reagents
Self-Replicating RNA
first genetic material was most likely RNA
RNA used for protein synthesis and enzyme-like catalyst
RNA catalysts called ribozymes
ribozymes can make complementary copies of short pieces of RNA if supplied with nucleotide building blocks
natural selection on molecular level has produced ribozymes capable of self replication
single stranded RNA molecules assume a variety of specific three-dimensional shapes mandated by nucleotide sequences
RNA molecules with certain nucleotide sequences may have shapes that enable them to replicate faster and with fewer errors than other sequences
RNA molecule with greatest ability to replicate will leave the most descendant molecules
occasional copying errors result in molecule shape that is more adept at self-replication
vesicle with self-replicating, catalytic RNA would differ from its many neighbours without those molecules
if vesicle could grow, split, and pass RNA molecules to daughters, daughters would be protocells
first protocells likely carried limited amounts of genetic information
inherited characteristics could have been acted on by natural selection
RNA could have provided template for assembly of DNA nucleotides
accurate replication advantageous for genomes
grow through gene duplication and other processes
DNA more chemically stable for storing genetic information
DNA replicated more accurately
25.2: The Fossil Record Documents the History of Life
fossil record based on the sequence in which fossils have accumulated in sedimentary rock layers
useful information also provided by insects preserved in amber and mammals frozen in soil
fossil record shows that there have been large changes in types of organisms on earth at different points in time
fossil record is also an incomplete chronicle of evolution
many organisms did not die in the right place and time to be preserved as fossils
many fossils layer destroyed by geologic processes
only a fraction of fossils have been discovered
record is biased in favour of species that:
existed for a long time
were abundant and widespread in certain kinds of environments
had hard shells, skeletons, or other parts that facilitated fossilization
How Rocks and Fossils Are Dated
radiometric dating = method for determining the absolute age of rocks and fossils based on the half life of radioactive isotopes
radioactive parent isotope decays to a daughter isotope at a characteristic rate
rate of decay is expressed by the half life
time required for 50% of the parent isotpe to decay
each radioactive isotope has a distinct half-life
not affected by temperature, pressure, or other environmental variables
fossils contain isotopes of elements that accumulated in organisms when they were alive
when organism dies, it stops accumulating isotope and amount in tissues does not change over time
method works for fossils up to 75 000 years old
geologic record = standard time scale that divides earth’s history into four eons and further subdivisions
phanerozoic eon
cenozoic era
quaternary period
holocene epoch
pleistocene epoch
neogene period
pliocene epoch
miocene epoch
paleogene period
eocene epoch
paleocene epoch
mesozoic era
cretaceuous period
jurassic period
triassic period
paleozoic era
permian period
carboniferous period
devonian period
silurian period
ordovician period
cambrain period
proterozoic eon
neoproterozoic era
edicaran period
archaean eon
hadean eon
The Origin of New Groups of Organisms
some fossils document the origin of new groups of organisms
mammals belong to group called tetrapods
having four limbs
mammals have a number of unique anatomical features that fossilize readily
lower jaw is composed of one bone (dentary) in mammals but several bones in other tetrapods
unique set of three bones that transmit sound in middle ear, whereas other tetrapods have one bone
teeth of mammlas differentiated (incisors, canines, premolars, and molars), other tetrapods have a row of undifferentiated single pointed teeth
fossil record shows these unique features evolved gradually over time
25.3: Key Events in Life’s History
key events include the origins of unicellular and multicellular organisms and the colonization of land
first three eons in the geological record (hadean, archaean, proterozoic) lasted about 4 billion years
phanerozoic eon represents most of the time that animals have existed on earth
divided into paleozoic, mesozoic, and cenozoic eras
each era is a distinct age in history of earth and its life
boundaries between eras correspond to major extinction events
The First Single-Celled Organisms
first organisms were single celled prokaryotes that lived in the ocean
earliest direct evidence comes from fossilized stromatolites
stromatolites = layered rocks that form when certain prokaryotes bind thin films of sediment together
stromatolites and other early prokaryotes were earth’s sole inhabitants for more than 1.5 billion years
Oxygen Revolution
most atmospheric oxygen gas (o2) is produced during the water-splitting step of photosynthesis
oxygen photosynthesis first evolved in photosynthetic prokaryotes
at first, free o2 produced probably dissolved in surrounding water until reaching a high enough concentration to react with other elements dissolved in water
incl. iron
iron would precipitate as iron oxide, which accumulates as sediments
sediments compressed into banded iron formations - red layers of rock containing iron oxide
once dissolved iron precipitated, additional o2 dissolved in water until seas and lakes became saturated with 02
then, o2 began to exit water and enter atmosphere
implies that bacteria similar to today’s cyanobacteria originated before 2.7 billion years ago
amount of atmospheric o2 increased gradually from 2.7 to 2.4 billion years ago, then shot up rapidly
oxygen revolution had an enormous impact on life
oxygen attacks chemical bonds and can inhibit enzymes and damage cells
rising concentration of atmospheric o2 probably resulted in the extinction of many prokaryotic groups
some species survived in habitats that remained anaerobic
diverse adaptations to changing atmosphere evolved
cellular respiration
The First Eukaryotes
eukaryotic cells have more complex organization than prokaryotic cells
nuclear envelope, mitochondria, endoplasmic reticulum, other internal structures
well developed cytoskeleton
enables eukaryotic cells to change their shape to surround and engulf other cells
eukaryotes originated by endosymbiosis
relationship between two species in which one organism lives inside the cell of another organism
when a prokaryotic cell engulfed a small cell that would evolve into the mitochondrion
small, engulfed cell is an endosymbiont that lives within another cell, the host cell
prokaryotic ancestor of the mitochondrion probably entered host cell as undigested prey or internal parasite
symbiosis could have become beneficial
world was becoming increasingly aerobic, anearobe host would have benefited from endosymbionts that could make use of oxygen
over time, host and endosymbionts would have become a single organism
serial endosymbiosis hypothesis: mitochondria evolved before plastids through a sequence of endosymbiotic events
all eukaryotes have mitochondria, but not all have plastids
mitochondria and plastids descended from bacterial cells
original host thought to have been an archaean
lots of evidence supports endosymbiotic origin of mitochondria and plastids
each organelle contains a genome that is often composed of a circular chromosome
genomes not associated with histones or large amounts of other proteins like bacterial chromosomes
mitochondria and plastids replicate y a process similar to certain bacteria
mitochondria and plastids have cellular machinery needed to transcribe and translate DNA into proteins
ribosomes of mitochondria and plastids more similar to bacterial ribosomes than to cytoplasmic ribosomes
size
protein and RNA sequences
sensitivity to certain antibiotics
inner membranes of both organelles have enzymes and transport systems homologous to plasma membranes of living bacteria
The Origin of Multicellularity
origin of structurally complex eukaryotic cells sparked the evolution of greater morphological diversity than possible for prokaryotic cells
after first eukaryotes appeared, great range of unicellular forms evolved
some single celled eukaryotes gave rise to multicellular forms
multicellularity evolved independently multiple times
accounts for large variety of algae, plants, fungi, and animals
Early Multicellular Eukaryotes
emerged about 1.3 billion years ago
oldest known fossis are of relatively small red algae from 1.2 billion years ago
larger more diverse multicellular eukaryotes dont appear until about 600 million years ago
fossils referred to as the ediacaran biota
soft bodied organisms
included both algae and animals and organisms of unknown taxonomic affinity
some over 1m long
rise of large eukaryotes in the ediacaran period represents an enourmous change in the history of life
prior to this, earth was a microbial world
diversification of the ediacaran biota ended about 541 million years ago
set the stage for the cambrian explosion
The Cambrian Explosion
many present day animal phyla appear suddenly in fossils formed early in the cambrian period
cambrian explosion = a relatively brief time in geologic history when many present day phyla of animals first appeared in the fossil record
fossils of several animal groups appear in rocks dating from the late proterozoic
sponges
cnidarians (sea anemones and relatives)
molluscs (snails, clams, relatives)
prior to cambrian explosion, all large animals were soft bodied
fossils of large pre-cambrian animals reveal little evidence of predation
animals appear to have been grazers, filter feeders, or scavengers
in relatively short period of time, predators over 1m in length emerged
had claws and other features for capturing prey
simultaneously defensive adaptations appeared in prey
sharp spines, heavy body armour
many animal phyla orginated long before cambrian explosion
sponges had evolved by 700 million years ago
common ancestory of arthropods, chordates, and other animal phyla lived 670 million years ago
fossils and DNA analyses suggest animals originated about 700 million years ago then remained small for over 100 million years
The Colonization of Land
milestone in the history of life
fossil evidence that some prokaryotes lived on terrestrial surfaces as early as 3.2 billion years ago
larger forms of life did not begin to colonize land until about 500 million years ago
gradual evolutionary venture out of aquatic environments associated with adaptations that made it possible to reproduce on land and helped prevent dehydration
land plants have vascular system for transporting materials internally, waterproof coating of wax on leaves that slows loss of water to the air
early signs of adaptations present 420 million years ago - small plants had a vascular system but lacked true roots or leaves
by 40 million years later, plants had diversified, included trees and many other plants with true roots and leaves
plants colonized land in the company of fungi
today roots of most plants are associated with fungi that aid in the absrption of water and minerals from the soil
root fungi (mycorrhizae) obtain organic nutrients from plants
mutually beneficial associations of plants and fungi evident in some of the oldest fossilized plants
most widespread and diverse land animals are arthropods (especially insects and spiders) and tetrapods
arthropods among the first animals to colonize land - 450 million years ago
earliest tetrapods found in fossil record lived about 365 milliion years ago, evolved from a group of lobe-finned fishes
tetrapods include humans
human lineage diverged from other primates around 6-7 million years ago
species originated about 195 000 years ago
25.4: The Rise and Fall of Groups of Organisms Reflect Differences in Speciation and Extinction Rates
rise or fall of any particular group is related to the speciation and extinction rates of its member species
rise of an organisms occurs when it produces more new species than are lost to extinction
changes in the fates of groups of organisms have been influenced by large-scale process (plate tectonics, mass extinctions, and adaptive radiations)
Plate Tectonics
three occasions when most of the landmasses of earth came together to form a supercontinent then later broke apart
1 billion years ago
600 million years ago
250 million years ago
each time this breakup yielded a different configuration of continents
estimated that a new supercontinent will form roughly 250 million years from now
plate tectonics = the theory that the continents are part of great plates of earth crust that float on the hot, underlying portion of the mantle
movements in the mantle cause the plates to move over time in a process called continental drift
move about a few centimetres per year
can measure the past locations using magnetic signal recorded in rocks at the time of their formation
as a continent shifts its position over time, the direction of magnetic north recorded in its newly formed rocks also changes
many important geologic processes, like the formation of mountains and islands occur at plate boundaries
some plates are moving away from each other
north american and eurasian plates drifting apart at a rate of about 2 cm per year
in other cases, two plates are sliding past each others, forming regions where earthquakes are common
ex. san andreas fault in california
two plates can be colliding
when an oceanic plate collides with a terrestrial plate, oceanic plate usually sinks below the terrestrial plate
oceanic plates are more dense than terrestrial plates
when two oceanic or two terrestrial plates collide with each other, causes violent upheavals and forms new mountains along the plate boundaries
ex. 45 million years indian plate crashed into the eurasian plate and started the formation of the himalayan mountains
Consequences of Continental Drift
alters the habitats in which organisms live
about 250 million years ago, plate movements brought previously separated landmasses together into supercontinent Pangaea
ocean basins became deeper, which lowered sea level and drained shallow coastal waters which most marine species inhabited
interior was cold and dry
organisms affected by the climate change that results when a continent shifts its location
southern tip of labrador once was located in the tropics but is now 40 degrees north over the last 200 million years
shifts in position cause organisms to adapt, move to a new location, or become extinct
continental drift also promotes allopatric speciation on a grand scale
when supercontinents break apart, regions that once were connected become geographically isolated
as the continents drifted apart each became a separate evolutionary arena with lineages of plants and animals that diverged from those on other continents
ex. marsupial mammals fill ecological roles in australia analogous to those filled by eutherians on other continents
fossils show masupial mammals in originated in asia and reached australia via south america and antartica while continents were joined
breakup of southern continents set australia afloat, marsupials diversified, few placental mammals became extinct
on other continents marsupials became extinct, eutherians diversified
can explain puzzling geographic distributions of extinct organisms
ex. fossils of the same species of permian freshwater reptiles found in brazil and ghana
two areas were joined when reptiles were living but are now separated by 3000 km of ocean
Mass Extinctions
the overwhelming majority of species that ever lived are now extinct
extinction can happen from physical factors in environment or biological factors
extinction occurs regularly, but certain disruptive changes have caused mass extinctions in which large numbers of species become extinct world wide
Five Mass Extinction Events
events are particularly well documented for the decimation of hard bodied animals that live in shallow seas
in each mass extinction 50% or more of the earths marine species became extinct
permian mass extinction
defines boundary between paleozoic and mesozoic eras
96% of marine animal species became extinct
8/27 known orders of insects became extinct
occurred in less than 200 000 years
during the most extreme episode of volcanism
1.6 million km² in siberia was covered in lava hundreds of metres thick
sibearian volcanism may have triggered combustion of coal seams and spread ash globally
ash detected in permian aged rocks in the arctic
eruptions produced enouch co2 to warm climate by 6 degrees celsius
rise in atmospheric co2 levels —> ocean acidification —> reduced availability of calcium carbonate
calcium carbonate used by reef building corals and many shell building species
eruptions added phosphorous to marine ecosystems
stimulate growth of microorganisms
microorganisms provide food for bacterial decomposers
bacteria use oxygen as they decompose dead organisms —> drop in oxygen concentrations
harmed oxygen breathers, promote growth of anaerobic bacteria that produce hydrogen sulphide gas as a metabolism byproduct
volcanic eruptions triggered series of catastrophic events resulting in permian mass extinction
cretaceous mass extinction
66 million years ago
extinguished over half of all marine species
eliminated many families of terrestrial plants and animals, all dinasours except birds
thin layer of clay enriched in iridium dates to the time of the mass extinction
iridium is rare on earth, common in metorites and other extraterrestrial objects
proposed that clay is fallout from a huge cloud of debris resulting from an asteroid or comet colliding with earth
cloud would block sunglight and cause sudden drop in global temperatures
chicxulub crater
right size to have been caused by an object with diameter of 10km
many species may have been vulnerable to extinction bc populations declined with volcanic eruptions
wildfires resulting from impact contributed to sharp rise in atmospheric co2 levels
another stress for populations in decline
Is a 6th Mass Extinction Under Way?
over a thousand species have become extinct in the last 400 years
100-1000 times the typical background rate seen in the fossil record
some species may be driven to extinction before we even learn of them
habitat loss, introduced species, overharvesting
climate change may hasten declines
extinction rates increase when global temperatures are high
Consequences of Mass Extinctions
takes 5-10 million years for diversity of life to recover to previous levels after a mass extinction
alter ecological communities
ex, after permian and cretaceous extinctions, % of marine organisms that were predators increased
rise in predators increases risks for prey and competition among predators
curtail lineages with novel and advantageous features
ex, late triassic period, group of gastropods could drill through shells of bivalves and feed on animals inside
wiped out during mass extinction at the end of the triassic
another group with this ability originated 120 million years later
mass extinctions —> adaptive radiations where new groups of organisms proliferate
Adaptive Radiations
diversity of life has increased over the past 250 million years
adaptive radiations = periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological roles or niches in their communities
large scale adaptive radiations occurred after each mass extinction
occurred in groups that possessed major evolutionary innovations
ex. seeds, armoured body coverings
occurred when organisms colonized regions where they faced little competition from other species
mammals underwent adaptive radiation after extinction of dinosaurs
mammal fossils older than 66 million years are small and show less morphological diversity than found today
many species were nocturnal (large eye sockets)
few early mammals were intermediate in size
early mammals restricted in size and diversity bc they were eaten or outcompeted by dinosaurs
disappearance of dinosaurs —> expansion of mammals
radiations in which groups of organisms increased in diversity as they played new ecological roles in communities
ex. rise of photosynthetic prokaryotes
ex. evolution of large predators in cambrian explosion
ex. radiations following the colonization of land by plants, insects, and tetrapods
organisms that arise in an adaptive radiation can serve as a new food source for other organisms
ex. diversification of plants stimulated series of adaptive radiations in insects that ate or pollinated plants
adaptive radiations can occur over limited geographic areas
initiated when a few organisms make their way to a new location where they face little competition
25.5: Major Changes in Body Form Can Result from Changes in the Sequences and Regulation of Developmental Genes
Effects of Developmental Genes
slight genetic differences can produce major morphological differences between species
large morphological differences can result from genes that alter the rate, timing, and spacial pattern of change in an organisms form
many evolutionary transformations are the resolution of heterochrony
heterochrony = an evolutionary change in the rate or timing of developmental events
organisms shaped depends on relaive growth rates of different body parts during development
ex. shape of human vs chimpanzee skulls
ex. increased growth rates of finger bones —> skeletal wing structure in bats
ex. slowed growth of leg and pelvic bones —> reduction and eventual loss of hind limbs in whales
heterochrony alterns timing or reproductive development relative to development of nonreproductive organs
if reproductive organs develop at same rate as other organs —> paedomorphis
paedomorphis = the retention in an adult organism of the juvenile features of its evolutionary ancestors
ex. some salamander species grow to adult size and become sexually mature while retaining gills and other larval features
produces animals that appear very different from ancestors
evolutionary changes result from alterations in genes that control placement and spatial organization of body parts
ex. homeotic genes (master regulatory genes) determine where a pair of wings, legs, flower parts, etc. will develop
products of Hox class genes provide positional information in an animal embryo
information prompts cells to develop into structures appropriate for a location
changes in hox genes have a large impact on morphology
Evolution of Development
set of genes sufficient to produce complex animals existed 25 million years before cambrian explosion
gene duplication events —> new developmental genes —> origin of novel morphological forms
study of developmental changes in divergens of six-legged insects from crustacean ancestors that had more than six legs
crustaceans and insects differ in expression and effects of Hox gene Ubx
Ubx suppresses leg formation in insects when expressed
artemia Ubx gene suppresses 15%, drosophila Ubx supresses 100%
identify mutations that would cause artemia gene to suppress leg formation
amino acid changes responsible for suppression of additional limbs in insects
particular changes in nucleotide sequence of a developmental gene can cause major evolutionary change
changes in gene regulation can be limited to one cell type, have less harmful side effects
changes in form of organisms may be caused by mutations in developmental gene regulation, not sequences
ex. stickleback fish
changes in gene regulation can alter the form of individual organisms and lead to evolutionary change in populations
25.6: Evolution is Not Goal Oriented
origin of new species affected by small scale factors and large scale factors
small scale = natural selection in a population
large scale = continental drift
evolution has led to three key features in the natural world:
organisms are suited for life in their environments
organisms have many shared characteristics of life
rich diversity of life
complex structures often evolved in increments from simpler versions that perform the same basic function
ex. human eye developed from simple eyes
simplest: patches of light-sensitive photoreceptor cells
single evolutionary origin
found in a variety of animals
enable animal to distinguish light from dark
complex mollus eyes (squids, octopuses) evolved independently of vertebrate eyes, but also from cluster of photoreceptor cells
independent bc vertebrate eyes detect light at back layer of retina and conduct nerve impulses toward the front, mollusc eyes are the opposite
eyes retain basic function of vision
evolutionary novelties arise when structures that originally played one role gradually acquire another
ex. as cynodonts —> early mammals, bones that comprised the jaw hing were incorporated into ear ragion, took on a new function
exaptations = structures that evolve in one context but become co-opted for another function
does not imply that a structure evolves in anticipation of future use
some evolutionary lineages exhibit trend towards larger or smaller body size
branching evolution can result in a real evolutionary trend even if some species counter the trend
species are analogous to individuals where speciation is birth, extinction is death, new species are offspring
species undergo species selection which determines the direction of major evolutionary trends
evolutionary trend does not imply an intrinsic drive towards a particular phenotype
evolution is the result of interactions between organisms and their current environments