Biology 102
Chapter 25 –
The History of Life on Earth
– Geological time scale – appearance of new species and evolutionary lineages, usually
take place over a geological time scale
– Strata – oldest layers of rock (lay at the bottom)
Principles of Stratigraphy –
– Fossils of similar organisms are found in widely separated places on Earth
– Certain fossils are always found in younger strata, and certain other fossils are found in
older strata
– Organisms found in younger strata are more similar to modern organisms than are those
found in older strata
Radioisotopes – radioactive isotopes of elements (they decay in a predictable)
Half-life – specific time interval, half of the atoms in a radioisotope decay to become a
different, stable isotope
Radiometric dating – the use of this knowledge to date fossils and rocks
Sedimentary rocks – formed from materials that existed for varying lengths of time before
being weathered, fragmented, and transported to the site of their deposition
Igneous rock – radiometric dating of rocks older than 60,000 years requires estimating
radioisotope concentrations in igneous rock, which is formed when molten material cools
Eons – scientists divided the history of life into 4 ions
Hadean eon – refers to the time where the earth was dark, before the earth was suitable for
life
Archean eon – early history of life occurred during this period
Proterozoic era – prokaryotic life diversified rapidly in this eon
(these three are collectively referred to as the Precambrian Period)
Alfred Wegener – created the idea of continental drift
– This idea was met with mad skepticism
– When plate tectonics were introduced as physical evidence then it grew more accepted
Lithosphere – earth’s crust consists of several solid plates
Asthenosphere – thick continental and thinner oceanic lithosphere plates overlie a viscous,
malleable layer of Earth’s mantle (weak sphere)
Subduction – when oceanic plates and continental plates converge, the thinner oceanic
plate is forced underneath the thicker continental plate
– It results in volcanism and mountain building on the continental boundary
When two oceanic plates collide, one is also subducted below the other, producing an
oceanic trench
Throughout Earth’s history, earth’s climate was considerably warmer than it is today
Oxygen concentrations in Earth’s atmosphere have changed over time
– The first big step occurred about 2.5 billion years ago, when certain bacteria gained the
ability to use water as the source of hydrogen ions for photosynthesis
– By chemically splitting H20, these bacteria generated O2 as a waste product
– They also made electrons available for reducing CO2 to form the carbohydrate end
products or photosynthesis
Geologists divided earth’s history into eons, eras, and periods based on distinct fossil
assemblages
– Biota – refers to the assemblage of all organisms of all kinds living at a particular time or
place
– Flora – all plants living at a particular time or place
Only a tiny fraction or organisms ever become fossils, and only a tiny fraction of fossils
are ever discovered by paleontologists
Most organisms live and die in oxygen rich environments, in which they quickly
decompose
Organisms are not likely to become fossils unless they are transported by wind or water
Plankton – small floating organisms, known collectively as plankton, were strained from
the water and eaten by slightly larger feeder fish
Cambrian explosion – life expanded rapidly
The Silurian –
– During this period the continents began to cluster together
– Marine life rebounded from the end of the Ordovian
– Scorpion and millipedes evolved around this time
The Devonian –
– Rates of evolutionary change accelerated in many groups of organisms during the
Devonian period
The Carboniferous –
– Large glaciers formed over high altitude portions of south land masses during the
Carboniferous period, but extensive swamp forests grew on the tropical continents
The Triassic –
– Many invertebrate groups diversified, and many burrowing animals evolved from group
diversified, and many burrowing animals evolved from groups living on the surfaces of sea
flower
The half life of carbon 14 is 5,700 years. If 10 percent of the original carbon 14 is left in a
structure, that structure is between 17,000 and 25,000
Friday janary 24th – 10 points and 5 questions
First lecture exam – Friday January 31 (25 21 23)
Major Events in Life’s History on Earth –
Climate has not be constant
We have had periods of very hot weather and cool weather and glaciation (and we have a
drop in sea level and mass extinction or marine life)
– Volcanism
– Meteorite hits
– Carbon dioxide
– Continental drift (can cause volcanism)
– Increase in oxygen concentration (more life on earth) (started to split hydrogen atoms)
Geologic time Scale – PRINT IT OUT AND STUDY IT
Eons are the largest period of time scales, split into 4
– The last eon is where we see diversification of life
The assemblage of all kinds of organisms alive at one time is called the biota
Flora – all the plants
Fuana – all the animals
Most organisms are decomposed quickly after death
– However, if they are transported to sites with no oxygen then decomposition is very slow
– Many geologic processes transform rocks and destroy the fossils they contain or bury
them to deeply
– Vast majority of fossil species are marine organisms that had hard shells or skeletons that
resist decomposition
– Insects and spiders are also well represented
STUDY FIGURE 25.12
– 4.6 billion years ago
Headean period
Archean period
– Life originated in the oceans
Proterozoic
– Increased oxygen
Phanerozoic
– See the fossils
3.8 billion years ago – life first started to appear
1.5 – protozoans – they diversified as O2 levels increased and Earth began to warm up,
leading to the Cambrian Explosion – diversification. Origin of multicellular eukaryotes.
Continental Drift-
– The idea that land masses have moved over time was first suggested by Alfred
Pangaea – super continent (because of this 96 percent of the population went extinct
because of volcanoes
When Europe collided with Asia, the Himalayas were formed
KNOW THE SEQUENCE OF THE PERIODS AND THE MAIN EVENTS OF WHAT
HAPPENED IN THESE PERIODS
Cambrian explosion
– Diversification of life
After Cambrian came the Ordovician Period
– A great radiation of marine organisms occurred, especially among the brachiopods and
mollusks
– At the end of this period, massive glaciers formed
– 75 percent of animals went extinct because of the glaciation
5 mass extinctions in our history
1st was the mass extinction due to the mass glaciation
Silurian Period
– Marine life rebounded from the late Ordovician extinction
– Life appeared onto land: the first vascular plants appeared (more light for
photosynthesis), as well as some terrestrial arthropods (scorpions and millipedes)
Devonian Period –
– Rates of evolutionary change accelerated in many groups
– There were evolutionary radiations of corals and squid like cephalopods
– Jawed fishes replaced jawless forms
End of the Devonian we see the 2nd mass extinction
– Two meteorite impacts
Carboniferous period
– A lot of carbon was deposited
– A lot of fossil fuels
– Swamp plants became fossilized as coal
– Snails, centipedes, and insects were abundant
– Insects evolved wings, flight gave them access to tall plants
Permian period –
– The continents came together to form the supercontinent Pangaea
– By the end of the period, the amniotes split into two lineages, reptiles and mammals
What percentage of the Permian period does 50,000 years
– .11 percent
Near the end of the Permian, massive volcanic
– About 96% of all species became extinct = 3rd mass extinction
Mesozoic era –
– The continents continue to drift apart
Triassic period –
– Pangea began to break apart
– On land, conifers and seed plants became dominant
A mass extinction eliminated about 65 percent of the species went extinct – maybe a
change in climate
Laurasia – in the north
Gondwana – in the south
Jurassic period –
– Pangea divided into Laurasia and Gondwana
Cretaceous period
– Many mammal groups had evolved
– Another mass extinction events happened, and the big animals usually die off
Quaternary Period –
– Divided into Pleistocene
– Homo sapiens arrived
Continuing of Chapter 21
Evolution – descent with modification
Evolution – change in genetic variation in population over time
Genetic variation – represented by allele frequency
In addition to natural selection there are four other mechanisms of evolution:
Mutation – primary change in genetic code (point mutations are the most common, change
in one nucleotide) (neutral is by far the most common mutation)
Gene flow – usually introduced through migration. Someone coming into a population and
then introducing variation into the population)
Genetic drift – bottleneck example
Nonrandom dating – it all comes down to survival and reproduction
Mutation adds new alleles to the gene
– Allele frequency – proportion of an allele in the gene pool
– Genotype frequency – proportion of each genotype in the population
– Calculation of these frequencies is used to measure evolutionary change
Much of evolution occurs through gradual changes in the relative frequencies of alleles in
a population from one generation to the next.
If a locus has two alleles, A and a, there could be three genotypes: AA, Aa, and aa. The
population is polymorphic at that locus.
P= _# of the alleles in a population
Total number of alleles in that population
90+40+70 = 200
0.2 = 40/20
90/200 = .45
70/200 = 0.75
N(AA) = 45
N(Aa) = 130
N (aa) = 25
N = 200
P(A) = 2(45)+ 130/400 = 0.55
q(a) = .45
The Hardy-Weinberg equilibrium describes a model situation in which allele frequencies
do not change
Genotype frequencies can be predicted from allele frequencies
Conditions that must be met for Hardy-Weinberg equilibrium (assumptions)
– No mutation
– No selection among genotypes
– No gene flow
– Population size is infinite (no genetic drift)
– Mating is random
If these conditions hold:
– Allele frequencies remain constant
– After one generation, genotype frequencies occur in these proportions:
