BIOL 105 Lecture 13: The Fossil Record & the History of Life on Earth

What are fossils ?

Preserved remains (usually mineralized) or impressions of organisms

Igneous

crystalline solids that form directly from cooling magma

• ex. granite

• DO NOT HAVE FOSSILS

• can be radiometrically dated

Sedimentary

the result of the accumulation of small pieces broken off of pre-esisting rocks that are deposited in layers that accumulate over time

• ex. sandstone

• almost all fossils are found here

• undisturbed layers are rare because they are usually degraded by erosion or other geological processes

Metamorphic

rocks that have been exposed to a rise in temperature and/or pressure that causes changes in the material structure

• ex. marble

• can contain fossils

• usually highly degraded

Stratigraphy

• indicates that one fossil is older than another NOT actual age

• based on simple and intuitive Principle of Superposition

  • upper units are younger

  • lower are older

• fossils both appear in older strata and disappear in younger strata

  • indicate both origins and extinctions

• sometimes strata can be folded or flipped over by various geological processes

4,500 mya

Earth formed

4,400 mya

First OCEANS formed as water vapor released into the atmosphere by volcanism cooled and then fell as rain

4,000 mya

First LIFE appeared on Earth : simple single-celled organisms

1,500 mya

OXYGEN began to accumulate as a product of photosynthesis

700 mya

First ANIMALS evolved: simple single-celled animals

530 mya

First VERTEBRATES (fish) evolved

500 mya

First LAND PLANTS

• oxygen in the atmosphere reacted to form ozone, which formed a layer that blocked some of the ultraviolet (UV) coming from the sun; this allowed plants to colonize the land

350 mya

First LAND VERTEBRATES evolved

• the first venture onto land were primitive amphibians and reptiles evolved soon afterwards

225 mya

First DINOSAURS evolved

65 mya

• Dinosaurs went extinct

  • metaroite impact caused earthquakes, tsunamis and volcanic erruptions that threw dust and acid into the atmosphere creating an impact winter

  • the dust blocked sunlight so plants could no longer photosynthesize, and food chains collapsed

• mammals flourished

0.13 mya (130,000 years ago)

Modern HUMANS evolved

Earth is old enough that

1. slow, gradual evolution as envisioned by Charles Darwin has plenty of time to occur

2. Lots of rare evets occured (asteroids,etc..)

How do we assign absolute dates to fossils ?

• Radioactive dating

• Radioactive elements: some mineral elements decay radioactively at a set rate from their time of origin

• Half-life (t1/2):

  • the time it takes for half of a radioactive substance's atoms to decay into another isotope

Uranium —> Lead ½ Life

0.7 Billion Years

Carbon-14 —> Carbon 12

dating half-life of 5,730 years

How complete is the fossil record ?

• very incomplete because of unusual conditions are required for efficient fossil formation

COnditions for Fossil Formation:

1. fast burial in anoxic soil (decomposition is minimized when little oxygen is present)

2. fine sediments must be present in high mineral content water

3. taxa with hard body parts that are protected from crushing (soft parts are rarely fossilized)

How can we estimate how incomplete the fossil record is

1. many time periods are poorly represented with sedimentary deposits

2. many lineages are known to have existed over vast spans of time but are only recovered sporadically (rare)

3. many large organisms are poorly represented as fossils

4. new fossils are being discovered at an increasing rate

The gradualist model of evolution

• suggests that character change is slow and continuous and that nothing unusual happens during speciation events

Coelacanth

“living fossil”

Perfect Stasis would be surprising because

1. random genetic drift is always occurring

2. environments are always changing and natural selection should cause the traits or organisms to “track” these changes

Punctuated Equilibrium

• Eldredge and Goud

  1. stasis was more common than we recognize

  2. change was concentrated around speciation

• gradualist model should be replaced with one in which changes were infrequent, but rapid, and concentrated at speciations

• if TRUE, it could explain the frequent lack of transitional forms in the fossil record

• more speciation = greater phenotypic evolution

Gradualism Model

• slow rate of change

• freq. of change is common

• various causes of change

• phenotypic divergence not correlated with speciation

• ancestral form was altered

Punctuated Equilibrium

• rapid rate of change

• frequency of change is rare

• caused of change: allopatric speciation

• phenotypic divergence correlated with speciation

• ancestral form retained

Criticisms of Punctuated Equilibrium

• because fossil record is so condensed a gradualc change across many generations can look instantaneous

• known as pseudoextinction

Bryozoan

• fossils provide evidence consisted with punctuated equilibrium

• shows abrupt morphological changes and few or no apparent intermediates

Foraminiferan

• fossils provide evidence of punctuated and rapid evolution

Foraminiferan fossils

• fossils provide evidence that consist wirh slow gradual evolution

The Verdict

• evidence supports multiple evolutionary patterns

• rate of speciation is not fixed

• rate of evolution is not constant and may or may not increase around speciation events

Miller & Ury Experiment

• setup stimulated an ancient, stormy atmosphere:

  • methane, ammonia, hydrogen gas, water

  • cycles of heating and cooling

• Yielded sugars, amino acids, purines and pyrimidines !!

  • building blocks of life

Requirments for ALL living things

1. ability to capture energy from the env.

2. ability to use energy to replicate reliably

3. ability to undergo evolutionary change

Origin of Life is

monophyletic origin:

• L-isomers of amino acids

• protein sysnthesis

Mycoplasma “laboratorium”

• J. Craig Venter

• the first fully synthetic genome was created un 2010

Life originated from simple replicating molecules

• 3.8 BYA

The origin of eukaryotes

• 1.5-2.7 BYA

Features of Eukaryotes

• membrane-bound organelles

• cytoskeleton

• nucleus w/ chromosomes & mitotic spindle

What groups are a clade

• eukaryotes

• fungi + animals

What groups are NOT a clade

• prokaryotes

• protists

Endosymbiotic Theory

• Margulis

• bacteria were engulfed by Archaea like cells to form organelles

• origins of mitochondria from purple-bacteria like ancestor

• 6-9 origins of chloroplasts from cynovacteria

• evidence from genomics supports endosymbiotic theory : organelles bear genomes that are closely related to specefic bacterial taxa

Cambrian Explosion

• 520-540 mya

• all the major body plans that we see on earth evolved over a short period and many of these lineages subsequently went extinct

• major source of cambrian fossils

  • the Burgess Shale

Burgess Shale

• discovered by Charles Walcott

• contained great deal of arthropod and mollusk diversity

• Is an incredible fossible becayse:

  1. many different types of fossil fauna recovered

  2. fossilis included soft body parts not just shells and exoskeleton

  3. some represented body plans are weird and have no modern counterparts

Some examples of Cambrian fossils

• Anomdacaris

• Wiwaxia

• Hallucigenia

Invasion of land (plants)

• plants arrived on land about 500 mya

• land plants are a monophyletic group (clade) that evolved from green algae

Three Requirements for life on land:

1. structural support

2. protection from desiccation

3. vascular tissues to transport water from ground

Invasion of land (Animals)

• vertebrates invaded land about 408 mya

Requirments for transmission ro land

1. structural support

2. ability to breathe air

3. limbs to move efficiently on land

lchthyostega

• is an example of mosaic evolution

• rest of body is fish like but limbs have evolved extensively

Radiation of the mammals - Cenozoic

• age of mammals

• least from 65mya to present

Mammals reinvade marine habitats - Cetacea

• 30-50 mya

• three separate invasions:

  1. cetacea (whales and dolphins) evolved hippo-like ancestors

  2. serenians (manatees, dugongs, sea cows) are related to elephants

  3. pinnipeds ( seals, sea lions, walruses) are related to bears (carnivores)

Adaptions related to return to water;

• hydrodynamic (smooth) shape

• sealed ear covering and nostrils near top of the head

• front and especially hind limbd may not be needed

The theory of plate tectonics

• explains some of the major geological events that have shaped evolution

  • theory of continental drift - Alfrew Wegener in 1915

Where two plates separate

new crust forms

Where plates collide

subduction zones form mountains

Where plates move over hot spots (thermal plumes)

volcanoes form

Continental drift is a key force in macroevolution

• important factors shaping distribution of organisms

• driver of massive climate changes and mass extinction events

• driver of mountain building events that separate fauna and foster speciation