The history of LifeSection 19.1: The fossil record

Fossils and Ancient Life

• Fossils provide information about extinct species

• Fossils can vary greatly• Different sizes, types and degrees of preservation

•Not every organism will become a fossil

Types of Fossils

Trace Fossils

Fossils in Sedimentary rocks

Why don’t all organisms become fossils?

What information can fossils tell you?

•The structure of an organism• Lead to ideas about evolutionary relationships

•The Environment an Organisms lived in• Fossilized plants and leaves give an idea of the ecosystem

•The way in which an organism lived• Footprints and bone structure give information about how an animal moved• Groups of fossils give information about how organisms interacted

How do researchers know the age of fossil specimens? •Two types of dating

• Relative dating• Radiometric dating

•Relative dating used the presence of an index fossil or other feature to determine whether a rock is older or younger than another rock• Can allow paleontologists to

tell the age of a rock

What is an index fossil?

• Index fossils are fossils used to establish the relative age of a rock layer• Often distinctive• Found in many places• Only existed on Earth for a small amount

of time

•Trilobites are a great example • 15,000 different species• Can be used to date nearly 300 million

years worth of rocks

What is radiometric dating?

• Radiometric dating uses the proportion of radioactive to stable isotopes to calculate the age of a sample• Different elements decay at different rates• The rate of decay is referred to as the half life

• Carbon 14 decays to Carbon 12 naturally in the atmosphere• When a plant photosynthesizes it will take in a small amount of

Carbon 14• Upon death, this amount of carbon is fixed – and will decay over

time to carbon 12• Carbon 14 will not be replenished, so by measuring the amount

of carbon 14 remaining, the age of the plant can be calculated • Animals also get carbon from eating plants so can be dated this

way• Half life of carbon is 5730 years

How do you date samples older than 60,000 years?

•Carbon-14 is not the only isotope used for radiometric dating

•Elements such as Rubidium (half life 48.8 billion years), Uranium (half life 4.5 billion years).

•By choosing the correct element you can essentially date anything on Earth

•These are often used alongside index fossils to determine the ages of geological units

Why don’t we just radiometrically date everything? •Because it is very

expensive and takes a very long time!

How old is the Earth?

•4.6 billion years old

•Divided into Eons, Eras, Periods and Epochs

•Precambrian is the first 3.5 billion years

•Our Geological period only started 1.8 million years ago

Geological time as a clock

• It is often hard to comprehend simply how old the Earth is, and for how little humans have occupied it

•This graphic puts it into perspective

How has the Earth changed through geological time?

What has affected conditions on Earth through time?

• Physical Forces• Climate is constantly changing

• Great ice age 10,000 years ago, average temperature was only 5 Celsius lower,

• During the Mesozoic, average temperatures were only 6 – 12 Celsius higher

• Earth is constantly changing – Mountains, Volcanoes, New Coastlines, which has knock on effects to habitats

• Plate Tectonics and Super-continental Cycles are a powerful driving force

Biological effects on Earth through time

• Life can have a large affect in shaping Earth• Early Oceans were full of dissolved Iron

• Atmosphere did not have much Oxygen

• Some of the first organisms to Evolve were photosynthetic bacteria• These took in CO2 and gave out O2

• CO2 is a greenhouse gas – the reduction in CO2 levels lead to drops in global temperatures

• As temperatures dropped, Iron reacted with atmospheric oxygen, was no longer soluble in the oceans and precipitated out

• This settled to the ocean floor – producing banded Iron formations

Key points

• Fossils are a powerful tool used to aid our understanding of the evolution of the Earth

•They can be dated through either relative or radiometric dating

•Earth is 4.6 billion years old

•The Earth’s surface has been shaped a number of different physical and biological factors over the course of it’s history

Patterns and processes of Evolution

Section 19.2

Why do some organisms live but others die?

• Some changes occur within a species

•Others can be over much larger scales

•When changes occur in clades greater than a single species they are referred to as Macroevolutionary patterns• Creation of species through speciation• Disappearance through extinction

•Emergence of large clades – dinosaurs, mammals – examples of macro evolutionary patterns

Classification of fossils

• Fossils can be classified into clades• Either clades only containing

extinct organisms

• Or clades also containing living organisms

•Cladograms illustrate how closely related organisms are

•Relation does not mean direct ancestor

Why do species go extinct?

•They fail to adapt to a changing environment

• Some clades appear more susceptible to extinction than others - why?

•The rate of speciation within a clade must equal to or greater than the rate of extinction for a clade to continue to survive over the long term

•Reptillia is an example of a highly successful clade • Living organisms continue to adapt and thrive• Most species in clade dinosauria are extinct, but birds survived because they

adapted to the new environment

What drives extinction of a species?

•Two types of extinction

•Extinction due to Natural selection – always occurring (background extinction)

•Extinction due to a mass Extinction• A mass extinction is when a many species

become extinct over a short period of time.• Entire ecosystems vanish, food webs

collapse…• Earth has experiences many of these

throughout it’s history

What causes mass extinctions?

Is there a good side to a mass extinction?

•Yes, there can be. It creates many new opportunities for survivors to adapt and speciate

•Recovery takes about 5 – 10 million years

Does evolution always occur at the same rate?

•No!

• Some organisms change consistently through time – gradualism

• Some organisms do not really change over time, until something happens to upset their equilibrium – ‘punctuated equilibrium’. • It has been proposed that most new species are

produced during this period of rapid change

Why does evolution occur at different rates?•Rapid evolution may occur if a small population gets separated from

the main populations• Small population will evolve faster as there are fewer individuals to spread

change to

•Migration to a new environment• Eg. Galapagos Islands

•Mass extinctions create many new ecological niches • Organisms that survive will fluorish

Macroevolution•There are two main types of macroevolution

• Adaptive Radiation• One ancestral species (or small group) evolves to produce a number of different species that

live in different ways

• Can be caused by migration, extinctions

• Species can also evolve a new feature allowing them to take advantage of the environment

• Example – dinosaurs and mammals • Mammals were around during dinosaurs but were not successful

• After dinosaur extinction, an adaptive radiation of mammals began.

• Convergent Evolution • Unrelated organisms in different places but similar environments evolve similar features

• Emus, Rheas and Ostriches

What is Coevolution?

• The process by which two species evolve in response to changes in each other over time

• Often they become so intertwined that neither can live without the other

• If one has an evolutionary change so will the other

• Examples• Specific flowers and pollinators • Plants and herbivores

• Plants produce poisons to stop themselves being eaten

• But insects resistant to the poisons will flourish

• Often these insects use the poisons to their advantage

Key Points

•Macro-evolutionary patterns are changes that affect many species of entire clades• Examples – speciation and extinction

•Earth has experienced a number of mass extinctions

•Evolution can occur at different rates • Gradualism vs punctuated equilibrium

•Paleontologists have identified patterns of macroevolution • Adaptive radiation and convergent evolution

Earth’s Early HistorySection 19.3

The Early Earth

• Earth formed through collision of a series of cosmic debris

• 4.2 billion years ago it cooled enough for rocks to form

• Early atmosphere had little to no oxygen, • Largely CO

2, water

vapor and nitrogen

• Very inhospitable!

The first organic molecules

• It was shown by Miller and Urey in 1953 that Earth’s early primitive atmosphere was capable of creating organic molecules – they created 21 amino acids!• Although the

composition for the Early Earth that they used was incorrect, but the general idea was correct

How do you go from organic molecules to living cells?

• A lot remains unknown about this step

• Evidence suggests 200 – 300 million years after the first rocks formed, cells similar to bacteria were common – how?

• Scientists have shown that under the correct conditions, large organic molecules can form protenoid bubbles called microspheres

• Not cells – but have some characteristics of cells

• Thought to acquire characteristics of living cells 3.8 billion years ago

Where did DNA and RNA come from?

•No one is sure, but a number of hypotheses exist

•RNA is thought to be able to form from simple organic molecules• Based on experimental evidence

•RNA is very versatile – lead to the RNA world hypothesis

•RNA existed before DNA, and lead to direct DNA directed protein synthesis

Where did the Oxygen come from?

• First fossils that resemble bacteria come from 3.5 billion years ago • Evolved in the absence of oxygen

•2.2 billion years ago, photosynthetic organisms evolved• Change the chemistry of the atmosphere• Cooled the Earth• Oxygen reacted with Iron in the Oceans – precipitating out

•Atmospheric composition changed over time – became toxic to many early organisms on Earth - adapt or die!

When did Eukaryotic cells originate?

•Proposed that 2 billion years, some ancient prokaryotes began to evolve internal cell membranes

•Endosymbiotic theory: prokaryotic cells entered these prokaryotes with an internal memberane • Intruders did not infect or be digested, instead they began living within larger

cells

• A symbiotic relationship developed between primitive eukaryotic cells and the prokaryotic cells within them

• Proposed due to similarities identified between membranes of chloroplasts and mitochondria and prokaryotes

Endosymbiotic theory explained

Modern Evidence

•DNA evidence shows that DNA from bacteria and DNA from mitochondria and chloroplasts are similar

•All have ribosomes of a similar structure and size

•They all reproduce by binary fission - division by mitosis

•All together these features suggest a common ancestor!

Sexual reproduction

• Sexual reproduction increases genetic variation and therefore sped yo evolutionary change • Asexual reproduction leads to a direct genetic copy

• Genetic variation is restricted to mutations

• In sexual reproduction, genetic material comes from two parents • Increased variation and increased chance of adapting to new or changing

environmental conditions

•Multicellular organisms evolved a few hundred million years after the evolution of sexual reproduction • Adaptive radiation lead to great diversity

Key points

• Earth’s original atmosphere was a harsh environment that could not support life as we know it

• The exact origin of life is unclear, but it is though to stem from the production of organic molecules such as proteins and RNA, leading to the formation of protenoid microspheres

• Photosynthetic bacteria lead to the production of an oxygen rich environment 2.2 billion years ago

• Eukaryotic cells developed through endosymbiotic theory – the ingestion of prokaryotes by a slightly altered prokaryote

• Sexual reproduction has lead to increased amounts of genetic variation and evolution