History of Life
A BIOLOGIST’S GEOLOGY
Formation of Earth
 Over 4.6 billion years old
 Was not “born” in a single event  pieces of debris
were most likely attracted to one another over the
course of 100+ million years
 Density determined which elements were located
where on the Earth – more dense @ core, less dense
@ surface
 More than 99% of all organisms that have ever lived
are now extinct
How Do We know?
 FOSSIL RECORD
 Evidence for life on Earth
 Evidence for evolution

Radioactive Dating


Calculate age of sample based on amount of remaining radioactive
isotope it contains based on the isotopes half-life (we know these)
Relative Dating

The age of a fossil is determined by comparing its placement with
that of fossils in other layers of rock.
Radioactive Dating
 Everything living thing contains… CARBON!
 When something dies, it starts to give off carbon – at
a predictable rate (the half life)
 Has a specific half-life 
the length of time
required for half of the
radioactive atoms in a
sample to decay
Relative Dating
 Older, less accurate method – still gives valid
information, but not a specific age a RELATIVE age
 Sedimentary rock is formed by the gradual accumulation
of layers of rock, sand and other sediments – these get
compacted over a long period of time and make rocks
 Sometimes, dead organisms (or prints, eggs, droppings,
etc) gets stuck in these layers and preserved… FOSSILS
 We use index fossils to identify specific layers – these are
organisms that were only alive for very short periods of
time and in a few layers of rock
Relative Dating
Earth’s Early Atmosphere
 Probably contained hydrogen cyanide, carbon
dioxide, carbon monoxide, nitrogen, hydrogen
sulfide, and water.

If you were to travel back in time, after a few short breaths you
would no longer exist.
 Around 3.8 billion years ago changes occurred in
Earth’s atmosphere.

Water could remain a liquid on the planet.
Origin of Life & Cells
• First life found to be around 3.5 billion years old.
What type of life?
–
Single-celled prokaryotic ancestors of modern bacteria.
• Little to no oxygen present in Earth’s atmosphere.
How is this possible?
–
Chemosynthesis  hydrothermal vents
How Do We Know?
 RUSTY ROCKS!
 2.2 Billion Years ago
 Organisms began producing oxygen, an end product
of…
 How could this be?

Observation of Oxygen reaction with iron and water. Many
rust deposits in rocks dated to this time.
Increase in Oxygen
 Organisms churning out Oxygen
 Chemosynthetic & Photosynthetic
 Phytoplankton played a HUGE role in the excess of oxygen
 What would this do to the atmosphere?
 Oxygen increase, methane and hydrogen sulfide
concentrations decreased.
 What would this do to the microbes on Earth?
 Remember we are talking about BILLIONS of
years!!
Increase in Oxygen
 Drove many organisms to extinction  did not adapt
fast enough to changes in environment

Oxygen is a poisonous (to some), reactive gas
 Those that remained developed NEW way to
metabolize… hello cellular respiration
Increase in Oxygen
 Stage set for the gradual transition to “modern” life
and the gradual transition to modern atmosphere.

Oxygen increase in the atmosphere took over HALF A
BILLION YEARS!
 Descendants of early life forms still exist today
 Use other chemicals (nitrates, sulfates, methane) for
metabolism
Oxygen increase in the atmosphere
took over HALF A BILLION
YEARS!
Origin of Eukaryotic Life
 2 billion years ago.
 Internal cell membranes began developing in
prokaryotes

these had been around for 1.5 billion years
 Ancestral eukaryote engulfed prokaryotes.
 Endosymbiotic theory
 Eukaryotes engulfed and formed a symbiotic relationship with
prokaryotic organisms. Engulfed prokaryotes are modern day
chloroplasts and mitochondria.
 Mitochondria and Chloroplasts share many similarities with
free-living bacteria.
THIS SHOULD SOUND FAMILIAR!
Geologic Time Scale
 We use divisions of the geologic time scale to
represent evolutionary time
 It contains three components: eras, periods & time


Time – how many millions of years ago
Eras – 4 main eras:


precambrian, paleozoic, mesozoic & cenozoic
Periods – subdivisions of eras
Precambrian Era: 4.6 bya – 540mya
 90% of Earth’s history
 Simple, anaerobic (without using oxygen) life
 Photosynthetic life forms developed
 Aerobic (with oxygen) life forms developed
 Eukaryotes developed
 Multi-cellular life forms developed
 Life ONLY existed in the sea
 Age of the Bacteria
Paleozoic Era: 540 mya – 250 mya
Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian Periods
 Diversity of marine (sea) life
 Cambrian Explosion  diversification of life
 Invertebrates ruled
 Trilobites were common
 Plants begin to adapt to drier habitats
 Insects appeared on land
 Age of the Fishes
 Vertebrates begin to invade land  four legged
amphibian
 Reptiles evolved
 Plants formed vast swampy forests  these remains
created the coal we use today
Paleozoic Era: The Cambrian Explosion
 Why the explosion of life?
 Development of the first major predator:
Anomalocaris (anom-o-lo-care-is)
 Evolutionary arms race between predators and prey
(predatory coevolution)  leads to huge diversity of
organisms; the biggest we’ve ever seen – even
compared to today!
 Video Clip:
Origin of Life: Conquest (6-8 mins)
MASS EXTINCTION #1
 250 mya
 At the end of the paleozoic era there was a mass
extinction – many organisms died out
 95% of complex life died


Land & Sea
Plants & Animals
 Cause? Unsure  Theories
 Volcanic activity
 Comets & meteoroids
Mesozoic Era: 250 mya – 65 mya
Triassic, Jurassic, Cretaceous Periods
 Increasing dominance of dinosaurs
 Appearance of flowering plants  seeds, fruits
 Age of the Reptiles
 Mammals first appear (small)
 Birds appear (flight, feathers)
MASS EXTINCTION #2
 65 mya
 Over 50% of life wiped out – all of the dinosaurs
 Allowed mammals to prosper  they didn’t have to
compete with dinosaurs
Cenozoic Era: 65 mya - present
Tertiary & Quaternary Periods
 Age of the Mammals
 Mammals evolved adaptations that allowed them to
live on land, water & air
 Flowering plants & insects flourished (coevolution)
 Grasses evolved  HERBIVORE evolution
 Series of ice ages; 20,000 years ago started to warm
again & glaciers melted
Present Day
 Still in quaternary period
 Big changes in climate & global patterns
 Age of the Humans