1. A Quick Look at the History of Life on Earth Part 2
The Phanerozoic Eon
Including the Paleozoic, Mesozoic and Cenozoic Eras

2. Paleozoic Era Paleozoic = old life
- bounded by two major events in life history
- begins with the Cambrian explosion 543mya
-ends with the Permian
extinction over 90% of life
disappeared 250mya

3. The Cambrian Explosion
a seemingly sudden appearance of large numbers of multicellular organisms in the fossil record.
- Rocks older than this appeared to have no fossils (because they are too small to see, and because the multicellular organisms didn’t have hard parts to preserve.
Most types of multicellular organisms present today appeared at this time.
Many other groups of organisms appeared and then went extinct during
this period
Successful forms tended to be large with shells or exoskeletons for support
Occurred in 5 – 10 million years

4. Other Paleozoic Highlights
Early Paleozoic dominated by marine invertebrates
Appearance and evolution of fish during the middle period of the Paleozoic – first fish had no jaws and cartilage skeletons
- Jaws developed
- cartilage skeletons give way to bones.

5. Paleozoic Highlights continued
Major change in vertebrates: tetrapods (animals with 4 limbs) appear in the mid-Paleozoic.
- First as paired fins on fish, then later as legs.
Amphibians and reptiles appeared.
- Reptiles were not common until the next era, the Mesozoic.

6. Paleozoic Highlights the move to land
Life moved from the sea to land during the middle of the Paleozoic Era.
- First bacteria and fungi – make soil
- followed by plants
- then arthropods (insects, etc.)
- finally vertebrates.
Large areas of swamp vegetation from this period were converted into coal after being buried and compressed.

7. Paleozoic Highlights- the move to land
Why now?
- The oceans had been the only shield life had against UV light
- UV light damages DNA and
threatens life
- by the mid Paleozoic, oxygen in the upper atmosphere formed ozone (O3)
- ozone blocks out much of the sun’s UV light – making life on land possible

8. Permian Extinction-the Dramatic End of the Paleozoic Era
Largest mass extinction in earth
history - 250mya
– 90% of species disappeared
Multiple ideas exist about the cause
- massive volcanic eruptions in Siberia lasting 1 million yrs coincided with the extinction
- climate change
- evidence for both global warming and glaciation exists
The mass extinction cleared the way for the rise of the dinosaurs
during the Mesozoic era
Evidence for massive volcanic eruption exists. Evidence to support global warming – animals with shells suffered most evidence for glaciation – glacial grooves etc in austrailia and other areas –pangea was found over south pole

9. Mesozoic Era 250mya – 65mya Mesozoic = middle life
Age of the dinosaurs
- reptiles were the largest animals on land and in the sea during this period.
major diversification of flowering plants late in the Mesozoic.
Supercontinent Pangea breaks into many continents allowed speciation to occur in many different habitats.
Birds and mammals appear but aren’t prominent.
Ends with a mass extinction – Cretaceous extinction

10. Cretaceous Extinction
Thought to have resulted from a large asteroid striking the earth
- drove huge amounts of dust to into the atmosphere and blocked out sunlight – reducing photosynthesis
- reduced primary productivity
- Evidence to support
- worldwide layer of iridium found at correct age in rocks
– very rare in earth rocks common in
meteors, asteroids, comets etc….
- massive impact crater found off of
the Yucatan penninsula of Mexico
- 100 mi in diameter
- correct age
Iridium- very rare element in earth rock but much more common in

11. Cenozoic Era- 65mya - present
Cenozoic = New Life
Mammals become the dominant life form on land. An adaptive radiation that took advantage of the sudden loss of dinosaurs.
Another large group evolves: the grasses.
Adaptive radiation of birds and flowering plants.
Fish, amphibians and reptiles don’t change much during the Cenozoic.
Geologically, continents that had been separated started to collide: Africa with Europe, North America with South America, India with Asia
Hominids appear 5 mya
–modern humans (genus Homo)2 mya
- Homo Sapiens approx 200,000 ya

12. Cenozoic Life

13. Summary Earth forms 4.6 billion years ago
Solid surface forms 4 billion years ago
Life starts (?) 3.8 billion years ago
Age of Bacteria: Archaean era
Oxygen atmosphere develops 2 billion years ago.
Eukaryotes develop. Proterozoic era
Edicarian life: 650 million years ago. First multicellular life, forms unknown today
Cambrian explosion: most current life forms appear 550 million years ago
Paleozoic era: 550 – 250 million years ago. Marine invertebrates, fishes, amphibians, invasion of the land. Coal formation.
Permian mass extinction: 250 million years ago. 95% of all life dies; end of Paleozoic
Mesozoic: million years ago. Age of the dinosaurs (reptiles). Mammals, birds, and flowering plants appear
Cretaceous mass extinction: asteroid hits the Earth, killing much of life, including the dinosaurs.
Cenozoic era: 65 million years ago till present. Mammals dominant