1. Chapter 12 The History of Life

2. I. Fossil Record A. Paleontologist – person who studies fossils
B. Fossil record – evidence about the history of life on Earth
C. More than 99 percent of species that
have ever lived on Earth have
become extinct

3. Types of Fossils
Premineralization- minerals carried in water deposited around a hard structure and may replace the hard structure itself.
Casts- impressions in sediment (no bone or tissue remains)
Trace fossils- includes leaf imprints, footprints, nests/borrows
Amber- preserved specimens in tree resin
Preserved- entire organism encased in ice, volcanic ash, etc.

4. V. How a fossil is made…

5. How old are fossils?
A. Relative Dating – the deeper the fossil is the older it is

6. Index fossils
Fossils of organisms that only existed during specific spans of time over large geographic areas
Help determine the age of rock layers and other fossils in the same rock layer (strata)

7. III. Radioactive (carbon) dating – How much Carbon-14 is left in a fossil determines age.
-Carbon-14 has a half-life of 5730 years.
- half of it’s atoms decay into nitrogen-14 every 5730 years.
-only good for fossils 60,000 years or younger
-Potassium-40 or argon-40 are used for older fossils – they have a half-life of 1.26 billion years.

9. IV. The Geologic Time Scale
A. Earth = 4.5 billion years old
B. Divided into eras, periods and epochs
You are here!

10. Eras- tens to hundreds of millions of years (consist of 2 or more periods)
Periods- most commonly used unit of time in geology lasting tens of millions of years (each associated with a particular rock system)
Epochs- smallest unit of geologic time, lasting several million years.

11. Eras Paleozoic means “ancient life” Mesozoic means “middle life”
Cenozoic means “recent life”

12. V. Earth’s Beginning
The Earth is estimated to be about 4.5 billion years old.
First 700 million years (Hadean eon) most likely violent and very hot.
Asteroids struck the planet releasing lots of heat = kept Earth in a molten state.
Hydrogen, carbon monoxide, and nitrogen filled the air and combined to form an atmosphere made of water vapor, methane, carbon dioxide and ammonia.

13. Hypotheses for the Origins of Life
Miler-Urey Experiment
1953 Stanley Miller and Harold Urey built an apparatus to model conditions they thought existed on early Earth.
Demonstrated that organic compounds could be made by passing an electrical current (to simulate lightning) through a mix of gases.
Methane, ammonia, hydrogen and water vapor.
The experiment resulted in the formation of amino acids!

14. Meteorite Hypothesis
A meteorite that fell in Australia in 1969 revealed that organic molecules could be found in space.
More than 90 amino acids identified
19 of these are found on Earth
Amino acids could have been on earth when it formed or maybe arrived on these meteorites

15. Early Cell Structure? Iron-sulfide bubbles hypothesis
1990s, scientists noted hot iron sulfide rising from ocean floor combines with cooler ocean water to make chimney-like structures made of compartments.
Bio-molecules may have combined in these compartments
Lipid membrane hypothesis
Lipids form enclosed spheres (liposomes)
Maybe formed around organic molecules acting as cell membranes

16. RNA before DNA
Proposal that RNA was the genetic material of early life on Earth, not DNA
Thomas Cech (1980s) and Sidney Altman independently discovered that RNA could catalyze reactions.
Ribozymes- RNA that can catalyze reactions
Can even replicate and synthesize without the help of enzymes!

17. VI. Life on Earth
The first life on the planet were primitive bacteria= Archaea
These prokaryotes were anaerobic
Many were chemosynthetic
Early photosynthetic cells= cyanobacteria
Produce higher oxygen levels allowing aerobic prokaryotes to survive!

18. Archaen Era 3.8-2.5 billion years ago
No continents, just small islands in a shallow ocean
High concentration of carbon dioxide in the atmosphere
Such levels of carbon dioxide would be toxic to the majority of animals alive today - as would the low oxygen levels.
Archaebacteria today

19. Endosymbiosis
Eukaryotic cells may have evolved from Endosymbiosis!

20. Evolution of Sexual Reproduction
Early cells = asexual
Eukaryotic cells began reproducing sexually
created genetic variation
increased the chance of mutations (and the rate of evolution)

21. Paleozoic Era-(542-251 mya) 1st multi-cellular life!!
Cambrian explosion= earliest part of the era
Great diversity of life
Earliest vertebrates (jawless fish), marine invertebrates, arthropods
Middle of the era
diversity on land including plants and amphibians
Era ended 251 mya with a mass extinction
90% marine and 70% land animals extinct

22. Mesozoic Era (251-65 mya) Triassic, Jurassic and Cretaceous periods
The Age of Reptiles
dinosaurs roamed
Birds, flowering plants and even mammals arose
Cretaceous period: Peak in dinosaur diversity and rise of mammals
Period ended with massive meteorite
Dust blocked light, plants die, no food!

23. Cenozoic Era (65 mya -present)
Tertiary Period ( mya)
Placental mammals/marsupials, and monotremes (mammals that laid eggs)
Birds, fish and flowering plants diversified
Earliest ancestors of modern humans evolved
Quaternary Period (1.8 mya-present)
Homo sapiens appeared about 100,000 yrs ago