1. The History of life on earth
Chapter 25

2. YOU MUST KNOW
A scientific hypothesis about the origin of life on Earth
The age of the Earth and when prokaryotic and eukaryotic life emerged
Characteristics of the early planet and its atmosphere
How Miller and Urey tested the Oparin-Haldane hypothesis and what they learned
Methods used to date fossils and rocks and how fossil evidence contributes to our understanding of changes of life on Earth
Evidence for endosymbiosis
How continental drift can explain the current distribution of species (biogeography)
How extinction events open habitats that may result in adaptive radiation

3. I. Conditions on early earth
Earth was formed about 4.6 billion years ago and life emerged about 3.8 billion years ago
1. For the 1st ¾ of its history, living organisms were microscopic a primarily unicellular

4. Current hypothesis about how life arose
Abiotic synthesis of small organic molecules such as amino acids and nitrogenous bases
Joining of these molecules into macromolecules such as proteins and nucleic acids

5. 3. Packaging of. these. macromolecules. into protocells. (membrane-
3. Packaging of these macromolecules into protocells (membrane- enclosed droplets)
The origin of self-replicating molecules that made inheritance possible (initially self-replicating RNA)
*Ribozymes – RNA catalysts

6. Laboratory Simulations
1. Oparin and Haldane
a. Hypothesized early atmosphere was thick with water vapor, nitrogen, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide and lightening
b. Energy was provided by lightening and UV radiation to form organic compounds

7. 2. Miller & Urey – experiment produced organic molecules from inorganic

8. II. The fossil record
Sequence in which fossils appear in layers of sedimentary rock
Studied by paleontologists
Record is incomplete – old, hard to find, generally requires hard shells or bony skeletons

9. Rocks and fossils are dated
1. Relative dating – using the order fossils are found in rock strata (older are lower)

10. Radiometric dating – uses the decay of radioactive isotopes to determine age
Half-life – time necessary for ½ of the parent isotope to decay

11. b. ex. C14 has a half-life of 5,730 years – living organisms take in both C12 and C14, but once dead, no new carbon is obtained. Calculate ratio of C14 to C12

12. Sample Problem The radioisotope potassium-40 can be used to date past events older than 60,000 years. Potassium-40 has a half-life of 1.3 billion years, decaying into Argon-40. If the igneous rock layer that scientists wish to date shows a ratio of Potassium-40 to Argon-40 at one-fourth the current ratio, what is the age of the rock layer? Express your answer in billions of years.

13. One-fourth – indicates 2 half-lives have passed (½ x ½ = ¼)
Sample Problem
The radioisotope potassium-40 can be used to date past events older than 60,000 years. Potassium-40 has a half-life of 1.3 billion years, decaying into Argon-40. If the igneous rock layer that scientists wish to date shows a ratio of Potassium-40 to Argon-40 at one-fourth the current ratio, what is the age of the rock layer? Express your answer in billions of years.
One-fourth – indicates 2 half-lives have passed (½ x ½ = ¼)
2 x 1.3 billion = 2.6 billion years

14. III. Origins of organisms
Earliest organisms were prokaryotes
2.7 billion years ago – increase in atmospheric O2 levels as a result of photosynthesis – caused the extinction of some prokaryotes and the evolution of others that could carry out cellular respiration

15. Eukaryotes appeared about 2.1 billion years ago
Endosymbiosis – larger prokaryotes engulfed smaller prokaryotes, developed symbiotic relationship
a. Mitochondria and chloroplasts have enzymes and transport systems homologous to those in the membranes of prokaryotes
b. They replicate in a way similar to binary fission
c. They contain single, naked, circular DNA chromosome
d. They have their own 70s ribosomes

16. Multicellular eukaryotes evolved about 1.2 billion years ago
Colonization of land – about 500 million years ago – plants, fungi, and animals began to appear

17. IV. The rise and fall of organisms
Continental Drift – movement of Earth’s continents on plates
Explains why same species can be found on different continents
Explains why no placental mammals in Australia (isolated early)

18. Mass Extinctions
1. Loss of large number of species in short time frame
2. Generally result from global climate changes
3. Alters ecological community
4. Causes niches to be vacated and open to new species (rise of mammals following loss of dinosaurs)
5. Leads to adaptive radiation

19. V. Evo-Devo
Slight genetic changes can be magnified into major morphological differences between species
Exaptations – structures that evolve in one context start functioning in another way (ex. bird feathers possibly originally for insulation but ended up being good for flying)

20. Heterochrony – evolutionary change in the rate or timing of development
1. Ex. increase rate of growth in bat finger bones to support wings or decrease rate of growth in leg bones of whales

21. Homeotic genes – master regulatory genes that determine location and organization of body parts
1. Hox genes – class of homeotic genes – changes cause profound effect on morphology
2. Ex. expression of Hox genes in a snake limb bud and a chicken leg bud
(chicken gets legs, snake does not)