1. The History of Life on Earth
Chapter 25

2. Brief History Earth is approximately 4.6 billion years old
The first organic molecules (simple carbs, amino acids, etc) probably appeared around 4 billion years ago (bya)
The first fossils of prokaryotes appeared about 3.5 bya
The first fossils of eukaryotes appeared about 2.1 bya
Question: How do you go from simple amino acids to a whole living organism in only 500 million years??

3. Conditions of Early Earth
4.6 bya Earth forms (Big Bang)
Bombarded with huge chunks of rock and ice left over from the formation of the solar system; collisions generate heat which vaporizes and water that would exist
3.9 bya collisions stop
Water vapor increases
Volcanoes erupt releasing N2, CO2, CH4, NH3, H2, H2S
Earth cools and water vapor condenses forming oceans
Energy from lightning and UV radiation could have allowed organic compounds to form such as simple carbs, amino acids, and nucleotides

4. Miller and Urey (1953)
Did an experiment recreating the conditions of early Earth in a lab setting
Demonstrated the abiotic synthesis of organic molecules was possible
Found that a variety of amino acids, simple carbs, and possibly nucleotides had formed

5. Key to Life
The key to a living organism is that is can replicate itself and it has a metabolism
How do you go from amino acids to a cell?
Somehow amino acids must have linked together to form early proteins which could act as catalysts for a variety of reactions such as DNA/RNA replication and metabolism
Protobionts are collections of abiotically produced molecules surrounded by a membrane-like structure of lipids
They can reproduce on their own by pinching off parts of its membrane to form a new protobiont molecule
They can also do simple metabolism
IF RNA were to enter into a protobiont you could have the first living cell

6. 25.2 Fossil Record Documents the History of Life
Fossils are dated through radiometric dating based on the decay of radioactive isotopes
Ex) living organisms contain C12 and C14. C12 will not decay even after death but C14 decays into N14 and it takes 5,730 years for half of the C14 to decay into N14 (called half-life). By measuring the ratio of C12 to C14 you can determine the age of a fossil
Only works on organic (once-living) organisms
Can only date fossils up to 75,000 year old
For older fossils it is a ballpark age after comparing the ages of the rock layers the fossil was found between
Electron antineutrino

7. 25.3 Brief Tour of Life
3.5 bya – fossilized stromatolites; fairly complex so life must have started earlier
2.7 bya – evidence of photosynthetic bacteria; iron banding
2.1 bya – evidence of eukaryotic organisms; endosymbiosis
1.2 bya – evidence of multicellular algae
565 mya – evidence of more complex multicellular eukaryotic organisms
535 mya – Cambrian explosion; cnidaria, porifera, mollusks, soft bodied larger organisms
500 mya – colonization of land; larger fungi, plants, and animals

8. 25.4 Continental Drift, Mass Extinctions, and Adaptive Radiation
Continental drift served as a means of allopatric speciation
Pangaea, Laurasia & Gondwana , present
Mass extinctions occurred due to environmental changes as continents relocated
Allowed for new niches to open up
New niches open the door for adaptive radiation

9. Adaptive Radiation
The evolutionary pattern characterized by a rapid increase in the number of kinds of closely related species
Results in an evolutionary explosion of new species from a common ancestor
Can occur when an organism invades a previously uninhabited environment (ie – Galápagos Islands) and has no competition or when a species has an adaptation enabling it to fill the niche of an extinct species
Ex) The adaptive radiation of mammals following the mass extinction of the dinosaurs at the end of the Cretaceous period 65 million years ago. Mammals coexisted with the dinosaurs but were small, nocturnal insect eaters. The extinction of the dinosaurs left environments full of opportunities for other types of animals. More than 4,000 mammal species evolved within 10 million years, including whales, bats, rodents, and primates.