1. Chapter 12, Lesson 1-2 How did life begin? Age of the earth: 4.5 bya
From a ball of fiery, molten rock, the Earth cooled over 500 million years
Water vapor in the early atmosphere condensed and fell as rain
Shallow oceans formed on Earth

2. Measuring Earth’s Age Radiometric Dating
Estimating the age of an object by measuring its content of radioactive isotopes
Isotope-form of an element with an atomic mass that is different from other atoms of the element
Radioisotopes are unstable isotopes that break down and give off energy called radiation

3. Measuring Method
As radioisotopes break down, radioactive decay results in other isotopes that are stable (e. g. potassium-40 becomes argon-40 and calcium-40)
The time it takes for one-half of a given amount of radioisotope to decay is called the isotope’s half-life
Scientists measure proportions of radioisotopes and their products of decay

4. Formation of Basic Life Chemicals
Molecules of nonliving matter probably reacted chemically during the first billion years of Earth’s history ( sun, volcanic heat)
These chemical reactions produced many different, simple organic molecules (molecules that contain carbon)
The hypothesis that organic molecules necessary for life can be made from nonliving matter has been tested and supported by recent laboratory experiments

5. Competing Models Primordial Soup 1920s Oparin, Haldane
Earth’s oceans filled with organic molecules
Gases in atmosphere
(ammonia, hydrogen, methane, water vapor)
reacted chemically and fell into oceans
This hypothesis tested by Miller and Urey in the 1950’s
Bubble Model 1986
Lerman
Undersea volcanos produced gases (ammonia, methane) that became trapped in bubbles underwater
Gases became concentrated and reacted
Bubbles burst at surface to release organic molecules into the atmosphere

6. Problems…
Reductant molecules could not have been in abundance/Early Earth---no ozone
UV radiation would have destroyed ammonia and methane
So, if not present in the atmosphere…where did these chemicals come from? Ocean vents? ???

7. Forming Cells The biochemistry of cells is complex
How did simple organic molecules link to form these highly complex molecules?
Scientists have not been able to make proteins or DNA form spontaneously
RNA, a second type of nucleic acid found in all cells and some viruses today, can form spontaneously in water

8. Role of RNA
Certain RNA molecules can act as enzymes and catalyze chemical reactions
Cech and Altman (1980s) hypothesized that RNA was the first self-replicating information storage molecule and it directed the formation of proteins
Since RNA is self-replicating, it could have evolved and changed over generations

9. Other Chemical Findings
Lipids (fats, oils) form tiny droplets whose surface resembles cell membranes
Chains of amino acids can gather in these small droplets = microspheres
Linked amino acids and sugars can gather in another type of droplet = coacervates
First step toward cellular organization?

10. Evolution of Cellular Life, Section 2
Prokaryotes
Unicellular
Microscopically tiny
No nucleus
No membrane-bound organelles
Eubacteria=true bacteria
Archaebacteria=old bacteria
Eukaryotes
Most are multicellular
Protists are only unicellular eukaryotes
Large and specialized
Nucleus
Many membrane-bound organelles
Protists, Fungi, Plants, Animals

11. Prokaryotes Oldest fossilized cells (2.5 bya)
Cyanobacteria= photosynthetic bacteria
Eubacteria are bacterial cells that cause disease and decay
Archaebacteria are bacteria that live in extreme earth environments (similar to those on prehistoric Earth) anaerobic forms, extreme temperatures, volcanic vents, sulfur and mineral springs

12. Eukaryotes Later in fossil record, 1.5 bya
All eukaryotic cells have mitochondria
Protists and plants have cholorplasts
Mitochondria and chloroplasts are about the size of a prokaryote and they contain their own DNA
Endosymbiosis (Margulis) Theory that mitochondria and chloroplasts are descendants of symbiotic eubacteria

13. Evidence for Endosymbiosis
Mitochondria are about the same size as most eubacteria
2 membranes
Circular DNA similar to bacterial cells
Contain genes that are different from the DNA in the nucleus
Reproduce by simple fission
Chloroplasts are similar in size to cyanobacteria
Surrounded by 2 membranes
Have thylakoids
Have circular DNA
Have genes that are different from DNA in nucleus
Reproduce by simple fission

14. Multicellularity Protists---oldest eukaryotes, 700 mya
Multicellular life has advantages—specialized cells for food capture, movement, protection
Three protistan ancestors evolved to produce very successful groups of multicellular organisms: Fungi, Plants, Animals

15. Origin of Modern Life
Most animal groups alive today probably originated during a brief period of time
lasting 10 to 100 million years = Cambrian Explosion
Cambrian period is a time of great evolutionary expansion due to changing geological and atmospheric conditions
New habitats = new opportunities
Burgess Shale (1909) is a rich collection of Cambrian fossils found in Canada that includes strange animals unlike anything alive today

16. Mass Extinctions First Mass Extinction Second Mass Extinction
Third Mass Extinction
Fourth Mass Extinction
Fifth Mass Extinction
440 mya
360 mya
245 mya
210 mya
65 mya