1. Unit 9: Evolution
9.2 Life over Time

2. Origins of Life
Earth formed 4.6 billion years ago (bya). At this time, conditions were not suitable for the formation of life. Life probably began between and 3.9 bya.

3. Origins of Life
The earliest evidence of life on Earth comes from fossils discovered in Australia that date back to about 3.5 bya. These fossils are structures know as stromatolites which are formed by the growth of layer upon layer of single-celled microbes.

4. Origins of Life
Oparin-Haldane Hypothesis (1920): That life on Earth could have arisen step-by-step from non-living matter through a process of “gradual chemical evolution.” It is suggested until early Earth’s conditions:
Simple inorganic molecules could have reacted with energy from lightning or the sun to form building blocks like amino acids and nucleotides, which could have accumulated in the oceans
The building blocks could have combined in further reactions, forming larger, more complex molecules (polymers) like proteins and nucleic acids.
The polymers could have assembled into units or structure that were capable of sustaining and replicating themselves.

5. Origins of Life
Miller-Urey experiment (1953): Provided the first evidence that organic molecules needed for life could be formed from inorganic components.
Built a closed system containing heated water with a mixture of gases thought to be abundant in the atmosphere of early Earth.
Simulated lightning by sending sparks of electricity through their experimental system
After running experiment for a week, found that various types of amino acids, sugars, lipids, and other organic molecules had formed

6. Image credit: "Miller and Urey's experiment," by CK-12 Foundation, CC BY-NC 3.0.

7. Current Hypotheses
Genes-First hypothesis: first life was self- replicating RNA and other elements were added later
Metabolism-First Hypothesis: first life began metabolic networks (chemical reactions that support the cell) before DNA or RNA
Some scientists think organic compounds might have come to early Earth on meteorites.

8. Life has been continuing to evolve since it first appeared
A timeline of Life’s Evolution:

9. Fossils Evidence
Fossils: the remains or impressions of prehistoric organisms that have been preserved.

10. Types of Fossil
Mold: Hollow impression left behind by a living thing

11. Types of Fossil
Cast: Solid mineral deposit that filled a mold leaving behind a copy of the living thing.

12. Types of Fossil
Imprint/trace: An impression in a rock made by a living thing during its life activities.

13. Types of Fossil
Petrification: Plant or animal tissues replaced by minerals

14. Types of Fossil
Whole Animal: An entire plant or animal encased and preserved in ice, sap, or another material

15. Fossils Most fossils are found in sedimentary rock.
Organisms that have died are then covered by sediments, which harden into rock

16. What Fossils Can Reveal
The fossil record contains an enormous amount of information for paleontologists, researchers who study fossils to learn about ancient life.
By comparing body structures in fossils to body structures in living organisms, researchers can infer evolutionary relationships.
Bone structure and trace fossils, like footprints, indicate how animals moved.
By dating fossils, scientists can determine what life on Earth was once like
When different kinds of fossils are found together, researchers can sometimes reconstruct entire ancient ecosystems.

17. Mass Extinctions
Mass Extinction: the extinction of a large number of species within a relatively short period of geologic time.
Thought to be due to factors such as catastrophic global event or widespread environmental change that occurs too rapidly for most species to adapt
At least five mass extinctions have been identified in the fossil record
Of all species that have existed on Earth, 99.9% are now extinct
Some biologists believe that we are currently in the 6th mass extinction