1. Origins of Life

3. How did life begin on Earth?
Remember...
According to the cell theory, life only comes from life, new cells only come from pre-existing cells. This is called biogenesis.
If this is true, then how did life on Earth first begin?
Quickly refresh students’ memories on the cell theory.

4. How did life begin on Earth?
There have been a few hypotheses for how life first appeared on early Earth.
However, even today’s strongest hypotheses are based on a relatively small amount of evidence.
Therefore, the gaps and uncertainties make it likely that scientific ideas about the origins of life will continue to change over time.
SC.912.N Identify what is science, what clearly is not science, and what superficially resembles science (but fails to meet the criteria for science)…..Remind students that one of the defining features of science its that scientific knowledge is based on evidence! Also note that test items assessing a scientific claim are limited to the scientific explanations of the origins of life on Earth.

5. Theory - Spontaneous Generation
Until the early 19th century, people generally believed in the “Spontaneous Generation” of life from non-living matter.
Due to early experiments, this idea was disproved
Francisco Redi (1668) – Rotten Meat Experiment
Louis Pasteur (1864) – Curved Flask Experiment
Use the figures to describe these two important experiments and how they disproved the idea of spontaneous generation. Test items may assess how contributions of scientists such as Pasteur, Oparin, Miller and Urey, Margulis, or Fox aided in the development of scientific explanation of the origin of life but WILL NOT assess what each scientist contributed.

6. HOW did life Begin? Hydrothermal Vents Electric Spark
These vents release important hydrogen-rich molecules
Mineral catalysts could have made critical reactions occur faster
Discovery of Archaebacteria may be evidence of this
Can generate amino acids and sugars from an atmosphere loaded with water, methane, ammonia and hydrogen
Demonstrated in the famous Miller-Urey experiment reported in 1953
New evidence suggest that it may have occurred in volcanic clouds
Video

7. HOW did life Begin? Panspermia Community Clay Ice Earth
Clay may have provided the foundation for first organic compounds.
Mineral crystals in clay could have arranged organic compounds into organized patterns.
Community Clay
Organic molecules could have come from outer space in a comet or meteorite.
Ice Earth
Just show student that there are different hypothesis out there…. No need to copy
3 billion years ago ice might have covered the oceans.
Protected from UV light, organic compounds may have formed and reacted with one another.

8. Early Earth The Earth was formed about 4.6 billion years ago.
The oldest fossils of microorganisms are about 3.5 billion years old.
But where did they come from???
Refer to this ”clock” analogy when explaining the timescale we are discussing. The entire clock represents 5 billion years. Be sure to point out the tiny sliver that represents the existence of the human species so that students can gain a sense of the amount of time we are dealing with. However, you should know that test items will not require specific knowledge of the age of Earth or its eras, periods or epochs.

9. Theory - Chemical Evolution
Conditions on the early Earth were very different from the Earth we know today.
The early atmosphere contained no free oxygen and probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water vapor.
Energy for chemical reactions between these gases could come from electric discharge in storms or solar energy (no ozone layer).
Volcanic eruptions probably released CO2 that warmed the atmosphere by absorbing outgoing heat.
Could organic molecules have evolved under these conditions?
This theory is also known as the “Primordial Soup Theory”. Test items may assess the conditions required for the origin of life.

10. Theory - Chemical Evolution
Mixture of gases simulating primitive atmosphere of early Earth
Water Vapor H2O
Ammonia NH3
Methane CH4
Alexander Oparin (1920’s), hypothesized that organic molecules could form from inorganic molecules under certain conditions.
In the 1950‘s Harold Urey and Stanley Miller tried to answer that question by simulating the conditions on the early Earth in a laboratory setting.
Hydrogen H2
Spark simulating
lightning storms
Cold water cools chamber, it condenses the gas causing droplets to form (Rain).
Stopcock for taking samples
Use the figure to explain chemical evolution and Urey and Miller’s experiment. They wanted to know if it would have been possible for organic molecules to evolve under “early earth” conditions.
Liquid containing Amino Acids (15) and other organic
compounds
Water
vapor

11. Theory - Chemical Evolution
Miller and Urey's experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth.
This formation of organic molecules (such as amino acids, sugars, fatty acids, and nucleotide bases ) from inorganic molecules is also known as abiogenesis.
Be sure to ask students what the difference between spontaneous generation and abiogenesis is! Many students confuse them. They should understand that Miller and Urey did not create life from non-life (like the idea of maggots coming from meat). They simply created the ingredients for life (organic molecules), from inorganic molecules. Scientists believe that we do not see this process occur in nature today because present day conditions on Earth are so different from the conditions present on early Earth.

12. Meat (Muscle), Fish, Cheese
Why were scientists trying to synthesize basic organic compounds in a simulated primitive Earth’s atmosphere?
Because basic amino acids are the building blocks of all Life on Earth. It gives evidence that living things could have started on Earth from these simple molecules.
In the “primordial soup”, small organic molecules could have reacted with each other to form more complex organic molecules.
Macromolecule
Where it is found
What it does
Protein
Meat (Muscle), Fish, Cheese
Enzymes, structural, transportation, hormones, to move, among many other functions in living organism.
Carbohydrate
Sugar, Breads, Fruits, Pasta, Rice
(thank you plants)
Short term energy
Lipid
Cell membranes
Fats
Long term energy storage; an important component of the cell membrane.
Nucleic Acid
RNA and DNA
Carries genetic information

13. The First Cells So how could the first cells on Earth have originated?
The stew of organic molecules suggested by Miller and Urey is a long way from a living cell.
So how could the first cells on Earth have originated?
Ask students what the defining characteristics of a cell are. Be sure to stress that in order to have a cell, you must have a membrane.

14. The First Cells Organic molecules have a tendency to aggregate.
Phospholipids form lipid bilayers when they are surrounded by water.
As a result, membrane-like vesicles called coacervates, form easily under certain conditions.
Quickly refresh students’ memories on the structure and function of the phospholipid bilayer that cell membranes are made of.
Click on the picture to play a short video clip of “proto-cell” formation. If the embedded video does not play, you can find it in the Topic 15 folder.

15. Coacervates Coacervates are made mostly of aggregated lipids.
They grow by adding new polymers.
They form a semi-permeable membrane.
When they get too big they divide.
Hypotheses suggest that structures similar to these might have acquired more characteristics of living cells.
Did not contain all the characteristics of life.
Lacked genetic material
Compare these characteristics of coacervates to the characteristics of cells. How are they similar? How are they different?

16. The Origin of Heredity
One of the characteristics of life, is that all living things contain hereditary information (DNA) which is passed from cell to cell during cell division.
Scientists speculate that RNA may actually have been the first hereditary molecule.
There are several hypotheses for how RNA could have evolved into modern cellular life.

17. Summary
The early Earth was very different from today. It had certain toxic gases and water vapor in the atmosphere and was highly charged with energy. There was NO free oxygen. These conditions allowed the formation of simple organic molecules from inorganic molecules. (Theory of Chemical Evolution)
Once simple organic molecules appeared in the oceans (aka “primordial soup”), they reacted together to form bigger, more complex organic molecules.
Coacervates, “bubbles” with lipid membranes that were able to divide and grow, formed from organic molecules.
Coacervates began reacting more with their environment and began metabolizing materials. These protocells where very similar to true cells, but lacked genetic material.
Once genetic material entered these protocells (RNA first, later replaced by DNA), they actually began to pass on their genetic material when they divided (reproduction). Now these structures had all of the characteristics of life and were the first living things: Prokaryotes (primitive bacteria-like cells).

18. Life Continues First cells were anaerobic prokaryotes
Diversification (through mutations and, eventually, sexual reproduction) led to different forms of cells.
About 2.7 b.y.a., cyanobacteria, or blue-green algae, began a primitive form of photosynthesis which added free oxygen to the environment.
Oxygen in the atmosphere killed off most cells. Survivors were aerobes.
Some prokaryotic cells became eukaryotic cells.

19. The First Eukaryotic Cells
The Endosymbiotic Theory, proposed by Lynn Margulis in 1967, proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms. According to the theory, eukaryotic cells formed from symbiosis among several different prokaryotes, where some lived inside of others, benefitting each other. After many generations of these cell communities living/reproducing together, they became single, more complex cells – eukaryotes.
Plants and plantlike protists
Aerobic
bacteria
Photosynthetic bacteria
Nuclear envelope
evolving
Mitochondrion
Test items may refer to the endosymbiotic theory but may not assess the term in isolation.
Primitive Photosynthetic
Eukaryote
Animals, fungi, and
non-plantlike protists
Ancient Anaerobic
Prokaryote
Primitive Aerobic
Eukaryote 19

20. Evidence of the endosymbiotic theory
Chloroplasts and mitochondria …
are approximately the same size as bacteria
Have ribosomes whose size and structure closely resemble those of bacteria.
divide independently of the cell (through binary fission instead of mitosis).
have a double membrane.
have DNA that is similar to bacterial DNA (and different from that in the cell’s nucleus).

21. Why can’t abiogenesis happen today ???