Ideas on the Origin of Life on Earth

General Biology

ANSWER KEY

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Early Ideas

• Spontaneous generation - the idea that nonliving material can produce life.Examples:– mud producing fish– grain producing mice– decaying meat producing maggots

Redi’s Experiment

• Effort to disprove spontaneous generation• Decaying meat in uncovered control jars vs.

covered experimental jars.• Results: maggots and flies filled the open jars but

not covered jars. Showed only flies produce flies.

People still didn’t believe it…

• Although Redi disproved spontaneous generation of large organisms, many scientists thought microorganisms must arise spontaneously, probably from a vital force in the air.

Pasteur’s Experiment (mid-1800s)

Two things must have occurred before life arose:

The formation of simple organic molecules

The organization of those molecules into more complex organic molecules such as proteins, carbohydrates, and nucleic acids.

Earth’s Early Atmosphere

• Contained: – Little oxygen– Mostly water vapor, carbon dioxide gas,

nitrogen gas, methane gas, ammonia gas

• Energy sources - heat from Earth’s formation, lightning, UV radiation

Oparin’s Hypothesis

• Energy from the sun, lightning, and earth’s heat triggered chemical reactions to produce small organic molecules from the substances in the atmosphere. Rain washed these into the oceans, forming a “primordial soup”.

Miller and Urey (1953)

• Set up a simulation of conditions on early earth.

• After 1 week, found they had produced several kinds of amino acids, sugars and other small organic molecules.

• Supported Oparin’s hypothesis.

How did simple organic molecules form complex organic molecules?

• Studies in 1950’s showed that if amino acids are heated without oxygen, they form proteins. A similar process produces ATP and nucleic acids from small molecules.

• Therefore, this may have occurred on early earth in the warm pools of water.

How did these molecules become CELLS?

• Sidney Fox: Protocells could be formed by heating solutions of amino acids.

• Protocell is a large, ordered structure, enclosed by a membrane, that carries out some life activities, such as growth and division.

~LIFE BEGAN!~

• We have found fossils of photosynthetic prokaryotic cells from 3.5 billion years ago. However, these were probably not the first cells.

• Reminder:– prokaryotic:

• example:

– eukaryotic:• examples:

~LIFE BEGAN!~

• We have found fossils of photosynthetic prokaryotic cells from 3.5 billion years ago. However, these were probably not the first cells.

• Reminder:– prokaryotic: simple cell structure, no nucleus

• example: bacteria

– eukaryotic:complex cell structure, has nucleus• examples: protists, fungi, plants, animals

The First Cells

• First cells were probably prokaryotes that evolved from protocells; didn’t need oxygen, used molecules in oceans for food.

• At some point, some cells developed the ability to make their own food– CHEMOSYNTHESIS - making glucose from

inorganic molecules, probably near deep sea vents or in hot springs. No light needed.

And later...

• Some cells developed the ability use light to perform PHOTOSYNTHESIS. This started increasing the amount of oxygen in the atmosphere.

• Some cells developed the ability to use oxygen through respiration.

• Lightning caused some oxygen (O2) to form ozone (O3).

– Protective layer, prevents most UV radiation from sun

– stopped origin of cells; enabled evolution of more complex cells.

Evolution of Eukaryotic Cells

• Lynn Margulis (1960s) - Endosymbiont Theory

– Ancient bacteria may have “taken in” other bacteria, which evolved to become cell structures such as mitochondria and chloroplasts.

– Supporting Evidence:• M & C have their own DNA (similar to

prokaryotes)

• M & C have ribosomes (similar to prokaryotes)

• M & C reproduce independently within cells

And finally...

• Over millions of years, these early cells evolved to the diversity of living things we have on Earth today!!

• This likely involved many, many different mutations over time. Survival was likely determined through natural selection (“survival of the fittest”).

Evolutionary Timeline

Time From the Present First Records of:

a 4.5 billion years ago origin of the Earth

b *3.8-3.5 billion years ago prokaryotic heterotrophs

c *2.5-3 billion years ago prokaryotic autotrophs, first evidence of photosynthesis

d 1.5 billion years ago unicellular (single celled) eukaryotes

e 650 million years ago multicellular eukaryotes

f 400 million years ago plants invade the land

g 300 million years ago animals invade the land

h 200 million years ago first mammals

i 150 million years ago first dinosaurs

j 100 million years ago last dinosaurs

k *100,000 years ago human development