Origins of Life

Chapter 12, Section 3

And parts of 12.4

Early Theories

Spontaneous generation = the idea that living things could come from nonliving things

Three experiments disproved this theory:

1. Francesco Redi (1665)

2. Lazzaro Spallanzani (1767)

3. Louis Pasteur (1862)

Redi’s Experiment

People thought maggots came from meat Redi showed that maggots came from flies laying

eggs on the meat

Spallanzani’s Experiment

People still thought that microorganisms could spontaneously generate

Spallanzani boiled two flasks of broth, then left one open and sealed one Bacteria grew in the open

flask The sealed flask

remained sterile

People convinced that spontaneous generation exists said that boiling the broth killed a “vital principle” in the air

Pasteur’s Experiment

Disproved spontaneous generation once and for all

Microorganisms only grew in the flask when the swan neck was broken

The swan neck prevented particles in the air from entering the broth

Animation

Biogenesis – Life from Life

A possible sequence:

1. Inorganic molecules form and make small organic molecules

2. Small organics join to form macromolecules / polymers

3. Origin of RNA / DNA to make inheritance possible

4. Packaging within membranes

Related Vocabulary

Inorganic – any substance that doesn’t contain both carbon (C) and hydrogen (H)

Organic – any substance that contains both C and H; usually comes from something that is, or once was, living

Polymer – substance made up of many repeating subunits (monomers)

Macromolecule – large molecules; biological examples include carbohydrates, lipids, proteins, and nucleic acids

Early Earth

For the first 700 million years, Earth was most likely very hot and in a molten state

Over time, the materials making up Earth separated into Earth’s layers (crust, mantle, core)

Gases released from Earth’s interior formed an atmosphere

Early Life on Earth – 4:37

Oparin and Haldane – 1920s

Theory for how life may have developed on early Earth; based on assumptions that: There was little or no oxygen present The atmosphere was mainly formed from volcanic

vapors – methane, ammonia, hydrogen, water vapor

Felt it would be possible for inorganic molecules to be converted to organic forms using energy from the sun and lightning

At the time, no effective way to test this

Miller and Urey – 1950s

Tested Oparin-Haldane hypothesis

Simulated atmosphere containing gases Oparin & Haldane thought were present

Exposed gases to electric shocks to simulate lightning

Produced small organic compounds – mainly amino acids

Animation

Follow-up to Miller/Urey

Based on the gases emitted from volcanoes today, scientists think the atmosphere would have been different from what Oparin & Haldane proposed More carbon monoxide (CO), carbon dioxide (CO2),

nitrogen gas (N2) and water vapor Similar experiments using this “updated”

representation of the atmosphere produced smaller amounts of amino acids

Meteorite Hypothesis

Analysis of meteorites indicate that organic molecules can be found in space

This suggests the following possibilities: Amino acids may have been

present when Earth formed Organic molecules may have

arrived on Earth through meteorite / asteroid impact

Formation of the First Cells

Once organic molecules / compounds are formed, how did they get packaged into cells? Iron-sulfide bubbles hypothesis Lipid membrane hypothesis

Iron-Sulfide Bubbles

Iron sulfide rising from deep sea vents combines with cool ocean water to form chimney-like structures with many compartments

Biological molecules may have combined inside these compartments, which acted as membranes

With the right combination of ingredients, the first organic cell membranes may have formed

Lipid-membrane Hypothesis

Lipids spontaneously form membrane-enclosed spheres called liposomes

Liposomes could act as membranes around a variety of organic molecules, separating them from the environment

The First Genetic Material

It has been hypothesized that RNA was the genetic material for the earliest life forms

Cech & Altman (1980s) discovered that RNA can: Catalyze reactions Copy itself

The First Eukaryotes

Fossil evidence indicates that the first living things were prokaryotes (bacteria) First appeared ~3.5 BYA Eukaryotes – cells with a nucleus and

other organelles – don’t appear in the fossil record until approx. 1.5 BYA

How did the first eukaryotic cells develop?

Endosymbiosis

Suggested by Lynn Margulis (1970s)

Idea that mitochondria and chloroplasts used to be simple prokaryotic cells that were engulfed by larger prokaryotes

Animation

Endosymbiosis, cont’d

What evidence supports endosymbiosis? Both mitochondria and chloroplasts:

Have their own DNAHave their own ribosomesCan copy themselvesAre about the same size as prokaryotesHave DNA in the shape of a circle, like

bacterial / prokaryotic DNA