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ORIGINS OF LIFE. Ultimate question for biologists Many theories/hypotheses/beliefs Cultural: How is all life connected? Religious: Is there a creator? Philosophical: Are we here at all? Scientific theory : must be based in unbiased research, use empirical evidence, tested over time.

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Ultimate question for biologists

  • Many theories/hypotheses/beliefs
    • Cultural: How is all life connected?
    • Religious: Is there a creator?
    • Philosophical: Are we here at all?
    • Scientific theory: must be based in unbiased research, use empirical evidence, tested over time
what is spontaneous generation
  • The hypothetical process by which living organisms develop from nonliving matter
  • An archaic theory, 4th century BC- late 19th century where many people(including scientists)utilized this process to explain the origin of life.
    • According to this theory, pieces of cheese and bread wrapped in rags and left in a dark corner were thought to produce mice, because after several weeks there were mice in the rags.
    • It was "common knowledge" that simple organisms like worms, frogs, and salamanders could come from mud, dust, and unpreserved food.
    • Many believed in spontaneous generation because it explained such occurrences as the appearance of maggots on decaying meat.
    • Malaria: “bad air” sleeping with windows open made one sick
experiments to disprove
Experiments to disprove…

Francesco Redi– 1668- One of the first to try to disprove spontaneous generation. An Italian doctor who proved maggots came from flies.


LazzaroSpallanzani- 1700s - Conducted an experiment with broth to show that the microscopic organisms entered from the air, not by means of spontaneous generation

Critics said by boiling the broth and sealing flasks “life force” was destroyed…


Rudolf Virchow – 1858- challenged the notion with his theory of “BIOGENESIS”, claiming “all cells come from preexisting cells”.

Although correct in his concept he lacked the experimentation to prove it…


Louis Pasteur - 1864 - Conducted an experiment with nutrient broth in curved-neck flasks to finally disprove spontaneous generation

7 origin theories
7 Origin Theories

Electric Spark

Electric sparks can generate amino acids and sugars from an atmosphere loaded with water, methane, ammonia and hydrogen, as was shown in the famous Miller-Urey experiment reported in 1953, suggesting that lightning might have helped create the key building blocks of life on Earth in its early days. Over millions of years, larger and more complex molecules could form. Although research since then has revealed the early atmosphere of Earth was actually hydrogen-poor, scientists have suggested that volcanic clouds in the early atmosphere might have held methane, ammonia and hydrogen and been filled with lightning as well.

community clay
Community Clay
  • The first molecules of life might have formed on clay, according to an idea elaborated by organic chemist Alexander Graham Cairns-Smith at the University of Glasgow in Scotland. These surfaces might not only have concentrated these organic compounds together, but also helped organize them into patterns much like our genes do now.
  • The main role of DNA is to store information on how other molecules should be arranged. Genetic sequences in DNA are essentially instructions on how amino acids should be arranged in proteins. Cairns-Smith suggests that mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.
deep sea vents
Deep-Sea Vents

The deep-sea vent theory suggests that life may have begun at submarine hydrothermal vents, spewing key hydrogen-rich molecules. Their rocky nooks could then have concentrated these molecules together and provided mineral catalysts for critical reactions. Even now, these vents, rich in chemical and thermal energy, sustain vibrant ecosystems.

chilly start
Chilly Start

Ice might have covered the oceans 3 billion years ago, as the sun was about a third less luminous than it is now. This layer of ice, possibly hundreds of feet thick, might have protected fragile organic compounds in the water below from ultraviolet light and destruction from cosmic impacts. The cold might have also helped these molecules to survive longer, allowing key reactions to happen.


In the world nowadays DNA needs proteins in order to form, and proteins require DNA to form, so how could these have formed without each other? The answer may be RNA, which can store information like DNA, serve as an enzyme like proteins, and help create both DNA and proteins. Later DNA and proteins succeeded this "RNA world," because they are more efficient. RNA still exists and performs several functions in organisms, including acting as an on-off switch for some genes. The question still remains how RNA got here in the first place. And while some scientists think the molecule could have spontaneously arisen on Earth, others say that was very unlikely to have happened.

simple beginnings
Simple Beginnings

Instead of developing from complex molecules such as RNA, life might have begun with smaller molecules interacting with each other in cycles of reactions. These might have been contained in simple capsules akin to cell membranes, and over time more complex molecules that performed these reactions better than the smaller ones could have evolved, scenarios dubbed "metabolism-first" models, as opposed to the "gene-first" model of the "RNA world" hypothesis.


Perhaps life did not begin on Earth at all, but was brought here from elsewhere in space, a notion known as panspermia. For instance, rocks regularly get blasted off Mars by cosmic impacts, and a number of Martian meteorites have been found on Earth that some researchers have controversially suggested brought microbes to Earth, potentially making us all Martians originally. Other scientists have even suggested that life might have hitchhiked on comets from other star systems.

However, even if this concept were true, the question of how life began on Earth would then only change to how life began elsewhere in space.

w here did the first cells come from
Where did the first cells come from?

The appearance of the first cells marked the origin of life on Earth. However, before cells could form, the organic molecules had to have united with one another to form more complex molecules called polymers…proteins, lipids, carbohydrates, nucleic acids

Most scientists agree that the biochemistry of life had to come before cellular structures.

example cell membrane
Example: Cell Membrane
  • Formation of phospholipid
  • nature of molecule gives it an end that is attracted to water and an end that is repelled by water
  • When these molecules are

placed into a watery

environment they form…..


first life 4 6 billion years ago
FIRST LIFE4.6 billion years ago

Earth Conditions

Earliest Life

Anaerobic heterotrophic prokaryotes consuming organic molecules

Anaerobic chemosynthetic prokaryotes(autotrophic volcanic vent archaebacteria)

Little or no free oxygen, toxic atmosphere, extremes in temperature, pH, cloudy humid, lightning


Photosynthetic Prokaryotes (cyanobacteria)

Autotrophic photosynthetic


Heterotrophic Eukaryotes


Photosynthetic Prokaryotes (cyanobacteria)

Temperatures regulate, less evaporation-more precipitation-more sun

Oxygen levels rise in atm, ozone forms and cuts UV levels

prokaryotic to eukaryotic endosymbiant theory
Prokaryotic to Eukaryotic?EndosymbiantTheory

Cells engulf large nutrient molecules in process called endocytosis…

Lynn Margolis’ theory

suggests small aerobic

prokaryotes were taken

in by endocytosis and

began to live inside a

larger anaerobic prokaryote in an endosymbiotic relationship….eventually they became the mitochondria and chloroplasts


what evidence supports theory
What evidence supports theory?

The mitochondria and chloroplasts:

  • Only organelles that have a bi-lipid membrane
  • Have their own genes, DNA different from nuclear DNA of cell
  • Replication cycle is independent from the replication cycle of cell that contains them
  • Both convert energy(autotrophic/heterotrophic)