1. Lecture 61 – Lecture 62 The Origin of Life Ozgur Unal
NIS - BIOLOGY
Lecture 61 – Lecture 62
The Origin of Life
Ozgur Unal

2. Earth’s Early History
People have speculated about life’s origin for centuries.
However, it has been only for the past sixty years or so that scientists have experimented to understand how life might have begun.
What environmental conditions do you think scientists might stimulate to investigate the origins of life?

3. Spontaneous Generation
The oldest idea about the origin
of life: Spontaneous generation
Spontaneous generation is the idea that life arises from nonlife.
Example: Waste food and mud give rise to worms, insects etc.
Make sense? Think about it...
Francesco Redi, in 1668, investigated spontaneous generation.
He hypothesized that flies produced other flies.
In his experiment, he observed maggots (larvae of flies) appeared only in flasks that were
open to flies.
Closed flasks had no flies and no
maggots  Figure 14.11!!

4. Theory of Biogenesis
Spontaneous generation was replaced by the theory of biogenesis in the 19th century.
The theory of biogenesis states that only living organisms can produce other living organisms.
Check out Figure for Pasteur’s experiment on the theory of biogenesis.

5. Origins: Modern Ideas
If life can arise only from pre-existing life, then how did the first life form appear?
We need to consider the first environmental conditions on Earth..
Most biologists agree that life originated through a series of chemical events early in Earth’s history.
During these events, complex organic molecules were generated from simpler ones.
Eventually simple metabolic pathways developed.

6. Origin: Modern Ideas
The primordial soup hypothesis was an early hypothesis about the origin of life.
In 1920, Alexander Oparin and John Haldane suggested that organic molecules could have been synthesized from simple reactions involving the gases in the early atmosphere of Earth.
UV radiation and lightning could have been the primary energy sources.

7. Miller and Urey Experiment
In 1953, Stanley Miller and Harold Urey showed that simple organic molecules could be made
from inorganic compounds.
Check out Figure for
Miller-Urey experiment!!

8. Origins: Modern Ideas
Later, other scientists found that HCN could be formed from even simpler molecules in early Earth atmosphere.
HCN can react with itself to
form adenine  nucleotide base
Making Proteins:
Once the amino acids were formed, how were proteins synthesized?
Check out Figure 14.14!!

9. Origins: Modern Ideas How about the genetic code? How was it formed?
RNA as the first coding system?..
And what processes made the transition from biological molecules to the cells?
Most of these details are still unknown..

10. Cellular Evolution
Compare and contrast prokaryotic cells and eukaryotic cells.

11. Cellular Evolution
There is chemical evidence of life from rocks that are 3.8 billion years old.
The earliest fossils are 3.5 billion years old (found in volcanic rocks)  life in volcanic environments
Scientists hypothesize that the first cells were prokaryotes.
Many scientists think that archaea are the closest relatives of Earth’s first cells.
Archaea are prokaryotic cells that often live in exteme environments, such as volcanic vents or hot springs.

12. Cellular Evolution
Scientists think that oxygen was absent from Earth’s earliest atmosphere.
Oxygen was first produced by cyanobacteria about 3.5 billion years ago.
The first oxygen produced combined with iron to form iron oxide. This iron oxide is today found in layers of rocks.
After Earth’s free iron was saturated, oxygen started to accumulate in the atmosphere around
1.8 billion years ago.
Why is free oxygen in the atmosphere
is so important?

13. Endosymbiont Theory
Eukaryotic cells first appeared 1.8 billion years ago.
How?
Let’s have a closer look at the some of the organelles found in eukaryotes: mitochondria and chloroplast
Mitochondria and chloroplasts are about
the same size as the prokaryotes.
They contain their own DNA.
They have ribosomes similar to the ones
in prokaryotes.
They reproduce by binary fission (like
prokaryotes) independent of the cell.

14. Endosymbiont Theory
In 1966, biologist Lynn Margulis proposed the
endosymbiont theory.
According to endosymbiont theory, the ancestors of eukaryotic cells lived in association with prokaryotic cells.
In some cases, prokaryotic cells might have lived inside eukaryotes and eventually become organelles inside eukaryotes.

15. Symbiogenesis
In addition to the endosymbiont theory, Lynn Margulis and Dorian Sagan suggested that cells acquired other cells and genomes from other organisms  symbiogenesis.
According to Margulis and Sagan, symbiogenesis is a major factor in the evolution of life on Earth.
They state that long term genomic mergers result in much greater evolutionary change than DNA mutations.
Do you agree with this hypothesis?