- Life first appeared on Earth through chemical reactions according to physical and chemical laws under conditions on the early Earth such as a reducing atmosphere without oxygen but with gases like hydrogen, methane, and ammonia. - Experiments by Miller and Urey simulated these early Earth conditions and produced amino acids and other organic compounds fundamental to life. - For centuries, the theory of spontaneous generation that life could arise from nonliving matter was believed but experiments by Redi, Needham, Spallanzani and especially Pasteur disproved this theory and established that life only arises from preexisting life.

1. Biochemical evolution

2. Biochemical evolution
Life appeared after a period of chemical reactions
According to physical and chemical laws.

3. The big bang is how astronomers
explain the way the universe
began.
It is the idea that the universe
began as just a single point, then
expanded and stretched to grow
as large as it is right now—and it
is still stretching!

4. Prebiotic Earth
Conditions on the primitive Earth were
not the same as those present today
No spontaneous generation of life today
(Pasteur)…
because the necessary conditions no
longer exist.

5. Conditions on early Earth
 Reducing atmosphere on the primitive Earth. No free
oxygen (O2)
 Free hydrogen (H2) and saturated hydrides (CH4, NH3
and H2O)
 Energy for chemical reactions between these gases
could come from electric discharge in storms or solar
energy (no ozone layer)
 The Earth’s surface temperature probably hotter than
today.

7. The formation of monomers
 Miller and Urey
recreate these
conditions in vitro
 The water is heated
and the mixture
circulates for many
days.
H2
NH3
CH4
H2O
H2O
Heat
Electric
discharge
Samples

9. Results
 After a week 15 amino acids in the
mixture
 Other biologically important
molecules had been formed
including ethanoic acid, lactic acid
and urea
 Later similar experiments were
done using CO2 that produced
nucleotides.

10. Conclusion
 These experiments cannot reproduce the exact
conditions on the primitive Earth
 We shall never know exactly what happened
 But it can be shown that the basic building
blocks for the large macromolecules can be
synthesised in vitro from inorganic
compounds.

11. How Do Microbes Arise?
 By 1860, the debate had become so heated
that the Paris Academy of Sciences offered a
prize for any experiments that would help
resolve this conflict
 The prize was claimed in 1864 by Louis
Pasteur, as he published the results of an
experiment he did to disproved spontaneous
generation in microscopic organisms

12. Spontaneous Generation
 For centuries, people based their beliefs on
their interpretations of what they saw going
on in the world around them without testing
their ideas
 They didn’t use the scientific method to arrive
at answers to their questions
 Their conclusions were based on untested
observations

13. Aristotle (384 –322 BC)
Proposed the theory of
spontaneous generation
Also called abiogenesis
Idea that living things can
arise from nonliving matter
Idea lasted almost 2000 years

14. Disproving
Spontaneous
Generation

15. Francesco Redi (1668)
In 1668, Francesco Redi, an Italian
physician, did an experiment with flies
and wide-mouth jars containing meat

16. Redi’s Experiment
 Redi used open & closed flasks which
contained meat.
 His hypothesis was that rotten meat does not
turn into flies.
 He observed these flasks to see in which
one(s) maggots would develop.

17. Redi’s (1626-1697) Experiments
 Evidence against spontaneous generation:
1. Unsealed – maggots on meat
2. Sealed – no maggots on meat
3. Gauze – few maggots on gauze, none on meat

18. Redi’s Findings
 He found that if a flask was closed with a lid so
adult flies could not get in, no maggots
developed on the rotting meat within.
 In a flask without a lid, maggots soon were
seen in the meat because adult flies had laid
eggs and more adult flies soon appeared.

19. Results of Redi’s Experiments
 The results of this experiment
disproved the idea of
spontaneous generation for
larger organisms, but people
still thought microscopic
organisms like algae or
bacteria could arise that way.

20. Needham’s Experiment

22. 22
Needham’s Results
copyright cmassengale
 Needham’s experiments seemed to support the
idea of spontaneous generation
 People didn’t realize bacteria were already
present in Needham’s soups
 Needham didn’t boil long enough to kill the
microbes

24. Lazzaro Spallanzani Experiment (1765)

25. Microbes come from cells of
organisms on dust particles in
the air; not the air itself.
Pasteur put broth into several
special S-shaped flasks
Each flask was boiled and placed
at various locations
Pasteur's Hypothesis

26. Pasteur's Experiment
S-shaped Flask
Filled with broth
The special shaped was
intended to trap any dust
particles containing bacteria
Flasks boiled
Microbes Killed

27. Flask left at various locations
Did not turn cloudy
Microbes not found
Notice the dust that collected in
the neck of the flask

28. Pasteur's Experiment
 Pasteur’s S-shaped flask kept microbes out but let air in.
 Proved microbes only come from other microbes (life
from life) - biogenesis