Transcript Great Ideas in Science: Lecture 3

Great Ideas in Science:
Lecture 13 – Evolving systems
Professor Robert Hazen
UNIV 301
Life originated on Earth billions of
years ago as a single cell, and has
been evolving by the process of
natural selection ever since.
Two Stages of Evolution
Chemical Evolution –
Lab experiments show
complex molecules arise
from simple ones
Natural Selection –
Once a reproducing cell
exists, complexity arises
from competition
The Origin of Life:
Four Possibilities
1.
2.
3.
4.
A miracle – an act of divine
intervention
An event consistent with chemistry
and physics, but extremely unlikely
An inevitable consequence of
chemistry, given an appropriate
environment and sufficient time
Intelligent design
Chemical Evolution
Life arose by a natural process
of “emergent complexity,”
consistent with natural laws.
This hypothesis predicts
that life began as a
sequence of chemical steps.
Intelligent Design
Life is “irreducibly complex.”
Therefore, a supernatural
designer must have formed it.
This hypothesis requires a
combination of natural and
supernatural processes.
Is ID Science?
ON THE ONE HAND:
ID makes predictions, albeit negative ones.
These predictions are falsifiable.
BUT:
ID is based on supernatural processes.
ID is therefore inherently untestable, and is
unsupported by observational evidence.
THE “DEBATE”
“Both sides ought to be properly
taught ... so people can understand
what the debate is about.” G. W. Bush
“Intelligent design should not be
taught in high school biology classes
as an alternative to evolution.”
American Chemical Society
How Should Science
Respond to ID?
Design a research program that
demonstrates the natural transition
from chemical simplicity to
emergent complexity.
If biological complexity can be
shown to arise spontaneously as
the result of natural processes,
then ID is unnecessary.
STONEHENGE
What is Emergent Complexity?
Emergent phenomena arise from
interactions among numerous
individual particles, or “agents.”
The Emergence of Slime Mold

Chemical
Potential
Gradients
Dictyostelium
The Emergence of Slime Mold
Dictyostelium
Emergent Phenomena – Life
Central Assumptions of
Origin-of-Life Research
The first life forms were carbon-based.
Life’s origin was a chemical process
that relied on water, air, and rock.
The origin of life required a sequence
of emergent steps of increasing
complexity.
Four Emergent Steps
1.
2.
Emergence of biomolecules
3.
Emergence of self-replicating
molecular systems
4.
Emergence of natural selection
Emergence of organized
molecular systems
Emergence of Biomolecules
The strategy is to use simple molecules
to build larger molecules.
The Miller-Urey Experiment
Organic synthesis near the
ocean-atmosphere interface.
Organic Synthesis in Interstellar
“Dense” Molecular Clouds
Experiments at NASA Ames simulate this environment.
The Hydrothermal Hypothesis
A “BLACK SMOKER”
Deep-Sea Vents

Reactants:
Pyruvic acid + CO2
+ H2O

Conditions:
o
200 C
2,000 atm
2 hours

Products: A
diverse suite of
organic molecules
Self-Assembling Amphiphile Molecules
Minerals and Chiral Selection
Mineral surfaces select chiral amino acids
The Emergence of SelfReplicating Molecular Cycles
The abiotic synthesis
of such a “metabolic”
cycle represents a
“Holy Grail” for our
experimental program.
The Emergence of SelfReplicating Molecular Cycles
Prof. Harold J. Morowitz
The Emergence of
Natural Selection
The emergence of competition and
natural selection appears to be
inevitable in any self-replicating
molecular system in which some
molecules have the ability to mutate.
CONCLUSIONS
The origin of life on Earth is best
understood in terms of a sequence of
emergent chemical events, each of
which added a degree of structure and
complexity to the prebiotic world.
While we don’t yet know all the details,
there is no compelling evidence to
suggest that life ’ s origin was other
than a natural process.
Chemical Evolution:
The Window of Opportunity



Great Bombardment: Ended
~4 billion years ago
Oldest Fossils: Evidence of
life 3.5 billion years ago
The first cell probably arose
4.0-3.5 billion years ago
Biological Evolution:
The First Cell
The first cell on Earth
 Had no competition
 Multiplied rapidly
 Special characteristics
 20 amino acids; DNA; RNA
 Ability to mutate, and thus
evolve

Three Definitions of Evolution
1.
2.
3.
Evolution as Change: Life on
Earth has changed over
billions of years.
Common Descent: All living
things on Earth descended
from a common ancestor.
Natural Selection: The process
by which life evolved is
Darwinian natural selection
First Definition of Evolution:
Change Over Time
Observational evidence overwhelmingly
supports the theory that life originated on
Earth billions of years ago as a single cell,
and has been changing ever since.

Fossils

Molecular Biology

Cellular biology

Comparative anatomy

Observations of nature and of breeding
Observational Evidence for an
Ancient Earth

Annual events:



Tree rings
Varve deposits
Ice cores
15,000 years
>500,000 years
1,200,000 years

Radiometric dating
4.5 x 109 years

Geological Processes
109 years

Astronomical data
1010 years
Half-Life
The average time for
decay of ½ of a batch of
radioactive isotopes


Wide range of half-lives




Radon-222: 3.8 days
Carbon-14: ~5,700 years
Uranium-234: 250,000 years
Uranium-238: 4.5 billion years
Life has Changed over Time:
The Fossil Record


Fossils: Evidence of past life

Tracks and trails

Organism’s hard parts

Turned to rock

Replaced by minerals
Fossil record

Fossils found, catalogued &
analyzed

Shows transitions

Incomplete
Life has Changed over Time:
The Fossil Record

A few examples:

Horn coral – 500 million years old

Trilobite – 350 million years old

Ammonites – 200 million years old


Primitive shark tooth – 100 million
years old
Whale bone – 15 million years old
Fossils
Strata
Younger = higher
Extinct Animals
TRILOBITES
If you accept observational evidence,
then the unambiguous conclusion is
that life has changed over time.
Calvert Cliffs
15,000,000 m.y.
If you accept observational
evidence, then the
unambiguous conclusion is
that life has changed over
time.
Fossils



Fossils prove that life on Earth has
changed over time; most are extinct.
Fossils demonstrate that these
changes are gradual and progressive
(simple to complex)
All known fossils fit into a pattern of
continuous evolution.
The Story of Life

First cell

Eukaryotes

Colonies

Hard shells (the
Cambrian explosion)

Vertebrates

Primates

Humans
Geological Time
The Evolution of
Human Beings
Evidence from Anatomy:
Vestigial Features

Vestigial features



Familiar structure
No useful function
Examples



Appendix: humans
Wings: penguins
Hind legs: whales
Evidence from Anatomy:
Homologous Structures
Compare forelimbs of vertebrates
How Did Evolution Occur?
Natural Selection
Populations exhibit
variations.
 More individuals are
born than will survive.
 The most fit individuals
are more likely to
reproduce.
