“There is grandeur in this view of life”
Natural selection and the complexity of life
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Earth is the most remarkable planet yet known
Scientists estimate there are at least 10 million species alive today.
We have yet to describe 86% of them.
An ancient debate
“[A god created life because] without them the universe will be incomplete, for it
will not contain every kind of animal which it ought to contain, if it is to be perfect.”
- Plato
“[When] everything turned out as it would have if it were on purpose, there the
creatures survived, being accidentally compounded in a suitable way.”
- Empedocles
Greek, Chinese, Roman, and Islamic philosophies, among others,
had notions of life as a mutable, changing phenomenon.
Our daily dose of Aristotle
Aristotle’s History of Animals was the earliest surviving zoological work:
Book 1:
The grouping of animals and the parts of the human body
Book 2-3:
The organs of vertebrates
Book 4:
Invertebrates
Book 5-7:
Reproduction of animals and humans
Book 8-9:
The behaviors of animals
Aristotle rejected Empedocles and embraced the ideas of Plato.
He would be the prime influence on Western science for 2000 years.
Not so static after all
Selective breeding showed that organisms could change form
dramatically over just hundreds of years.
Buried monsters
Fossils refute the notion that the Earth has remained static over its
history.
Darwin finds the evidence
Natural selection
A simple equation:
Variation + DIfferential Survival = Evolution
Living species
Extinct species
There are likely billions of extinct species.
Natural selection
The basic mechanism:
1.
2.
3.
4.
5.
Life arises
As reproduction happens, slight variations occur through random chance
Some variations help an organism survive, others do not
Organisms with detrimental traits are less likely to reproduce
As variation accumulates, groups of organisms become distinct
Three key facts:
1.
2.
3.
All life is constantly evolving
Evolution by natural selection is slow
Evolution does not always increase complexity
The origin of life
96% of
human body
Most common elements by mass:
Human
body
Earth’s
crust
Earth’s
oceans
Earth’s
atmosphere
Universe
Oxygen
Oxygen
Oxygen
Nitrogen
Hydrogen
Carbon
Magnesium
Hydrogen
Oxygen
Helium
Hydrogen
Silicon
Chlorine
Argon
Oxygen
Nitrogen
Iron
Sodium
Carbon
Carbon
The origin of life is one of the great unsolved mysteries of science.
Two possible origins
●
●
Endogenous
Exogenous
Life arose on Earth
Life arose elsewhere in the Universe
All the ingredients are right here
Life appears very quickly after the
Earth is capable of supporting it
●
●
Sometimes called panspermia
We observe material transfer
between the planets
Early quirks
Homochirality
Chirality is the handedness of a
molecule. Chiral molecules are mirror
images of one another.
Amino acids
Amino acids are basic organic chemical
compounds. More than 5o0 exist in
nature.
→ Life is virtually exclusive in its use of
one chirality or the other
Left: Amino acids
Right: Nucleotides, Sugars
Of these ~500, the same 23 are used in
all life on Earth.
Protocells and the RNA world
Laboratory experiments demonstrate that the ingredients for life could form in the
conditions present on the early Earth. The formation of the membrane was likely a
critical step in the development of life.
Membranes:
●
●
●
Constrain important chemicals so that reactions happen more quickly
Ensure the products of these reactions do not disperse
Reduce the likelihood of a reaction with compounds from the outside world
With the formation of a membrane, the protocell could form. Within the protocell,
RNA provided a fragile mechanism for information storage.
Once the protocell forms, natural selection guides the development of life.
The mechanism of variation
Darwin, Wallace, and other pioneers of natural selection could not explain how
variation was passed from parent to child...
We now know that that information is carried by a substance known as
Deoxyribonucleic acid (DNA)
How DNA works
●
●
●
●
Regions of DNA that encode
information about the construction of
an organism are called genes.
Genes can be copied into RNA,
which serves as a template for
proteins, a fundamental part of the
cell.
During DNA replication, mistakes,
called mutations, can occur.
If the organism lives long enough to
reproduce, these mutations can be
passed on to the next generation.
A simple example
A population
has a variable
trait
Members with a
certain
expression of
that trait survive
less frequently
Less frequent
reproduction
means fewer
expressions of
that trait
Eventually that
trait disappears
Natural selection in action
From one to many
Eventually, the accumulation of traits
means that two populations that started
the same have become too different to
reproduce with one another. This is
known as speciation.
Different species maintain much of
the same DNA.
Humans share 7% of their DNA with
bacteria. This is the machinery of
life.
A slow, but powerful force
Modern primates began to diverge about 8 million years ago.
When things go terribly wrong
90-96% of
species die
60-70% of
species die
75% of species die
70-75% of
species die
Millions of years ago
Rapid changes in the environment lead to mass extinction.
The question of complexity
An active increase in complexity occurs
when simple organisms always evolve into
more complex ones.
→ Over time, most organisms become complex
A passive increase in complexity occurs
when organisms evolve along a random
walk.
→ Over time, few organisms become complex
The question of complexity
So which process is at play?
→ No conclusive answer
SImpler organisms are more
difficult to discover, making
species numbers difficult to
estimate.
→ Number of bacteria species
estimated between tens of
thousands and billions!
The tree of life
Finding the root of the tree
Because all life on Earth
appears to function on a
cellular level using the same
basic machinery of life, we
believe today that all living
things can trace their
evolution back to a single
species, the last universal
ancestor (LUA).
LUA
Small group activity
The search for life elsewhere in the Universe is perhaps the ultimate scientific
undertaking. But, how will we know when we’ve found it?
In a small group, discuss the following:
1)
2)
What specific aspects of possible extraterrestrial life could distinguish it
from life here on Earth?
Is life elsewhere in the Solar System likely to be more similar to Earth
life than life elsewhere in the MIlky Way or is all life likely to be unique?
What evidence supports your postulate?
Be prepared to share your group’s answers with the class!