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Transcript
This recent highresolution picture
from Mars
Reconnaissance
Orbiter shows
twisting dark trails
criss-crossing the
martian surface.
These are now
known to be the
work of miniature
wind vortices
known as dust
devils. On Mars,
dust devils can be
up to 8 kilometers
high.
Homework #5
due Wednesday, October 21, 10:00 pm
The Earth owes its habitability primarily to
(red) the chemical composition of its surface
(blue) a combination of its size and its distance
from the Sun
(yellow) its size
(green) its distance from the Sun
The Earth owes its habitability primarily to
(blue) a combination of its size and its distance
from the Sun
Which of the following processes is believed to
have been an important source of the Earth’s
atmosphere?
(red) outgassing by volcanoes on the Earth’s
surface
(yellow) gas trapped from the solar nebula
(blue) matter blasted from the surface of the
Moon
(green) charged particles trapped from the Sun
Which of the following processes is believed to
have been an important source of the Earth’s
atmosphere?
(red) outgassing by volcanoes on the Earth’s
surface
How old is the Earth?
(red) 4.6 billion years old
(yellow) 3.85 billion years old
(green) 100 million years old
(blue) 4.0 billion years old
(orange) 6000 years old
How old is the Earth?
(red) 4.6 billion years old
Cells
 All life on Earth is made of cells - microscopic
units in which living matter is separated from the
outside world by a membrane.
 All cells on Earth share common characteristics
(e.g., use of ATP, DNA, …), leading to
conclusion that they share a common ancestor
 All cellular life is carbon based (organic
molecules)
Components of Cells

Carbohydrates: energy needs and structures
 Lipids: Source of energy & major component of
membranes. Lipids can spontaneously form membranes
in water.
 Proteins: participate in a vast array of functions;
structural, enzymes, catalysts. Built from long chains of
amino acids.
 Nucleic acids: instructions for reproduction
 70 amino acids known to
exist; only 22 are found in
life on Earth.
 Only left handed versions
are found in living
organisms
 Both of these traits
suggest a common
ancestor for life on Earth.
Based upon the cellular structure of an
organism, living cells come in two types:
Prokaryotes
Eukaryotes
The prokaryotes
 lack a cell nucleus
 Most are unicellular
 two domains: bacteria &
archaea
 asexual reproduction
The Eucaryotes
 cells are organized into complex
structures enclosed within membranes.
Have a nucleus.
 typically much larger than prokaryotes
 May be unicellular, as in amoebae, or
multi-cellular, as in plants and humans.
 both sexual and asexual reproduction
Tree of Life
Cells & Metabolism
Metabolism takes place in cells.
Four different approaches to metabolism
have been identified.
Carbon and Energy Sources
●
Carbon:
–
–
●
Heterotroph: eat other organisms
Autotroph: self-feeding by converting atmospheric CO2
Energy:
–
Photoautotrophs (plants): photosynthesis: CO2 + H2O +
sunlight
sugar
–
Photoheterotrophs (rare prokaryotes): carbon from food
but make ATP using sunlight
–
Chemoheterotrophs (animals): energy from food
–
Chemoautotrophs (extreme prokaryotes): energy from
chemicals and not sunlight
Extremophiles
●
Life that exists under “extreme”
conditions, conditions that until recently
were thought to be inhospitable to life.
Extremophiles
●
Volcanic vents:


Water temperature
reaches 400°C
(750°F), possible
because of the large
pressure
Black smokers: mixed
with volcanic chemicals
Extremophiles
●
Antarctic dry valleys:
 Microbes in small pockets of water in rocks
Extremophiles
●
Lithophiles (rock lovers):
 Several kilometers below the surface
 Chemical energy from rocks
 Carbon from CO2 filtering down
Extremophiles
●
Endospores (e.g., anthrax)
 Can lay dormant for long periods
 Can survive lack of water, extreme heat and
cold, and poisons
 Some can survive in vacuum
Implications for Extraterrestrial Life
 Oxygen for eukarya on Earth for only ~10% of its life
 What is the probability that eukarya-like organisms
would develop?
 We are more likely to find extremophiles elsewhere
 Extremophiles may be the norm, not the exception
All organisms have finite lifetimes and
eventually die. Which of the basic
characteristics of life corrects for this?
(red) energy utilization
(blue) response to the environment
(green) evolutionary adaptation
(yellow) reproduction
All organisms have finite lifetimes and
eventually die. Which of the basic
characteristics of life corrects for this?
(yellow) reproduction
Energy utilization in living organisms is
(red) one of the most basic requirements of life,
without which organisms could not maintain order,
grow, and reproduce
(yellow) only important for organisms like plants which
receive their energy directly from the Sun
(green) neither a necessary nor a sufficient condition
for life
(blue) not important for organisms that have adapted
to survive extremely low temperatures (psychrophiles)
Energy utilization in living organisms is
(red) one of the most basic requirements of life,
without which organisms could not maintain order,
grow, and reproduce
Of the six basic properties of life, biologists
consider the most fundamental to be
(red) response to environment
(yellow) reproduction
(blue) order
(green) evolutionary adaptation
Of the six basic properties of life, biologists
consider the most fundamental to be
(green) evolutionary adaptation
The basic biological structures of life on Earth
are called
(red) cells
(blue) bacteria
(yellow) molecules
(green) atoms
The basic biological structures of life on Earth
are called
(red) cells
The fact that all cells used in life on Earth are based
on the same biochemistry suggests that
(red) life originated from a variety of sources
(yellow) life on Earth was created by some
omnipotent being
(blue) all life on Earth shares a common
ancestor
(green) the biochemistry of life on Earth is
universal
The fact that all cells used in life on Earth are based
on the same biochemistry suggests that
(blue) all life on Earth shares a common
ancestor
Origins of Life
 Certain chemical processes are energetically
favored given specific circumstances, i.e., in the
presence of specific elements, energy, liquid(s)
 Can the presence of specific elements and
energy inevitably lead to the formation of life?
 We know it can lead to the building blocks (as
will be discussed)
●
●
●
Searching for the Origin of Life
–
When Did Life Begin?
–
DNA Molecules as Living Fossils
–
Where Did Life Begin?
How Did Life Begin?
–
Early Organic Chemistry
–
Chemistry to Biology
–
Migration of Life to Earth?
Early Evolution and the Rise of Oxygen
–
Early Microbial Evolution
–
Photosynthesis and Oxygen
–
Rise of Oxygen
When Did Life Begin?
●
Stromatolites
–
–
–
●
Living: colonies of bacteria living in outer layer of sedimentary rocks
3.5 Byr old rocks: almost identical layered structure
Inconclusive evidence: sedimentation layering may mimic stromatolites
Fossil evidence
–
–
–
–
3.5 Byr old Australian rock shows “cells”
Could this form naturally from minerals?
Younger sites: at least two more (3.2-3.5 byr old)
Older sites: sedimentary rock too altered to be useful
●
13C/12C
–
–
–
●
ratio
Normal abundance ratio 1/89
Living tissue and fossils show
less 13C
Some rocks older than 3.85 byr
show the low 13C abundance
Sterilization
–
Last sterilization: 3.9-4.2 byr
ago
The evidence indicates life formed
quickly after the Earth formed.
Within a few
100 million
years,
Perhaps as
short as 100
million years
Where Did Life Begin?
Unlikely on land
–
Solar UV radiation: protection today by ozone (O3)
–
But no atmospheric oxygen in the early Earth
–
In water: no problem, UV absorbed effectively
Shallow ponds
–
First evidence from Miller-Urey experiment
–
Recent evidence: incorrect atmospheric content
Thermophiles
–
DNA evidence shows early thermophiles
–
Have advantage of more chemical energy
–
Deeper sea vents better protected against bombardment
Early Organic Chemistry
●
●
●
No atmospheric oxygen
–
Helps: Oxygen destroys many organic compounds
–
Atmosphere is reducing, not oxidizing
Miller-Urey experiment
–
Can form amino-acid soup from methane (CH4) and ammonia (NH3) with electric
energy (mimicking lightning)
–
Current thinking: early atmosphere was dominated by CO2
–
Low yield
–
Shallow ponds close to surface -> UV sterilization
Alternative sources of amino acids
–
Extraterrestrial: amino acids are abundant in meteorites
–
Deep sea vents: abundant chemical energy & protected from UV
Chemistry to Biology
●
●
●
Organic soup of amino acids
–
Has to be the initial step, however the amino acids formed
–
Perhaps in special locations, not ubiquitous initially
Short strands of RNA
–
Required physical catalysts: clay or other minerals
–
Some had to be self-replicating
–
Initial simulations in the laboratory
Spontaneous membranes  “pre-cells”
–
●
●
Protect chemicals and allow faster reactions
Slow initial natural selection
–
Gradual increase of complexity
–
Fast mutation but slow natural selection at first
Rapid natural selection
–
Complexity  risk  faster natural selection
–
Probably the stage when DNA formed and took over
●
●
●
Against:
–
No atmosphere or water
–
Solar and stellar radiation
Migration of Life
to Earth?
For:
–
Fact: amino acids found in meteorites
–
Question is not “could” but “did”
Difficult formation of life
–
Argument: too little time to form life on Earth
–
Counter argument: other sites in the solar system no better
–
Another stellar system?
●
Travel time much longer and more dangerous
●
All meteorites found in the solar system have the same age
Migration of Life to Earth?
●
Easy formation of life
–
Life could have formed on Earth, but also elsewhere
in the solar system
–
Life was then transported to Earth, before life on
Earth had a chance to form
●
●
Who came first?
Early Microbial
Evolution
–
Fossils of Eukarya go back only 1-2 byr
–
But hard to detect cell nuclei in fossils
–
Tree of life suggests that prokaryotes came first, but how much earlier?
–
Whoever came first, microbes still rule the Earth, even by mass
Early microbes
–
Rudimentary metabolism: at least a few enzymes
–
Anaerobic: there was no oxygen
–
Chemoautotrophs: obtained energy from inorganic chemicals
–
Thermophiles favored: Abundant energy & chemicals (H, S, Fe)
–
Simple creatures -- faster mutation and evolution
–
Complexity: hard to form but also more stable and survives
Photosynthesis and Oxygen
●
●
Photosynthesis:
–
Key step to harnessing sunlight
–
May have begun as a pigments protecting against
UV radiation, then started using the energy
–
Some bacteria, even today, use H2S instead of H2O
in photosynthesis
Cyanobacteria
–
Exist today (blue-green algae)
–
Earliest fossils resemble modern cyanobacteria
–
Release oxygen in photosynthesis
Rise of Oxygen
 O is highly reactive: would disappear from the atmosphere in a
few million years
 To maintain oxygen, it therefore needs to be replenished
constantly
 Today: living creatures consume most of the O
 Early Earth: inorganic reactions, mainly rusting iron, suffice,
and for a long time prevented the rise of O
 Banded iron formations (2-3 byr old) suggest very low
atmospheric O (less than 1% of today)
 Rise of O began about 2 byr ago
 Evidence for O at today’s levels: only 0.2 byr ago
 Intermediate O level uncertain