This recent high-resolution 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:– 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

Carbon and Energy Sources

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

Migration of Life to Earth?

● Against:– No atmosphere or water– Solar and stellar radiation

● 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

Early Microbial Evolution

● Who came first?– 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