We are now mostly finished with planetary environments, so..

Before proceeding to life…

A brief visit to a missed topic…

The Origin of the Earth’s Moon

The Earth-Moondouble planet does not fit well into the nebular theory

planetesimal accretion predicts both should have the same chemical composition.

They don’t - there are subtle but significant differences

Moon is composed of less dense material than Earth

The general view is that the Earth’s moon was created as a result of the impact of a large object, perhaps as large as Mars, with the Earth very early in its existence.

The moon was formed from the debris of this collision, which included lower density “mantle” material from the Earth.

“Life” How is life defined?

What is needed for life?

How hard it is for life to form?

What environments are suitable for life?

How is “LIFE” defined?

Order - life has structure

This is extremely difficult. We can look at commonalities of what we have defined as living…

How is “LIFE” defined?

Order - life has structure Reproduction Growth & development Energy utilization Senses & reacts to environment Evolutionary adaptation

This is extremely difficult. We can look at commonalities of what we have defined as living…

All six properties of life are important, but biologists consider evolutionary adaptation to be the most important.

Evolution: “change with time”

Organisms need to be able to encode their structural information in order to reproduce.

In Earth-based life, this encoding is accomplished through DNA.

DNA Replication● Steps:

– Complete double helix– Strands separate into 2 helices– Free floating bases join open strands– Fill in deoxyribose and a phosphate group– Two identical copies of the DNA in the cell– Cell division: one copy to each daughter cell

● Complexity:– Enzymes: a dozen or more needed– Heredity: ensured by exact copying, but– Errors: occur occasionally -> evolution– Origin of Life: need simpler mechanism (RNA?)

Will Life Elsewhere Use DNA?

Heredity and evolution are essential

DNA does the job on Earth today

RNA may have been the first mechanism

Who’s to say that the same complex mechanism is universal?

Some type of molecule has to provide the mechanism for heredity and evolution

ERRORS ARE IMPORTANT!

Changes (mutations) in this encoding will lead to changes in the organism.

Mutations and Evolution

Causes of mutations:– Ultraviolet (UV) light– Chemical agents (carcinogens)– Nuclear radiation (mostly natural cosmic rays)

Effect of mutations:– Harmless– Fatal– Evolution

If the change produces an organism better suited to its environment, it is more likely to be passed on, i.e., the organism changes (evolves).

Natural selection

Artificial selection

What are the necessities of life?

Nutrient source Energy (sunlight, chemical reactions, internal heat) Liquid water (or possibly some other liquid)

Common Characteristics of Life?

Carbon based

Have a protective membrane

Need water

Use energy to maintain internal state

Can get energy from environment

Conduct metabolic processes (use stuff, make waste)

Responds to stimuli

Grow, reproduce (replicate)

Evolve and adapt to the environment as a population

Obtaining Energy

Living organisms can obtain energy through

“eating”, energy & nutrients from other organisms

extraction from chemical reactions in the environment (black smokers - ocean vents)

extraction from radiative energy (e.g., photosynthesis)

Metabolism

Metabolism: chemical reactions within living organisms. It takes place within cells.

Why in cells? Chemical reactions much faster than in the open Collects the raw materials for the chemical

reactions Provides the energy for the reactions Provides enzymes to catalyze the reactions Instructions for enzymes encoded in DNA

Synthesis reactions = “building up” reactions

Example: Making starch from glucose - requires energy

Decomposition reactions = “breaking down” reactions

Example: Breaking glucose into CO2 & H2O - releases chemical energy

Metabolism and Cells Metabolism:

– Four forms of metabolism defined by:● Sources of carbon (direct or indirect)● Sources of energy (light or chemical)

– The four forms of metabolism are quite general and should apply to life anywhere

Cells:– Needed environment for metabolism at acceptable

rate Origin of Life (on Earth and elsewhere):

– Look for cells as sites of metabolism

Why Carbon based?

Can bond to as many as 4 atoms at a time. Can form skeleton of long chains of atoms

(polymers). The complexity of life requires complex

molecules.

Silicon can also form 4 bonds and is relatively abundant, however…

Bonds are weaker than those of carbon (fragile: complex Si-based molecules don’t last long in water)

Does not normally form double-bonds like Carbon; this limits the range of chemical reactions and molecular structures.

Carbon is more mobile in the environment - it can travel in gaseous form, e.g., CO2

Environmental limits to life (as we know it) ?

Is the planet of interesting missing any of the key ingredients? (water, energy, nutrients)

Are temperatures below –15 or beyond +115 C?

Is it really cushy? – does it have an atmosphere

Importance of Water● Importance:

– Contact: organic chemicals float in the cell and find each other

– Transportation: bring chemicals in and out of cells– Participant in reactions:

● ATP● Photosynthesis

● Necessity:– Life on Earth: all use water– Dormant without water: for a limited time only– Elsewhere: need a liquid– What are the alternatives?

Water

Liquid water plays a fundamental role in life:

Make chemicals available (dissolved)

Transports chemicals

Plays a role in many metabolic reactions

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

Origins of Life

Certain chemical processes are energetically favored given specific circumstances, i.e., 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