Homework #5 Due Tuesday, October 25, 6:00 pm Exam #2 date Wednesday, November 2 Exam #3 tentative date Wednesday, November 30

Extremophiles Life that exists under “extreme” conditions, conditions that until recently were thought to be inhospitable to life. Life that exists under “extreme” conditions, conditions that until recently were thought to be inhospitable to life.

Extremophiles Volcanic vents: 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: Antarctic dry valleys:  Microbes in small pockets of water in rocks

Extremophiles Lithophiles (rock lovers): Lithophiles (rock lovers):  Several kilometers below the surface  Chemical energy from rocks  Carbon from CO 2 filtering down

Extremophiles Endospores (e.g., anthrax) 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  Sufficient atmospheric oxygen for eukarya has existed for only ~10% of the Earth’s lifetime – complex life not possible before this time  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

The Origin and Evolution of Life on Earth When Did Life Begin?

Stromatolites 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 Fossil evidence – 3.5 Byr old Australian rock shows “cells” – Could this form naturally from minerals? – Younger sites: at least two more ( byr old) – Older sites: sedimentary rock too altered to be useful

13 C/ 12 C ratio 13 C/ 12 C ratio – Normal abundance ratio 1/89 – Living tissue and fossils show less 13 C – Some rocks older than 3.85 byr show the low 13 C abundance Sterilization Sterilization – Last sterilization: byr ago (Late Heavy Bombardment)

The evidence indicates life formed quickly after the Earth became habitable. Within a few 100 million years, Perhaps as short as 100 million years

The Origin and Evolution of Life on Earth How Did Life Begin?

No Life Life ? How did life get here? few hundred million years

Simplest organisms today and those dated 3.5 billion years ago are remarkable advanced Simplest organisms today and those dated 3.5 billion years ago are remarkable advanced What are the natural chemical processes that could have led to life? What are the natural chemical processes that could have led to life? Assumptions Assumptions –Life began under chemical conditions of early Earth –Life did not migrate to Earth

 Certain chemical processes are energetically favored given specific circumstances, i.e., in the presence of specific chemical elements, energy sources & 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

 Simulated conditions thought to exist on the early Earth  Tested for the occurrence of chemical evolution  Considered to be the classic experiment on the origin of life

 After one week, Miller and Urey observed that 10–15% of the carbon was now in the form of organic compounds.  Recent re-analysis of Miller's archived solutions found 22 amino acids

Miller-Urey Experiments Different mixes of gases used for atmosphere Different mixes of gases used for atmosphere Different energy sources, like UV (sunlight) Different energy sources, like UV (sunlight) Results: With the exception of when oxygen was present, ALL VARIATIONS PRODUCE AMINO ACIDS AND COMPLEX ORGANIC MOLECULES Results: With the exception of when oxygen was present, ALL VARIATIONS PRODUCE AMINO ACIDS AND COMPLEX ORGANIC MOLECULES –Proper mix produces less than original experiment –THERE MUST be additional sources of organic material – the atmosphere is not sufficient!

The Origin and Evolution of Life on Earth Where Did Life Begin?

Sources of Organic Molecules Chemical reactions in atmosphere Chemical reactions in atmosphere –Lab experiments show this is probably insufficient Organic material brought by impacts Organic material brought by impacts –Chemical analysis of comets and carbonaceous chondrites show that they have organic molecules Chemical reactions near deep-sea vents Chemical reactions near deep-sea vents –Heat from undersea volcanoes and vents can fuel chemical reactions between water and minerals

Early Organic Chemistry No atmospheric oxygen No atmospheric oxygen – Helps: Oxygen destroys many organic compounds – Atmosphere is reducing, not oxidizing Miller-Urey experiments Miller-Urey experiments – Can form amino-acid soup from methane (CH 4 ) and ammonia (NH 3 ) with electric energy (mimicking lightning) – Current thinking: early atmosphere was dominated by CO 2 – Low yield – Shallow ponds close to surface -> UV sterilization Alternative sources of amino acids Alternative sources of amino acids – Extraterrestrial: amino acids are abundant in meteorites – Deep sea vents: abundant chemical energy & protected from UV

Where Did Life Begin?  Unlikely on land – Solar UV radiation: protection today by ozone (O 3 ) – 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

Chemical reactions near deep- sea vents: Considered the most likely sites where life on Earth originated Chemical reactions near deep- sea vents: Considered the most likely sites where life on Earth originated –“Black smokers”

The Origin and Evolution of Life on Earth Chemistry to life

Search for Self-Replicating Molecule Work backward from organisms that live today Work backward from organisms that live today DNA is double-stranded = complicated DNA is double-stranded = complicated RNA obvious candidate, simpler than DNA RNA obvious candidate, simpler than DNA –Hereditary information –Can serve as template for replication –Fewer steps to produce backbone structure

Problem: RNA and DNA require enzymes to replicate (chicken and egg problem?) Problem: RNA and DNA require enzymes to replicate (chicken and egg problem?) In 1980’s determined that RNA might catalyze their own replication instead of other enzymes In 1980’s determined that RNA might catalyze their own replication instead of other enzymes Conclusion: Early Earth-life probably used RNA to encode its structure Conclusion: Early Earth-life probably used RNA to encode its structure Search for Self-Replicating Molecule

Early Earth: short strands of RNA-like molecules produced spontaneously partially or completely Early Earth: short strands of RNA-like molecules produced spontaneously partially or completely RNA-like molecules that could replicate faster with less errors soon dominated population RNA-like molecules that could replicate faster with less errors soon dominated population Copying errors introduced mutations, ensuring the production of many variations of successful molecules Copying errors introduced mutations, ensuring the production of many variations of successful molecules Allowed molecular evolution to continue Allowed molecular evolution to continue RNA-world gave way to DNA-world RNA-world gave way to DNA-world –DNA less prone to copying errors –DNA more flexible hereditary material –RNA kept some of its original functions

Assembling Complex Organic Molecules Organic soup was too dilute to favor the creation of complex organic molecules Organic soup was too dilute to favor the creation of complex organic molecules Lab experiment with possible solution: When hot sand, clay or rock is placed in dilute organic solution, complex molecules self-assemble Lab experiment with possible solution: When hot sand, clay or rock is placed in dilute organic solution, complex molecules self-assemble –Organic molecules stick to surface of clay –Increases density and likelihood of reactions –Strands of RNA up to 100 bases have been spontaneously produced this way

Other inorganic minerals may have also had a similar role Other inorganic minerals may have also had a similar role Iron pyrite (fool’s gold) Iron pyrite (fool’s gold) –Positive charges on surface which allows organic molecules to adhere –Formation of pyrite releases energy which could be used as fuel for chemical reactions

Early Cell-like Structures There are advantages to enclosing enzymes with RNA molecules There are advantages to enclosing enzymes with RNA molecules Their close proximity increases the rate of reactions between them Their close proximity increases the rate of reactions between them Also, isolates contents from outside world Also, isolates contents from outside world

Lab experiments suggest that membrane structures existed on early Earth Lab experiments suggest that membrane structures existed on early Earth These form spontaneously when… These form spontaneously when… –Cool down warm-water solution of amino acids –Mix lipids (fats) with water

Nonliving Pre-Cells have Lifelike Behavior Grow in size until unstable then split to form a ‘daughter’ cell Grow in size until unstable then split to form a ‘daughter’ cell Selectively allow other types of molecules to pass in/out of membrane Selectively allow other types of molecules to pass in/out of membrane Store energy in the form of electric voltage Store energy in the form of electric voltage

Brief Summary  Synthesis of organic precursor molecules  Origin of self-replicating RNA  Origin of enclosed pre-cells  Origins of true cells with RNA genome  Evolution of modern cells with DNA genome

Migration of Life to Earth? “Panspermia”  Proposal: “Seeds of life” exist everywhere around the universe  Life on earth started when these ‘seeds’ came here, probably by a meteor.  It also suggests that these seeds are taken to other habitable places in the universe.

Panspermia argument:  Against: – No atmosphere or water in space – Solar and stellar radiation hazards in space  For: –organic material is everywhere, and some bacteria can withstand large amounts of radiation and go dormant under low atmospheric conditions – Fact: amino acids are found in some meteorites – Question is not “could” but “did” life originate elsewhere

Panspermia: 2 schools of thought School 1: life did not evolve as easily as imagined on early Earth in timescales we’ve determined School 1: life did not evolve as easily as imagined on early Earth in timescales we’ve determined School 2: life evolved easily and was everywhere with suitable conditions School 2: life evolved easily and was everywhere with suitable conditions

Problem: entire solar system was under heavy bombardment at the same time - hard to form life quickly in another location in Solar System Problem: entire solar system was under heavy bombardment at the same time - hard to form life quickly in another location in Solar System Other possibility: interstellar migration Other possibility: interstellar migration Problem: rock to be ejected out of its own system, then fall into ours and hit the tiny planet of Earth - very dificult Problem: rock to be ejected out of its own system, then fall into ours and hit the tiny planet of Earth - very dificult School 1: life did not evolve as easily as imagined on early Earth in timescales we’ve determined School 1: life did not evolve as easily as imagined on early Earth in timescales we’ve determined

Earth was not first planet with suitable conditions Earth was not first planet with suitable conditions Migration of life from another planet (say Mars) dominated before early life on Earth could Migration of life from another planet (say Mars) dominated before early life on Earth could –We’re Martians!!!! School 2: life evolved easily and was everywhere with suitable conditions School 2: life evolved easily and was everywhere with suitable conditions

Martian meteorites Martian meteorites Fossil evidence of life on Mars or geochemical structure? Fossil evidence of life on Mars or geochemical structure?

Early Evolution and Rise of O 2 First organisms had simple metabolism First organisms had simple metabolism Atmosphere was O 2 free, life must have been anaerobic Atmosphere was O 2 free, life must have been anaerobic Probably chemoheterotrophs Probably chemoheterotrophs –Obtained nutrients from organic material –Obtained energy from inorganic material  Modern archaea appear to be close to the root of the tree of life  Obtaining energy from chemical reactions involving hydrogen, sulfur and iron compounds (all abundant on early Earth)

Early Evolution Natural selection probably resulted in rapid diversification Natural selection probably resulted in rapid diversification Modern DNA has enzymes that reduce the rate of mutations Modern DNA has enzymes that reduce the rate of mutations RNA is not so lucky, more likely to have copying errors RNA is not so lucky, more likely to have copying errors Higher mutation rate in early evolution than now Higher mutation rate in early evolution than now