origins of life - austin community college district 1409/1409lecnotes/lnexami/origins of... ·...

Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009 1

Origins of Life

Why is there such a diversity of living forms yet alllife shows remarkable similarities:

1. genetic code = DNAsame basic code for all forms of life

2. common chemicals & metabolic pathwayseg. 20 amino acids used by all life

3. all made of cellscell structures are amazingly similar

only two basic forms:procaryotes and eucaryotes

Darwin provided a mechanism that showed all lfe was interrelated

But., how did it all start?

Darwin called this the “mystery of mysteries”

since Darwins time we have learned how to very accurately date rocks & fossils by radioactive decay

Decay System Half Life Useful TimespanRubidium Strontium 1.42X10-11 yr-1 48.8 BYLutetium Hafnium 1.94x10-11 yr-1 35.7±1.2BY

Uranium Lead 1.55 X10-10 yr-1 4.47BYPotassium Argon 0.581 X10-10 yr-1 1.93BY

14C 5568±30 yr3

from the Fossil record we have learned:


all forms of life didn’t appear at the same time

life progressed from small, simple forms tomore complex and larger forms

the early earth was very different from today

Previous ideas on the Origin of Life

Until mid 1800’s it was generally thought that life couldarise by spontaneous generation (600BC-1800’s)

=Nonliving matter had capacity within itself toturn into certain types of living organisms

eg: moist soil toads, snakes, micemanure fliesfruits molds

believed there was some “vital force” in the elementsthat could transform matter

Late 1600’s: Redi performed experiments that dealt1s t major blow to this theory

Later: Spallanzani -- sealed flasks

Pasteur: Final downfall of the ideashowed it included microrganisms

BUT: This left biologists with no plausible hypothesisfor the Origin of Life until fairly recently


How quickly did it happen?

some believe that the spontaneous origin of life wouldbe impossible:

the odds of life arising on the early earth are about the same as the odds of a tornado going through a junkyard and randomly assembling a fully functional jumbo jet, 747

others believe that life began as something verysimple and only after billions of years did itachieve the great complexity exhibited today

if the origin of life occurred relatively quickly after wecan surmise that conditions were suitable it wouldindicate that it did indeed begin very simply andthat natural laws are conducive to its origin giventhe suitable conditions and materials; that theorigin of life is inevitable under the properconditions

if life didn’t appear on earth for several billion yearsafterwards then it would imply that the origin of life is an extremely unlikely event and may have implications for finding life on other planets


Major Steps Needed for Life to Originate:

1. Suitable Environment

2. Formation of Basic Building Blocks

3. Metabolism & the Formation of LargePolymers

4. Compartmentalization; ie. Cells

5. Operating Instructions

These are probably not sequential events:

most or all probably occurred at the same time

eg. interstate system: none fully developed

not: first roads, then gas stations, state police system, then dealerships, etc

Our thoughts on what the first cell were like can onlybe based on what cells are like today

it is possible the the first cells were considerably more simple andinefficient and perhaps very little like cells around today;

over billions of years more complex and more efficient cells would have evolved while such early primitive cells went extinct with out leaving any fossil record


1. Suitable Environment

4.6 BY:

the sun is a second or third generation star

even today, a star explodes each second with abrightness greater than a galaxy

all matter on earth has been through 1 or 2previous cycles of “stellar alchemy”

sun and planets form from previous supernovaexplosion producing cloud of dust and gas

heavy bombardments by comets and meteorites(size of Ohio) – each impact would havecaused any liquid water to boil awaycompletely

when the moon formed by a grazing impact of a mars-sizedbody it would have completely destroyed any life that mighthave arisen

even today 40,000 tons of space debris fall to earth eachyear, mainly as dust

surface temperatures up to 1000-3000º C

no solid groundno liquid water (no oceans, lakes)only steam from geysers, volcanoes

4.0 -3.9 BY:


intensive bombardment slowed

crust cooled and solidified

cooled enough for liquid water to collect in basins

intense storms

atmospheric water condensed to form oceans

most of this water came from volcanic activity

up to 1/3rd of water on the earth (and in us) may havecome from comets

even today 10 M small comets hit earth each yr each comet = wt of 60 compact cars together

if this is same as rate throughout earths history it wouldequal the volume of today’s oceans

earth’s early atmosphere was derived mainlyfrom volcanoes:

contained: H2O, CO2, N2, CO, H2, H2S,HCl, HCN (reducing atmosphere)

[todays atm: 78% N2, 20% O2, 4% H2O, 0.03% CO2]

CHON all major atoms were available

early earth still very energetic:lots of volcanic activitysevere lightning stormsunshielded solar radiation: xrays and UV


more radioactive materials

most agree there was a suitable environment for life by 3.9 BY ago

How quickly did life happen?

unfortunately, there is no rock left on the surface ofthe earth older than 3.85 BY due to earth’s active geology

everything older has been lost to weathering andcontinental drift

some suggest that the best place to look for signs for theorigin of life are on the moon

some estimate there are thousands of tons of earth rock on the moon

if we can find rocks older than 3.5 BY we might be ableto find fossils or signs of life in them

oldest “trace fossils” = 3.85 BY, Greenland

1s t true fossils appear 3.5 BY

life must have begun 4.0 to 3.8 BY ago


2. Formation of Basic Building Blocks

~4.0-3.9 BY ago there were ideal conditions forchemical reactions that could produce smallorganic molecules:

sugars, amino acids, nucleotides, fatty acids, etc

The comets and meteorites that helped to form theearly earth also brought organic molecules

eg. amino acids are fairly common in these objects

Miller and Urey (1953) and others later modeled the composition of the early ocean & atmosphere andused electrical sparks to mimic energy

produced:all 20 amino acidsseveral sugarslipidspurines and pyrimidinesshort chains of nucleotidesATPshort chains of nucleotides

Miller and others (early 1950’s) could produce buildingblocks of virtually all small organic moleculesgiven early atmosphere and energy

these same reactions could also have happened atdeep ocean vents on early earth

but generally cannot occur in the presence of O2


3. Metabolism & the Formation of LargePolymers

even a single living cell consist of 1000’s of highlyorganized chemical reactions, all occurring at the same time

all these reactions together are called metabolism

metabolism consists of 2 main kinds of reactions:

synthesis reactions = building larger molecules(polymers) out of smaller ones

eg. sugars starchesamino acids proteinsfatty acids lipidsnucleotides nucleic acids (DNA & RNA)

decomposition reactions = breaking apart largemolecules into smaller ones

synthesis reactions are used for building cell parts,growth, reproduction, etc

requires energy to to synthesis

decomposition reactions are used to get rid of wornout parts, to produce building blocks for synthesis and to produce energy for the synthesis reactions

in living cells today, each of these reactions requires aspecific enzyme (= organic catalyst)


but enzymes are proteins (large polymers);proteins weren’t around yet

without enzymes, need high temperatures or somekind of catalyst to do these reactions

there were many high energy sources in theseearly days:

lots of volcanic activity

horrendous thunder & lightning

lots of UV radiation (earth had no ozoneshield)

meteorite impacts produce enough energyto form polypeptides

some believe, given enough time, such reactions wereinevitable on early earth

eg. near volcanic activity?

eg. hydrothermal vents?

also, HCN was common then and is commonly used as a catalyst in chemistry labs

over millions of years large polymers could conceivably be constructed


the main problem is that such large organic polymersare unstable and under these intense conditions would have tended to break down fairly quickly

there had to be some way to make thesepolymers more stable???

another question is how any reactions got organizedinto something we could call metabolism

one of the most basic, most important, and mostwidespread set of reactings in metabolism in all cells is something called the Krebs cycle or citric acid cycle

this is a set of 11 reactions between very smallorganic molecules

it is central to most other metabolic pathwaysin all cells

and it can lead to the formation of aminoacids, sugars, lipids and nucleic acides (all major kinds of organic molecules)

it is conceivable that if this small pathway wasrandomly formed it could have fairly easily evolved into many of the much more complex pathways we see in cells today


4. Compartmentalization; ie. Cells

Compartmentalization and isolation of chemicalstew

the internal chemical environment must beisolated from external environment

large organic molecules must be enclosed andcontained or at least collected together in one place

must have high concentrations of interactingorganic molecules to approximate metabolism in living organisms

this container must be able to absorb nutrients and getrid of waste products

= selective permeability

possible kinds of early compartments:

a. protein &/or lipid spheres (proteinoidmicrospheres)

lipids, especially phospholipids can formbilayered membrane similar to cellmembranes

can self assemble into lipid bilayercan grow


can engulf other spheres

produces a selective permeable membrane

some can store energy; ie. membranepotential

some of the proteins can act as catalysts

b. ocean foam (bubbles)

mixture of organic chemicals

5% of ocean’s surface today is covered with foam

can collect and concentrate many materials

c. clay particles

could absorb and concentrate them

eg. clay concentrates amino acids

eg. clay also collects iron and zinc which can help tocatalyze such reactions

Iron Pyrite around hydrothermal vents couldalso do that

evolving procell would require a controlled andconstant source of energy to synthesize complexorganic molecules


sugars are used today to make ATPmust assume ATP was available then

can be made if phosphates are present

Evolution must have started “before” lifeactually arose

even without life would get a kind of chemicalnatural selection

those droplets that were most stable and bestable to accumulate organic molecules wouldgrow and split

other droplets would fall apart or fail to growand divide

the “environment” would “select” for someover others

may have happened in tidepools, on clay, at deepocean vents??


5. Operating Instructions

need instructions = blueprint

must be highly organized

must be reproducible

Origin of heredity

main problem today in understanding origin of life isexplaining how these collections of organicchemicals could become organized into living selfautonomous, reproducing cells

Life as Information Storage

All life consist of chemicals in very organized patterns

yet laws of nature says that things tend towarddisorder, not order

one way to get an idea of the likelihood of lifeappearing is to consider how ordered life iscompared to nonliving matter

what is needed to make order out ofdisorder

we can view the genetic code as “information storage” of data bytes (1 byte = 8 bits)


Tandy 1000 256 kilobytes 100,000’s bytesearly Pentiums & power PC’s megabytes millions of bytestoday’s computers gigabytes billions of bytes

eg. DNA in human cells contains ~1 gigabyteof RAM in each cell

eg. E. coli contains ~1 megabytes

eg. the simplest organism capable ofindependent existence has 200 genes;therefore ~ 60 kilobytes

eg. George Church of Harvard MedicalSchool calculated that the theoreticalminimum number of genes a cellneeds is 151 (no junk DNA) or ~45 kilobytes

eg. even viruses with 2% as much geneticinfo as E coli have ~15 kilobytes

eg. one typical gene contains ~ 300 bytes

based on natural laws, the early pre life earth couldhave accumulated only up to ~25 bytes ofinformation by pure chance

if the first cell needed 50 kilobytes or more ofinformation it is highly unlikely that this wouldhave occurred on the early earth by pure chance

in laboratory biologists have succeeded in producing very small DNA molecules of about 50 nucleotides from a mixture of simple building blocks

much less than that neede for even 1 gene

if life on earth had a spontaneous origin there musthave been some intermediate state that sped up


the ability to accumulate information content.

In every cell today:

DNA stores the genetic code

RNA uses that code to make either

enzymes for metabolismor

new copies of DNA for reproduction

enzymes metabolismDNA RNA

new DNA reproduction

A simpler genetic code?

metabolism is the result of the activity of 10’s of1000’s of different enzymes

enzymes are some of the largest, most complexorganic molecules in a cell

computer analysis indicates that today’s enzymescould have originated from a very few, muchsimpler enzyme molecules constructed with a much simpler genetic code than the one in cells today

A simpler molecule than DNA?


many believe that for life to appear some simpler genetic material had to be there 1st

probably RNA appeared 1st

1. RNA is a much simpler molecule than DNA

2. RNA carries the same kind of geneticinformation as does DNA

3. RNA is able to replicate itself

4. all life requires RNA to make DNA

5. all life requires RNA to make proteins(enzymes)

6. RNA can act directly as enzyme or catalyst

7. some viruses have only RNA as geneticmaterial and are able to function

8. all components of RNA: sugar, phosphate and Nbases could have been formed under primitiveearth conditions

the packaging of RNA “genes” and their enzymeproducts within some kind of container would havebeen a significant milestone in the origin of life:


would now have had all basic requirements for life together and interacting

these simple cells could then “evolve” as units

favorable interactions could have been selected,preserved and perpetuated

Natural Selection could occur

RNA “life” could have arisen ~ 4BY ago and lasted for~ 200MY

once DNA appeared, natural selection would havegreatly favored it over RNA

eventually ALL the RNA only cells would havegone extinct, leaving no trace


What were the first cells:

1. 1st cells were Prokaryotes

the earliest fossils we find are prokaryotes

the only fossils we find for the first 2 Billionyears of life are prokaryotes

small, simple, inefficient metabolism, littleinternal structure

2. First cells were heterotrophs

all life requires nutrients and energy

all life produces the energy they need by breakingdown sugars and other organic molecules= respiration

some organisms are able to make their ownsugars

= Autotrophs (=”self feeders”)

other organisms must consume preformed organicmolecules to break down for energy

= Heterotrophs ( ~”feed on others”)

of the two, autotrophs require considerably morecellular “machinery” to make their own sugars

more genes are required; more complex


3.9 BY ago the oceans were thick with 100’sMillions of years of accumulated organicchemicals

There were plenty of organic molecules after 100’sof millions of years of earth history

life didn’t require primary producers the planet was awash in “food”

First cells were simpler heterotrophs able to capitalize on this abundance of

nutrients

3. First cells produced energy anaerobically

there was no free oxygen (O2) on the early earth,either in atmosphere or in oceans

the breakdown of sugars to produce energy= respiration

respiration can occur with or without O2

with O2 the process is very efficientbut more complex

requires additional pathways and enzymes, ie. more genes

= aerobic respiration


without O2 some energy can be made

it is a much simpler process, fewer enzymes required, fewer genes

= anaerobic respiration

but nowhere near as efficient

( 2 units of energy vs 34 units of energy per sugar molecule)

first cells were anaerobic heterotrophs whobroke down sugars without the use of O2 gas

they took organic molecules, extractedenergy from them and produced organicmolecules as waste products

this is the simplest form of energyproduction

= fermentation

fermenters excrete acids, alcohols, etc. (chemicals that contain less energy than the food ingested)vs aerobic respiration that produces CO2 & H2O

4. 1st cells probably used RNA as genetic instructions

[what is life? bacteria and their descendants]


Where did life begin?

A. shallow warm waterB. deep ocean thermal ventsC. some other planet or solar system

Evidence:

A. Shallow warm water

on soft clay sediments

habitat closest to suspected energy source neededfor “prelife” events

some of earliest fossils are stromatolites: bandeddomes of sedimentary rock and bacterial mats

these form today in shallow salt marshes andwarm lagoons

consist of colonies of bacteria and cyanobacteria injellylike secretions interspersed with sedimentlayers

produces a banding pattern

B. Deep Ocean Thermal Vents

less exposed seafloor and


molecular biology supports idea that earliest lifethrived in hot conditions and may have usedsulfur compounds – common conditions in ventcommunities

structure of proteins and genes resembles thosefound in bacteria in such habitats today seemto be precursors to other genes

C. some other planet or solar system

=panspermia; ie. seeded from space

100’s to 1000’s of meteorites and cometsbrought organic molecules from space whichcould have jump started the process

some suggest bacteria may have traveledfrom other places (Mars meteorite-nowdisproven)

bacteria, esp spores, can survive very harshconditions

eg. some bacteria (Microbispora) that were on the shuttleColumbia, survived the fiery reentry

but: but only traveled about 1/5th the speed of ameteorite

still survived extreme heat and impact

that was 6 times faster than anything tested before


just defers the problem

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