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4. Origin and development of life on Earth

Some organic molecules arrived on Earth from space.How well could they also have formed on Earth itself?

Synthesis of Organic Molecules on Earth

Energy supply: mainly the Sun, but also locally from lightning, radio active decay, volcanoes (primordial heat), shock waves (entry of meteorites)

Note that the rate of meteor impacts 4 Gyr ago was much higher.This provide organic molecules (which may vaporize on impact), but also energy. This could result in further complex organic molecules

Miller’s origin of life experiment (1950s)

• Atmosphere of methane, ammonia, and H2• Energy from sparks (lightning)• Boiling water as an ocean

Experiment produced amino acids easily

[Current knowledge of Earth’s early atmosphere (CO2 + N2) suggests it

is much more difficult.]

Hints from chirality of amino acids

Some molecules can exist in right-handedand left-handed forms. This is called chirality

• 19 of the 20 amino acids can exist in left- and right-handed forms• Chemical reactions that make amino acids generally create equal numbers of R and L handed isomers• Mixing R and L isomers would hinder proteins from performing their biological functions• Life on Earth only uses left-handed amino acids• Early in its history, life must have begun with L-amino acids, locking • biology in its preference• Was this random? Maybe not! Meteorites have an excess of L-amino acids.Prebiotic material may have come from meteores• UV circular polarized light seen in star forming regions destroys one type of isomer easier than the other, which could have caused the bias

4.2 Making more complex molecules

The rate of chemical reactions increases with theconcentration of the reactants

How can organic matter be concentrated?

• Lagoons and tidal pools, with solutions evaporating out

• Freezing of the solution (water freezes first)

• Mineral surfaces can trap organic matter. Clays can incorporate molecules in their structure

Creating polymers and macromolecules

Important chemical reactions see section 2.1

The high level of order and complexity of macromoleculesIn living organisms need more sophisticated methods,

making use of catalyzers (enzymes)

4.3 Boundary layers and cell structure

At some point, the complex chemicals need to be kepttogether, for protection and confinement, to stop them

from wondering about.

How have these structures formed?

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Lipid monolayersLipids are amphiphiles, molecules with

Hydrophilics heads and hydrophobictails

• Amphiphiles float on water, heads in water tails in the air, creating a single layer of molecules, a monolayer

• Shake the water, and they form spherical structures, called micells

• The double layer equivalent is called a bilayer vesicle.

• These structures are formed spontaneously, and can trap molecules

Grouping together without cellstructure

When polymers are added to water, theyForm droplets called coacervates.

This has its origin in the polarity of moleculesAnd the formation of H-bonds with water

Sidney Fox experiment: • polymerization of amino acids can occur by

heating them• Dissolving these in water they form small,

double walled spheres• They can absorb more protein material

from the solution

Deamer (1985): amphiphilic molecules are alsopresent in the Murchison meteorite, forming

boundary layers when added to water.

The role of minerals

To reach structure and complexity,minerals could play an important role

Protection: from dispersal and distruction

Support: structure for molecules to accumulate and interact

Selection: crystal phases can select left and/or right handed amino acids.

Catalysis: Nitrogen gas can flow over a metallic surface to form ammonia, becoming a valuable source for biological reactions.

4.5 The first biological systems

• As seen in the previous sections, many possible pieces of the jigsaw puzzle of how to form a living cell are proposed

• No one has yet synthesized a protocell using basic Components

• How a combination of organic molecules can form a Protocell is unknown.

L

I

F E

There are many hypotheses

Most important ones:

• RNA world hypothesis

• Metabolism first hypothesis

The RNA world hypothesis

It proposes that RNA was the first life form on Earth,later developing a cell membrane around itself, andbecoming the first prokaryotic cell

Why? RNA can store, transmit and duplicate information,But it can also act as enzyme, performing the tasks ofboth DNA and proteins.

• The nucleotides in RNA are more easily synthesized than those in DNA.

• One could imagin that DNA evolved from RNA, taking over its role due to its greater stability

• RNA are likely to have evolved before proteins. How could the latter otherwise replicate?

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Evolution in the RNA world• Free floating nucleotides regularly form bonds that break up easily• Certain sequences are more stable - the longer the chain the more it attracts matching nucleotides.• Natural selection makes the more efficient catalyzer and replicator to survive more easily and could evolve into modern RNA• Further competition may have favored cooperation between chains, and giving advantage to those that could serve as ribozymes.• Eventually DNA, lipids, and carbohydrates were recruited to form cells

Retroviruses: RNA life is still present today!

• They consist of fragments of nucleic acid with a protein coating

• In the outside world they do not carry out functions of life

• Within the cell of an organism they take the cell’s energy to make proteins and nucleic acids to copy them selves

• Retroviruses (e.g. HIV) reverse the cellular process by transcribing RNA into the cells DNA, to make more viral RNA

H IV

Some problems with the RNA world theory

1. Large RNA molecules are very fragile they strongly absorb UV radiation

2. Prebiotic simulations show that nucleotides can not be made at the same time as sugars. They must have been synthesized separately and brought together

Metabolism first modelsA cycle of chemical reactions that produce energy that is used subsequently by other processes.

Example: Iron-Sulfur world theory (J. Wachterhauser) Life occurred first on mineral surfaces near black smokers (high T and P)

One black smoker would operate like one big cell.

Primitive metabolic cycles produce more complex compounds (amino acids and peptides)

The temperature and chemical gradient around the smoker wouldplay an important role

How would it evolve into cellular life?

Other hypotheses

Lipid world hypothesis, clay theory, bubble theory, Auto-catalyses theory,

deep hot biosphere model,...

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The top-down approach

Life does not reject what evolution has created,but builds on what has gone before

Try to extrapolate back to the origin of life by using theInformation contained in DNA in organisms

All present-day life is related - last common ancestor seemsto be a heat loving chemosynthetic organism that lived near

hydrothermal vents

Phylogenetictree

Alternative hypothesis: PanspermiaCan life itself survive in space, and could it Havearrived on Earth from a distant planet?

Why not? Large dosis of fatal radiation(?)

Alternatively, panspermia could have come on ameteor.

May not be completely stupid... (see ALH 84001),although some versions of it are

4.7 Diversity of life in extreme environmentsTo know under what extreme range of environments life can thrive on Earth, may help us to understand how widespread

life me be in the universe

Extremophiles: organisms that tolerate or thrive in extreme environments.

• Hyperthermophiles• Psychrophiles• Radiation extremophiles• Vacuum tolerant• Pressure extremophiles• Salinity, pH, chemical extremes, etc...

5. Prerequisites for lifeelsewhere

Life on earth can exist under extreme environments

However, for more complex life (that uses oxygen) todevelop, the conditions are more narrow.

It would require an ocean, some dry land, O2, low CO2To allow O3, and a seasonable stable climate.

The circum-stellar habitable zone

Liquid water seems crucial for the development of life on Earth

The circumstellar habitable zone: the range of distances to a star for which liquid water can exist on the planet’s surface. For a planet like Earth this is between 273 and 373 K

Effective Temperature of a planet, Teff: The average T of a planet determined by the balance of incoming solar radiation and the planet’s outgoing thermal emission

Teff = (1-A) Lstar16 π d σ

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Time-dependent habitable zone

The solar luminosity (or that of any star) is not constant in time. 4 Gyr ago the Sun was about 30% weaker thantoday, moving the habitable zone inwards

Continuous habitable zone: The region in which a planet may reside and maintain liquid water throughout most of the star’s lifetime.

H.Z. as function of stellar type

The luminosity of a star is a strong function of its mass.

The greenhouse effect

The surface temperature on a planet with an atmospherecan be significantly higher than Teff

Teff (Earth, Venus) = 255 K, 2?? K. Tsur (Earth, Venus) = 288 K, 733 K.

Atmosphere passes light fromthe Sun, but is optically thickfor reradiated heat from the planet

Acts as a warm blanket

Prominent greenhouse gases areH2O and CO2

Stability of greenhouse effect

• CO2 is a very important factor in the Earth’s greenhouse effect.

• It is removed from the atmosphere through chemical weathering

• It is released in the atmosphere through volcanoes

• Life on Earth also removes CO2 by photosynthesis

• A huge anount of carbon is stored in the Earth. On Venus this balance is almost completely shifted to one direction (96.5% of its atmosphere is CO2

Was the young Earth a habitable planet?

• Oldest rocks on Earth are 4 Gyr old, which have been deposited in water.

• This means that there was liquid water, and that the temperature was about like now

HowHow is that possible - the sun was 25-30% less luminous?is that possible - the sun was 25-30% less luminous?

[Teff =-33 c]

How could it be so warm on the young Earth?

• Plate tectonics was particularly violent on the young Earth. Energy from: 1. Primordial heat 2. Radio-active decay 3. Iron and Nickel sinking to the core

• The enhanced plate tectonics keep the CO2 in the air.

Where does the Earth’s water come from?Comets? (but do have a different isotope ratio)

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Other energy sources

We assumed that the main heat source of a planet is its star

Other potential energy sources:

radio-active decay primordial heat/core formation tidal interactions

This makes other planets outside the habitable zone still interesting for the search for life, such as The moons of Jupiter and Saturn