Visions of the Cosmos

Life Among the Stars

The Nature of Life

 

Like a blue jewel in the depths of space, the Earth spins on its axis as it orbits round the fireball of its yellow star.   It is indeed a very special planet, an oasis in the cosmos, teeming with superabundant life.  Even if primitive life exists beneath the ice-cap of Europa or lies hidden under the permafrost of the cold and arid deserts of Mars, it is only on Earth that highly evolved organisms are to be found in our Solar System. To discover another world similar to Earth we should have to travel across the vast abyss of interstellar space to an extra-solar planet beyond the Empire of the Sun.  

 

Although we only know about life on our own planet, we can make intelligent hypotheses about the conditions necessary for life to arise and of what it may be like elsewhere. We shall assume for the purposes of this web-site that many, if not all forms of life will be governed by a similar biochemistry to our own.  With this in mind it is proposed to outline the principles upon which life on Earth is based and then to move on and discuss possible variations that may exist on planets belonging to other star systems.

 

Life on Earth

 

The "Black Smokers"

In 1977, an American deep-sea research vessel was investigating underwater volcanoes near the Galapagos Islands. They found that three kilometres beneath the surface of the ocean hot water was spouting from volcanic vents on the ocean floor.  Under high temperatures and pressures a cocktail of chemicals was liberated that would normally be lethal to most known forms of life.  Here at temperatures far higher than most enzymes could operate and in a brew of deadly poisons, such as hydrogen sulphide, a whole new ecosystem was discovered.  As time went by other regions of the ocean floor were found where similar conditions existed.  These undersea volcanoes are called "black smokers" and their discovery has led to a revolution in the science of biogenesis.  Great concentrations of ‘bacteria’ were discovered in the vicinity of the volcanic vents. They depend on chemical reactions taking place around the vent - such organisms are called CHEMOTROPHIC.  In the stygian darkness of the abyssal depths, where sunlight never penetrates, strange unknown fish and blind white crabs were clustered around the worms and clams.  They depend for their food supply upon the bacteria and archae that are at the bottom of the food chain.  The theory has been advanced that it was in such hydrothermal vent environments that life began rather than in the shallow waters of some ancient shoreline when the world was young.  

Recent studies of genetic sequences of living organisms suggests that the most probable ancestors of living organisms were in fact sulphur-loving microbes that lived at high temperatures.  It has long been known from the study of organisms living in hot-springs that there are life forms capable of thriving at much higher temperatures than had previously been believed possible.  They are called THERMOPHILIC ORGANISMS.  Organisms living on the ocean floor near the vents do not rely on photosynthesis but derive their energy from chemical reactions, which take place between sulphur compounds.  Much information can be from NOAA.

 

The Archaea

The Domain Archaea wasn't recognized as a major domain of life until quite recently. Until the 20th century, most biologists considered all living things to be classifiable as either a plant or an animal. By the 1970s, a system of Five Kingdoms had come to be accepted as the model by which all living things could be classified. At a more fundamental level, a distinction was made between the prokaryotic bacteria and the four eukaryotic kingdoms The distinction recognizes the common traits that eukaryotic organisms share, such as nuclei, cytoskeletons, and internal membranes.

In the 1970s Dr. Carl Woese and his colleagues at the University of Illinois were studying relationships among the prokaryotes using DNA sequences, and found that there were two distinctly different groups. Those "bacteria" that lived at high temperatures or produced methane clustered together as a group well away from the usual bacteria and the eukaryotes. Because of this vast difference in genetic makeup, Woese proposed that life be divided into three domains: Eukaryota, Eubacteria, and Archaebacteria. He decided that the term Archaebacteria was a misnomer, and shortened it to Archaea.

Most Archaea don't look that different from bacteria under the microscope, and that the extreme conditions under which many species live has made them difficult to culture, so their unique place among living organisms long went unrecognized. However, biochemically and genetically, they are as different from bacteria as human beings are!

Illustration Family Tree of Life on Earth.

Many thanks are due to Dr Bharat Patel for permission to reproduce the above diagram Website http://trishul.sci.gu.edu.au/~bharat/courses/ss13bmm/archaea... Dr Bharat Patel, Associate Professor Microbial Technology and Director Clinical Microbiology Programme, Faculty of Science and Technology. Griffith University (Nathan Campus), Brisbane, Australia 4111

 

Work was also carried out on the sequencing of a certain type of RNA molecule (16S RNA molecules) that supported the classification of life into the three domains.  The new taxonomy includes a new form of classification involving DOMAINS

 

Eukarya     Domain – four kingdoms  -  Protista   Fungi  Plants  Animals

Prokaryota Domain –one kingdom Bacteria

Archaea     Domain- probably three kingdoms  Many of them are extremophiles : that is to say they live and thrive under conditions that are very extreme

     Thermophiles can live at 85-115 degrees Celsius in superheated water under pressure.

Cryophiles can live at minus 5 degrees in freezing water

Halophiles can live in water containing very high concentrations of salts

Acidophiles can tolerate very high acids (low pH)

Alkalinophiles can tolerate very high alkaline conditions (low pH)

Most archae and some bacteria derive their energy from chemical reactions in environments that would be very highly toxic to oxygen 'breathing' organisms.  Substances such as cyanides and hydrogen sulphide are common in such environments.  They are said to be Anaerobic.  Free oxygen is a deadly poison to such organisms.  Organisms that depend on Oxygen are said to be Aerobic.

It is now believed by many scientists that the archaea are the common ancestors to all forms of life on the planet and originated at high temperatures and pressures in volcanic vents.

If this is how life started on our own planet, it is possible that similar processes took place on Mars and Europa.   Deep beneath the permafrost of the cold Martian surface there may yet be places where chemotrophic organisms still exists.  The exploration of the red planet has only just started but as detailed surveys are carried out in the next few decades Mars may yet yield up fascinating secrets to future explorers.  Much interest too is focussed on Europa and plans are already being made to investigate the possible existence of present day life in an ocean beneath the ice-cap.

In searching for life on Mars or Europa there are two precautions that must be vigorously observed.  Both concern cross-contamination.  We must be certain that any forms of life found on either of those two worlds are not contaminated by something that has come on a spacecraft from Earth.  Even more important any samples brought back from other planets to Earth must not be allowed to contaminate our environment with an alien life form.

 

“The Oxygen Revolution”

            Near the southwest coast of Australia a biologist by the name of Linda Moore is investigating a group of primitive photosynthetic organisms known as CYANOBACTERIA (formerly called blue-green algae).  They grow on rocks and the rock/bacterial formations are called STROMATOLITES.

            Working in another part of Australia, J. William Schopf discovered some 3.46 billion year old fossils in a rock called the Apex rock.  He claims that they are very similar to the present day organisms studied by Linda Moore.  He believes that in those far off times stromatolites were clustered round the coasts of volcanic islands.  The Earth was a very different place then to what it now is.    A pale yellow Sun noticeably dimmer than it is at present raced rather quickly across the heavens and the day lasted less than 18 hours.  At night a huge brilliant Moon far closer to the Earth than it is now hung in a sky filled with a totally different pattern of star constellations.  There was no oxygen in the atmosphere but the blue-green organisms that covered the rocks were just starting to grow and were about to alter all that.  Very slowly but inexorably the cyanobacteria brought about a momentous change in the planet’s environment.  Sometimes it is referred to as THE OXYGEN REVOLUTION.

            The cyanobacteria had found a way of utilising the light of the Sun as a source of energy.  Photosynthesis had begun.  Molecular oxygen was produced as a bye-product.  For any ANAEROBIC organisms the atmosphere and the sea-water became contaminated with a very toxic gas.  Most of the organisms died except for the cyanobacteria and a few others that had found ways of defending themselves against the toxic effects of oxygen.  These oxygen-resistant bacteria (AEROBIC organisms) adapted and actually began to use the ‘poison’ to their enormous advantage.  They were able to extract about sixteen times as much energy from the ‘food’ as the anaerobes.   It appears to have taken a very long time for oxygen to accumulate to high levels in the atmosphere so it was a very slow revolution.   The oxygen initially formed reacted with reducing substances in the environment, the most important of which was iron in its divalent ferrous (Fe++) form.    Sea-water used to contain a lot of ferrous iron in solution.  Evidence for its reaction with oxygen is seen in the banded rocks of alternating green ferrous and yellow brown ferric (Fe++) iron found in strata laid down during the early years of the oxygen revolution.

 

Stromatolite Rock Formations

 

 

In the hyper-saline water of Hamelin Pool at the base of Shark Bay in Western Australia the rocks aren’t quite what they appear to be. They are living things, Stromatolites, which are a very ancient form of life on the planet. Stromatolites are the result of primitive life forms that first existed on Earth 3.5 billion years ago. The dome shaped structures reach up to 60cm in height and are formed by cyanobacteria.
Hamelin Pool is the location of the best example in the world of living marine stromatolites. The water of Hamelin Bay is twice as saline as usual sea water because of a bar across the Bay's entrance and rapid evaporation from the shallow water. Most living animals, which feed on the bacteria and algae of which stromatolites are composed, cannot tolerate such saline conditions. As a result stromatolites can grow here successfully, undisturbed. Most stromatolites are extremely slow growing. Those in Hamelin Pool grow at a maximum of .3mm a year, so those which are up to a metre high are hundreds of years old.
Acknowledgement of Picture ‘Discover West Holidays – Western Australia Holiday Planning’

 

 

For at least three-quarters of the Earth's history stromatolites were the main reef building organisms However their most important role in Earth’s history has been contributing oxygen to the earth's atmosphere. The organisms which construct stromatolites are photosynthetic. They take carbon dioxide and water to produce carbohydrates, and in doing this they liberate oxygen.  When stromatolites first appeared on earth about 3.5 billion years ago there was little or no oxygen in the atmosphere. It was through the oxygen-generating activity of stromatolites that animal life on earth was able to develop.
There is evidence that Stromatolite fossils were found in very early rocks. Western Australia perhaps has the best stromatolite fossils, giving a record through the eons of time.  The present day stromatolites at Hamelin Pool gives an indication of what the earth may have looked like 3.5 billion years ago when stromatolites were widespread. Because of their range and numbers it is a place of great interest to botanists and geologists alike.
It's a humbling thought that the great change in life, which is believed to have started with oxygen hating anaerobic archaea and bacteria was probably brought about by the stromatolites

A wide range of stromatolite fossils can be seen in the Western Australian Museum at Perth.

 

 

The Atoms of Life

Twenty-four chemical elements are known to play a role in life processes on our planet.    Six of them predominate and help to build the very large molecules, which make up living matter.  They are CARBON, HYDROGEN, OXYGEN, NITROGEN, SULPHUR and PHOSPHORUS.   Five other elements occur in significant quantities. They are the metals SODIUM, POTASSIUM, MAGNESIUM and CALCIUM  largely in the form of their cations and the non-metal CHLORINE in the form of the chloride anion.

            Although only present in tiny amounts, trace elements are also essential for most living organisms.  For example, IRON is needed for blood haemoglobin and a number of important enzymes. Other trace elements are VANADIUM, CHROMIUM, COBALT, NICKEL, COPPER, ZINC, MOLYBDENUM, BORON, FLUORINE, SILICON, SELENIUM and IODINE

 

The Molecules of Life

There is a vast repertoire of carbon compounds that play a role in living processes.  Despite their vast diversity, all the living organisms on Earth use similar types of molecules.    They use small molecules such as sugars, fatty acids, amino acids and nitrogen bases.  These small molecules join together to form very large assemblages of atoms called polymers or MACROMOLECULES (very large molecules).   All forms of life on Earth depend upon a few basic types of macromolecules of which the most important are the nucleic acids and the proteins.

 

The Genetic Code.

The Nucleic Acids

The nucleic acids carry the genetic code.  There are two main types, RNA and DNA.  Many scientists consider that life began using RNA as the primary carrier of the code. A few viruses called retroviruses use RNA as the genetic material but DNA carries the genetic messages for all cellular life forms. 

DNA is an extremely stable molecule and it is used to pass on the genetic messages from one generation to the next. 

DNA also plays its part in the moment-to-moment work of all living cells.  To ensure that the correct proteins are produced, molecules of messenger-RNA are synthesized in the cell nucleus on the surface of the DNA.

          DNA is always double stranded - hence the name of the famous book by Watson 'The  Double Helix'.  

 

 

Because DNA is an extremely stable molecule, the same messages can be passed from one generation to the next.   'Mistakes' (mutations) are rare but very occasionally occur a base change or a deletion can occur which leads to a mutation. 

DNA is used to pass the genetic message on to future generations of living cells.  Before a cell can divide to produce a daughter cell, the DNA molecules in the nucleus of the cell have to produce a full complement of daughter molecules.  When a DNA molecule reproduces a copy of itself, the double helix is progressively zipped open and the nucleotide sub-units are connected in the same order as the mother molecule using as the catalyst a system of enzymes called collectively DNA polymerase.

Besides its role in passing on the genetic message from one generation to the next, DNA also plays its part in the moment-to-moment work of all living cells.  To ensure that the correct proteins are produced, molecules of messenger-RNA are synthesized in the cell nucleus on the surface of the DNA.

Messenger RNA molecules are single stranded.  In both forms of nucleic the main strand is composed of a helix of a repeating sugar unit (Ribose or deoxyribose) joined by a phosphate group.  Each sugar unit is connected to a nitrogen base.   There are four mononucleotide nitrogen bases and it is these bases that carry the genetic code. 

 

Nitrogenous Base Mononucleotide Full Abbreviation Short Abbreviation Found in DNA Found in RNA
Adenine Adenosine monophosphate AMP A Found in both Found in both
Cytosine Cytosine monophosphate CMP C Found in both Found in both
Guanine Guanine monophosphate GMP G Found in both Found in both
Thymine Thymine monophosphate TMP T Found in DNA only  
Uracil Uracil monophosphate UMP U   Found in RNA only

 

DNA is an extremely stable molecule and it is used to pass on the genetic messages from one generation to the next. 

DNA also plays its part in the moment-to-moment work of all living cells.  To ensure that the correct proteins are produced, molecules of messenger-RNA are synthesized in the cell nucleus on the surface of the DNA.   The messenger-RNA then carries the code out of the cell nucleus to a RIBOSOME.  The word ribosome is the name given to a cellular ‘factory’ where proteins are made.  At the ribosomes small amino acid molecules are fused together to an exactly genetically determined recipe to produce specific proteins.  A randomly produced protein would be of no biological use whatsoever.

Most energy reactions in living organisms use a biochemical system involving Adenosine triphosphate and adenosine diphosphate (abbreviated to the ATP/ADP system).   It should also be noticed that it is one of the important sub units in DNA and RNA.

Their are 4 different LETTERS in the DNA code.  Three letters are required to code for a protein.  Thus their are 64 different DNA words.   The DNA molecules do not code for proteins DIRECTLY but pass on their messages in the nucleus of the cell to messenger RNA molecules.  This is done by a process analogous to the production of a positive picture from a negative in ordinary photography.  It is this positive message that is transported across the membrane of the nucleus to the protein factories of RIBOSOMES.  Here by a series of chemical reactions the messenger RNA passes on the genetic code to another type of RNA called ribosomal RNA.  This is a long protein molecule is formed from amino acids in the correct genetically determined sequence.   The protein takes up the correct shape required and is then ready to carry out its function with in the organism.

The Diagram  lists the 64 code words used by messenger RNA and the names of the amino acids for which each word codes.   The technical term for the code word is CODON.

Table : RNA codon table

This table shows the 64 codons and the amino acid each codon codes for.
  2nd base
U C A G
1st
base
U UUU (Phe/F)Phenylalanine
UUC (Phe/F)Phenylalanine
UUA (Leu/L) Leucine
UUG (Leu/L)Leucine
 
UCU (Ser/S)Serine
UCC (Ser/S)Serine
UCA (Ser/S)Serine
UCG (Ser/S)Serine
 
UAU (Tyr/Y)Tyrosine
UAC (Tyr/Y)Tyrosine
UAA Ochre (Stop)
UAG Amber (Stop)
 
UGU (Cys/C)Cysteine
UGC (Cys/C)Cysteine
UGA Opal (Stop)
UGG (Trp/W)Tryptophan
 
C CUU (Leu/L)Leucine
CUC (Leu/L)Leucine
CUA (Leu/L)Leucine
CUG (Leu/L)Leucine
 
CCU (Pro/P)Proline
CCC (Pro/P)Proline
CCA (Pro/P)Proline
CCG (Pro/P)Proline
 
CAU (His/H)Histidine
CAC (His/H)Histidine
CAA (Gln/Q)Glutamine
CAG (Gln/Q)Glutamine
 
CGU (Arg/R)Arginine
CGC (Arg/R)Arginine
CGA (Arg/R)Arginine
CGG (Arg/R)Arginine
 
A AUU (Ile/I)Isoleucine
AUC (Ile/I)Isoleucine
AUA (Ile/I)Isoleucine
AUG (Met/M)Methionine Start
 
ACU (Thr/T)Threonine
ACC (Thr/T)Threonine
ACA (Thr/T)Threonine
ACG (Thr/T)Threonine
 
AAU (Asn/N)Asparagine
AAC (Asn/N)Asparagine
AAA (Lys/K)Lysine
AAG (Lys/K)Lysine
 
AGU (Ser/S)Serine
AGC (Ser/S)Serine
AGA (Arg/R)Arginine
AGG (Arg/R)Arginine
 
G GUU (Val/V)Valine
GUC (Val/V)Valine
GUA (Val/V)Valine
GUG (Val/V)Valine
 
GCU (Ala/A)Alanine
GCC (Ala/A)Alanine
GCA (Ala/A)Alanine
GCG (Ala/A)Alanine
 
GAU (Asp/D)Aspartic acid
GAC (Asp/D)Aspartic acid
GAA (Glu/E)Glutamic acid
GAG (Glu/E)Glutamic acid
 
GGU (Gly/G)Glycine
GGC (Gly/G)Glycine
GGA (Gly/G)Glycine
GGG (Gly/G)Glycine
 

 

The second table shows the amino acids and the codons which code for each amino acid in the table

Table: Reverse codon table

This table shows the 20 standard amino acids used in proteins, and the codons that code for each amino acid.

Alanine GCU, GCC, GCA, GCG Leucine UUA,UUG,CUU,CUC,CUA,CUG
Arginine CGU, CGC, CGA, CGG, AGA, AGG Lysine AAA, AAG
Aspartic acid AAU, AAC Methionione AUG
Asparagine GAU, GAC Phenylalanine UUU, UUC
Cystein UGU, UGC Proline CCU, CCC, CCA, CCG
Glutamic acid CAA, CAG Serine UCU, UCC, UCA, UCG, AGU, AGC
Glutamine GAA, GAG Threonine ACU, ACC, ACA, ACG
Glycine GGU, GGC,GGA,GGG Trptophane UGG
Histidine CAU, CAC Tyrosine UAU, UAC
Isoleucine AUU, AUC, AUA Valine GUU, GUC, GUA, GUG
Start AUG Stop UAG, UGA, UAA

 

Many codons are redundant, meaning that two or more codons can code for the same amino acid. e.g., both GAA and GAG code for the amino acid GLUTAMINE

This is analogous to the letters C and K.  C is a redundant letter and could always be replaced by K.  Thus there is no reason why the word CAT should not be spelt KAT Degenerate codons may differ in their third positions.

There a few variations that do exist - a few 'dialects' in a few organisms.  However, despite the variations that do exist, the genetic codes used by all known forms of life on Earth are very similar. Since there are many possible genetic codes that are thought to have similar utility to the one used by Earth life, the theory of evolution suggests that the genetic code was established very early in the history of life.

The RNA Hypothesis

It is believed that before the coming of cellular life there was a gradual molecular evolution.  The build up of complex chemicals was most likely catalysed by clays containing iron and smaller amounts of other metals.  A strongly held opinion, known as the RNA Hypothesis suggests that simple molecules containing carbon, hydrogen, oxygen, nitrogen and phosphorus reacted at the catalytic surfaces of minerals, possibly clays, to form a group of large complex molecules called RIBONUCLEIC ACIDS (RNA).  For this purpose energy was required and this was obtained from chemical reactions which took place in the environment.  One of the most important energy donating chemicals even at this early stage of evolution was probably ADENOSINE TRIPHOSPHATE or ATP.  According to the theory these early forms of RNA acquired the capacity to catalyse the production of molecules identical to themselves without the need of inorganic substances such as the clays.  The first stage of biochemical life began by the production of ‘daughter molecules’ identical to the ‘mother molecules’.  Occasionally ‘mistakes’ were made in the copying mechanism giving rise to ‘mutant molecules’.  Eventually there were a large number of different types of RNA competing with one another in the environment. Chemical evolution had begun.  The RNA molecules were both the first biochemical catalysts (enzymes) and the first carriers of genetic messages.

 RNA molecules consist of long strands of alternating units of a sugar called RIBOSE and of PHOSPHATE IONS.  These form the spine of the RNA molecule.  Each of the sugar sub-units carries a nitrogen base.  Only four bases are found in naturally occurring RNA.  For simplicity these bases are denoted by letters A, G, C and U where A=ADENINE, G=GUANINE, C=CYTOSINE and U=URACIL.  The alternating phosphate and ribose sub-units form the spine of a rod-like spiral molecule, which carries an extremely long sequence of the nitrogen bases.  RNA molecules only differ from one another in the length and the sequence of their nitrogen bases.

Further stages in this pre-biotic chemical evolution eventually resulted in the synthesis of proteins and DNA.  The proteins were more efficient catalysts than RNA and took over the enzyme functions.  Another change that occurred was that DNA took over the genetic role.  DNA differs from RNA in that it uses DEOXYRIBOSE instead of ribose And THYMINE (T) instead of uracil as shown in the above table.

An even more recent theory suggests that the first self-replicating molecules did not belong to the RNA group but to a group of related chemicals called PEPTIDE NUCLEIC ACIDS or PNA.  These are a kind of hybrid group of molecules in between RNA and proteins in which the sugar ribose is replaced by peptide links attached to the A, G, C and U bases.  Unlike RNA these compounds are stable at 100oC.  It is likely that the early Earth was very hot.  Also black smokers are surrounded by extremely hot water..

It is supposed that at some stage the RNA, DNA and proteins were encapsulated in protected membranes and simple cellular life began.  Early theories about the origins of life suggested that it began in shallow sunlit pools.  This theory has however been very strongly challenged, since without an ozone layer early life forms would have been constantly destroyed by the ultra-violet radiation from the Sun.

Proteins

Proteins are composed of amino acids linked together to form long chains of atoms. 

 They play a vital role in the functioning of biological systems from the simplest viruses and bacteria to the most complex plants and animals.

Two amino acids can be joined together by what chemists call a peptide link to form a dipeptide molecule and a water molecule.  Several peptides can be united to give a polypeptide.  A water molecule is split out when each peptide link is formed.  
Proteins are formed by the union of a large number of amino acids.  As previously mentioned to be of ‘biological use’, proteins are ‘manufactured ‘ according to exact genetic recipes carried by the messenger-RNA molecules.
 

Proteins are composed of amino acids linked together to form long chains of atoms. 

 They play a vital role in the functioning of biological systems from the simplest viruses and bacteria to the most complex plants and animals.

 Amino acids possess an amino group, a carboxylic acid group, a hydrogen atom and a fourth group, which varies from one amino acid to another.   They are all attached to the same carbon atom. Where they differ from one another is in the nature of the group of atoms, which occupy the fourth valency position on the carbon atom.  

            

The possible number of amino acids must run into millions but only a few of them are of biological interest.  Only twenty of them are usually found in proteins.
They are listed below:-
 

Alanine      Leucine  Isoleucine

Valine    Proline Phenylalanine
Tryptophan        Methionine   Glycin
Serine                                          Threonine Tyrosine
Cysteine                                         Glutamine                                            Arginine
Aspartic acid      Glutamic acid   
Lysine    Histidine
Asparagine

 

A few more do occur rarely in proteins but these will not be discussed here.
 
The number of proteins produced by the differing arrangements of these amino acids is enormous.

 

The general structure of all amino acids found in proteins can be represented by the formula

            H              

  R -C -COOH      where R denotes the relevant group of atoms

                     NH2

 

One of the simplest amino acids is ALANINE which has the formula

                                                                                                       

                                             H

 H3 C  -  C  -  COOH            R is a -CH3 group.  It is called a methyl group.            

                                    NH2

 

When a carbon atom is surrounded by four different atoms or groups of atoms it can exist is two different arrangements in space.  The two types are called optical isomers of one another or two handed (chiral) varieties. All the amino acids occurring in proteins with the sole exception of glycine can exist in the two forms.
The two forms of the amino acid alanine are shown below
.     However only one form is found in proteins  See web-page on van't Hoff for diagram of two forms of alanine

Although, only L- amino acids such as L-alanine are used to build up into proteins on our planet, there seems to be no apparent reason why one form should be preferred over the other.   There might well be biochemical systems on other planets where only the D-forms of amino acids are used in the synthesis of proteins.  The fate of a hypothetical Earthman or Earthwoman, after travelling through a wormhole to such a mirror world, would indeed be a sad one.  However good the food seemed to be they would soon die of starvation since the mirror image 'food' would be of no use whatsoever!

 
Also alien proteins may well have a different mix of amino acids to those on Earth

 

Carbon is the fourth most abundant atom in the cosmos and is a substance of enormous versatility.   The process of the building up of molecules containing long chains and rings of carbon atoms together with hydrogen, oxygen and nitrogen starts early, even at the freezing temperatures of the interstellar dust clouds.  Some prebiotic molecules such as carbon monoxide, hydrogen cyanide, ethyl alcohol, amines and amino acids have been detected in the Orion Nebula and similar nebulae where planetary systems appear to be forming. 

Ever since the Renaissance in Europe many members of the scientific community have suspected that apart from our Sun, some other stars may be accompanied by planets. As early as 1698 Christiaan Huygens attested his belief in the existence of other worlds inhabited by plants and animals in his book 'The Celestial Worlds'.   In 1600 ' Giordanno Bruno was burnt at the stake as a heretic.  One of his heresies was to postulate that the stars were suns and some of them were accompanied by planets on which there were people living.  

Already there is evidence that nearby stars are accompanied by large planets as massive as or more massive than Jupiter.  These large planets have been detected by the wobble they produce in the star during the course of their orbit.  It requires the use of extraordinarily sensitive measurements by what are called ASTROMETRIC techniques.  They are called Extra-solar Planets and are dealt with in another section of this web site.  It is believed that there are many smaller terrestrial type planets and some of these may be abodes of life.  The Darwin project and other future advances within the next decade are planned and it is possible that spectroscopic evidence of ozone, carbon dioxide, methane and water may be found on some of these other worlds.

In considering the possibility of life on planets of other star systems it is necessary to consider both the planet and its star. Mathematical studies on planetary evolution and the habitable zones around stars suggest that advanced life may be restricted to planets similar to the Earth orbiting stars similar to the Sun.

 
The Star

It is generally considered that life-sustaining stars will lie in a range between 80% and 120% of a solar mass.   It has been suggested that planets orbiting very close to small red dwarf stars could play host to life (see New Scientist 27 January 2001 pages 28-31).

However there would seem to be a number of limitations.

1)      If the star is variable and subject to violent flares this may make evolution very difficult.  Recently compiled evidence suggests that some stars of a similar size and composition to our Sun are subject to erratic and violent flare activity.  This may cut down considerably the number of suitable planets where life could comfortably evolve.

2)      If the stellar system is very deficient in higher elements large terrestrial planets may be unlikely to form (by higher elements we mean elements above helium in the periodic table such as oxygen, carbon, nitrogen etc which astronomers persist in calling ‘metals’).

 3)     If the stellar system is too near the galactic centre conditions may be too violent and variable for life to develop.

 
The Planet

1)  any planet suitable for the evolution of advanced life forms would have to have a mass high enough to retain an atmosphere.  This depends upon the gravitational force at the surface.

2) It would need to have an adequate water supply and be within the right temperature range.  It is widely and almost universally believed that biochemical life requires water in the liquid phase.  The only other candidate is liquid ammonia NH3.   This will not be discussed here and for a number of reasons it is most improbable that worlds exist where there are large amounts of ammonia and very little water.

3) a planet must orbit its Sun in a relatively narrow 'continuously habitable life zone'.  Had the Earth been only a few percent closer to the Sun it may have become a furnace like Venus early in its history.  If it had been a few percent further out like Mars, it may have become frozen in a permanent ice age.

Thus life may be restricted to fairly large sized terrestrial planets orbiting stable stars in ‘Goldilocks Orbits’.    Although this cuts down the number of places where life may evolve there are still a very large number of possible star systems where life as we know it may exist.  Also there may be a number of planets such as Europa where large quantities of liquid water may be present but where the energy to keep the water in the liquid phase is derived from tidal or other forces.

4) There is another problem that may arise in a planet where the angle of inclination of the axis to the ecliptic plane is rapidly undergoing wild variations.  Such a situation would cause devastating changes in the weather conditions and may make it more difficult for life (or at any rate highly evolved life) to develop.   In this connection the possession of a large moon may be of great help in maintaining fairly stable conditions.  If a terrestrial planet in a suitable orbit (‘Goldilocks orbit”) also needs a large moon then this would cut down considerably the number of planets where advanced life would be likely to develop.

It seems likely that simple bacterial-type life may be relatively common in other ‘solar’ systems but highly evolved life such as animals and plants may be less common.  Nevertheless one day perhaps we shall have some sort of contact with other intelligent beings who share with us the stars.  It is interesting to speculate what form the chemistry of life would be likely to take on other planets

Alien Proteins

If a similar situation exists on another planet like our own we may expect a variation on the theme.  For example, there could be a  different repertoire of amino acids making up the proteins.  

Mirror Image Molecules

 One of the mysteries of molecular biology is that there is a bias in the shape of the molecules that make up living matter. In the second half of the nineteenth century Louis Pasteur and Johannes Siliceous, discovered that lactic acid, a simple molecule that is found in living organisms, existed in two optically active forms.   It soon became apparent to them and other workers that most other compounds that were involved in biochemical processes were similar to the lactic acids.  In most respects, the substances were chemically similar.  It was however found that in general only one of them was biologically active. It took the scientific world many years to discover the true reasons for the phenomenon, which is called optical isomerism or chirality.   In 1875, two young chemists, van't Hoff and Le Bel independently offered a satisfactory explanation for the existence of the pairs of optical isomers.

They realised that in order to fully show the way in which carbon atoms bond to other atoms it is necessary to draw a diagram showing the orientation of the atoms in three dimensional space.

They discovered the simple fact that if a carbon atom is connected by four single covalent bonds to four different atoms or groups of atoms A, B, C, and D then two different arrangements in space are possible.  This results in the existence of two forms of the molecule which are called optical isomers, which are mirror images of one another.

In particular van't Hoff postulated that the four valencies of a carbon atom are directed towards the corners of a tetrahedron with the carbon atom at its centre.   Geometrical considerations demand that compounds in which a central carbon atom is bonded to four groups, all of which are different to each other, must exist in two forms.  These are known as the D-and L-forms or right and left handed forms.  Such compounds are called CHIRAL compounds from the Greek word meaning hand.  The molecules of the two chiral forms are mirror images of one another and the existence of chiral molecules is of great importance in biochemistry. 

 With the exception of glycine all the amino acids found in proteins can exist in two optically active isomeric forms.   They are called the D- and the L- forms. 

Many scientists now believe that life on Earth may have evolved from pre-biotic molecules originally formed in the gas clouds and incorporate into comets and meteorites. The presence of amino acids has now been found in the Murchison meteorite. A sample of the amino acid alanine from the meteorite showed a higher 13C/14C ratio than is normally found on Earth thus indicating its extra-terrestrial origin.  This proves that amino acids can originate outside the Earth.  Although both isomers are present, the 'left-handed' variety predominated just as it does in earthly organisms.

Recent work on the Orion Nebula has detected the presence of circularly polarised ultra violet radiation.  In some regions of the gas cloud it is polarised clockwise and in others anti-clockwise.

Chemical experiments have shown that the decomposition of a 50/50 mixture of an optically active substance by circularly polarised ultraviolet light is affected by the direction of the polarisation.   The rate of decomposition of the two forms vary, according to the direction of the polarisation.   This would suggest that in some regions the concentration of the L-form would predominate whilst in other regions the D-form would predominate. Although, only L- amino acids such as L-alanine are used to build up into proteins on our planet, there seems to be no apparent reason why one form should be preferred over the other.   There might well be biochemical systems on other planets where only the D-forms of amino acids are used in the synthesis of proteins.  The fate of a hypothetical Earthman or Earthwoman, after travelling through a wormhole to such a mirror world, would indeed be a sad one.  However good the food seemed to be they would soon die of starvation since the mirror image 'food' would be of no use whatsoever!

 

Energy Sources

Assuming the theories of planetary formation to be more or less correct it would seem that all terrestrial planets would have volcanic activity.   Planets with seas or oceans would be likely to have underwater volcanoes.  It would seem highly probable that chemotrophic organisms would form colonies around volcanic vents just as they do on Earth.

 The most obvious source of energy is from starlight.  At some stage in the development of life on Earth, organisms similar to blue green algae developed a way of trapping and utilising sunlight.  On Earth, photosynthesis has developed using a complex enzyme system that includes a group of compounds called chlorophylls to trap the solar energy.  It seems reasonable to assume that similar systems will have evolved on other planets.  It could well be that the choice of light-trapping catalysts may be different to chlorophyll.  It could be that such a compound may not be green in colour. There may indeed be planets where the predominant colour of the plants is blue, yellow, red or orange depending on the colour of the pigment that plays the main role in the photosynthetic processes.

Another point with regard to biochemistry on Earth is that most energy transfer changes in biochemical systems rely on the use of a system in involving the donation of energy from a molecule of adenosine triphosphate (ATP) which is converted to adenosine diphosphate (ADP) when it parts with its energy..  It would be interesting to know if other life forms used a different energy transfer system.   In economic analogy would be that different nations use different currencies - the American Dollar, the Euro, The Pound sterling.

Alien Genetic Codes

All organisms found on Earth appear to use the same genetic code..   One of the most intriguing questions is whether or not organisms, which have evolved on other planets, use the same code, a similar code or something completely different.

Genetic processes are likely to be similar in principle to ours.  There must be large and stable molecules capable of holding a huge biochemical library.  DNA on Earth is a remarkably stable macromolecule capable of holding an enormous amount of information.  Because of its stability it is able to hold and pass on its message to the next generation.

Nevertheless, because it can occasionally become slightly changed mutations can occur.   By natural selection many mutations will be less successful than their the parent organism or, in sexual reproduction, the parent organisms but a few will be more successful so life will not only change but become more and more complicated and versatile as time goes by.  It is possible that there are a number of possible macromolecules similar to RNA and DNA.  They may well have different sub-units and different sugars.  It would be interesting to conjecture on the nature of the messages encapsulated in the genetic molecules of an alien life form. By a process of natural-selection, a similar type of coding to our own may well develop elsewhere, although different sub-units may be used in the actual DNA-like molecules. If panspermia turns out to be true after all then the genetic code could even be similar on other inhabited planets.

 

Panspermia

Following the discovery of organic molecules in space, particularly ones such as formaldehyde and the amino acid glycine, the old theory of Panspermia has been revived.  At first sight, it seems an extremely improbable set of ideas.    It was first put forward in a coherent form by the Swedish chemist Svandte Arrhenius.  He postulated that primitive life forms were present in interstellar space and were carried on comets and meteorites.  When these bodies collided with a planet whose environment was suitable they seeded it with life which then went on to evolve and produce the higher organisms.  If such a theory were correct, he argued organisms would have to be able to survive great cold.  To support his ideas Arrhenius quoted in his book 'Worlds in the Making', published in 1908, the results of experiments carried out with bacteria at the Jenner Institute in London.  Bacterial spores were subjected to a temperature of -252 degrees Celsius using liquid hydrogen for 20 hours.  They still maintained the power to germinate.

Evidence as to the ability of bacteria to survive extremely harsh environments was discovered after the Moon landing by one of the early robot space vehicles.  In April 1967 Surveyor 3 landed on the Moon. Over two years later in November 1969, members of the Apollo 12 crew retrieved it.  On return to Earth the TV camera was examined in quarantine and found to contain living bacteria of the species Streptococcus mitis.  The bacteria seemed to have survived two years of exposure to the harsh lunar conditions at almost zero pressure and at temperatures which varied between plus 100 and minus 150 degrees Celsius. 

In the 1960s by Hoyle and Wickramasinghe again began to support the 'heretical idea' of 'Panspermia'.  They argued that bacteria and viruses could probably survive the cold of interstellar space in the vast molecular gas and dust clouds that gave rise to the birth of stars. Opponents of the theory argued that, even if bacteria survived in interplanetary space near a star, they would burn up on entering the atmosphere of a suitable planet.  However, experiments showed that bacterial spores could survive temperatures of 700 degrees Celsius.  Very small particles the size of bacterial cells would only be heated to temperatures of around 500 degrees Celsius on entry into the Earth's atmosphere. Also this temperature would only be maintained for a very short time.  Hoyle and Wickramasinghe went even further in their book 'Our place in the Cosmos' and suggested that bacteria and viruses are still reaching the Earth from outer space.

            Whether panspermia is true or not, it would seem that organic molecules, such as amino acids, do occur in space and have reached the Earth on comets and meteorite.  They must still be doing so to a much lesser extent.

 

The Search for Civilisation

            There may be many planets on which primitive life exists.  On the other hand, to provide a stable habitat for highly advanced forms of life more stringent requirements may be necessary.  Planets on which advanced technological civilisations similar to ours have formed might be relatively rare.

Whether it may ever be possible to visit planets orbiting other stars depends upon one important consideration.  That is whether the laws of physics permit us to do so.  If the speed of light is a totally limiting factor and there is no other way of overcoming the gigantic distances between the stars then, however brilliant our technology becomes, we will never be able to visit civilisations existing in other star systems. 

On the other hand if the universe is stranger than we have hitherto imagined, then future generations may indeed discover a means of travelling in some way, as yet unenvisaged at 'warp speed' through wormholes in space-time' to planets orbiting other suns.

Even if interstellar travel proves impossible, we may be able to communicate with relatively nearby civilisations by radio and even television. Judging by the number of sun-like stars there is a strong possibility that there are planets with advanced civilisations near enough for this to be possible.  If this ever comes about it will certainly answer many of our questions about the nature of life.  It will indeed alter our whole philosophy even more than the revolution of Copernicus if we learn for certain that we are not alone in the Universe.

Numerous attempts have been and are still being made to try and detect signals from other civilisations through projects such as SETI (the Search for Extra-terrestrial Intelligence).   This is discussed in the section of this web-site dealing with SETI.

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