Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


 

7.3.1 Prebiotic Synthesis

For many years it was known that mixtures of carbon dioxide, ammonia and water vapor would produce small amounts of simple organic chemicals if energy was supplied. But the results of these experiments were generally very discouraging and the yields miniscule under these oxidizing conditions. To originate life in such a poor, thin broth would be well-nigh impossible.

In 1953 a graduate student named Stanley Miller, working under Nobelist Harold C. Urey at the University of Chicago, constructed an apparatus to imitate the conditions of the primitive Earth (Figure 7.1). Previous investigators had always assumed the atmosphere to be oxidizing or neutral. Miller and Urey, following the suggestions of A. I. Oparin in the Soviet Union and J. B. S. Haldane in Britain during the 1920’s, took the unprecedented step of devising a reducing environment instead.2258

 


Figure 7.1 Miller Apparatus for Prebiotic Synthesis2315

In this schematic of the apparatus used in Stanley Miller's s historical experiment, a variety of organic compounds are synthesized as the atmosphere of methane (CH4), ammonia (NH3), hydrogen (H2) and water vapor (H2O) is subjected to an electric spark discharge. Circulation is maintained in the system by the boiling water on one end and the condensing jacket on the ether.

After one week of continuous operation, the water was removed and tested by paper chromatography. A great abundance of amino acids and other organics was detected.


 

Miller mixed together methane, hydrogen, ammonia and water, and carefully eliminated all oxygen from the system. This gaseous concoction was then circulated past an electric spark discharge, followed by a water bath to simulate the primitive sea. After about one week of continuous operation, the "ocean" had turned a deep reddish-brown.

The experiment was halted and the contaminated water removed for analysis. Miller discovered to his amazement and delight that many amino acids had been produced in surprisingly high yields. Two percent of the total amount of carbon in the system was converted into glycine alone. Sugars, urea, and long tarlike polymers too complex to identify were also present in unusually high concentrations.

Of course, electrical energy was only one of the many sources of energy available on the primitive Earth (Figure 7.2). In fact, ultraviolet radiation was probably the principle source: UV would have been able to penetrate to the surface be cause the protective ozone layer in the upper atmosphere did not yet exist. A Miller-type experiment using ultraviolet rays and a reducing atmosphere was performed in 1957 by the German biochemists W. Groth and H. von Weyssenhoff at the University of Bonn.2307 Their results closely paralleled those obtained at the University of Chicago half a decade earlier.

 


Figure 7.2 Prebiotic Chemical Evolution on the Primitive Earth

 


 

Countless prebiotic simulations have since been achieved which confirm Miller’s original conclusions. One bibliography, current through 1974, lists more than three thousand papers on the subject.1679 An exhaustive treatment of all of them is clearly beyond the scope of this book, but the interested reader in encouraged to dive into the literature (Table 7.1).

 


Table 7.1 Summary of Prebiotic Synthesis Experiments through 1975

 

Year
Reactants 
Energy Source
or Catalyst
Products
Ref. #
AMINO ACIDS, FATTY ACIDS, AND SIMPLE ORGANICS  
1828
Ammonium sulfate, potassium cyanate
Thermal reaction
Urea
1586 
1913
 
Electrical discharge
Glycine
2257
1926
CO2, CO, CH4 or H2
a-rays
Resinous organic material
2224
1937
CO + H2O
UV
Formaldehyde, (HCO)2 (glyoxal)
2317,2266
1951
CO2, H2O, Fe++
a-rays
Formaldehyde, formic, succinic acids
2226
1952
formic acid + H2O
a-rays (40 MeV)
(COOH)2
2279
1953
acetic acid + H2O
a-rays
Malonic, malic, capric acids
2278
1953
CH4, NH3, H2, H2O
Electrical discharge
Glycine, alanine, aspartic & glutamic acids
2258
1954
CH2O, H2O, KNO3, ferric chloride
Sunlight
Amino acids
2261
1955
Ammonium fumarate 
Heat
Aspartic acid, alanine 
2267
1955
CH4, NH3, H2, H2O
Electrical discharge
Amino acids, hydroxy acids, urea, HCN
2259
1956 
CH4, NH3, CO2, H2O
 
Amino acids
2260
1957
CH4, NH3, H2, CO2, H2, CO, N2
X-rays
Amino acids
2262
1957
CH4, NH3, H2O
UV
Simple amino acids, fatty acids
2269
1957
Ammonium acetate
b-rays
Glycine, aspartic acid, diaminosuccinic acid 
2223
1957
Glycine
Heating w/quartz sand
Alanine, aspartic acid
2264
1957
Ammonium carbonate
g-rays
Glycine
2227
1959
Formaldehyde, hydroxylamine
Thermal reaction
Glycine, alanine, serine, and others
2265
1960
Hydroxy acids ammonia or urea
Heat
Glycine, alanine, aspartic & glutamic acids
2263
1960
CO2, H2O
g-rays
Formic acid
2277
1961
CH4, NH3, H2
Accelerated protons
Urea, acetone, acetamide
2316
1961
NH3, HCN, H2O
Heat (70 C)
Amino acids
2270
1961
CO2, C2H4
g-rays, or high pressures
Long chain fatty acids (C-40)
2276
1962
CH4, NH3, H2
b-rays
Simple aliphatics incl. amino acids
2272
1963
CH4, NH3, H2, H2O
Hypersonic shock wave
Many unidentified organic compounds
2318
1964
CH4, NH3, H2O + silica
Heat (850 C)
Amino acids
2273
1964
CH4
Electrical discharge
Higher aromatic hydrocarbons
2274
1966
CH4 + silica contact
Heat (1000 C)
Higher aromatic hydrocarbons
2271
1966
Cyanoacetylene, HCN, NH3, H2O
Heat (100 C)
Aspartic acid
2275
1970
CH4, C2H6, NH3, H2O
Shock waves ("thunder")
Amino acids
1664
1974
CH4, NH3, H2, H2O
Electrical discharge
Amino acids and simple organics
521
SIMPLE SUGARS AND CARBOHYDRATES     
1924
Formaldehyde + H2O
UV
Hexoses, hydroxy acids
2280
1959
Monosaccharides
g-rays
Polysaccharides
2310
1962
CH2, Ca(OH)2, CH3CHO, CH2HCHOHCHO
Heat (50 C)
2-deoxyribose, 2-deoxyxylose, etc.
2281
1963
Formaldehyde + H2O
UV
Ribose, deoxyribose
2243
1965
Glucose
Heat (130 C)
Polyglucose
2314
1965
Formaldehyde
UV
Ribose, deoxyribose, other sugars
2282
1965
Monosaccharides
UV
Disaccharides
2313
POLYPEPTIDES, AMINO ACID POLYMERS, PROTEINOIDS  
.
1954
Amino acids
Heat
Amino acid polymer proteinoids
2268
1954
Glycine
Electrical discharge
Polyglycine 
2283
1959
Amino acids
X-rays, UV, or g-rays
Amino acid polymers
2310
1959
Hot proteinoid material
Heat, slow water cooling
Proteinoid microspheres
2286
1960
Glycinamide
Heat (100 C)
Polyglycine (up to 40-unit strands)
2311
1961
Asperine in water
Heat
Amino acid polymers
2285
1961
Glycine
Heat (140-160 C)
Polyglycine (up to 18-unit strands)
2312
1964
Glycine + Glucose + H2O
Heat
Amino acid polymers
2284
1964
Amino acids
Heat + cold quenching
Proteinoid microspheres
1702
1969
Alkanes, Mg++, PO4=
UV
n-Hexadecane membranes
1415
1974
   
Coacervates (a review)
1432
1974
Proteinoid
 
Higher bonding in proteinoid microspheres
1435
1975
HCN, H2O
 
Heteropolypeptides
1438
NUCLEIC ACID BASES: PURINES AND PYRIMIDINES    
1926
Malic acid, urea, strong mineral acid
Thermal reaction
Uracil
2292
1960
Amino acids
 
Purines
2289
1961
Malic acid, urea, polyphosphoric acid
Heat (100-140 C)
Uracil
2244
1961
Amino acids
 
Purines
303
1962
Urea + AICA
Heat
Guanine, xanthine 
2281
1962
HCN, etc.
 
Purine intermediates
2290
1963
CH4, NH3, H2O
b-rays
Adenine
2237
1963
Urea + CH2CHCN or NH2CH2CH2CN
Heat (130 C)
Uracil
2293
1963
CH4, NH3, H2
b-rays
Adenine
304
1963
 
Heat
Guanine
2238
1964
Aspartic acid, glutamic acid 16 others
Heat (180 C)
Guanine
2239
1966
Urea + AICA
Heat
Guanine, xanthine
2291
1971
   
Thymine
2246
1971
   
Adenine, Guanine, Cytosine
2241
1972
 
Zeolite catalyst
Purines
2247
NUCLEOTIDES AND POLYNUCLEOTIDES    
1962
Nucleotides
g-rays
Polynucleotides
2256
1963
Adenine, ribose, ethyl metaphosphate
UV
Adenosine triphosphate (ATP)
2294
1965
Nucleosides + phosphates
Heat (160 C)
Nucleotides
2242
1965
Bases + metaphosphate ester
 
Polynucleotides
2235
1967
Cytidylic acid + polyphosphoric acid
 
Cytosine nucleotide
2236
1967
Nucleosides + polyphosphoric acid
Heat (22 C)
Nucleotides, nucleoside triphosphates
2295
1970
Imidazole + cyanamide
 
Mononucleotides
2252
1971
 
Heat
Polynucleotides
2251
1971
Cyanamide 
 
Mononucleotides 
2253
1971
Adenine nucleotide
UV
Adenine polynucleotide 
1628
1971
Cyclonucleoside 
Heat
Polynucleotides 
1627
1971
Nucleotide
Heat
Polynucleotides
1626
1971
   
Cytosine nucleotide
2254
1972
Thymine nucleoside 
 
Thymine nucleotide 
2245
3972
Ammonium cyanide + water
 
Guanine nucleoside
2240
1972
 
Apatite catalyst
Mononucleotides
2250
1973
Cyanamide + AICA
 
Mononucleotides
2248
1973
Cyanogen + water
Apatite catalyst
Mononucleotides 
2249
1974
Nucleotides
 
Polynucleotides 
1435
1974
Nucleosides
Heat
Oligonucleotides
1429
1975
Nucleoside + ammonium oxalate 
Heat
Nucleotide 
1439


 

Table 7.2 lists the sources of energy believed to be present during the first eon or so of Earth’s history. Ultraviolet radiation leads the pack. Carl Sagan and others have completed experiments with UV which seem to indicate rather high yields for prebiotic amino acids, the building blocks of proteins. Over the first billion years of chemical evolution on this world something like a hundred kilograms of amino acids per square centimeter may have been produced, resulting in a "soup" of about 1% concentration. This is the approximate consistency of chicken bouillon.

 


Table 7.2 Energy Available for Synthesis of Organic Compounds on the Primitive Earth
Source of Energy
Energy Available
(joules/meter2/year)
Solar radiation, all wavelengths
1.1 1010
Ultraviolet, l < 3000 Ang
1.4 108
 
l < 2500 Ang
2.4 107
 
l < 2000 Ang
3.6 106
  l < 1500 Ang
1.5 105
Electrical discharges
1.7 105
Decay of crustal K-40, 4 eons ago
1.2 105
(Decay of crustal K-40, today)
(3.4 104)
Shock waves
4.6 104
Heat from volcanoes
5.4 103
Meteoritic impact
4.2 103
Cosmic rays
6.3 101


 

But ultraviolet radiation is a two-edged sword. While it may be the most abundant form of energy for molecule building, it is also the most destructive. Early researchers were concerned that organics would be destroyed as fast as they were created. Fortunately, the primitive oceans probably turned opaque like the brownish glop in Miller’s apparatus rather quickly. Vital chemicals newly synthesized and carried a short distance beneath the surface of the soup by convection undoubtedly escaped decomposition.

Of the remaining energy sources, electrical discharge was the most potent. As much as 5-15% of the carbon in a mixture of methane, ammonia and water may be converted to amino acids and other organics by the energy of the discharge. Various forms of ionizing radiation give high yields as well. a particles, b particles, and g rays were common on the surface of the primitive Earth because of the presence of intense natural radioactive sources in the crust -- such as potassium-40, thorium-232, and isotopes of uranium.

Volcanic heat was another prebiotic power supply.2368,2380 It has been shown that lava-heated seawater and underwater volcanoes may be effective in producing biologically important compounds. Heat and sonic energy would have been released by infalling meteorites -- certainly a significant factor in the environment of the primitive solar system.1417,2375 In fact, experiments performed recently by Bar-Nun and others have conclusively demonstrated that as much as 30% of the nitrogen in an ammonia atmosphere can be converted into amino acids in this manner.315,1664,2375 Torrential rains have even been suggested as a possible source of energy for prebiotic synthesis, and experiments have shown that a flask of formaldehyde, allowed to stand for a few days at room temperature, will produce some simple sugars.

The great lesson appears to be that the exact nature of the power supply is relatively unimportant. Amino acids, sugars, and other chemical precursors to life probably arise on any planet possessing an initially reducing atmosphere and quantities of hydrogen, carbon, nitrogen and oxygen in gaseous reduced form -- regardless of the particular source, or sources, of energy available.*

 


* Other factors may also be important. For instance, early-type stars (F) are more likely to emit ultraviolet radiation in copious quantities than are late-type stars (K, M). The speed of chemical evolution in primitive planetary environments may actually slow as we move from class F through classes G to K stars among habitable solar systems.

 


Last updated on 6 December 2008