June 16, 2005 | David F. Coppedge

Miller Time Party Drags On

Astrobiologists threw a party when a team of researchers decided there was more hydrogen in the early earth’s atmosphere than thought (see “In the beginning, hydrogen: was it Miller Time?, 04/22/2005).  While this was good news for those wishing for better conditions on the early earth for chemical evolution, a few are staying sober enough to warn against letting the celebrations get carried away.
    Last month, veteran origin-of-life researcher Christopher Chyba, buoyed by the announcement, was nevertheless cautious about how much it helps the Miller scenario.  He wrote in Science:1

In 1952, Stanley Miller, working with Harold Urey, simulated the atmosphere of early Earth with a gas mixture of methane (CH4), ammonia (NH3), molecular hydrogen (H2), and water.  When he introduced an electrical spark to represent lightning, he observed the formation of amino acids, the building blocks of proteins….
    However, by the 1960s, the validity of hydrogen-rich (and hence reducing) model atmospheres for early Earth, such as the CH4-NH3 atmosphere used by Miller and Urey, was under attack.  Since the 1970s, carbon dioxide (CO2)-rich atmospheres have been favored.  Miller has shown that the production of amino acids and other organic molecules is orders of magnitude less efficient in such atmospheres.  For this and other reasons, the Miller-Urey approach to the origin of life has fallen out of favor with many researchers.  But on page 1014 of this issue, Tian et al.2 argue that the early-Earth atmosphere might have been hydrogen-rich after all.
(Emphasis added in all quotes; see also 05/02/2003 entry on the history of the Miller experiment.)

Chyba described the Miller-Urey scenario in more detail, but admitted it was “probably largely wrong.”  Such a reducing atmosphere would have been hard to form or sustain.  If, however, there was a sustainable hydrogen abundance of 30% or more, as suggested by the Colorado team, conditions favoring higher production of amino acids might have existed.  Still, “Many uncertainties and problems remain,” Chyba said, and they seem serious, indeed:

  1. Rinse:  Tian et al. focus on the oceans as the “birthplace of life,” but polymerization of amino acids into proteins (or nucleotides into RNA) is thermodynamically unfavorable in liquid water.
  2. Salt:  Furthermore, in an early ocean as saline as that of today, the salt inhibits key prebiotic reactions.3  The bulk ocean may thus have been one of the worst places to try to originate life.
  3. Toss:  After making life’s building blocks in the ocean, one needs to look elsewhere to carry the chemistry further.

He suggested that meteor impacts “may also have been a major driver of organic production in an early H2-rich atmosphere,” but with all the hope and hype, Chyba advises sobriety:

These are tumultuous times in the study of the origin of life.  The early ocean may have been even less hospitable for prebiotic chemistry than previously thought, and claimed evidence for the earliest signatures of life on Earth is being strongly challenged.  Now a 30-year, albeit shaky, consensus on the nature of the early atmosphere may have to be reexamined, and the geochemical implications of an H2-rich early atmosphere will need to be scrutinized.  This turmoil makes it a great time for young scientists to enter the field, but it also reminds us that some humility regarding our favorite models is in order.  As Jacob Bronowski noted, “Science is a tribute to what we can know although we are fallible.”

These week in Science,4 Richard Kerr also wrote about the higher hydrogen estimate:

Thirty years ago, geochemists took away the primordial soup that biologists thought they needed to cook up the first life on Earth.  Now, some atmospheric chemists are trying to give it back.
    Creating the primordial organic goo used to be easy.  If you combined the methane and ammonia seen in the still-primordial atmosphere of Jupiter, passed lightninglike sparks through the mixture, and added some water, voilà, complex organic compounds such as amino acids formed.  But then in the 1970s geochemists spoiled the party by insisting that Earth’s earliest atmosphere was nothing like Jupiter’s.  Earth’s carbon would have been part of oxygen-rich carbon dioxide, and its nitrogen part of inert nitrogen gas, they said.  And hydrogen seeping from the planet’s interior would have quickly escaped to space.  That left chemists with a thin gruel indeed.

Kerr summarizes the new estimate and what it means: “Overall, hydrogen would have escaped at 1/100 the rate previously assumed, the group says…. That would make for a far more productive atmosphere than chemists have been coping with for 30 years” – allowing vast amounts of organics to form into the ocean “ to make a soup.”
    Kerr hastens to make clear that there is still disagreement.  While the announcement “is going to make the biologists a lot happier,” another doesn’t feel that Tian et al. adequately dealt with all the factors that contribute to hydrogen escape; “a more sophisticated model would show that hydrogen escaped the early Earth at least as fast as it does today.”  (Kerr does not even mention the problem with salts in the ocean.)  Is the Miller party running out of food?  He ends, “Time will tell whether too many cooks spoil the primordial broth.”


1Christopher Chyba, “Rethinking Earth’s Early Atmosphere,” Science, Vol 308, Issue 5724, 962-963 , 13 May 2005, [DOI: 10.1126/science.1113157].
2Tian et al., “A Hydrogen-Rich Early Earth Atmosphere,” Science, Vol 308, Issue 5724, 1014-1017, 13 May 2005, [DOI: 10.1126/science.1106983].
3Monnard et al., “Influence of ionic inorganic solutes on self-assembly and polymerization processes related to early forms of life: implications for a prebiotic aqueous medium,” Astrobiology 2002 Summer;2(2):139-52.  They write that concentrations of salts anything like those in our contemporary oceans inhibits formation of amino acids and completely disrupts primitive membrane systems.  Conclusion: “These observations suggest that cellular life may not have begun in a marine environment because the abundance of ionic inorganic solutes would have significantly inhibited the chemical and physical processes that lead to self-assembly of more complex molecular systems.”
4Richard Kerr, “A Better Atmosphere for Life,”, Science, Vol 308, Issue 5729, 1732, 17 June 2005, [DOI: 10.1126/science.308.5729.1732].

Same comment as in 04/22/2005: too little, too late.  The good news is no better than that in the Geico commercials: “I have good news and bad news.  The jury has found you guilty, you have to go into the slammer for life, your wife and kids have left you and are changing their names, your stocks went bust, and you have cancer.”
“What’s the good news?”
“I just saved $400 on my car insurance by switching to Geico.”
Everything is against the astrobiologists: the chemistry, the sources, the geology, the salt, the water, the information, the probability, the thermodynamics, the philosophy.  Does the good news really matter?  “I just saved 30% on my hydrogen when switching to the Tian et al. model.”  It would make any knowledgeable astrobiologist want to hold his head and groan, “Oh, shut up.”

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Categories: Origin of Life

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