Last week I described how Fred Hoyle, in 1946, came up with the idea that carbon is synthesized in hot stars toward the end of their lifetime, and we now know that carbon and the other elements of life are strewn into interstellar space when the star explodes. In his later career, Hoyle was never able to match his earlier triumph of carbon nucleosynthesis, but he certainly tried. Together with his colleague Chandra Wickramasinghe, now at Cardiff University in Wales, he co-authored a series papers and books that proposed an alternative hypothesis for one of the remaining great questions of science, which is the topic of these columns: How did life begin on the Earth?
The scientific consensus is that life began as a chance event in which just the right mix of organic compounds was acted upon by an energy source so that growth and reproduction could occur. The earliest life would not resemble today’s highly evolved version, but more likely was a kind of scaffold that had the essential properties of life. The scaffold was left behind when more efficient living systems evolved.
Hoyle and Wickramasinghe did not subscribe to this view. Instead, they elaborated a version of panspermia, an older idea championed in 1903 by the great Swedish chemist Svante Arrhenius. Arrhenius proposed that life exists everywhere in the universe and was delivered to the Earth when frozen extraterrestrial bacteria or spores, drifting as interstellar dust through the galaxy, happened to land here four billion years ago and found our planet to be habitable. Hoyle took it a step further when he claimed that this was still happening, that epidemics such as the flu pandemic of 1918 were actually caused by extraterrestrial organisms in the tails of comets.
I met Wickramasinghe in 1986 at the Tidbinbilla radio telescope observatory near Canberra, Australia, and asked whether he and Hoyle really thought that interstellar space was infested with bacteria. He was quite certain of it, he said, noting that the infrared spectrum of interstellar dust closely matched that of dried, frozen bacteria. I mentioned that I was working with the astronomer Lou Allamandola at NASA Ames Research Center, who had demonstrated that the infrared spectrum could be reproduced by ordinary non-living compounds called polycyclic aromatic hydrocarbons (PAHs for short). This seemed a much more plausible explanation than a galaxy full of bacteria. Wickramasinghe had a ready retort: “It is up to you to prove that they are not bacteria.”
This was my first experience with someone who is not swayed by Occam’s Razor and the weight of evidence. Scientists are like investors, but instead of money, the capital they have to spend is time, limited to roughly 40 years of active research. Good scientists are constantly making judgment calls to decide where to invest their time. They hope their investment will be profitable, not necessarily in monetary terms (that rarely happens) but rather in revealing significant new knowledge. But a few scientists spend their lives seeking unusual explanations that others would immediately discard as implausible. Most often the ideas turn out to be not just implausible, but wrong. However, once in awhile a wild idea is beautifully, wonderfully correct, and overturns a paradigm. George Gamow had one such idea, which Hoyle jokingly referred to as the "Big Bang", and in a later column I will tell you about Peter Mitchell, another maverick whose implausible idea taught us how energy is made available in every living cell.
The game of life
Getting back to the main story, carbon and the other biogenic elements synthesized in stars can be delivered to planetary surfaces like the Earth and Mars during planet formation, mostly in the form of organic carbon compounds and carbon dioxide. These are chemically processed into a variety of other organic molecules which in turn assembled into the first living systems of self-reproducing molecules. How could this possibly happen? After all, a living cell is incredibly complex.
I think the answer lies in the fact that even though a list of life’s atomic and molecular components is relatively short, complexity can be produced by the exponentially large number of possible interactions among those components, subject to fairly straightforward laws of chemistry and physics. Think about the game of checkers. The parts are very simple, just black and red pieces on a checkerboard with 64 squares, and the rules that govern the way the pieces move about on the board are also easy to understand. However, the situations arising during an actual game of checkers are so complex that only in 2007, using the most powerful computer, was the game completely analyzed. (It turns out that if two players make perfect moves, the game always ends in a tie.)
In living organisms, as in checkers, immense complexity arises from the way specific rules govern a few basic pieces. Instead of two colors of checker pieces, life is based on six elements abbreviated CHONPS, as described in last week’s column. Carbon ( C ) phosphorus (P) and sulfur (S) are solids at ordinary temperature, and hydrogen (H) oxygen (O) and nitrogen (N) are gases. These elements comprise over 99 percent of the water and organic matter in a living cell. One of the chemical rules of life is that the six biogenic elements combine into four basic kinds of molecules, which in turn assemble into the structures that make a cell. This is the reason that CHONPS are the biogenic elements. No other group of six elements could be assembled into a set of simple molecules that can readily be linked together into chains called proteins and nucleic acids.
Starting with elemental carbon, it is an interesting exercise to construct four basic kinds of biomolecules, adding one more biogenic element at each step to show how complexity increases.
Carbon by itself: Nada, unless you like graphite and diamonds.
Hydrogen and carbon are easy: they compose the hydrocarbon chains of fat, cholesterol and phospholipids, collectively referred to as lipids.
Carbon, hydrogen and oxygen: Another easy one -- carbohydrates, or “watery carbon.”
Carbon, hydrogen, oxygen and nitrogen: Amino acids of course.
Carbon, hydrogen, oxygen, nitrogen and phosphorus: A little more challenging, but there is only one biomolecule left: nucleic acids.
Oops... forgot sulfur -- a couple of amino acids contain sulfur, and one class of lipids.
And there you have it, the main players in the game of life. But where did they come from for the game to begin?
Two of my scientific colleagues – Bill Irvine and Lou Allamandola – introduced me to a fundamental yet little known fact of life: We live in an organic universe. Twenty years ago, when I first heard Bill speak at a conference on the origin of life, I naively wondered why a radio astronomer would be invited. But then as he began to show his slides, the mystery was solved.
Really cold matter in molecular clouds emits radiation in the microwave region, with each chemical bond in a compound producing a specific wavelength. If the radiation is detected with a radio telescope and then analyzed, it is possible to decipher the kinds of bonds present and determine the nature of the compound. Bill presented clear evidence that dense molecular clouds, the nurseries of stars and solar systems, had nearly a hundred kinds of organic compounds present. Some of these are familiar, such as cyanide, formaldehyde, methanol, ethanol, formic acid (named after formica, Latin for ants that release formic acid as a sour spray when disturbed), and acetic acid, the sour component of vinegar. Others are truly exotic, including one with a chain of nine carbon atoms and one nitrogen atom at the end. Such a weird compound could not exist on Earth, but in the cold of outer space it is stable.
The connection between radio astronomy and the origin of life became obvious as I listened to Bill. If molecular clouds give rise to stars, planets and solar systems, maybe some of the organic matter in the clouds was delivered to the early Earth four billion years ago to help life get started. Next week I will describe an experiment in which the synthesis of organic compounds on interstellar dust particles was simulated in Lou Allamandola’s lab at NASA Ames.