Strangers In Our Midst
I am optimistic about the prospect of detecting alternative life. All life that we know, as different as it may appear in size and shape, shares a common heritage at the biochemical level. From amoebas to zebras, familiar life is dominated by two types of large molecule—nucleic acids and proteins. This biochemical similarity, which extends to many other features as well, implies that we are all products of a single life-starting event.
If this event was extremely improbable, then Earth may be the only seat of life in an immense and barren universe. This picture would be little changed if our kind of life has drifted across empty space to fertilize our nearest planetary neighbors. As Jacques Monod commented: "The universe was not pregnant with life nor the biosphere with man. Our number came up in the Monte Carlo game."
As no firm evidence exists that supports or denies this package of gloom, we are not obliged to purchase it. A different scientific position holds that the generation of life is written into the laws that govern the universe. If a limited set of environmental requirements are satisfied—a supply of useful energy, fertile material to absorb and use the energy, and a fluid medium to support the transaction, then life will emerge. In the words of biologist Stuart Kauffman: "If all this is true, life is vastly more probable than we have supposed. Not only are we at home in the universe, but we are far more likely to share it with unknown companions."
The issue can be settled by scientific exploration. The discovery of life forms sufficiently different from our own to indicate a separate origin would tilt the debate decisively in favor of a fertile universe. The search for such life has traditionally been directed to extraterrestrial locales. Spacecraft have conducted preliminary surveys of Mars, Europa, Titan and Enceladus, and discovered that one or more of the necessary requirements have been met. As human have not yet traveled further than the Moon, the search for novel life forms on these worlds is likely to be carried out will be carried out by robots. If any creatures are encountered, then their biochemical characterization will also be conducted with the use of pre-programmed instruments, but weight limitations will constrain the versatility of the tools that can be landed on these distant worlds. The Viking missions of 1976 illustrated the ambiguities that can arise in such explorations. Even if encouraging data was returned to Earth, a sample return mission would most likely be needed to settle a question of such magnitude. Considerations of safety would make stringent quarantine measures mandatory for any returned samples.
Extensive planning and testing would be needed in advance to design a weight-limited apparatus capable of identifying alternative life. As astrobiology budgets are often under pressure, some delays would also be likely before such an apparatus was launched. Further, all of the above listed sites except Mars would require a number of years of travel time to bring the instrument package to its destination. Thus, even if the fertile universe view was correct, many decades might pass before the issue was settled.
Fortunately, a new strategy has emerged that is capable of providing much more rapid returns. One world exists that is known to have all of the capabilities needed to generate and sustain life. It is close at hand, so that any possible samples of alternative life could quickly be subjected to examination in depth, using the best instruments that science can provide. Human scientists would supervise the studies directly, and modify them as needed. That world is Earth.
The suggestion that alternative, novel life forms might be found on our own planet runs of course directly into the obstacle of an entrenched paradigm. Biologists have characterized hosts of life forms, particularly at the microbial level, and encountered the familiar nucleic acid-protein based system every time. Our type of life reigns on this planet. If alternative creatures ever existed, then surely they were eliminated during the intense combat of evolution. The fact that no such creatures had turned up despite the intense efforts that biologists have expended in studying life on Earth has served to reinforce this widely accepted conclusion.
Recently, however, two papers have challenged this assumption. One, written in Australia, was provided by physicist Paul Davies and mathematician Charles Lineweaver. The other was authored by Colorado-based philosopher Carol Cleland and microbiologist Shelley Copley. Three of these writers and a number of other scientists who have been interested in the question of extraterrestrial life (myself included) gathered at Arizona State University to discuss this possibility. A central conclusion that emerged was that alternative Earth life may simply have been overlooked because microbiological search techniques were targeted at our own kind of life. Many diverse cell-like objects can be observed when samples taken from soil or water are examined under the microscope. Only about 1 % of them choose to multiply when conventional growth media are added to the mixture and these colonies are the easiest to characterize. In some cases, newer techniques based upon nucleic acid sequencing have been used to identify additional species. Some of them represent early and unexpected branches from our presumed universal tree of life. The existence of truly different organisms in that mix, for example ones that lacked nucleic acids entirely and stored their hereditary information in some other way, was hardly considered. If such organisms existed, they would most likely be products of that speculative second origin.
How could such organisms have survived the competition of our robust nucleic acid based life? In one scenario, they may have preferred to dine upon alternative food stuffs not favored by familiar life, selecting for example arsenate in place of phosphate, unfamiliar amino acids or mirror image forms of conventional biomolecules. The most extreme example of this type may be the speculative mineral-based life forms suggested by Scottish chemist Graham Cairns-Smith.
A different strategy would also allow alternative Earth life to flourish without direct competition. The organisms may have selected environments that are uninhabitable by conventional life. Conventional terrestrial organisms have shown great versatility in adapting to extremes of acidity, temperature, dryness, saltiness, radiation and other variables. Even so, their adaptability is not unlimited and some niches yet remain which they cannot utilize. Yet organisms with a very different set of internal chemicals might find them to be ideal dwelling places. One such locality that was mentioned at the conference was Iron Mountain, California, from whose interior extremely acidic waters emerge.
Ironically (forgive the pun) a front page story in the New York Times of Dec. 23, 2006, derived from a paper published in Science a day earlier reported the isolation of novel microorganisms from the waters of Iron Mountain. Their novelty arose from the record-breaking smallness of the cells, rather than from a difference in their internal biochemistry sufficient to suggest a separate origin. Yet the group of California-based scientists that had made the discovery also noted the presence of "rounded objects" that "were not shown to contain DNA."
How could such objects, or others that may turn up if a deliberate search for biochemically novel organisms is made, be shown to be offspring of a second origin? My own suggestion is that an inventory be made, as complete as possible, of their chemical contents. Many advanced instruments have been devised in recent years that can perform a microchemical analysis of tiny samples. No questions of instrument payload, robotic analysis or sample return need be considered, in contrast to the case of specimens taken on other worlds. If the analysis should reveal a chemical suite that differed notably from those derived from conventional life and from the near-random mixtures produced by abiotic processes, then we would have a strong indication that we may have hit a scientific jackpot. Another approach to the identification of alternative life would involve the use of unorthodox culture media, toxic to conventional life, that induce the alternative organisms to grow. More ingenious strategies may emerge when the energies of additional scientists are turned toward this question.
Of course, a thorough search of this planet may yet return empty-handed. My own optimism is based on my particular outlook on the mechanisms involved in the origin of life. But that is another story.