1. The Search for Life: Initial Thoughts.

Now that "marvelous feat" is left to 21st century science. For decades, some of us have dreamed of detecting intelligent life elsewhere in the Universe (Crater, 2009). Failure of searches like SETI (Search for Extraterrestrial Intelligence) may have several causes. Electronic signals traveling through space from one or more advanced civilizations may come from too far away to have arrived during our listening time of a few decades. Or, such emissions might have arrived, but we cannot recognize them. Perhaps they are too weak or in a form we cannot distinguish against cosmic background noises. Or, intelligent life elsewhere may be in early stages akin to the several million years when humans, Homo erectus followed by Homo sapiens, had only stone age technologies. Over the last century we have inadvertently sent a bubble of electronic signals outward into space. It is now only a hundred or so light years wide, but steadily expanding. Some day another planetary civilization may find our bubble.

In contrast, our chance of discovering life in a broader sense on one or more extrasolar planets, "life planets" as I like to call them, is increasing. The search for extrasolar planets is underway, as shown by the growing catalogue of over 370 presented on planetquest.jpl.nasa.gov.

However, the first Earth-like planet is yet to be found. We know of stars in the Milky Way that potentially have planets in "habitable zones" and that a long "galactic habitable zone" stretches away through our arm of the Milky Way. Among billions of galaxies similar places surely exist. When I speak of life on other planets in a broad sense, I’m thinking of life originating in simpler forms and undergoing development into more complex and diverse kinds under the forces of reproduction, heritable variation, and genetic and physiological change under natural selection. Just as it did on Earth. Accepting this possibility leads to these questions.

(1) Will some of these living forms on other planets resemble life on Earth? To this question I answer: "Very likely."

(2) Will other life forms be so different that we cannot detect their presence? Here I say: "Possibly, but not likely we’ll miss them if the organisms have become abundant and physiologically diverse."

(3) Is there life on or in some planets, or their moons, in our solar system, for example Titan (Lorenz, et al. 2008; Takeshi and Yasuhito, 2010), or Europa, Ganymede, Callisto, and Enceladus (Khurana, et al. 1998; Waite, et al. 2009)? If the answer to this question is "yes," I think we will find convincing evidence (see University of Arizona press release, 2009).

Finding additional life in our solar system will be exciting, and especially so if we obtain samples to learn if DNA and RNA are present and whether such DNA and RNA molecular sequences are related to those of organisms on Earth. Do they use the same amino acids, or different ones?

The first two questions provide the focus for the rest of this article.

2. Life On Earth, One Model For Life Arising On Other Planets.

We don’t know exactly how life began on Earth. It may have started here or arrived as precursors transported through space (i.e., panspermia; Crick, 1981; Joseph, 2009; Wickramasinghe, et al. 2009). Desiccated and frozen viral-like RNA or DNA replicons might have arrived shortly after the Earth formed. But on thawing they would have to fall into an environment with a supply of the specific sugar molecules and nucleotides required for their replication. It has been suggested that bacterial spores arrived. I think this unlikely even though spores made by some Bacteria are extremely resistant to harsh environments. However, based on DNA gene sequence data, the Archaea, possibly the oldest of the prokaryotes, played a crucial role, along with Bacteria, in the evolution of life on Earth, as we’ll learn shortly. But none of the many known Archaea species make spores.

Hence, I prefer the hypothesis that carbon-based life on Earth began with the synthesis of ribonucleic acid (RNA) from inorganic and organic molecules in primordial milieus shortly after the origin of the Earth. Numerous milieus, almost certainly holding nucleotides, sugars, amino acids, phosphorus, nitrogen, and lipids, probably formed according to laws of physics and chemistry among constituents from volcanos, comets, meteors, or other material striking Earth (Botta, et al. 2008; Glavin, et al. 2008; Martins, et al. 2008). These molecules may have accumulated in hot or cold springs, ocean upwellings, or salty lakes (Gilbert, 1986; Szathmary, 1999; Nelson, et al. 2000; Orgel, 2000). Some prebiotic chemical syntheses may have occurred in the solar nebula (Hill and Nuth, 2003). Photochemistry of methane and CO₂ might have created a haze in the atmosphere of the early Earth that protected these milieus from intense UV irradiation (Hearth, 2009).

But first, let’s look back at work demonstrating what might form in such chemical milieus. Laboratory experiments by Stanley Miller (1953) and Miller and Urey (1959) explored this possibility. (See also critique by Fox, 1959). Miller and Urey used a simple system of two interconnected flasks where a small heated one sent vapors from ocean water to a large one with H₂O, CH₄, and H₂CO. Periodic electric discharges (pseudo lightening) in the larger flask caused liquid samples to drop down under a condenser. The result? Eventually 22 amino acids, with glycine most common, were found along with sugars, lipids, and subunits of nucleic acids. Subsequently, in experiments with varying ingredients, others obtained additional intriguing results. For example, Oro (1961) found the nucleotide base adenine, and much later Johnson, et al. (2008) provided supporting analyses from residues in stored vials left from unpublished experiments done by Stanley Miller.

Experimental evidence published in the last year provides the most compelling evidence for earthly biogenesis (Szostak, 2009). Powner, et al. (2009) have demonstrated how ribonucleotides, RNA subunits, might form spontaneously. With phosphate as both a pH buffer and catalyst, reactions between the sugar glycolaldehyde, cyanamide, cyanoacetylene, and glyceraldehyde produced a complex mixture of products. The phosphate prevented unwanted reactions and allowed rapid synthesis of 2-amino-oxazole. As the reactions proceeded, ribocytidine formed and subsequent UV irradiation (pseudo sunlight) caused ribouridine to appear. Thus, two of the four nucleotides of RNA structure emerged. Quite different experiments using UV irradiation of mixtures of ice and pyrimidine under conditions simulating interstellar environments yielded uracil, another part of the RNA genetic code (Nuevo, et al. 2009). These achievements in small laboratory systems are extraordinary when we think about the innumerable chemical reactions that must have occurred on the early Earth in far flung and highly variable chemical and physical environments, and over thousands or millions of years―systems chemistry on enormous geographical and temporal scales.

Complete RNA strands may have arisen in water or on moist surfaces. Once formed, replicating RNA molecules could undergo mutational change and natural selection for faster and more reliable replication. Such a process has been experimentally demonstrated by Lincoln and Joyce (2009). They showed that cross-replicating RNAs provisioned with the four requisite oligonucleotide substrates undergo continuous replication with a doubling time of one hour. Next, Lincoln and Joyce created populations of different cross-replicating RNAs competing for nucleotides, and recombinant forms emerged and grew to dominate their populations—they had evolved. No proteins were present to serve as enzymes. The RNAs served as their own replication enzymes. This result has been made less surprising because of experiments by Vaidya and Lehman (2009). They showed that a single RNA fragment is capable of performing three different functions: self-assembly, self-replication, and cooperation in construction of a catalyst molecule, with no other catalysts present. Earlier studies suggested such complex RNA behaviors (Ancel and Fontana, 2000; Schultes and Bartel, 2000).

RNA molecules can also act like enzymes to conduct chemical reactions other than those involved in replication. Association with naturally occurring lipid molecules to form surrounding membranes may have led to the first cells, then to association with DNA, and the evolution of the Bacteria and Archaea with DNA in their chromosomes (Zimmer, 2006). There is solid molecular evidence that fusion of members of these procaryote groups, possibly including an Eocyte branch of the Archaea, led to the Eukarya with nuclear membranes surrounding a DNA chromosome (Bergsland and Haselkorn, 1991; Gupta and Golding, 1996; Davidov and Jurkevitz, 2009; Foster, et al. 2009; Ragan, et al. 2009; Zimmer, 2009). It is clear from shared DNA gene sequences that Bacteria subsequently entered eukaryotic cells as endosymbionts to become mitochondria, the numerous energy factories in the cytoplasm. Cyanobacteria also entered eukaryote cells to provide chloroplasts for photosynthesis (Tomatani, et al. 1999; Lake, 2009).

Photosynthesis probably arose early in the history of life, but until about 2.4 billion years ago oxygen in the atmosphere did not reach a level high enough so that the anaerobic life that thrived under the early Earth’s reducing atmosphere was driven into oxygen-free locations (Canfield, 2005). About 850 million years ago Precambrian plant communities on land added additional oxygen (Knauth and Kennedy, 2009). By 600 million years ago the grand evolutionary explosion and diversification of plant and animal life was beginning. Vast, complex, ecological communities formed across the Earth’s surface and through all the oceans and fresh waters. The atmosphere, hydrologic and nutrient cycles, aquatic chemistry, as well as the colors and physical appearance of much of the Earth dramatically changed under biological control. The law of evolutionary and ecological dominance had prevailed. This was not a defeat of the second law of thermodynamics. It was simply the way a complex, highly evolved life planet, an open thermodynamic system, traded successfully in entropy for as long as its sun would support it (Schrödinger, 1944).

Viruses were no doubt diverse and abundant from the beginning, some (bacteriophages) even serving as intermediates in lateral genetic exchange among Bacteria (Pantastico, et al. 1992). Lateral genetic transfer probably played a major role throughout procaryotic evolution (Ragan, et al. 2009), just as it can at present (Graham and Istock, 1979; Duncan, et al. 1989, 1994). Much more is being learned about complex relationships of viruses with Archaea, Bacteria, and Eukarya. Molecular similarity among numerous topoisomerases that manage the unwinding and opening of DNA during gene expression in all four domains have revealed complex, shared relationships (Forteere and Gadeelle, 2009).

Here is my shorthand image of the progression as carbon-based life began and evolved on Earth.

However, as we’ve learned, it was not a simple linear sequence. Complex networks of change involving lateral gene exchange, fusion of cells and replicons, copying of genes or sequences from RNAs to DNA in the nucleus, and much more, created life with many elaborations (Ragan, et al. 2009).

3. Alternative Forms Of Biosynthesis.

Another life planet might differ from Earth in its stereochemistry (chirality). Rather than having three-dimensional amino acids of the left-handed or L form, or sugars of the right-handed or D form, as on Earth, opposite forms might have been selected. Learning this will require biological samples. Suppose we obtain such samples from Mars, or some other body in our solar system, and they are of the opposite forms. We would know such life originated independently. In a small scientific literature and a large body of science fiction, alternative ways that life might be organized without carbon have been suggested. Most commonly, silicon is imagined to take the place of carbon in organic molecules like those on Earth. Curiously, silicon is far more abundant on Earth than carbon, yet carbon was the choice when earthly life originated. Silicon has a larger mass and atomic radius, making formation of double and triple covalent bonds more difficult, and long chain silicane molecules are less stable than hydrocarbons. Another choice might be nitrogen and phosphorous as a basis for long-chain and ring molecules. Ammonia and hydrogen fluoride have been suggested as substitutes for water to provide an environment for life. Another interesting possibility is photosynthesis based on colored pigments other than green chlorophyll, say orange, red, brown, or yellow. Some thinkers interested in panspermia have envisioned such kinds of life. One wonderful excursion into fantasy is Fred Hoyle’s (1957) novel The Black Cloud, where life exists in interstellar dust, and another is Carl Sagan’s story in Contact (1985) telling of communication received from extraterrestrials.

4. Why other Planets Harbor Life, And Finding Them.

Try to imagine our world with no life. What would it look, sound, and smell like? Walk out and look around anywhere except the rolling dunes of the Sahara, or barren reaches of the high altitude Atacama Desert in Chile, where it only rains tiny amounts every few years. Walk out where the world is green, even the Sonoran desert of Arizona and northern Mexico. Without life what would the atmosphere be like, an atmosphere with no oxygen, and feeble circulation of water without vegetation? It would look like Mars and possibly smell like it―a world with no renewing cycles of carbon, nitrogen, and phosphorus driven by microbes, plants, and animals, no flowers, mushrooms, or tree-covered hills and mountains. There would be rushing sounds of winds and waves, glaciers cracking, occasional roaring volcanos, or crashing meteorites, and thundering rock slides—but no songs or calls from birds, frogs, insects, and mammals, no rustle of grass and leaves in trees, no sounds of engines and machinery. Even the procaryotes would be absent from soils, oceans, and freshwater.

Starting with the known extrasolar planets, study of a large sample of them can catalog their properties and appearances: size, proximity to the mother star, gaseous or rocky, colors of their surfaces or atmosphere, temperatures, age of mother star, atmospheric chemistry and composition, orbit, proximity to other planets in their solar systems, and properties of these companion planets. Thereby, we will obtain a reference collection of background material for comparison with newly found planets that look promising as life planets (see University of Puerto Rico at Arecibo news release, 2009). Such reference studies will also facilitate the search for "cosmic anomalies." One interesting "anomaly" would be a planet that developed clear ecological dominance entirely through the evolution of life forms with chemosynthetic metabolism, perhaps like earthly methanogens, and possibly with photosynthetic organisms like kinds known on Earth that do not release oxygen. Other "anomalies" will emerge, like COROT-7b, a rocky planet discovered in February 2009. It has a silicate atmosphere containing ingredients that condense into small pebbles that rain down on lakes of molten lava. COROT-7b is less than twice the size of Earth, and orbits an orange dwarf star in the constellation Monoceros (Lutz, 2009; Schaefer and Fegley, 2009). Could it sponsor life? Not likely with a surface temperature of 1800o to 2600o C.

It is my guess that life arising on many other planets will begin in an RNA-like world, though not necessarily with RNA per se. Here is an abstract rendering for the origin and evolution of such life.

When another planet reaches between the third to fifth stages in this progression we should be able to tell that it is a life planet.

At a truly fundamental level, we already know that antecedents exist for chemical milieus (stage 1) that can engender the origin of life. Using the infrared Spitzer Space Telescope, it has been possible to "see" that these chemicals are plentiful in the Universe, particularly in protoplanetary disks (Bergin, 2009). Equally remarkable are chemical assemblages in dark nebulas that have hydrogen and carbon compounds, e.g., the Barnard 68 molecular cloud 500 light years away, and soon (in ~100,000 years) to give rise to a new star (Maret and Bergin, 2007; Maret, et al. 2009). Here we encounter dazzling displays of cosmic systems chemistry—providing conditions for life arising here and there across the Universe.

Once unequivocal proof of life on an extrasolar planet is found, it will forever change humanity. All of science, philosophy, and no doubt art and literature, will be affected. That "marvelous feat" will change how we think about living worlds in ways we can only imagine now. But, I think for many people the Cosmos will become more enthralling, more beautiful.