Even a single data point is useful information. A detailed study of Earth based life as we know it can tell us much about the potential for finding life elsewhere. Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. Let us examine each of these points in turn.
How did life on Earth begin?
Right away, we stumble upon a huge unanswered question. Science has yet to come up with a satisfactory theory of abiogenesis, or how life could have arisen from inanimate matter. Darwin's theory of evolution does not address the origins of life, merely the change of species over time. In a letter to a colleague Darwin did suggest a "warm little pond" of charged chemicals with the potential to become life. The idea of a primordial soup has been around for over a century, but no one can figure out the definitive list of ingredients.
Many researchers have attempted to simulate chemical environments that could have existed on the early Earth, the most famous being the Miller-Urey experiment in the early 1950's. A glass flask was filled with water (the ocean) and attached to another flask filled with nitrogen, methane, and carbon monoxide gasses (the atmosphere). The liquid was heated so the ocean could evaporate into the atmosphere, where an electric spark discharge simulated the energetic effects of lightning; a condenser cooled the gas mixture back to a liquid, completing the cycle. The experiment was run continuously for several weeks and accumulated a brown tar. Analysis of the tarry residue revealed amino acids and sugars necessary for life.
Current thought suggests the conditions of the Miller-Urey experiment did not accurately model the actual environment of the early Earth, but the notion of chemical evolution seems well established. Large, complex molecules can easily be synthesized from smaller precursors under prebiotic conditions. Many biologically relevant molecules have even been found in comets and meteors, suggesting the building blocks of life are fairly common throughout the universe.
A singular living unit is the cell: a membrane bound chemical system, capable of absorbing nutrients from the environment, and making a nearly exact copy of itself. The main hypotheses regarding the origins of life tend to focus on 'information first' or 'metabolism first', although the notion of 'membranes first' deserves attention. [idea for future entry... what are the differences between the competing hypotheses of life's origins?]
Whatever the sequence of events, life seems to have established itself on Earth roughly 3.5 billion years ago. The exact date is under debate, due to the difficulty in demonstrating the existence of fossilized microbes in rocks so ancient and altered. Considerable amounts of research attempt to identify definitive biosignatures in the rock record. These studies will also help future work on Mars or other rocky bodies, in the search for possible fossilized extraterrestrials.
How does life evolve?
The theory of evolution is the cornerstone of modern biology. Evolution operates on populations over time, requiring variation in the genetics between individuals and a selective pressure that grants differing degrees of reproductive success based on the underlying genetics. "Nothing in biology makes sense except in the light of evolution," according to the evolutionary biologist (and Russian Orthodox Christian) Theodosius Dobzhansky.
What evidence do we have for evolution? All life on earth uses the same four DNA nucleotides to code for the same set of twenty amino acids that make up all the proteins required for the chemistry of life. Proteins central to metabolic processes are nearly identical across all species. Correlating the sequences of homologous proteins between species has allowed the construction of a phylogenetic tree of life, and suggests a theoretical Last Universal Common Ancestor (LUCA) to every living thing on the planet. Whether LUCA represents the first life on Earth or a later genetic bottleneck is still up for debate.
For the first three billion years, single celled organisms ruled the Earth. Only 500 million years ago did the first multi-celled organisms emerge. A key event in the transition to multicellularity was the capture and incorporation of a previously free living organism into another, or endosymbiosis. Mitochondria are membrane bound organelles that specialize in the production ATP (adenosine triphosphate), the energetic 'currency' of life; they also contain their own DNA, which replicates independently of the DNA in the cell's nucleus. Chloroplasts are the chlorophyll containing organelles in plants responsible for photosynthesis, and carry their own DNA, like mitochondria. Genetic sequencing has identified the free living bacteria that were probably involved in these capture events. Large, multi-celled organisms require greater amounts of energy than simple bacteria, and would not have been able to flourish without these specialized endosymbiotic organelles.
What is the distribution of life on Earth?
Over the course of 3.5 billion years, living things have spread across the entire planet, filling almost every niche imaginable. Even environments once considered inhospitable have been shown to harbor thriving communities of microorganisms. Life forms that thrive in conditions outside what we would expect are broadly classified as extremophiles, or 'extreme loving'. (although, some have pointed out this term is a misnomer, since the bacteria that live in hot springs are quite comfortable in that environment -- to the bacteria, WE are the extremophiles...) Thermophiles like high temperature, some living close to the boiling point of water. Psychrophiles prefer low temperature, and can be found living within ice. Halophiles require an environment with high salinity. Piezophiles need high pressure, and live at the bottom of the ocean or in deep sea trenches.
Astrobiologists study extremophiles to determine the absolute limits of life, the window of tolerance that defines the Habitable Zone (HZ). Some off-Earth locations within our solar system are known to have conditions within the range of habitability of some extremophiles. Dry windswept valleys in Antarctica have been examined as analogous to some spots on Mars, and revealed cryptoendolithic ('hidden within rock') microbes thriving in a seemingly barren wasteland. Almost everywhere we look on Earth, we can find an organism suitably adapted to the environment.
What is the future of Earth-based life?
Death comes to all living things. Not even the Sun or the Earth are eternal. Over the next several billion years the Sun will use up it's supply of hydrogen and expand, eventually consuming the orbits of the inner planets, including Earth. Any life will have long since perished, as the gradually increasing temperatures boil off the oceans and sterilize the planet. Geologists estimate higher life has about 500 million years remaining before conditions on Earth become too extreme. That will not be the end of all life on Earth; bacteria once ruled the planet for three billion years, and it is expected they will again reign supreme long after we are gone. A billion years from now, Earth could look very similar to what it looked like a billion years ago.
If Earth is the only planet in the universe to have developed life, the event will pass unremarked upon. Another star goes nova and all record of a self-aware universe reflecting back upon itself incinerated in an instant. Personally, I doubt such a cold and empty fate.
Giordano Bruno, heretical Dominican monk, was one of the first to read Copernicus and realize the stars were other other suns like our own with other solar systems. He saw an infinite universe, filled with planets like ours, teeming with life. The first confirmed extrasolar planet was announced in 1992. As of today, July 11, 2010, there are 464 confirmed exoplanets. The Kepler mission recently announced over 700 exoplanet candidates seeking confirmation from other telescopes, only a single year into its 3.5 to 6 year long planned operation.
Within twenty to thirty years, it is expected we will have telescopes capable of directly imaging the surfaces of distant worlds, observing the weather patterns and mapping continents. As our instrumentation improves, we will undoubtedly seek evidence of extraterrestrial life; and we will probably find it.
The Earth formed roughly 4.5 billion years ago, and suffered frequent sterilizing impacts from large meteors up until the Late Heavy Bombardment, approximately 4.1 to 3.8 billion years ago. Life took hold almost as soon as the Earth was habitable, around 3.5 billion years ago, and it has proven tenacious, filling every possible niche from deep sea vents to snow capped peaks.
It seems highly likely, from studying this single example of a planet with life, that life will find a way wherever conditions are favorable. Astrobiology is the emerging synthesis of many scientific disciplines meant to address these questions. The first aliens discovered will probably be microbes, but such an announcement could still profoundly affect the perception of our place in the universe. What would it mean to look up at the stars and know, not merely suspect, that there are other worlds with life out there? We may face such a night sky within our lifetimes.
