By any measure relating our corporeal dimensions to those of the universe, elsewhere is a vast domain containing fabulous variety. We know of one life-bearing planet and have a working grasp of its molecular biology. If we seal in a flask the first thirty elements in their local relative abundances and add any energy source there will be formed within a few weeks a red-brown mess that includes amino acids, fatty acids, sugars, nucleotides... in short, a comprehensive sampling of the fundamental chemical inventory of life. Life is the default status of any planet orbiting a star so as to obtain temperatures between the freezing and boiling points of water. Are we doomed to stare out into space and one fine day confront what mostly sums to ourselves staring back?
The freezing and boiling points of water have a lot of slack in their gears. Water freezes at -22°C under 30,000 psi. Pore water freezes at -40°C. Above its critical pressure of 3200 psi, water is simply a fluid. Deep sea hyperthermophile Methanopyrus kandleri strain 116 tolerates 122°C.
Biochemistries based upon silicon, liquid ammonia, and so on do not exist even on paper. Attempts to alter abiotic syntheses by varying ratios and conditions have failed. Primary planetary biochemistry may be constrained within the boundaries of what we know to be terrestrial. Can alternate life find its way?
The freezing point of bulk water can be nicely depressed by biochemical "antifreeze" sugars, zwitterions, and proteins - but not before they evolve into existance. Arctic and Antarctic organisms exist taht make their own antifreeze but are otherwise ordinary. Reaction rates roughly halve for every ten degrees Celsius drop in temperature. Very cold life is unpromising. Poikilothermic (without regulated body temperature) carnivores have found ways to keep warm and active - tuna are vicious. Herbivores are not contenders. What adequacies are required to stalk and capture a leaf?
The boiling point of water is variable, increasing with pressure. Hot springs host algae and bacteria preferring temperatures of 70° to 90°C. Above its critical pressure of 218.3 atmospheres (2.2 kilometers of ocean depth) water does not boil at any temperature - it only varies in density. That density is exquisitely sensitive to pressure and temperature. Life under hyperthermal circumstances can be at the mercy of an invariant environment. Deep sea hydrothermal vents support archeobacterial populations demanding 110 degrees and more. Their enzymes are heavily crosslinked with disulfide bonds and ionic zinc fingers to prevent thermal denaturation. Their cell membranes substitute the ubiquitous lipid bilayer of undergraduate biology with double-stranded polyisoprenyl ethers. Life under these bizarre circumstances, though it will not perform in our gentler climes, is nevertheless founded upon remarkably unremarkable biochemistry.
Is this all that there is? Have we endured 386 words to go home without Star Trek or Alien? Don't bet on it.
Genetic information, the blueprint for protein synthesis, is encoded within DNA and RNA. The polymeric sugar phosphodiester backbones of these molecules is a unique and, in the case of RNA, labile construct containing no information. The composition of these supports can be varied without degrading overall structure. The promise of endless varieties of evolved functions of proteins can be accessed. If there exists life more robust and varied than the local product, we have a model of how it might work.
The nucleic acid sugar phosphodiester backbone uniformly and rigidly spaces and orients the five bases (adenine, thymine (DNA only), cytosine, guanine and uracil (RNA only)) whose triplet sequences encode genetic information. It is plausibly supplanted by anything which in turn can identically space and orient those bases - like a peptide chain. Linear poly[N-(2-aminoethyl)] glycine has precisely the right pitch spacing to do the job. If we acetylate each non-amide secondary nitrogen group and hook a nucleotide base to that spacer's alpha carbon, we obtain peptide nucleic "acids." They are a commercial synthetic product for nucleic acid research. They outperform the natural product in every possible way including resistance to environmental and enzymatic degradations. Protein is limitlessly plastic in its evolvable abilities to perform structural or metabolic functions. Were these man-made materials accompanied by a suite of synthetic enzymes, life as we know it (that's us, buster) would be pushed aside by a more able competitor.
Terrestrial evolution has strictly proceeded through modification of protein structure encoded by nucleic acid base sequence mutation. Even archeobacteria with their profoundly disparate proteins and cell membranes would recognize the genetic structure of an accountant. Imagine a world wherein life could evolve through both protein and nucleic acid structure modification. Our most virulent viruses - wandering nucleic acid strands seeking to suborn cellular apparatus for their reproduction - would be beneath notice. Natural selection would fight to the death with the molecular equivalent of nuclear weaponry.
The alien conquest of planet Earth is always depicted as physical frightfulness visited upon an awesome scale. It might be nothing more than a friendly handshake and a dirty spacesuit. Perhaps the time has come to contemplate building a better life than a putatively omniscient deity saw fit to manage.