Life as we know it builds simple organic molecules that are used as building blocks for large molecules. Amino acids are used to construct the long chains of proteins; simple sugars combine with the purine and pyrimidine bases and phosphate to construct the nucleic acids. It seems logical that the evolution of any organic-based life form would similarly result in the construction of complex molecules as repeating structures of simple subunits. Indeed, it seems likely that the basic building blocks of life anywhere will be similar to our own, in the generality if not in the detail. Thus, the 20 common amino acids are the simplest carbon structures imaginable that can deliver the functional groups used in life, with properties such as repeating structure (the peptide unit), reactivity with water, and intrinsic chirality. Moreover, amino acids are formed readily from simple organic compounds and occur in extraterrestrial bodies such as meteorites, so are likely to form in any setting that results in the development of chemical complexity necessary for life.
Similarly, the five-carbon sugars used in nucleic acids are likely to be repeated themes, perhaps in part because they are the smallest sugars that can cyclize and thereby confer spatial orientation on other molecules, for instance the purines and pyrimidines that comprise the genetic information of terrestrial organisms. Further, because of the unique abilities of purines and pyrimidines to interact with one another with particular specificity, these subunits, too, or something very similar to them, are likely to be common to life wherever it occurs. Differences in evolutionary systems likely will lie at the higher-order levels: the structures of the large molecules assembled from the simple units, and the mechanisms by which they are assembled and in which they participate.
Themes that are probably common to life everywhere extend beyond the building blocks. Energy transformation is a critical issue. The processes of life require the capture of adequate energy, from physical or chemical processes, to conduct the chemical transformations requisite for life. Based on thermodynamics there are only two such energy-capturing processes that can support "primary productivity," the synthesis of biological materials from inorganic carbon dioxide. One process, termed lithotrophy, involves the oxidation and concomitant reduction of geochemical compounds. For instance, methanogenic organisms gain energy for growth by the use of hydrogen (H2) as a source of high-energy electrons, which are transferred to carbon dioxide (CO2), forming methane (CH4). Other microbes might use hydrogen sulfide (H2S) as an energy source, respiring with oxygen (O2), to produce sulfuric acid (H2SO4). It is thought that the earliest life on Earth relied on lithotrophic metabolism.
The second general process for obtaining energy, photosynthesis, captures light energy and converts it into energetic electrons that can be used to accomplish biochemical tasks. Photosynthesis arose early in the history of terrestrial life and probably drives most primary productivity on Earth today. The contribution of lithotrophy to terrestrial primary productivity remains unknown, however, because there currently is little information on such organisms that may be distributed throughout the Earth's crust, wherever the physical conditions permit.
Although terrestrial life and life that might arise independently of Earth are expected to use many similar, if not identical, building blocks, they also are expected to have some biochemical qualities that are unique. This expectation is based on the fact that different evolutionary lines of terrestrial evolution also have engendered novelties unique to those lines. Thus, the biochemistry of methanogenesis arose uniquely in Archaea, whereas the property of chlorophyll-based photosynthesis was invented among the phylogenetic domain Bacteria (below). The cytoskeleton, which is probably a requirement for large and complex cell structure arose in the eukaryotes. Considering the variety of Earth's life, novelty, as well as commonality, must be expected elsewhere.