A Genetic Signature of Terrestrial Life
All life on Earth is genetically related through an evolutionary past that extends beyond 3.8 billion years ago. We see this relatedness in the many common structural and mechanistic features that make up all cells. The relationships between different terrestrial life forms are quantitatively explicit in the now-emerging maps of the course of evolution, phylogenetic trees based on DNA sequences. Even if potentially alien organisms were to present the same biochemistry as seen in terrestrial organisms, genetic sequences could provide criteria to distinguish them if they are of different evolutionary sources.
The gene sequence-based overview of terrestrial biological diversity is embodied in phylogenetic trees, relatedness diagrams such as that shown in Fig. 1 (9). The construction of a phylogenetic tree is conceptually simple. The number of differences between pairs of corresponding sequences from different organisms is taken to be some measure of the "evolutionary distance" that separates them. Pair-wise differences between the sequences of many organisms can be used to infer maps of the evolutionary paths that led to the modern-day sequences. The phylogenetic tree shown in Fig. 1 is based on small-subunit ribosomal RNA (rRNA) gene sequences, but the same topology results from comparing sequences of any other genes involved in the nucleic acid-based information-processing system of cells, the core of genetic continuity. On the other hand, phylogenetic trees based on metabolic genes, those involved in manipulation of small molecules and in interaction with the environment, sometimes are not congruent with the rRNA topology. Such genes do not offer any consistent alternative to the rRNA tree, however. Consequently, patterns that deviate from the rRNA tree probably are best interpreted to reflect lateral transfers of genes or even the intermixing of genomes in the course of evolution (10). The genome of any particular organism is comprised of genes derived evolutionarily from both vertical and lateral transmission.
Phylogenetic trees are rough maps of the evolution and diversification of life on Earth. From the standpoint of both sequence divergence and complexity, most of Earth's life is seen to be microbial in nature, which is surely what we need to expect of life that might occur elsewhere in our solar system. Some of the conclusions that can be drawn from the molecular trees verify previously advanced biological hypotheses. For instance, the molecular trees confirm what was once a hypothesis, that the major organelles, mitochondria and chloroplasts, were derived from bacteria, proteobacteria and cyanobacteria, respectively. The biochemical trait of oxygenic photosynthesis arose with the cyanobacterial radiation (indicated in Fig. 1 by the lines leading to Synechococcus, Gloeobacter, and chloroplast). Because the cyanobacterial radiation is peripheral in the tree, early life must have been anaerobic and most of bacterial diversification must have happened before the availability of oxygen.
Other conclusions from the molecular trees clarify relationships among terrestrial life. The molecular trees show that Earth's main lines of descent fall into three relatedness groups, the "domains" of Archaea, Bacteria, and Eucarya (eukaryotes). The point of origin of the lines leading to the modern domains cannot be determined by using rRNA gene sequences alone. Comparison of gene family sequences such as the protein synthesis factors Ef-Tu and Ef-G, that diverged before the last common ancestor of life, however, indicate an origin deep on the bacterial line as shown in Fig. 1. This relationship means that Archaea and Eucarya shared common ancestry subsequent to the separation of their common ancestor from Bacteria. Biochemical properties of the organisms are consistent with this conclusion. For example, the transcription and translation machineries of modern-day representatives of Archaea and Eucarya are far more similar to one another than either is to corresponding functions in Bacteria. This result shows that the eucaryal nuclear line of descent is not a relatively recent derivative of symbiosis, rather, is as old as the line of Archaea. This result also indicates that the common textbook presentation of life as divided into two categories, prokaryote and eukaryote, is incomplete. Rather, terrestrial life is of three kinds: archaeal, bacterial, and eucaryal, distinct from one another in fundamental ways.
Gene sequences that are common to all organisms are incisive signatures of terrestrial origin. This is because organisms with independent origins are unlikely to have evolved identical genetic sequences, even if the chemical structures of the subunits that comprise the genetic information were identical. Thus, in gene sequences we can recognize terrestrial life, distinguish it from life derived from a different evolutionary origin even in the face of substantial biochemical similarity. This would become a significant issue if lifeor its remainswere discovered on another body in the solar system.
Because planetary systems are formed by accretion, I think it unlikely that life on another body in the solar system arose independently of terrestrial life. It is now clear from meteorite studies that bodies can be transported from one planet to another, for instance from Mars to Earth, without excessive heating that would sterilize microbial organisms (11). Although such transfer events are now rare, they must have been far more frequent during the accretion of the planets. Large-scale infall, blasting ejecta throughout the forming solar system, probably extended until at least about 4 billion years ago and so probably overlapped with the processes that resulted in the origin of life. In principle, life, regardless of where it arose, could have survived interplanetary transport and seeded the solar system wherever conditions occur that are permissible to life. So, if we go to Mars or Europa and find living creatures there, and read their rRNA genes, we should not be surprised if the sequences fall into our own relatedness group, as articulated in the tree of life.