Our perspective on the nature of the geochemical processes that gave rise to life on Earth has improved enormously over the past decade. Astronomical and geochemical studies provide an increasingly clear view of the character of the early Earth, at the time of the origin of life. Microfossil work is increasingly sophisticated in its interpretations and has provided solid evidence for life at 3.5 billion years ago or before. Perhaps most dramatically, we can now infer with increasing confidence the biochemical nature of the earliest life, which also provides information on the environment in which that life came to be. The record that we interpret for the nature of the earliest life is a quantitative one, phylogenetic trees based on molecules that are found in all of modern-day life and so must have been present in the earliest genetic cell. Together, the biological and physical information paint a harsh cradle for life, a violent world of volcanism and hydrothermal interaction with the molten bones of the forming planet. We must couch our speculation on the origin of life in terms of these findings.

The Phylogenetic Record

The geophysical record tells us that the early Earth was hot and volcanic, but it tells us nothing about the nature of the earliest organisms. By analysis of molecular sequences, we can infer maps of the course of evolution. In this analysis, "homologous" (of common ancestry) genes from different organisms are compared, and the number of differences in their nucleotide sequences is counted. The greater the number of differences between the genes, the more evolutionarily separated are the pairs of organisms. The "evolutionary distance," the fraction of sequence differences between pairs in a collection of sequences, can be used to infer "phylogenetic trees," maps of evolutionary diversification. Because of their slow rates of evolutionary change, small-subunit (16S or 18S) ribosomal RNA (rRNA) sequences have been used most extensively for phylogenetic classification of all life.

The molecular phylogenetic studies provided, for the first time, a clear view of the evolutionary scope of life on the grand scale. The figure shows a phylogenetic tree based on rRNA sequence comparisons. It can be viewed as a roadmap of evolution. Evolutionary distances between organisms (i.e., the number of sequence-changes separating them) are represented by the lengths of line-segments (see scale bar in the figure). The phylogenetic results show that known biodiversity falls into three primary groupings, or 'Domains': Archaea (formerly archaebacteria), Bacteria (eubacteria), and Eucarya (eucaryotes). Although both are procaryotic (i.e., lack a nuclear membrane), Archaea and Bacteria are phylogenetically distinct from one another. Indeed, there is strong evidence indicating that Archaea and Eucarya are more closely related to each other than either is to Bacteria: the origin of the extant evolutionary lineages, the root of the tree of all life (indicated as "origin" in the figure), appears to separate the Bacteria from the other two forms.

Some of the lessons gleaned from the universal tree of life contradict long-held beliefs. For instance, the recognition of the deep evolutionary divergence between Archaea and Bacteria shattered the entrenched notion of evolutionary unity among procaryotes. The phylogenetic results also show that the eucaryotic nuclear line of descent, Eucarya, is as old as the procaryotic lines, Archaea and Bacteria. The idea that the eucaryotic cell arose relatively recently (1-1.5 billion years ago was a common assumption based on no credible data), from the fusion of two procaryotes, proved essentially incorrect. To be sure, the rRNA-based data confirm that mitochondria and chloroplasts are of endosymbiotic bacterial origin, as postulated long ago (note in the figure the association of the mitochondrial and chloroplast lineages with Bacteria). The organellar lineages emerge from subgroups of the Bacteria, however, so their evolution must have occurred relatively late in the history of life. Some microbial eucaryotic lineages seem never to have had mitochondria and chloroplasts so may have diverged from the eucaryotic line of descent prior to the incorporation of the organelles.

Although phylogenetic trees are based on modern-day organisms, we can, in principle, infer properties of ancestral organisms by "parsimony": if two organisms have a common trait, then it is likely that their common ancestor had the same trait. With that perspective, we can infer that the last common ancestor of all life possessed DNA, a rudimentary transcription system, ribosomes similar to the modern ones, and many other traits--all the commonalities of today's life. More specialized traits of the earliest organisms also are suggested by the phylogenetic results. In the figure, organisms shown in bold are high-temperature organisms, extreme thermophiles. It is noteworthy that all of the most deeply diverging lineages in both Bacteria and Archaea are thermophiles. This suggests that their common ancestor, the common ancestor of all life, was thermophilic as well. Additionally, all of these types of organisms have physiologies consistent with a geothermal setting. All thrive by metabolizing compounds such as hydrogen, sulfur, and iron.

The Setting for the Origin of Life

We can now imagine, based on solid results, a fairly credible scenario for the terrestrial events that set the stage for the origin of life. It seems fairly clear, now, that the earliest Earth was, in essence, a molten ball with an atmosphere of high-pressure steam, carbon dioxide, nitrogen, and the other products of volcanic emission from the differentiating planet. The cooling curve of the molten Earth would have reached the critical point of water by roughly 4.2-4.4 billion years ago; then it would have rained, depositing during an unknown time period 3-4 kilometers of water over the hot crust. It seems unlikely that any landmass would have reached above the waves, to form the "tidal pools" invoked by some theories for the origin of life. This is because the early crust would have been dynamic and thin and consequently probably incapable of supporting a continental mass sufficient to reach the surface of the overlying ocean. Instead, the conspicuous feature of that early Earth would have been the zone of interaction between the water and the cooling crust. Water would have taken the main role of conducting heat from the interior of the Earth, as it does now along the Mid-Ocean Ridge; the hydrosphere would have been in dynamic interaction with the forming crust through convective hydrothermal circulation. We can imagine that this early hydrothermal circulation would have stirred up a thick sludge of volcanic elements, probably highly enriched with reactive kerogens and organic tars extracted by convection from the differentiating planetary mass. It was in this sludge, a perfect setting for complex organic surface-chemistry supported by pyrite and basaltic glasses, that life probably came to be. This physical scenario is probably a common one in the formation of rocky, iron-rich planets. If so, then the origin of life may be a common consequence of planetary orgins. Perhaps the question is not the probability of the origin of life but rather the probability that life, having arisen, survives and comes to dominate a planet.

The Chemistry of the Origin of Life

We know very little about the course of chemical events that gave rise to life's molecules. Since the setting was geothermal, the most fruitful arena for experimentation would seem to be the interactions of iron, sulfur, and hydrogen with carbon compounds. This notion is also indicated by the biological record: iron and sulfur are the basis for many universally conserved biological catalysts which, because of their universality, probably were employed by the earliest life. Since evolution builds upon the pre-existing, iron and sulfur are indicated as main proto-physiological catalysts. For the same reason, I think it unlikely that clays or like minerals played much role in the origin of life. If they had, we might expect materials such as aluminum and silicate to be widely distributed in biology. Instead, these compounds are rare.

Considerable attention has focused on RNA as the first catalyst with genetic relevance, but the evidence for this is hypothetical: RNA is the only (known) biological molecule that is capable of serving as both catalyst and genetic information. As argued previously, however, the fragile chemical nature of RNA is inconsistent with the hydrothermal fluids that bathed early life. There probably was no "first self-replicating catalyst." Rather, following the tenets of biology, it seems much more likely that RNA, DNA, and the other molecules of cells co-evolved, as generally is the case with interacting organisms today.

Image of Tree of all Life

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Last updated Feb-11-1997

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