Origin of Life

From the myths of antiquity to the era of modern science, questions concerning origins have a special fascination. How did the universe come into being? The earth and the solar system? Life itself? While the origin of life on earth is an obvious fact, given that we are here to pose the question, how life came to be is critical to any speculations regarding the probability of life on other worlds. Studies of the genesis of life fall into two broad categories. The first involves direct observation and documentation of the occurrence of fossils of the earliest life forms. These data provide the basis for reconstructing the early evolutionary history of life on earth.

Evidence from the fields of cosmology, astronomy, chemistry, physics, and geology all converge to indicate that the primordial crust of the planet was formed between 4.6 and 4.5 billion years ago (4.6 to 4.5 Ga.). The most critical events leading to the origin and early diversification of life fall in the first half of this long history, resulting in a challenge to investigators seeking direct fossil evidence for these events. Rocks of this great age are rare, confined to limited exposures in the interior of ancient continental shields. Much of the rock from this remote period is igneous in origin or has been subject to such intense metamorphism that any fossils that once might have been present are absent or altered beyond recognition. With the exception of layered columnar fossils called stromatolites, most of the fossils are microscopic cells about the size of modern bacteria, ranging from isolated cells to simple chains or filaments. Most are observed by microscopic study of thin sections of promising rock samples. The major challenge in studies of this type is differentiating actual microscopic fossils from inclusions or other small structures in the rock that might resemble simple cells (see Schopf and Walter, 1983).

3.5 Ga filamentous prokaryote from western Australia

The earliest cellular remains recognize to date all date from approximately 3.6 to 3.5 Ga, and include material from the North Pole Dome area of Australia (3.56 Ga) and the Onverwacht Group of South Africa (3.54 Ga). All the microfossils in these deposits appear to represent simple prokaryotic organisms, cells characterized by very small size and relatively simple structure, lacking an organized nucleus or other complex internal structure. In form, the fossils include isolated cells, small clumps or aggregates of cells, simple filaments, and laminated sheets of cells, each layer being one cell thick, accumulating in multiple layers that result in the characteristic columnar fossils known as stromatolites. The fossils appear to document a moderately diverse arays of organisms including bacteria that obtain their energy from the breakdown of organic molecules (heterotrophic metabolism) to photosynthetic cyanobacteria that utilized solar energy to synthesize complex organic molecules (autotrophic metabolism). The degree of diversity represented by the earliest known microfossil assemblages would suggest that the actual origin of life occurred somewhat prior to 3.6 Ga.

The diversity of both heterotrophic and autotrophic prokaryotic organisms increases steadily in somewhat younger rocks, with a corresponding increase in the diversity of filamentous and colonial growth forms.

A diverse array of cellular growth forms from the Gunflint chert (Schopf and Barghoorn).

A stromatolite from the Upper Peninsula of Michigan. The striated appearance typical of these fossils results from the accumulation of layers of sheet-like cyanobacterial colonies.

An artist's impression of how groups of cyanobacterial mats (stromatolites) would have appeared along a shallow marine coastline approximately 2 Ga ago.

Stromatolites become particularly abundant beginning about 2.8 Ga. Approximately 1.5 Ga. there is an increase in the size and complexity of cell fossils, marking the appearance of eukaryotic cells (cells with a true nucleus and relatively complex internal structure) of the type found in most organisms today.

Individual fossil specimens from the Bitter Springs Formation of Australia (Schopf) arranged in a sequence that suggests cell division. The fact that these cells contain a small, darker mass inside the body of the cell, suggests that these may be remains of an early eukaryote.

By perhaps 1 Ga. these more complex cells begin to become organized and specialized in true multicellular plants and animals. At the same time, stromatolites, which had dominated the macrofossil record for almost 2 Ga., begin a precipitous decline in abundance, possibly as a consequence of predation by increasing complex animal-like organisms. While direct observation of microscopic fossil remains can provide a basic time-frame for the earliest stages in the history of life, data from a diverse array of disciplines are required to reconstruct how the first organisms came to be and what factors impacted the course of their evolution.

Three attributes of the pre-biotic earth were critical in the chain of events that would lead to the appearance of the first life forms. These included the composition of the ancient atmosphere, the planetary temperature regime, and the presence of oceans comprised of liquid water. The earliest or primary atmosphere was undoubtedly like that of the outer "gas giants" of the solar system, consisting primarily of hydrogen and helium, with lesser quantities of methane, ammonia, and water vapor. Early attempts to reconstruct the origin of life assumed that it was this type of atmosphere that was present when life appeared. More recent work suggests that much of this primary atmosphere may have been lost to space during the short molten period following the initial accretion of the planet itself. According to this view, the primary atmosphere was replaced by out-gassing from the interior of the planet as it cooled. Based on the analysis of present-day outgassing from volcanoes, the secondary atmosphere was made up primarily of nitrogen, carbon dioxide, and water, with relatively little hydrogen, compared to the primary atmosphere. Secondary reactions would have added gasses such as ammonia and methane to this atmospheric "mix". The actual ratio of the various gasses that made up the atmosphere at the time of the origin of life may never be known, but the proportions of the various gasses does not seem to be critical to the events which followed. What is critical is that none of the models for the atmosphere include free oxygen. A lack of oxygen is consistent with the known atmospheres of any of the planets in the solar system.

As the crust formed with the initial cooling of the earth, much of the water that would make up the oceans was resident in the atmosphere as water vapor. Astronomical evidence suggests that the sun was sufficiently less energetic in terms of output during this period that, as the surface cooled, much of this water might have accumulated as ice. Fortunately, both methane and carbon dioxide are "greenhouse gasses", which helped to retain solar energy as the planet cooled, resulting in the formation of liquid oceans as water vapor condensed from the atmosphere. The mode of ocean formation (a single sustained condensation event or multiple events) and timing of ocean formation are conjectural. The oldest sedimentary rock record (metamorphosed) from the Ishua Series in Greenland suggests that significant liquid water was present by 3.8 Ga.

Two primary models for the origin of life have been proposed. The most popular assumes that life appeared as the final consequence of a period of pre-biotic chemical and biochemical evolution, based on the reaction of elements and compounds in the early environment. In contrast, a second possibility, generally known as "panspermia", suggests that the early earth may have been "seeded" by viable spore-like bodies drifting through space, much as spores of fungi and bacteria will contaminate a culture dish that is opened in the laboratory.

As late as the 1970s, most paleobiologists assumed that the origin of life might have required a billion years or more, following the initial formation of the oceans. With the actual fossil evidence of the earliest cells (3.6 Ga.) gradually being pushed back in time toward the origin of the oceans (3.8 Ga.), whatever the mechanism for life's origin, it is clear that the process may have occurred in as little as 200 to 300 million years. This narrowing of the time "window" for the origin of life might appear to support the panspermia model, but the extreme conditions of open space make it unlikely that living cells could retain their viability for the long ages required for such particles to drift between solar systems. While extraterrestrial sources for living cells seem unlikely, studies of meteorites and comets confirm that a wide range of simple to moderately complex organic molecules do arrive from space (see Chyba et al, 1990). Such chemical "seeding" might well speed up the chemical evolution sequence leading to the first cells. [NOTE ADDED TO THE ORIGINAL MANUSCRIPT: the recent reports of cell-like bodies in meteorites of possible Martian origin revives the panspermia debate, since the arrival of viable cells from Mars does not represent a major problem, compared to passage between star systems.]

The first stage in the origin of life involves non-biological synthesis of simple biochemical "building blocks" (monomers), followed by the linkage of these units into more complex biochemical molecules called polymers. Numerous experiments, involving water, gas mixtures analogous to the early atmosphere, and various energy sources (electrical discharge, heat, ultra- violet radiation, etc.) suggest that a diverse array of monomers can be spontaneously formed by reactions between elements and simple compounds that were present in the pre-biotic earth. These experiments, often called "Miller Experiments" in recognition of the pioneering work of Stanley Miller in 1953, produce varying mixtures of amino acids, nucleotides, simple carbohydrates and fatty acids, and critical energy-rich molecules such as adenosine tri-phosphate (ATP). The basis of the energy metabolism of all living cells is the breakdown of ATP molecules.

A diagram of the apparatus used in a basic "Miller Experiment". The precise gas mixture does not appear to be important nor does the source of energy (Campbell, 1996, Biology)

In living things, simple monomers are linked to form larger polymers, made up of repetitive sequences of monomer sub-units. Amino acids are linked to form proteins, simple carbohydrates polymerize to form complex sugars and starches, fatty acids make up lipids, and nucleotides link to form nucleic acids such as RNA and DNA. Simple monomers, in a dilute biochemical "soup" are unlikely to polymerize effectively without additional concentration and precise spatial orientation. Recent studies suggest that clay minerals may have played a critical role in polymerizing more complex molecules. Charge distribution on the surface of exposed clay minerals can attract and concentrate monomers from the surrounding water, simultaneously orienting the molecular sub-units and catalyzing their linkage. Such linkages would be accelerated if ATP molecules were also present to serve as an energy source. None of these processes would have occurred at significant rates on land or in shallow water due to the disruptive effects of intense ultra-violet radiation from the sun (with no oxygen, there was no protective ozone shield in the upper atmosphere). Submarine vents have been suggested as one possible site that would have provided protection from U.V. exposure while providing a rich chemical environment.

A living cell is more than a simple collection of polymers and additional processes must have facilitated the formation of structures analogous to cell walls and membranes. Heating and cooling and wetting and drying of amino acid mixtures has been shown to result in the formation of small cell-like structures called "proteinoid microspheres". When lipids are present, the walls of these proto-cells consist of a proteinoid-lipid complex with some of the differential permeability found in cell membranes. These microspheres can differentially accumulate various biochemical molecules, leading to the initial stages in the evolution of membrane-bound chemical systems. Although these microspheres can "grow" and even bud off new microspheres, they are not alive since they cannot replicate with precision. There were probably at least two crucial steps in the evolutionary transition from non-living microspheres to the simplest prokaryotic cells. The first was the evolution of protein catalysts (enzymes) that would facilitate a wider range of chemical reactions, followed by the evolution of a simple genetic system to assure reliable duplication of these enzymes.

In all modern cells, genetic coding is a three-element process. The primary coding is in the form of double-stranded DNA. Simpler, single strand RNA molecules are constructed from the DNA template, and the RNA units serve as the pattern for protein/enzyme synthesis. The recent discovery of simple RNA sequences that will catalyze their own replication suggests that the earliest genetic code may have used RNA as the primary information storage with direct synthesis of proteins from RNA strands. DNA molecules are more stable than RNA however, and selection may well have favored the synthesis of DNA copies of the RNA units, resulting in DNA assuming the primary information storage function.

The development of a stable, DNA-based genetic system was probably the critical even in the transition from non-living membrane-bound chemical entities to the simplest prokaryotic cells. While multiple life origin events are a theoretical possibility, the universal nature of the DNA genetic code and the extreme specificity of living cells with respect to chemical isomers, suggests that all living cells today are probably descended from a single primordial cell type. Other proto-cell types may have evolved, but, if they did, they were ultimately displaced by the descendants of the single cell lineage.

Structurally and chemically, the simplest living cells are heterotrophic, bacteria-like cells that synthesize ATP (the universal energy "currency" of life) using energy obtained from the breakdown of carbohydrates and other organic polymers. The first cells probably functioned in this manner and used molecules from the surrounding "organic soup" as their food source. In the absence of oxygen, such breakdown reactions are inefficient, in that the "food" molecules are only partially disassembled and the ATP yield is minimal, but the process does work. This simple reaction system, known as "glycolysis" or "anaerobic respiration", is still used as the preliminary step in cell respiration in all living prokaryotic and eukaryotic cells.

Limitations in the availability of "food" molecules might well have resulted in the eventual extinction of these first life forms were it not for the appearance of autotrophic prokaryotes that could use an outside energy source to synthesize energy-rich molecules. The most successful of these early autotrophs were the cyanobacteria, which used light (photosynthesis) as their energy source. The evolution of the cyanobacteria may well be linked to the intense U.V. levels near the ocean surface as a result of the lack of oxygen and an ozone shield. If cells were to survive in shallower water, one solution would be internal shields that would protect the cell from U.V. exposure. Chlorophyll is a pigment that absorbs blue and U.V. light and chlorophyll pigments, incorporated into membranes, would make an effective U.V. shield. Early cyanobacteria, with such shields, could survive in sunlit waters. Chlorophyll, when excited by U.V. exposure, generates "high-energy" electrons. The development of very simple electron transport systems would allow some of this energy to be channeled into ATP synthesis. With ample supplies of ATP, synthesized from solar energy, cyanobacteria could essentially operate anaerobic respiration reactions "in reverse", producing energy rich molecules like sugars from simple molecules such as carbon dioxide. This is a very simple form of photosynthesis which does not produce oxygen, and is essentially identical to a little-used form of photosynthesis (cyclic photo-phosphorylation) found in all plant-like cells. The early appearance of photosynthetic cyanobacteria represented the first stable balance between photosynthetic "producers" and heterotrophic "consumers" that has been the basis for ecosystem structure since that time.

Gleocapsa (upper) and Anabaena (lower) two examples of living cyanobacteria.

The most successful cyanobacteria were the stromatolites and their photosynthetic activity provided the resource base for diversification of heterotrophic prokaryotes. Their photosynthesis did require sources of hydrogen, which initially would have been available directly from the atmosphere either as hydrogen gas or hydrogen sulfide. Hydrogen, the lightest of the elements, was the gas most prone to leakage into space and, by about 2.5 Ga., cyanobacteria faced a hydrogen crises. The solution was to adapt to the use of a new hydrogen source - water! Elaboration of the light reactions of photosynthesis led to a new reaction (photolysis) that split water into its component hydrogen and oxygen. The oxygen was an unneeded (and toxic) byproduct that was released into the surrounding environment.

As a result of this new variant of photosynthesis, oxygen gradually began to be added to the atmosphere between 2.5 and 2.0 Ga., creating a serious biological crisis. Oxygen is a reactive gas that is toxic to cells and, as oxygen levels increased (essentially oxygen pollution of the atmosphere), all cells were increasingly at risk. The solution to his crisis was the evolution of new variations on respiratory metabolic pathways that stripped hydrogen from "food" molecules and linked them up with any oxygen molecules entering the cells. The evolution of this new metabolic variant, known as aerobic respiration, had two consequences. The first was the protection of the cell from oxygen (the water produced when hydrogen was combined with oxygen was harmless) while the second was a huge increase in metabolic efficiency. Being able to strip hydrogen from complex molecules meant that food molecules such as sugar could be broken completely down to carbon dioxide and water with an ATP yield that was ten to fifteen times greater than what could be accomplished with anaerobic metabolism! Aerobic cells that could metabolize oxygen were thus not only protected from its toxic effects, they also enjoyed a ten-fold increase in available energy with the same food intake.

The large increase in energy yields brought about by aerobic metabolism provided the energy resources for rapid evolutionary innovation between 2.0 and 1.5 Ga., leading to the appearance of larger and more complex eukaryotic cells and, after 1 Ga., the evolution of complex multicellular life forms. In effect, photolysis created and maintains the earth's oxygen-rich atmosphere, a chemical anomaly with respect to all other atmospheres in our solar system. Aerobic respiration, a metabolic solution to the problem of increasing oxygen levels in the atmosphere, led to the evolution of complex, multicellular life forms. Together, the two evolutionary innovations have led to the paradox where almost all organisms in the biosphere require constant supplies of a biochemically toxic gas to maintain their sophisticated cell structure. The descendants of prokaryotic bacteria that did not evolve pathways to disable oxygen within their cells (obligate anaerobes) became progressively less common as oxygen levels continued to rise through the late Precambrian and Paleozoic and are now confined to rare environments where oxygen is essentially absent. In effect, the entire fabric of life reflects events and innovations dating back to the dawn of life itself.

Chyba, C.F., P.J. Thomas, L. Brookshaw, and C. Sagan, 1990. Cometary delivery of organic molecules to the early Earth, Science 249:366-373.

Schopf, J.W. and M.R. Walter. 1983. Archean microfosssils: new evidence of ancient microbes, pp. 214-239 in Schopf, J.W. (ed.), Earth's earliest biosphere: its origin and evolution. Princeton University Press, Princeton, N.J.

Ralph E. Taggart (taggart@msu.edu)