The origin of life on earth

The Earth is thought to have formed 4.5-4.6 billion years ago, along with the other planets in our solar system, from a protoplanetary disc of gas and dust surrounding the sun (Figure 7.1). A period of intense bombardment from meteorites, asteroids and other protoplanets, swept up by the gravitational pull of the Earth, would have prevented the formation of life during the early stages. Life is thought to have arisen between 4.0 and 3.8 billion years ago. There is convincing fossil evidence for life 3.5 billion years ago and plausible evidence for life 3.85 billion years ago. This suggests that life developed on Earth as soon as there were conditions suitable for its existence. The major systems that needed to develop in order to produce something that we would recognise as being an organism are: a method of harvesting energy from some available source, the formation of the complex organic molecules found in organisms from inorganic or simple organic molecules, the development of an information-encoding and self-replicating molecule capable of supporting a process of evolution (such as DNA or RNA), and the formation of a membrane which encloses the organism.

Complex organic molecules may have formed on the Earth itself or have been carried to Earth from elsewhere. It is thought that the first organisms were heterotrophs, utilising these pre-existing organic compounds as a source of energy, rather than autotrophs, which produce organic compounds from inorganic compounds using the energy from o o 2 < 2

PHANEROZOIC

PROTEROZOIC

first animals eukaryotes aerobic atmosphere photosynthesis

ARCHEAN

oldest fossils origin of life formation of Earth

CENOZOIC

diversification of mammals

CRETACEOUS

IC OI

dinosaurs, spread of mammals

Z

s

JURASSIC

dinosaurs, first birds

TRIASSIC dinosaurs, first mammals

PERMIAN

mammal-like reptiles

CARBONIFEROUS

IC

first reptiles

DEVONIAN

O Z O

first terrestrial vertebrates

J

SILURIAN first terrestrial arthropods

ORDOVICIAN first vertebrates

CAMBRIAN

origin of many invertebrate phyla

figure 7.1 Geological timescale showing the major geological periods and some of the major events during the history of life on Earth.

sunlight or the oxidation of chemicals. The first attempt to simulate the formation of organic compounds on the early Earth was made by Stanley Miller in 1953, then a graduate student working under the supervision of the chemist Harold Urey. The Miller-Urey experiment consisted of a sealed glass apparatus in which a reservoir of liquid water was heated in the presence of an 'atmosphere' consisting of ammonia, methane, water vapour and hydrogen gas (Figure 7.2). Energy was supplied to the gas phase via an electrical discharge to simulate the effects of lightning. After running for several weeks, the apparatus was found to contain a wide variety of complex organic molecules, including hydroxy acids, aliphatic acids, urea and amino acids. Amino acids are the building blocks of proteins and they comprised about 4 per cent of the organic compounds formed in the experiment. Other sources of

HEAT

products dissolve in water

HEAT

products dissolve in water figure 7.2 Diagrammatic representation of the Miller-Urey experiment.

energy, such as ultraviolet (UV) light, produce similar results. The production of organic compounds, however, is low if the conditions in the vessel are not strongly reducing (high in hydrogen, low in oxygen). If the carbon is present as carbon dioxide (rather than methane) and the nitrogen as nitrogen gas (rather than ammonia), the yield of organic compounds is reduced a 1000-fold. If the conditions on the early Earth were strongly reducing, one can imagine the accumulation of organic compounds in the oceans to form the famous 'primordial soup' from which life could develop. In recent years, the most commonly held view, however, is that the early atmosphere was not strongly reducing but a weakly reducing mixture of mainly carbon dioxide, nitrogen gas and water vapour. More strongly reducing conditions could still have existed on a more localised basis, or for limited periods of time, allowing the accumulation of organic compounds.

The discovery of deep-sea hydrothermal vents in the late 1970s (see Chapter 2) suggested these as a potential site for the production of organic compounds. Water enters these systems from the sea by percolating through cracks formed as the Earth's crust cools and moves away from the mid-ocean spreading centres. It gets heated to high temperatures within these systems, which assists it to dissolve materials from the surrounding rocks. The water is injected back into the ocean at the vents at temperatures up to 370°C, but rapidly cools as it mixes with ocean water. Several mechanisms for the production of organic compounds in these systems have been suggested and experiments that, it is claimed, simulate their conditions have produced organic compounds similar to those formed in the Miller-Urey experiments. Some, however, doubt whether these experiments simulate prebiotic conditions and point out that the high temperatures of vents are likely to destroy, and not create, organic compounds.

There are potential extraterrestrial sources of organic compounds. Comets, asteroids and meteorites impacted with the Earth at a high rate during its early history and material continues to do so today - fortunately at a lower rate. A wide variety of organic compounds have been detected in interstellar dust clouds by radioastronomy, which has detected nearly 100 different molecules in these clouds. These include the amino acid glycine. Organic compounds have been detected in comets and also in meteorites collected on Earth, including purines and pyrimidines which are components of nucleic acids. A class of meteorites called carbonaceous chondrites can contain more than 3 per cent carbon compounds with a variety of organic compounds, including amino acids and polycyclic aromatic hydrocarbons. It might be expected that organic molecules in meteorites, and other extraterrestrial bodies, would be destroyed by the heat generated during entry through the Earth's atmosphere. Micrometeorites (up to half a millimetre in diameter) and the dust from cometary tails are likely to have a soft landing, which would preserve their organic compounds. These are thought collectively to deliver 100 times more material to the Earth than do larger, but rarer, meteorites. They, together with the portion of larger bodies which survives cataclysmic impact with the Earth, could deliver significant quantities of organic compounds.

There are thus several potential sources of organic compounds for the early Earth. It is hard to judge their relative importance. If the atmosphere was strongly reducing, the sorts of mechanisms highlighted by the Miller-Urey experiment were likely to be predominant. Under weakly reducing or oxidising conditions, hydrothermal systems and extraterrestrial sources may have been more important. Whatever the mechanism of their formation, these relatively simple organic compounds would need to interact to form the more complex compounds found in organisms.

Organic compounds dissolved in the seas of the early Earth were likely to form a fairly weak solution. In order to interact to form complex molecules, they needed to become more concentrated. One possibility is that they were absorbed onto the surface of clays or other minerals. Assuming the Earth was not completely covered by oceans, the margins of the land would have provided plenty of opportunities for the formation of pools or lagoons in which the organic material could become concentrated by the evaporation of water. Stanley Miller, who has maintained his interest in the question of the origin of life since his graduate studies in the 1950s, favours this 'prebiotic beach' hypothesis. The Moon and the Earth were much closer together early in their history and tides may have been 30 times greater than they are today. This would have allowed the formation of numerous tidal pools in which life could begin.

RNA is likely to have preceded DNA as the self-replicating informational molecule of life, since it is a simpler molecule and has catalytic properties that may assist its functioning. In modern cells, RNA catalysts, called ribozymes, facilitate the formation of new RNA and a variety of reactions involved in protein synthesis. Since RNA can catalyse its own formation, it could have formed self-replicating molecules at a time when there was no DNA and no protein enzymes (referred to as the 'RNA world'). RNA is itself likely to have been preceded by simpler self-replicating organic molecules and/or self-replicating non-organic systems such as clays or minerals. Membranes may have developed from bubbles or froth formed in the primordial soup, by gas bubbles in hydrothermal systems or by the tendency of lipids, and other amphi-philic molecules (which have water-attracting and water-repelling parts), to assemble spontaneously into membrane-like structures. Such structures have been formed from amphiphilic compounds extracted from a meteorite. Membranes restrict the passage of materials through them and organisms have sophisticated transport systems which control the exchanges between cells and their surroundings. Primitive cells may have had membranes that were more permeable than those found today or that may have allowed the incorporation of materials from their environment during cycles of desiccation and rehydration.

As you will have gathered, there are plenty of competing theories of the origin of life among scientists (quite apart from religious interpretations!). Can our knowledge of the biology of organisms under extreme environmental conditions shed any light in this area? Conditions on the early Earth were likely to be much more extreme than they are today. Oxygen was lacking from the atmosphere and the first organisms would have needed to metabolise under anaerobic conditions. High concentrations of oxygen in the atmosphere only developed after the evolution of photosynthetic organisms, which were responsible for its production. Anaerobic bacteria are found today but are restricted to sites where oxygen is absent since they cannot deal with the potentially toxic products of an oxygen metabolism (see Chapter 6). Perhaps they are the survivors of the preoxygen world.

Temperatures on the early Earth may have been much hotter than they are today. The techniques of molecular systematics have given us an overall view of the tree of life (see Chapter 1). The most ancient groups of bacteria and archaea are hyperthermophilic. This suggests that the first organism (the last common ancestor) was also a hyper-thermophile. This argues for a hot origin of life and for hot conditions on the early Earth. Of course, this is not accepted by some, who argue for a cool origin of life, as in the tidal pools envisaged by the 'prebiotic beach' hypothesis.

Anhydrobiosis may also be a primitive property of life. The primitive beach hypothesis envisages the concentration of materials in drying ponds and lagoons. These would presumably frequently dry out completely. Desiccation/rehydration cycles have been suggested to be important in allowing the entry of materials through primitive membranes. The involvement of anhydrobiosis in the origin of life is not a new idea. In the early 1960s, the late Howard Hinton (once Professor of Entomology at Bristol University) suggested that anhydrobiosis was a primitive property of cytoplasm. He thought that life arose on land rather than in the sea since not only would desiccation concentrate materials, but it occurred in numerous small ponds. This would allow more variation in both physical conditions and in the mixtures of chemicals contained within them than would be possible in the more uniform conditions of the sea. Such ponds could have provided an enormous number of experiments, just one of which could have resulted in the origin of life.

Life got started on the Earth once the period of heavy bombardment ceased. Even after this, occasional impacts from large asteroids, or other bodies, may have sterilised the Earth and required life to start again. Extremophiles can live in extreme conditions while cryptobi-otes, in a desiccated state, can survive extreme environmental stresses that would be fatal to the fully hydrated organism. Extreme organisms would have had more chance of surviving large impacts and could have provided a remnant able to recolonise the Earth following such a cataclysmic event. Organisms may also have been protected at the bottom of the sea, around hydrothermal vents, deep within the Earth or under a covering of ice.

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