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The first cells may have originated by chemical evolution on a young Earth:an overview

Most biologists favor the hypothesis that life on Earth developed from nonliving materials that became ordered into molecular aggregates that were eventually capable of self-replication and metabolism.

Resolving the Biogenesis Paradox

From the time of the ancient Greeks until well into the 19th century, it was common "knowledge" that life could arise from nonliving matter. This idea of life emerging from inanimate material is called spontaneous generation. Experiments with flies and other organisms in the late Renaissance period convinced scientists to reject the notion of spontaneous generation for macroscopic life. However, the idea persisted well into the 19th century as an explanation for the rapid growth of microorganisms in spoiled foods. Then, in 1862, Louis Pasteur’s famous experiments with broth completed the overturn of spontaneous generation, even for microorganisms (FIGURE 26.9). As far as we know, all life today arises only by the reproduction of preexisting life. This "life-from-life" principle is called biogenesis.


Fig 26-9. Pasteur and biogenesis of microorganisms. In the early 1860s, Louis Pasteur (inset) conducted a series of experiments to test whether microorganisms emerge by spontaneous generation or by reproduction of existing microorganisms (biogenesis). His research contributed to the germ theory of disease, which connected infections to the spread of microorganisms and led to improvements in hospital hygiene and public sanitation. The legacy of these famous experiments is also manifest in the term pasteurization . Pasteurized milk, for example, has been heated to destroy potentially harmful microorganisms and then sealed to maintain the sterility.


But what about the first organisms? If they arose by biogenesis, then they couldn’t have been the first organisms. Although there is no evidence that spontaneous generation occurs today, conditions on the early Earth were very different. For instance, there was relatively little atmospheric oxygen to tear apart complex molecules. And such energy sources as lightning, volcanic activity, and ultraviolet sunlight were all more intense than what we experience today. The resolution to the biogenesis paradox is that life did not begin on a planet anything like the modern Earth, but on a young Earth that was a very different world.

A Four-Stage Hypothesis for the Origin of Life

Most biologists now think that it is at least a credible hypothesis that chemical and physical processes in Earth’s primordial environment eventually produced very simple cells through a sequence of stages. There is much debate about the nature of those stages.

According to one hypothetical scenario, the first organisms were products of chemical evolution in four stages: (1) the abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nucleotides; (2) the joining of these small molecules (monomers) into polymers, including proteins and nucleic acids; (3) the origin of self-replicating molecules that eventually made inheritance possible; and (4) the packaging of all these molecules into "protobionts," droplets with membranes that maintained an internal chemistry different from the surroundings. This is all speculative, of course, but what makes it science is that the hypothesis leads to predictions that can be tested in the laboratory. Let’s take a closer look at some of the evidence for each of these four stages.















Abiotic synthesis of organic monomers is a testable hypothesis

In the 1920s, A. I. Oparin, of Russia, and J. B. S. Haldane, of Great Britain, independently postulated that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors present in the early atmosphere and seas. This cannot happen in the modern world, Oparin and Haldane reasoned, because the present atmosphere is rich in oxygen produced by photosynthetic life. The oxidizing atmosphere of today is not conducive to the spontaneous synthesis of complex molecules because the oxygen attacks chemical bonds, extracting electrons. Before oxygen-producing photosynthesis, Earth had a much less oxidizing atmosphere, derived mainly from volcanic vapors. Such a reducing (electron-adding) atmosphere would have enhanced the joining of simple molecules to form more complex ones. Even with a reducing atmosphere, making organic molecules would require considerable energy, which was probably provided by lightning and the intense UV radiation that penetrated the primitive atmosphere. The modern atmosphere has a layer of ozone produced from oxygen, and this ozone shield screens out most UV radiation. There is also evidence that young suns emit more UV radiation than older suns. Oparin and Haldane envisioned an ancient world with the chemical conditions and energy resources needed for the abiotic synthesis of organic molecules.

In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by creating, in the laboratory, conditions comparable to those that scientists had postulated for the early Earth. Their apparatus produced a variety of amino acids and other organic compounds found in living organisms today (FIGURE 26.10; also see FIGURE 4.1).


Fig 26-10. The Miller-Urey experiment. A warmed flask of water simulated the primeval sea. The "atmosphere" consisted of H2O, H2, CH4, and NH3. Sparks were discharged in the synthetic atmosphere to mimic lightning. A condenser cooled the atmosphere, raining water and any dissolved compounds back to the miniature sea. As material circulated through the apparatus, the solution in the flask changed from clear to murky brown. After one week, Miller and Urey analyzed the contents of the solution and found a variety of organic compounds, including some of the amino acids that make up the proteins of organisms.


The atmosphere in the Miller-Urey model was made up of H2O, H2, CH4 (methane), and NH3 (ammonia), the gases that researchers in the 1950s believed prevailed in the ancient world. This atmosphere was probably more strongly reducing than the actual atmosphere of early Earth. Modern volcanoes emit CO, CO2, N2, and water vapor, and it is likely that these gases were abundant in the ancient atmosphere. Hydrogen gas was probably not a major component, and traces of O2 may even have been present, formed from reactions among other gases as they baked under the powerful UV radiation. Many laboratories have repeated the Miller experiment using a variety of recipes for the atmosphere. Abiotic synthesis of organic compounds occurred in these modified models, although yields were generally smaller than in the original experiment.

The Miller-Urey experiments still stimulate debate on the origin of Earth’s early stockpile of organic ingredients. Today, one line of research focuses on where chemicals needed for organic syntheses came from and where the reactions most likely occurred. Some scientists now doubt that the early atmosphere played a significant role in early chemical reactions. Instead, submerged volcanoes and deep-sea vents--gaps in Earth’s crust where hot water and minerals gush into deep oceans--may have provided the essential resources. Evidence is also building that life could have begun in a much simpler chemical environment than formerly thought. For instance, the first cells may have used inorganic sulfur and iron compounds as energy sources to make their own ATP instead of taking it up from their surroundings.

It is also plausible that some organic compounds reached Earth from space. In 2000, Indian scientists reported computer models showing how molecules such as adenine, an ingredient of DNA, could form by reactions of cyanide in the clouds of gas between stars. These simulations would explain why some meteorites that have crashed to Earth contain organic molecules. But whether the primordial Earth was stocked with organic monomers made here or elsewhere, the key point is that the molecular ingredients of life were probably present very early.







Protobionts can form by self-assembly

The properties of life emerge from an interaction of molecules organized into higher levels of order. Living cells may have been preceded by protobionts, aggregates of abiotically produced molecules. Protobionts are not capable of precise reproduction, but they maintain an internal chemical environment different from their surroundings and exhibit some of the properties associated with life, including metabolism and excitability.

Laboratory experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds. For example, droplets called liposomes form when the organic ingredients include certain lipids. These lipids organize into a molecular bilayer at the surface of the droplet, much like the lipid bilayer of cell membranes. Because the membrane is selectively permeable, the liposomes undergo osmotic swelling or shrinking when placed in solutions of different salt concentrations. Some of these protobionts also store energy in the form of a membrane potential, a voltage across the surface. The protobionts can discharge the voltage in nervelike fashion; such excitability is characteristic of all life (which is not to say that liposomes are alive, but only that they display some of the properties of life). Liposomes behave dynamically, sometimes growing by engulfing smaller liposomes and then splitting, other times "giving birth" to smaller liposomes (FIGURE 26.12a). If enzymes are included among the ingredients, they are incorporated into the droplets. The protobionts are then able to absorb substrates from their surroundings and release the products of the reactions catalyzed by the enzymes (FIGURE 26.12b).

Fig 26-12. Laboratory versions of protobionts.


Unlike some laboratory models, protobionts that formed in the ancient seas would not have possessed refined enzymes, which are made in cells according to inherited instructions. Some molecules produced abiotically, however, do have weak catalytic capacities, and there could well have been protobionts that had a rudimentary metabolism that allowed them to modify substances they took in across their membranes.

This post was edited by Abstraction on Dec 11 2008 12:52am

Life on Earth originated between 3.5 and 4.0 billion years ago

Life began remarkably early in Earth’s history, and those first organisms were ancestral to the great diversity of life we observe today. The organisms most familiar to us are macroscopic and multicellular -- mainly plants and animals. However, for the first three-quarters of evolutionary history, Earth’s only organisms were microscopic and mostly unicellular.

The planet Earth formed about 4.5 billion years ago. However, it is unlikely that life could have survived on Earth for the first few hundred million years, because the planet was bombarded by huge rock bodies left over from the formation of the solar system. The impacts were colossal; one of them may have dislodged a chunk of Earth that became the moon. During this period of bombardment, the pounding generated enough heat to vaporize all the available water and prevent seas from forming. Most geologists now agree that this bombardment phase ended about 3.9 billion years ago.

The oldest known rocks on Earth’s surface, located at a site called Isua, in Greenland, are 3.8 billion years old. Although there are some chemical clues in these rocks that life may have existed at the time, no one has yet found fossils of microorganisms in rocks so old. The oldest fossils of organisms that biologists have found so far are embedded in rocks from western Australia that are 3.5 billion years old. These microfossils resemble certain bacteria that still exist today (FIGURE 26.3). For bacteria so complex to have evolved by 3.5 billion years ago, it is a reasonable hypothesis that life originated much earlier, perhaps as early as 3.9 billion years ago, when Earth began to cool to a temperature at which liquid water could exist. We do know that prokaryotic life was already flourishing when Earth was still relatively young.

Fig 26-3. Early and modern prokaryotes. Fossils in rocks older than about 500 million years escaped notice until about 40 years ago, partly because they are microscopic.















Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago

One might guess from the relatively simple structure of the prokaryotic cell, compared with the eukaryotic cell, that the earliest organisms were prokaryotes. The fossil record supports that presumption. There is a rich fossil history of prokaryotic life, without solid evidence of eukaryotes, spanning 1.5 billion years, from about 3.5 to 2.0 billion years ago. Relatively early in that prokaryotic world, two main evolutionary branches, the bacteria and the archaea, diverged. Diverse species of these two main prokaryotic groups continue to thrive in various environments today.

Many of the oldest fossils of prokaryotes are found in stromatolites, fossilized mats similar to layered microbial mats that certain groups of prokaryotes still form today in salt marshes and warm lagoons (FIGURE 26.4). Researchers have also discovered fossils of prokaryotes in Australian sediments that formed around hydrothermal vents about 3.2 billion years ago. Hydrothermal vents are hot volcanic outlets in the deep-sea floor. Prokaryotes that inhabit such vents today are very different in their metabolism from prokaryotes making up algal mats such as those in FIGURE 26.4b. We’ll explore the metabolically diverse prokaryotes in detail in Chapter 27. For now, the important point is that considerable metabolic diversity among prokaryotes living in various environments had already evolved over 3 billion years ago.

Fig 26-4. Bacterial mats and stromatolites.




























Photosynthesis evolved early in prokaryotic life

All forms of nutrition and nearly all metabolic pathways evolved among prokaryotes before there were any eukaryotes. As early prokaryotes evolved, they were met with constantly changing physical and biological environments. In response to these changes, new metabolic capabilities evolved that in turn changed the environment faced by the next community of prokaryotes. All the major metabolic capabilities seen among prokaryotes (and hence among eukaryotes) probably evolved in the first billion years of life. And during this expansion of metabolic diver sity, photosynthesis evolved relatively early--a hypothesis supported by molecular systematics, comparisons of energy me tabolism among extant prokaryotes, and geologic evidence.

The energy metabolism of even the simplest photoauto trophs is relatively complex. Thus, it seems reasonable to postulate that the very first prokaryotes were heterotrophs that obtained their energy and carbon skeletons from the pool of organic molecules available in the "primordial soup" of early Earth (see Chapter 26). Glycolysis, which can extract energy from organic fuels to generate ATP in anaerobic environments, was probably one of the first metabolic pathways. That would account for the existence of glycolysis in almost every group of modern organisms. As heterotrophs depleted the supply of organic nutrients in the environment, natu ral selection would have favored any prokaryotes that could harness the energy of sunlight to drive the synthesis of ATP and generate reducing power to synthesize organic compounds from CO2.

What evidence is there for an early evolution of photosynthesis? First, photosynthetic groups are scattered among diverse branches of prokaryote phylogeny. It is possible that photosynthesis evolved independently many times in various prokaryotic lineages (FIGURE 27.12a). This seems unlikely, however, since the molecular machinery required for photosynthesis is very complex. Applying the principle of parsimony we discussed in Chapter 25, the most reasonable hypothesis is that photosynthesis originated once in an ancestor common to the diverse prokaryotic groups where we find photosynthesis today. The heterotrophic groups that are closely related to photosynthetic ones could represent a loss of photosynthetic ability during evolution (FIGURE 27.12b). This would explain molecular data suggesting that in nutritionally diverse taxa of prokaryotes, the autotrophic species are generally older lineages than the heterotrophic ones. Although the very first organisms may have been heterotrophs from which autotrophs evolved, the diversity of heterotrophs we observe today probably descended secondarily from photosynthetic ancestors.

Fig 27-12. Contrasting hypotheses for the taxonomic distribution of photosynthesis among prokaryotes.


Further evidence for an early origin of photosynthesis is the antiquity of cyanobacteria. These are the only autotrophic prokaryotes that release O2 by splitting water during their light reactions. The geologic evidence for accumulation of atmospheric oxygen beginning at least 2.7 billion years ago suggests that cyanobacteria were already a major part of the biosphere by then. And fossils of prokaryotes that look very much like modern cyanobacteria have been found in stromatolites as old as 3.5 billion years, pushing the possible origin of oxygenic photosynthesis back considerably further.

Oxygenic photosynthesis is especially complex because it requires two cooperative photosystems (see FIGURE 10.12). Some groups of modern prokaryotes perform a simpler mode of photosynthesis, using a single photosystem to extract electrons from compounds such as H2S instead of splitting water. But certain components of the photosynthetic machinery are common to such nonoxygenic photosynthesis and the oxygenic version in cyanobacteria. A logical inference is that cyanobacteria evolved from ancestors with simpler, nonoxygenic photosynthesis. This would push the origin of photosynthesis back very close to our earliest fossil evidence of life.

The evolution of cyanobacteria changed the Earth in a radical way, gradually transforming its atmosphere from a reducing one to an oxidizing one. In Chapter 25, we discussed how the oxygen revolution posed an environmental crisis for life, which originated in a mostly anaerobic world. The most elegant adaptation to the changing atmosphere was the evolution of cellular respiration, which uses the oxidizing power of O2 to increase the efficiency of fuel consumption. Photosynthesis and cellular respiration are actually closely related, both using electron transport chains to generate proton gradients that power ATP synthase machines (see FIGURE 10.15). Given the early origin of photosynthesis, it is likely that cellular respiration evolved by modification of the photosynthetic equipment for a new function.

Now that we have applied this book’s theme of evolution to the origin of metabolic diversity in prokaryotes, let’s survey the diverse groups of archaea and bacteria that continue to have enormous impact on Earth and its life.

This post was edited by Abstraction on Dec 11 2008 12:57am

Oxygen began accumulating in the atmosphere about 2.7 billion years ago

Photosynthesis probably evolved very early in prokaryotic history, but in metabolic versions that did not split water and liberate oxygen (see Chapter 10). We’ll see examples of such nonoxygenic photosynthesis among modern prokaryotes in Chapter 27. The only photosynthetic prokaryotes that generate O2 are called cyanobacteria. Plentiful and diverse today, they probably evolved over 2.7 billion years ago.

Most atmospheric oxygen is of biological origin, from the water-splitting step of photosynthesis. When this oxygenic photosynthesis first evolved, the free O2 from the cyanobacteria probably dissolved in the surrounding water until the seas and lakes became saturated with the oxygen. Additional O2 would then react with dissolved iron and precipitate as iron oxide. These marine sediments were the source of banded iron formations, red layers of rock rich in the iron oxide that is a valuable source of iron ore today (FIGURE 26.5). Once all the dissolved iron had precipitated, additional O2 finally began to "gas out" of the seas and lakes to accumulate in the atmosphere. This change left its mark in the rusting of terrestrial rocks rich in iron that began oxidizing about 2.7 billion years ago. This chronology implies that cyanobacteria may have originated as early as 3.5 billion years ago, when the microbial mats that left stromatolites began forming.

Fig 26-5. Banded iron formations are evidence of the vintage of oxygenic photosynthesis. These bands of iron oxide at Jasper Knob in Michigan are about 2 billion years old.


The accumulation of atmospheric O2 was gradual from about 2.7 to 2.2 billion years ago, but then shot up relatively rapidly to more than 10% of its present level. This oxygen revolution had an enormous impact on life. The "corrosive" O2, which attacks chemical bonds, doomed many prokaryotic groups. Some species survived in habitats that remained anaerobic, where we find their descendants still living today as obligate anaerobes (see p. 533). Among other survivors, a diversity of adaptations to the changing atmosphere evolved, including cellular respiration, which uses oxygen to help harvest energy stored in organic molecules.

The early rise in atmospheric O2 was associated with the photosynthesis of early cyanobacteria. But what caused the accelerated rise in O2 a few hundred million years later? One hypothesis is that it followed the evolution of eukaryotic algae containing chloroplasts.

I already posted the same concept of what you did. Interesting finds though *continues reading the massive amounts of text*.

The Miller-Urey experiment is flawed, simply because most scientists today believe that early Earth's atmosphere did not contain Hydrogen because it would have escaped into space. Geo-chemists since the 1960's have said that the experiment did not reflect an accurate atmosphere.

Science magazine has even dismissed the experiment.

Early Earth's atmosphere, it is postulated, was made up of Carbon Dioxide, Nitrogen, and water vapor. Which is conducive to organic molecules such as Formaldehyde and Cyanide, but it is essentially chemically impossible for this atmosphere to have formed amino acids.

then i doubt a college grade textbook would be publishing it in 2001

also, Why is it impossible to form amino acids in that atmosphere?

This post was edited by Abstraction on Dec 15 2008 03:48am

Just because an experiment is flawed, doesn't mean it's not worth noting. Most textbooks have a disclaimer somewhere in the book about how the experiment "might not have been an accurate representation" of early Earth atmosphere.

I explained that to you in the other topic.

Water vapor would contain the hydrogen needed if H2 by itself was not present. I get what you're saying though and we might not never know. That is why these things are called theories ><.

Thanks for posting that I saw a video related to this, and I was impressed by how quickly organisms moved around these vents once they started pumping out food sources.

Abstraction: nice posts in here. PM me sometime if you'd like to talk about potential sticky topics related to some of the basic science you've neatly described in this topic, or about other possible projects. That goes for anyone, really, but now everyone sees some examples. I don't actually plan to put up any guides for basic or advanced matieral just yet, but I'll be collecting snippets in case it could be seen as useful later For now, let's just focus on building good discussions

The Formaldehyde fumes alone would have destroyed the protein.

Moreover, the oxygen in the water vapor would have prevented amino acids from forming.