Science Literature

Hot on the heels of an arthropod with complex compound eyes from the Emu Bay Shale (in Australia) has come an even more dramatic discovery! The same lagerstatten has yielded some fossil eyes, attributed to Anomalocaris, that show just how much 'modernity' can be traced back to the Cambrian fauna.

"The number of ommatidia in the Anomalocaris eyes would almost certainly have greatly exceeded the count based on the exposed surface of the eye alone. [. . .] the total count could be substantially greater than the observed 16,000+ lenses. If this is indeed the case, few living arthropods have as many ommatidia, and these eyes would certainly have functioned with a high degree of acuity. [. . .] Throughout the geological history of Arthropoda, compound eyes have rarely exceeded this size; very large Siluro-Devonian pterygotid eurypterids and some Jurassic thylacocephalans represent some of the rare examples with eyes larger than those of Anomalocaris."

Anomalocaris and its compound eyes (source here)

Inevitably, this raises questions about the way the evolution of compound eyes has been presented in the past. Plots of ommatidia density vs geological time have been used to defend gradualism. However, most of the data was related to trilobites, which are dominated by benthic forms in the Lower Cambrian, diversifying to include nektonic species in the Late Cambrian and subsequent Periods. Now that the eyes of nektonic animals are being discovered and documented, the picture looks rather different. A whole range of eye designs were present during the Early-Middle Cambrian.

"The eyes of Anomalocaris expand the known diversity of visual adaptations in the early Cambrian: low-resolution organs with >100 ommatidia (eodiscoid trilobites), higher-resolution eyes with a distinct bright zone that might have functioned in low light, and very large eyes with a uniformly dense visual field adapted to bright environments."

It should always be remembered that eye complexity means nothing without neuronal processing capability. Light signals have to be transmitted, analysed and decoded as visual images. Modernity in these aspects must be inferred also.

"The very large size of anomalocaridid compound eyes and the visual acuity inferred from the elevated lensnumber and low interommatidial angles suggest that processing of visual information would have required the optic neuropils and brain to be of comparable complexity to crown-group (that is, modern) arthropods. In the crown group, two optic neuropils are reconstructed in the most recent commonancestor, transmitting to a protocerebrum with a median unpaired neuropil, the central body."

Discoveries like this create major challenges for advocates of Darwinian gradualism. Again and again, the source of novelty is pushed earlier into the undocumented past. Gradualism as a working concept is sustained, not by data, but by inference - but the gaps available for gradualist change are ever shrinking!

"Dense, hexagonal packing of ommatidia in compound eyes has been demonstrated to have been unequivocally present in Schinderhannes bartelsi, a Devonian species resolved as the immediate sister group to the arthropod crown group. The eyes of Schinderhannes resemble those of Anomalocaris in being large, stalked, having an ovoid outline of the visual surface, and a highly elevated number of lenses. The finding that Anomalocaris, resolved more basally than Schinderhannes in the arthropod stem group, possesses the same kind of ommatidial packing as in Schinderhannes and crown-group arthropods pushes the origin of compound eyes further down the arthropod stem group. As such, compound eyes evolved earlier than the origin of a hardened tergal exoskeleton and biramous trunk limbs (the latter characters being present in Schinderhannes but not anomalocaridids). We infer that the stalked eyes of all Radiodonta (that is, anomalocaridids) are arthropod-type compound eyes."

The same general analysis applies to all the evolutionary stories that are developed around the fossil record. As in so many research papers, the evolutionary comments are tacked on and are little more than table-talk. The last sentence of the abstract and the last sentence of the paper make the same point: the sophisticated visual system of Anomalocaris would have the consequence of enhancing selection pressures and "probably helped to accelerate the escalatory 'arms race' that began" with the Cambrian fauna.

However, justification of the 'arms race' and the way it is supposed to affect the course of evolution is left to the imagination. As an alternative scenario, consider an ecological perspective. The more we know of Cambrian faunas, the more we are finding evidence of ecosystems adapting to environmental change over time. The changes documented in the fossil record are better explained by reference to ecological concepts combined with the phenomenon of phenotypic plasticity.

Acute vision in the giant Cambrian predatorAnomalocaris and the origin of compound eyes
John R. Paterson, Diego C. Garcia-Bellido, Michael S. Y. Lee, Glenn A. Brock, James B. Jago & Gregory D. Edgecombe
Nature, 480, 237-240 (08 December 2011) | doi:10.1038/nature10689

Until recently, intricate details of the optical design of non-biomineralized arthropod eyes remained elusive in Cambrian Burgess-Shale-type deposits, despite exceptional preservation of soft-part anatomy in such Konservat-Lagerstatten. The structure and development of ommatidia in arthropod compound eyes support a single origin some time before the latest common ancestor of crown-group arthropods, but the appearance of compound eyes in the arthropod stem group has been poorly constrained in the absence of adequate fossils. Here we report 2-3-cm paired eyes from the early Cambrian (approximately 515 million years old) Emu Bay Shale of South Australia, assigned to the Cambrian apex predator Anomalocaris. Their preserved visual surfaces are composed of at least 16,000 hexagonally packed ommatidial lenses (in a single eye), rivalling the most acute compound eyes in modern arthropods. The specimens show two distinct taphonomic modes, preserved as iron oxide (after pyrite) and calcium phosphate, demonstrating that disparate styles of early diagenetic mineralization can replicate the same type of extracellular tissue (that is, cuticle) within a single Burgess-Shale-type deposit. These fossils also provide compelling evidence for the arthropod affinities of anomalocaridids, push the origin of compound eyes deeper down the arthropod stem lineage, and indicate that the compound eye evolved before such features as a hardened exoskeleton. The inferred acuity of the anomalocaridid eye is consistent with other evidence that these animals were highly mobile visual predators in the water column. The existence of large, macrophagous nektonic predators possessing sharp vision - such as Anomalocaris - within the early Cambrian ecosystem probably helped to accelerate the escalatory 'arms race' that began over half a billion years ago

See also:

Marshall, M. First top predator was giant shrimp with amazing eyes, New Scientist (7 December 2011)

The earliest example of a domestic dwelling built from bone has been discovered in the Ukraine. The structure is considered to be 44,000 years old and the builders were Neanderthals. The significance of this is that Neanderthals are supposed to be lacking in creativity and aesthetics, so they are usually portrayed as pragmatists dwelling in caves and rock shelters, largely devoid of characteristics we associate with humanity. Yet again, the evidence base shows the iconic Neanderthal to be a figment of the imagination.

"Neandertals are stumping for bragging rights as the first builders of mammoth-bone structures, an accomplishment usually attributed to Stone Age people. Humanity's extinct cousins constructed a large, ring-shaped enclosure out of 116 mammoth bones and tusks at least 44,000 years ago in West Asia, say archaeologist Laetitia Demay of the National Museum of Natural History in Paris and her colleagues. The bone edifice, which encircles a 40-square-meter area in which mammoths and other animals were butchered, cooked and eaten, served either to keep out cold winds or as a base for a wooden building." (source here)

Caves are portrayed as dwelling places and burial locations for Homo neanderthalensis. This scene is from Hannover Zoo. (Source here)

Finding a building made of mammoth bones is suggestive of a lifestyle that involved creativity, forward planning, cooperation and language. To build a dwelling is suggestive of them living in one place for extended periods of time. In addition, many of the bones had been decorated with carvings and ochre pigments, revealing an aesthetic sense in Neanderthals. Here is a selection of comments from Laetitia Demay, who led the research:

"It appears that Neanderthals were the oldest known humans who used mammoth bones to build a dwelling structure.
"This mammoth bone structure could be described as the basement of a wooden cover or as a windscreen.
"Neanderthals purposely chose large bones of the largest available mammal, the woolly mammoth, to build a structure.
"The mammoth bones have been deliberately selected - long and flat bones, tusks and connected vertebrae - and were circularly arranged.
"The use of bones as building elements can be appreciated as anticipation of climatic variations. Under a cold climate in an open environment, the lack of wood led humans to use bones to build protections against the wind."

People have been talking about revising our assessment of Neanderthals for some time now, but we still find them portrayed as brutish; still as sub-human; still as making noises to communicate rather than using speech. In a report on the research, Richard Gray says that the new finds "add to the growing view that Neanderthals were in fact quite advanced humans who had their own culture and may have even used language to communicate". Another comment is provided by Simon Underdown, an academic who researches Neanderthals at Oxford Brookes University, who said:

"It's another piece in the newly emerging Neanderthal jigsaw puzzle. Far from being the stupid cavemen of popular image it's becoming increasingly clear the Neanderthals were a highly sophisticated species of human. We can now add shelter building to the list of advanced behaviours that includes burying the dead, spoken language, cooking and wearing jewellery."

What is it that stops us thinking that Neanderthals were humans like us? For more on this, and an answer, go here.

Mammoths used as food and building resources by Neanderthals: Zooarchaeological study applied to layer 4, Molodova I (Ukraine)
Laetitia Demay, Stephane Pean, Marylene Patou-Mathis
Quaternary International, In Press, Available online 26 November 2011

Abstract: Considering Neanderthal subsistence, the use of mammoth resources has been particularly discussed. Apart from procurement for food, the use of mammoth bones as building material has been proposed. The hypothesis was based on the discovery made in Molodova I, Ukraine (Dniester valley). In this large multistratified open-air site, a rich Mousterian layer was excavated. Dated to the Inter-Pleniglacial (MIS 3), it has yielded 40 000 lithic remains associated with ca. 3000 mammal bones, mostly from mammoth. Several areas have been excavated: a pit filled with bones, different areas of activities (butchering, tool production), twenty-five hearths and a circular accumulation made of mammoth bones, described as a dwelling structure set up by Neanderthals. Attested dwelling structures made of mammoth bones are known in Upper Paleolithic sites, from Ukraine and Russia, attributed to the Epigravettian tradition. [. . .] Based on anthropogenic marks, mammoth meat has been eaten. The presence of series of striations and ochre on mammoth bones are associated with a technical or symbolic use. Furthermore, mammoth bones have been deliberately selected (long and flat bones, tusks, connected vertebrae) and circularly arranged. This mammoth bone structure could be described as the basement of a wooden cover or as a wind-screen. The inner presence of fifteen hearths, lithic artifacts and waste of mammal butchery and cooking is characteristic of a domestic area, which was probably the centre of a residential camp recurrently settled. It appears that Neanderthals were the oldest known humans who used mammoth bones to build a dwelling structure.

Most biological students think that adaptive radiations and Darwinism go together, and that the mechanisms of genetic mutation and natural selection explain all the data. However, in most cases, this explanation is assumed and not supported by evidence. It is assumed because of the dominance of neodarwinism in evolutionary biology and because students are very impressed with the "mountains of evidence" claimed to support the consensus. Happily, there are some biologists prepared to step outside the paradigm, and one of them is Austin Hughes from the University of South Carolina. In preparing the ground for his iconoclastic analysis, he writes:

"I will refer to this mechanism as the Neo-Darwinian mechanism; and, following general usage, I will refer to an allele that has been fixed by this process as one that has been fixed by positive Darwinian selection. The Neo-Darwinian mechanism is often assumed by biologists to be the only source of adaptive traits of organisms, to the point where 'adaptive evolution' and 'positive (Darwinian) selection' are treated as interchangeable terms in the literature."

Cichlids are a textbook example of rapid speciation - but not of Darwinian evolution! (For more, go here. Image source here)

Also, by way of preparation, he refers to the significant theoretical and evidential base for neutral evolution (Kimura, 1976). There is a phenomenon known as genetic drift involving neutral or nearly neutral mutations. There are mechanisms for fixing these genetic changes - all invisible to natural selection. These are considered to be more important than we realise, evidenced by the continuing scarcity of advantageous mutations. This was recognised in 1976 and it continues to this day.

"In the ensuing decades, a vast amount of molecular sequence data, including complete genome sequences of many organisms, has become available to test for the evidence of positive selection at the molecular level. However, the number of well-established cases has not increased greatly in comparison with those known in the mid-1970s. It is true that a very large number of papers have been published in recent years purporting to show evidence of positive selection on the basis of various statistical methods. However, the vast majority of these cases cannot be considered well established. [. . .] Moreover, in almost all of the putative cases of positive selection identified by statistical analysis of sequence data alone, the biological basis of the supposed selection and even the phenotypic effects, if any, of the supposedly selected nucleotide substitutions have not been addressed."

Hughes has previously pointed out difficulties in identifying evidence for positive selection, yet there appears to be plenty of evidence for purifying selection (the elimination of deleterious variants).

"The predominance of purifying selection was predicted by Kimura and Ohta (1974), and the fact that their prediction has been proved to be correct is the cornerstone of many routine methods of modern bioinformatics, whereby evolutionary conservation of a sequence element (the consequence of purifying selection) is taken as evidence of that element's functional importance."

So, in view of the meagre evidential support for "the Neo-Darwinian mechanism", Hughes turns his attention to the thought that adaptive phenotypes might arise by alternative routes. This has been considered by a few other authors, but the field is wide open. Consequently, Hughes proposes one such mechanism: the plasticity - relaxation - mutation (PRM) mechanism. He argues that the evidential base for this concept is already in existence in the biological literature.

"I examine some predictions of this theory and summarize evidence relating to those predictions. The present hypothesis does not deny that the Neo-Darwinian mechanism operates in certain cases. Rather, based on what we can learn from the known cases of positive selection, I conclude that the phenomenon of positive selection may be of relatively minor importance in phenotypic evolution. Instead, phenotypic plasticity and changes in the direction and nature of purifying selection, combined with the chance fixation of neutral or nearly neutral mutations, are proposed to be the major factors in the evolution of adaptive phenotypes."

Much of the paper provides clarification of the PRM mechanism and justifies the claim that the concept has a track record in the literature. The emerging scenario is that organisms typically have an ability to adapt to environmental inputs in ways that change and fix the phenotype. This is a variability that does not depend on mutations for new genetic information, although mutations may be involved in the fixing of the phenotype. The genomic architecture is already present that supports adaptation in a variety of directions. Influences may come from neutral mutations and genetic drift, and they may involve epigenetic mechanisms. After reviewing a variety of evidences, Hughes concludes:

"The PRM mechanism provides unification to the biological sciences by uniting observations at the genomic level (where purifying selection and genetic drift predominate) with those at the phenotypic level (where adaptive characters are well known). As mentioned above, some known examples are suggestive of the action of the PRM mechanism, but it is not yet known how widespread this mechanism is. However, I would predict that the PRM mechanism is likely to be a major mechanism for the origin of evolved adaptations, and perhaps more common than the Neo-Darwinian mechanism."

Let's look at some of the applications of the PRM concept.
First, a comment on the general picture. Adaptive radiations in the fossil record appear to have been rapid, followed by stasis. This pattern is quite unlike the branching illustration found in On the Origin of Species.

"In some cases, the time frame seems rather short for a Darwinian process to have occurred, and in other cases, the effective population sizes of the species in question are small, suggesting that there is unlikely to have been extensive genetic variation in the population prior to selection. However, none of these factors are problematic if these cases of apparent rapid evolution in fact represent cases of phenotypic plasticity, perhaps in some cases rendered heritable through germline DNA methylation. Thus, rather than the paradoxical observation of Darwinian evolution over ecological time, we may be merely seeing incipient evolution by the PRM mechanism, which is expected to operate over ecological time."

Second, the specific case of cichlid fishes is of interest, because these radiations do not have the luxury of extensive time.

"The PRM mechanism provides a simple explanation of such comparatively recent adaptive radiations as that of the cichlids of the East African Great Lakes. The oldest of these lakes, Lake Victoria, is no more than 200,000 years old, and others are still more recent. The diversity of species in these lakes is problematic for Neo-Darwinism, but is easily explained by the PRM mechanism if prior to the divergence of ecotypes the ancestral species showed a phenotypic plasticity similar to that described in sticklebacks."

Third, consider that classic example of adaptive radiation: the Galapagos finches.

"Perhaps ironically, the PRM mechanism can likewise account readily for the radiation of Darwin's finches in the Galapagos Islands. The natural history of Darwin's finches provides many examples where it is plausible that phenotypic plasticity preceded morphological change; a striking example involves the sharper bill shape of a population of the ground finch Geospiza diffilis that feeds on the blood of boobies."

Fourth, the topic of artificial selection is of interest - not least because Darwin (and modern textbooks) portrayed artificial selection as directly relevant to natural selection, whereas Wallace thought it was irrelevant. There is no doubt that artificial selection produces rapid phenotypic change, but we already know that most of this does not involve mutations.

"The same process [incipient evolution by the PRM mechanism] might also be involved in rapid responses to artificial selection, for instance in accelerated domestication."

The significance of this research for the study of biological variation surely deserves our attention. We are not here considering a theory that claims to explain the concept of common descent from a single cell, but it has the more modest aim of explaining adaptive radiations from ancestral populations. However, the main critique that has been advanced is that the PRM hypothesis "does not explain why the ancestral state should be phenotypically plastic, or why this plasticity should be adaptive in the first place." The critique is not a fair one, because the new theory is proposed to explain observations of biological variation, rather than to explain the origin of all species.

The perspective provided by Hughes is one that is based on both theory and empirical data, and it stands up to testing very well. This model of variation provides an understanding that differs markedly from the Darwinism and the Neo-Darwinism of most textbooks. It is time for evolutionists to cease claiming all examples of variation and adaptation as evidence for Darwinian mechanisms of evolution. This is bad science and it is the perpetuation of a consensus by repetition rather than by hypothesis testing and validation. Hughes concludes thus:
"The hypothesis proposed here has the advantage of explaining the available data regarding adaptive evolution on the levels of genomics, ecology and paleontology, without invoking any mechanisms other than the commonly observed phenomena of phenotypic plasticity, purifying selection, mutation and genetic drift. Although it may represent a new perspective to biologists schooled in Neo-Darwinism, this view of life in its own way is not without 'a certain grandeur.'"

Evolution of adaptive phenotypic traits without positive Darwinian selection
A L Hughes
Heredity, advance online publication 2 November 2011 | doi: 10.1038/hdy.2011.97

Recent evidence suggests the frequent occurrence of a simple non-Darwinian (but non-Lamarckian) model for the evolution of adaptive phenotypic traits, here entitled the plasticity-relaxation-mutation (PRM) mechanism. This mechanism involves ancestral phenotypic plasticity followed by specialization in one alternative environment and thus the permanent expression of one alternative phenotype. Once this specialization occurs, purifying selection on the molecular basis of other phenotypes is relaxed. Finally, mutations that permanently eliminate the pathways leading to alternative phenotypes can be fixed by genetic drift. Although the generality of the PRM mechanism is at present unknown, I discuss evidence for its widespread occurrence, including the prevalence of exaptations in evolution, evidence that phenotypic plasticity has preceded adaptation in a number of taxa and evidence that adaptive traits have resulted from loss of alternative developmental pathways. The PRM mechanism can easily explain cases of explosive adaptive radiation, as well as recently reported cases of apparent adaptive evolution over ecological time.

See also:

Levi, P.J. No Positive Selection, No Darwin: A New Non-Darwinian Mechanism for the Origin of Adaptive Phenotypes, Evolution News & Views (November 14, 2011)

Tyler, D. Rodents evolve - but does the evidence suggest phenotypic plasticity? ARN Literature Blog (18 November 2011)

Tyler, D. A call for an end to Pseudo-Darwinian hype, ARN Literature Blog (11 September 2008)

It is universally claimed that the early Earth had a reducing atmosphere. Models have been proposed for the gases to have accumulated after outgassing of volatiles from volcanism. This reducing atmosphere was originally thought to have been dominant throughout the Precambrian, but signs of oxygenation have pushed it back earlier than the earliest rocks that researchers have discovered. The earlier claims for a reducing atmosphere have other explanations, such as resulting from the action of hydrodynamic fluids. This has put severe constraints on theories of abiogenesis, because the proposed mechanisms typically presuppose a reducing atmosphere. By the earliest Archaean, the atmosphere was at least neutral - so abiogenesis is inferred to have occurred even earlier. But moving back earlier brings us to the Late Heavy Bombardment which is generally deemed to have obliterated all traces of any life that may have been present. So there is a little window in the Hadean that is deemed to have offered a reducing atmosphere free from the destructive bombardment.

"For decades, scientists believed that the atmosphere of early Earth was highly reduced, meaning that oxygen was greatly limited. Such oxygen-poor conditions would have resulted in an atmosphere filled with noxious methane, carbon monoxide, hydrogen sulfide, and ammonia. To date, there remain widely held theories and studies of how life on Earth may have been built out of this deadly atmosphere cocktail." (Source here)

Artist's impression of the Hadean Earth (source here)

The evidence for a Hadean reducing atmosphere has been entirely theoretical. It does not rest on empirical evidence because there has been so little to work with. However, a new study of zircon crystals has reported some fascinating results that allow speculation about the Hadean black box to be replaced by empirical evidence. Zircons have been identified that carry signatures identifying them with the Hadean - and zircons are remarkably stable once formed. Using zircons dated to almost 4.4 Ga, the researchers have analysed their redox state (a measure of the degree of oxygenation of the mineral). This gives a handle on the type of gases that would have been outgassed by the magmas, and so, according to these models of Earth history, the type of atmosphere that would have been formed.

"Unlike other materials that are destroyed over time by erosion and subduction, certain zircons are nearly as old as Earth itself. As such, zircons can literally tell the entire history of the planet - if you know the right questions to ask. The scientists sought to determine the oxidation levels of the magmas that formed these ancient zircons to quantify, for the first time ever, how oxidized were the gases being released early in Earth's history. Understanding the level of oxidation could spell the difference between nasty swamp gas and the mixture of water vapor and carbon dioxide we are currently so accustomed to, according to study lead author Dustin Trail, a postdoctoral researcher in the Center for Astrobiology. "By determining the oxidation state of the magmas that created zircon, we could then determine the types of gases that would eventually make their way into the atmosphere," said Trail." (Source here)

It is important to realise what was predicted by prevailing theories: the redox state of the magmas with which the zircons were associated was expected to be strongly reducing. This prediction is a necessary part of the Earth having a reducing atmosphere in the Hadean. The research findings did not confirm the prediction. Here is the comment of the authors of a News & Views commentary in Nature:

"[In] this issue, Trail et al. report their analysis of the sole mineral survivors of the Hadean, zircon samples more than 4 billion years old. Their findings allowed them to determine the 'fugacity' of oxygen in Hadean magmatic melts, a quantity that acts as a measure of magmatic redox conditions. Unexpectedly, the zircons record oxygen fugacities identical to those in the present-day mantle, leading the authors to conclude that Hadean volcanic gases were as highly oxidized as those emitted today."

To keep the reducing atmosphere theoretical approach, the timescales must again shrink. The window is now less that 150 Ma - right at the beginning of Earth history - preceding the Late Heavy Bombardment. If life appeared so early, it must have been pulverised before the Archaean provided an environment stable enough for single-celled organisms to survive.

"Their findings extend the mantle's oxidized realm to almost 4.4 billion years ago. Although somewhat tenuous, this is the first direct evidence of the redox state of the earliest Earth. If the zircons analysed by the authors are representative of the Hadean eon, this result shrinks the duration of the reduced era of Earth's mantle to less than 150 million years. It also increases the lag time between the oxidation of the mantle and the subsequent oxidation of the atmosphere [. . .]." (source here)

The authors are well aware of the implications of their research. We need to discard theories that require a reducing atmosphere on Earth - if interest in these theories is to be perpetuated, then locations should be sought outside the Earth.

"The calibrations reveal an atmosphere with an oxidation state closer to present-day conditions. The findings provide an important starting point for future research on the origins of life on Earth. [. . .] Despite being the atmosphere that life currently breathes, lives, and thrives on, our current oxidized atmosphere is not currently understood to be a great starting point for life. Methane and its oxygen-poor counterparts have much more biologic potential to jump from inorganic compounds to life-supporting amino acids and DNA. As such, Watson thinks the discovery of his group may reinvigorate theories that perhaps those building blocks for life were not created on Earth, but delivered from elsewhere in the galaxy." (Source here)

There are two points worth making here. The first concerns the importance of empirical evidence in developing theory. The problem for any historical science is that it is relatively easy for speculation to become dominant because testing hypotheses by reference to empirical data is often a challenge. Abiogenesis is a case in point. The reducing atmosphere scenario and the mechanisms for turning simple chemicals into self-replicating cells have received theoretical development that has gone far beyond the evidential base. So confident have researchers become that they have created the delusion that it is unscientific to even challenge the consensus! Yet they have had to retreat before the evidence. The Archaean atmosphere was realised not to be reducing, so the theorists retreated to the Hadean where data is almost non-existent. They could just about live there - until this week! Now, they must revise their theories to make it all happen in the first 150 Ma of Earth history (and somehow miraculously survive bombardment) or move it "elsewhere in the galaxy". If you are aware of "god of the gaps" reasoning, this case seems to fit the pattern pretty well - the argument is from theory unsupported by evidence and there is a progressive retreat in response to evidence to a place where the theory looks untenable.

The second point is that science has not demonstrated self-correction as it is supposed to do. Evidence has been around for 30 years that the Earth's early atmosphere was not reducing. Jonathan Wells has summarised the research evidence against the reducing atmosphere in Icons of Evolution (2000). He refers to geologists who declared the concept to be mere "dogma" in 1982. Yet the reducing atmosphere has persisted in textbooks, the media, and in the research community to this day! The new research findings bring a renewed challenge to the science community: it is time to revise the textbooks and to follow the evidence wherever it leads.

The oxidation state of Hadean magmas and implications for early Earth's atmosphere
Dustin Trail, E. Bruce Watson & Nicholas D. Tailby
Nature, 480, 79-82, (01 December 2011) | doi:10.1038/nature10655

Magmatic outgassing of volatiles from Earth's interior probably played a critical part in determining the composition of the earliest atmosphere, more than 4,000 million years (Myr) ago. Given an elemental inventory of hydrogen, carbon, nitrogen, oxygen and sulphur, the identity of molecular species in gaseous volcanic emanations depends critically on the pressure (fugacity) of oxygen. Reduced melts having oxygen fugacities close to that defined by the iron-wustite buffer would yield volatile species such as CH4, H2, H2S, NH3 and CO, whereas melts close to the fayalite-magnetite-quartz buffer would be similar to present-day conditions and would be dominated by H2O, CO2, SO2 and N2. Direct constraints on the oxidation state of terrestrial magmas before 3,850 Myr before present (that is, the Hadean eon) are tenuous because the rock record is sparse or absent. Samples from this earliest period of Earth's history are limited to igneous detrital zircons that pre-date the known rock record, with ages approaching ~4,400 Myr. Here we report a redox-sensitive calibration to determine the oxidation state of Hadean magmatic melts that is based on the incorporation of cerium into zircon crystals. We find that the melts have average oxygen fugacities that are consistent with an oxidation state defined by the fayalite-magnetite-quartz buffer, similar to present-day conditions. Moreover, selected Hadean zircons (having chemical characteristics consistent with crystallization specifically from mantle-derived melts) suggest oxygen fugacities similar to those of Archaean and present-day mantle-derived lavas as early as ~4,350 Myr before present. These results suggest that outgassing of Earth's interior later than ~200 Myr into the history of Solar System formation would not have resulted in a reducing atmosphere.

Redox state of early magmas
Bruno Scaillet & Fabrice Gaillard
Nature, 480, 48-49 (01 December 2011) | doi:10.1038/480048a

A study of cerium in zircon minerals has allowed an assessment of the redox conditions that prevailed when Earth's earliest magmas formed. The results suggest that the mantle became oxidized sooner than had been thought.

See also:

Setting the Stage for Life: Scientists Make Key Discovery About the Atmosphere of Early Earth, ScienceDaily (30 November 2011)