Ancient CO2 estimates worry climatologists

Concerns about impending, indeed actual, anthropogenic climate change brought on by rapidly rising levels of the greenhouse gas carbon dioxide have spurred efforts to quantify climates of the distant past. Beyond the CO2 record of the last 800 ka established from air bubbles trapped in glacial ice palaeoclimate researchers have had to depend on a range of proxies for the greenhouse effect. Those based on models linking plate tectonic and volcanic CO2 emissions with geological records of the burial of organic matter, weathering and limestone accumulation are imprecise in the extreme, although they hint at considerable variation during the Phanerozoic. Other proxies give a better idea of the past abundance of the main greenhouse gas, one using the curious openings or stomata in leaves that allow gases to pass to and fro between plant cells and the atmosphere. Well preserved fossil leaves show stomata nicely back to about 400 Ma ago when plants first colonised the land.

Stomata on a rice leaf (credit: Getty images)

Stomata draw in CO2 so that it can be combined with water during photosynthesis to form carbohydrate. So the number of stomata per unit area of a leaf surface is expected to increase with lowering of atmospheric CO2 and vice versa. This has been observed in plants grown in different air compositions. By comparing stomatal density in fossilised leaves of modern plants back to 800 ka allows the change to be calibrated against the ice-core record. Extending this method through the Cenozoic, the Mesozoic and into the Upper Palaeozoic faces the problems of using fossils of long-extinct plant leaves. This is compounded by plants’ exhalation of gases to the atmosphere – some CO2 together with other products of photosynthesis, oxygen and water vapour. Increasing stomatal density when carbon dioxide is at low concentration risks dehydration. How extinct plant groups coped with this problem is, unsurprisingly, unknown. So past estimates of the composition of the air become increasingly reliant on informed guesswork rather than proper calibration. The outcome is that results from the distant past tend to show very large ranges of CO2 values at any particular time.

An improvement was suggested some years back by Peter Franks of the University of Sydney with Australian, US and British co-workers (Franks, P.J. et al. 2014. New constraints on atmospheric CO2 concentration for the Phanerozoic. Geophysical Research Letters, v. 41, p. 4685-4694; doi:10.1002/2014GL060457). Their method included a means of assessing the back and forth exchange of leaf gases with the atmosphere from measurements of the carbon isotopes in preserved organic carbon in the fossil leaves, and combined this with stomatal density and the actual shape of stomata. Not only did this narrow the range of variation in atmospheric CO2 results for times past, but the mean values were dramatically lessened. Rather than values ranging up to 2000 to 3000 parts per million (~ 10 times the pre-industrial value) in the Devonian and the late-Triassic and early-Jurassic, the gas-exchange method does not rise above 1000 ppm in the Phanerozoic.

The upshot of these findings strongly suggests that the Earth’s climate sensitivity to atmospheric CO2 (the amount of global climatic warming for a doubling of pre-industrial CO2 concentration) may be greater than previously thought; around 4° rather than the currently accepted 3°C. If this proves to be correct it forebodes a much higher global temperature than present estimates by the Intergovernmental Panel on Climate Change (IPCC) for various emission scenarios through the 21st century.

See also: Hand, E. 2017. Fossil leaves bear witness to ancient carbon dioxide levels. Science, v. 355, p. 14-15; DOI: 10.1126/science.355.6320.14.

Amazonian forest through the last glacial maximum

Accelerated evolution may occur when a small population of a species – whose genetic variability is therefore limited – becomes isolated from all other members. This is one explanation for the rise of new species, as in the Galapagos archipelago. Creation of such genetic bottlenecks encourages rapid genetic drift away from the main population. It has been suggested to explain sudden behavioural shifts in anatomically modern humans over the last hundred thousand years or so, partly through rapid and long-distance migrations and partly through a variety of environmental catastrophes, such as the huge Toba eruption around 74 ka. Another example has been proposed for the teemingly diverse flora and fauna of the Amazon Basin, particularly among its ~7500 species of butterflies, which has been ascribed to shrinkage of the Amazonian rain forest to isolated patches that became refuges from dry conditions during the last glacial maximum.

Top: Arid ice age climate Middle: Atlantic Per...

Potential forest cover inferred from global climate models for the last glacial maximum (top) the Holocene thermal maximum and at present.. (credit: Wikipedia)

A great deal of evidence suggests that during glacial maxima global climate became considerably drier than that in interglacials, low-latitude deserts and savannah grasslands expanding at the expense of humid forest. Yet the emerging complexity of how climate change proceeds from place to place suggests that evidence such continental drying from one well-documented region, such as tropical Africa, cannot be applied to another without confirming data. Amazonia has been the subject of long-standing controversy about such ecological changes and formation of isolated forest ‘islands’ in the absence of definitive palaeoclimate data from the region itself. A multinational team has now published data on climatic humidity changes over the last 45 ka in what is now an area of dense forest but also receives lower rainfall than most of Amazonia; i.e. where rolling back forest to savannah would have been most likely to occur during the last glacial maximum (Wang, X. et al. 2017. Hydroclimate changes across the Amazon lowlands over the past 45,000 years. Nature, v. 541, p. 204-207; doi:10.1038/nature20787).

Their study area is tropical karst, stalagmites from one of whose caves have yielded detailed oxygen-isotope time series. Using the U/Th dating technique has given the data a time resolution of decades covering the global climatic decline into the last glacial maximum and its recovery to modern times. The relative abundance of oxygen isotopes (expressed by δ18O) in the calcium carbonate layers that make up the stalagmites is proportional to that of the rainwater that carried calcium and carbonate ions dissolved from the limestones. The rainwater δ18O itself depended on the balance between rainfall and evaporation, higher values indicating reduced precipitation. Relative proportions of carbon isotopes in the stalagmites, expressed by δ13C, record the balance of trees and grasses, which have different carbon-isotope signatures. Rainfall in the area did indeed fall during the run-up to the last glacial maximum, to about 60% of that at present, then to rise to ~142% in the mid-Holocene (6 ka). Yet δ13C in the stalagmites remained throughout comparable with those in the Holocene layers, its low values being incompatible with any marked expansion of grasses.

English: View of Amazon basin forest north of ...

Amazonian rain forest north of Manaus, Brazil. (credit: Wikipedia)

One important factor in converting rain forest to grass-dominated savannah is fire induced by climatic drying. Tree mortality and loss of cover accelerates drying out of the forest floor in a vicious circle towards grassland, expressed today by human influences in much of Amazonia. Fires in Amazonia must therefore have been rare during the last ice age; indeed sediment cores from the Amazon delta do not reveal any significant charcoal ‘spike’.

See also: Bush, M.B 2017. The resilience of Amazonian forests. Nature, v. 541, p. 167-168; doi:10.1038/541167a

Some observations on scientific publication

For most scientists research brings many pleasures: exercising curiosity and ingenuity; the moment of discovery, sometimes an esprit de corps; showing that you were right, and so on. Anthropologists might say that it is a form of playfulness, the ‘scientific method’ being the rules of the game. Telling people about your results at conferences also has its bright moments: showing off; making new acquaintances and renewing old ones; a wider esprit de corps; globetrotting, all expenses paid. Communicating data and discussing results formally before that part of the academic world that you inhabit is a pain by comparison, even for the most gifted writer.  Have all avenues of enquiry and interpretation been exhausted? Is your paper a model of clarity, and will/can anyone read it? Is what you have to recount actually new and/or important? Have you missed something that has already been published? Have you committed plagiarism unconsciously. Are your references bang up to date? The anonymous peer-review system can be merciless, and so can journal editors. Writing up and awaiting reviews are among the most stressful periods in the professional lives of most researchers, because so many boxes have to be ticked to glide effortlessly into print.

The greatest of all literary bugbears is tailoring the style of your list of references to that of the target journal. Very, very occasionally the publisher will employ kindly sub-editors who make sure that all is well in this the most arcane of all academic rituals. The problem is that every academic publishing house and even different journals that each produces have subtly different rules for references cited, in the text and in the list at the end. There are so many permutations and combinations: a comma before the year; v. before volume number, including the issue or not, bold or plain font; journal name in full or one of several kinds of abbreviation; each author’s initials separated by a space or not (the former for the Journal of Geology if you have been wondering – ‘Their given names in full are separated by a space, so it is only polite’!). And there is much, much more in each journal’s ‘Information for Authors’. Quanmin Guo of the University of Birmingham (Correspondence, Nature, v. 540 22/29 December 2016, p. 525) makes the obvious point that every journal should conform to a uniform style – within vividly distinctive bindings what is the need for arcane house rules?

But there is another, more serious grouse about the vast majority of journals. If your institution or you as an individual cannot afford to subscribe to a journal you will inevitably come up against the ‘paywall’ when you try to read an article on line ($10 – 50 per article) even at a time when well-heeled academics are paying for their papers to be open-access (in most cases still behind the paywall for 6 months following publication). The irony is that less well-off researchers also cannot afford to make their work available to all.  Alexandra Elbakyan of Alamaty, Kazakhstan, set out to circumvent the paywall barrier to scholarly exchange, and succeeded in the foundation of Sci-Hub (Van Noorden, R. 2016. Paper pirate. Nature, v. 540, p. 512-513), which hosts about 60 million papers and encompassed about 3% of all PDF downloads in the last year (simply by pasting in a paper’s DOI. Alexandra has been widely praised and thanked, served with a writ for breach of copyright (by Elsevier), had Sci-Hub shut down by order of a US judge (there are proxies), and is currently incommunicado (except for encrypted e-mail) for fear of a demand for multi-million dollar damages. Chances are that she has opened a floodgate to future universal open access. In the meantime, unless I am hopelessly mistaken, there is a perfectly legal work around for you to get must-read papers shortly after they appear at no cost. Email the corresponding author (usually in the free online Abstract in a journals latest issue list of contents) and ask for an offprint in the form of a PDF ‘for the purpose of scholarly exchange’.

Human penis bone lost through monogamy?

The baculum or penis bone is arguably the most variable of mammalian bones, present in some species but not others. Among those in which it does occur the baculum varies enormously in shape, length and breadth relative to body size. This makes it likely to have been subject to the most divergent evolution among mammals. Yet its evolution has remained somewhat puzzling until recently. Observation has shown that the width of the baculum in male house mice is positively correlated with reproductive success. So one factor in the bone’s evolution may be postcopulatory sexual selection: female mice seem to favour males well endowed in this department once they have mated with them, a notion supported by careful laboratory experimentation. The physical role of the penis bone is to support and protect the penis during sexual intercourse. Sturdy dimensions are increasingly efficacious the longer the duration and the greater the frequency of copulation, particularly among polygamous and seasonally breeding species. They also tend to delay or inhibit a female mating with another male after copulation.

Walrus baculum, approximately 22 inches long

Walrus baculum, approximately 0.6 metres long (credit: Wikipedia)

Matilda Brindle and Christopher Opie of University College London have applied advanced phylogenetic statistical analysis to data on the dimensions of penis bones among 2000 mammal species (Brindle, M. & Opie, C. 2016. Postcopulatory sexual selection influences baculum evolution in primates and carnivores. Proceedings of the Royal Society, B, v. 283, doi: 10.1098/rspb.2016.1736) and suggest that the baculum first evolved in mammals between 145 to 95 Ma ago, earlier mammals likely having no penis bone. Ancestral primates and carnivorous mammals, however, were so endowed. Yet some mammalian species have lost the baculum.  Among the primates human males do not have one whereas male chimpanzees and bonobos, with which we share a last common ancestor, do: both are boisterously promiscuous whereas humans are pair-bonded to a large degree.

The issue of polygamy versus monogamy among human ancestors, and when the latter emerged, continues to exercise palaeoanthropologists. The former in other living primates is often associated with a marked contrast in size between males and females – sexual dimorphism. The earliest hominins, such as species of Australopithecus, did exhibit such dimorphism whereas species of Homo show significantly less size contrast, which some have taken to mark the emergence of pair-bonding amongst members of the earliest human species to be passed on to their successors. Another indicator of competitiveness among primate males for females, and their dominance over the latter, is the near universal possession of large canine teeth among males of polygamous primates; an odd feature for species whose diet is dominantly and often exclusively vegetarian. Not only do living humans not have prominent canines, neither do any known fossil hominins. Despite the views of a small minority of anthropologists who demand that modern human females won social parity with males only in the last 100 thousand years, only to lose it following the Neolithic ‘revolution’, the physical evidence suggests that a trend towards that emerged with other distinct characteristics of hominins and concretised in early Homo. An assiduous search for fossil hominin penis bones may yet reveal the moment of monogamy.

Geoscience academic under threat

While she was US Secretary of State from 2009 to 2013, Hilary Clinton the 2016 Democrat candidate for presidential office habitually used her private email server to send and receive messages, both personal and concerning affairs of state. She did not activate a state.gov email account for the official stuff, saying that it was ‘for convenience’ as her Blackberry smartphone could only access on account. More than 30 thousand undeleted e-mails were hacked by ‘persons unknown’ and appeared on Wikileaks in early 2016, with more in late October 2016, to become one of several issues central to the 2016 US presidential campaign. This practice was twice exonerated by the FBI, despite her account proving to be insecure and the risk to state secrets.

Hans Thybo, President of the European Geophysical Union and a widely esteemed professor of seismology at the University of Copenhagen, was not so lucky. He was fired by the University authorities, allegedly for using his private email account for work-related issues and advising a postdoctoral fellow that criticising the University’s management was ‘legitimate’. More than 1000 academic colleagues have petitioned the University of Copenhagen to reverse its decision and reinstate Thybo, and his case was central to the lead editorial A creeping corporate culture in Nature of 15 December 2016.

Anyone connected for more than a few years with academic life in probably every university on the planet will be conscious of the spread of a culture of bureaucratic control, corporatism and commodification in what formerly were largely self-governing institutions of higher education and research. The trend is in line with increasing, omnidirectional economic pressures stemming from the aftermath of the 2007-8 global financial crisis. But it is not entirely new. My own experience suggests it is partly a logical outcome of leading academics becoming increasingly prone to saying ‘Yes’; at best simply disengaging from dispute with a growing managerial caste within education and research, at worst by opportunistically joining it. A serious disjuncture has developed between teachers and researchers and the managers and business administrators in institutions of higher education. Symptomatic of this kind of schism was the recent passing of a motion of no confidence in the Vice Chancellors Executive of Britain’s Open University by its unionised academic and academic-related staff, following disastrous bungling of a new means of tuition of its entirely non-residential undergraduates in the current academic year. Those who were to implement the measures were inadequately consulted by the leading managers, most of whom had little experience of how the OU had functioned successfully since it received its Charter in 1969.

In Britain, checks and balances on the requirements of management and faculty historically centred on their Senate, once the primary academic authority of universities, in which all members of both academic and non-academic sectors freely debated and passed judgement on new directions and the abandonment of practices that had been found wanting. In most institutions, the Senate has been reduced since the mid 1980s to a mere fraction of staff, who, after nomination, are elected by the various components of the institution, together with unelected, ex officio, members of senior management. In practice, Senates now generally act as a ‘rubber stamp’ for decisions of the top echelons, much in the manner of business corporations.

Part of the new culture attempts to regulate electronic communications. An example of such an IT regulation states that the institution ‘… may monitor all data, systems and network traffic at any time …’, i.e. it claims ownership of work-related communication. No wonder Hans Thybo fell foul of his university. Should outside pressure persuade the authorities of the University of Copenhagen to reinstate him, that would be a significant blow against what has become an unwholesome aspect of learning and scholarship.

‘Big data’ on water resources

 

Two petabytes (2×1015) is a colossal number which happens to approximate how much data has been collected in geocoded form by the Landsat Thematic Mapper and its successors since it was first launched in 1984. In tangible form these would occupy about half a million DVDs, weighing in at about 8 metric tonnes; ‘daunting’ comes nowhere near describing the effort needed to visually interpret this unique set of multi-date imagery. Using the Google Earth Engine, the free cloud-computing platform for big sets of image data which hosts all Landsat data and much else (but not yet the equally daunting ASTER data – roughly a million 136 Mb scenes) the 32 years-worth has been analysed for its content of hydrological information by the European Commission’s Joint Research Centre in Italy, with assistance from Google Switzerland. Using the various spectral characteristics of water in the visible and infrared region, the team has been able to assess the position on the continents of surface water bodies larger than 900 m2, both permanent and ephemeral, and how the various categories have changed in the last 32 years (Pekel, J.-F. et al. 2016. High-resolution mapping of global surface water and its long-term changes. Nature, v. 540, p. 418-422; doi:10.1038/nature20584). The results are conveniently and freely available in their entirety at the Global Surface Water Explorer, an unparalleled and easy-to-use opportunity for water resource managers, wetland ecologists and geographers in general.

Among the revelations are sites and areas that have been subject to gains and losses in water availability, the extents of new and vanished permanent and seasonal water bodies and the conversion of one to the other. A global summary gives a net disappearance of 90 thousand km2 of permanent water bodies, about the area of Lake Superior, but exceeded by new permanent bodies totalling 184 thousand km2. There has been a net increase in permanent water on all continents except Oceania with a loss one percent (note that Antarctica and land north of the Arctic Circle were not analysed). More than 70 % of the losses are in the semi-arid Middle East and Central Asia (Iran, Iraq, Uzbekistan, Kazakhstan and Afghanistan), due mainly to overuse of irrigation, dam construction and long-term drought. Much of the increase in water occurrence stems from reservoir construction, but climate change may have played a part through increased precipitation and melting of high-altitude snow and ice, as in Tibet.

The Aral Sea in Uzbekistan and Kazakhstan has suffered dramatic loss of standing and seasonal water cover due to overuse of water for irrigation from the two main rivers, the Amu (Oxus) and Syr, that flow into it. Note the key to the colours that represent different categories of changes in surface water. (Credit: Global Surface Water Explorer)

The Aral Sea in Uzbekistan and Kazakhstan has suffered dramatic loss of standing and seasonal water cover due to overuse of water for irrigation from the two main rivers, the Amu (Oxus) and Syr, that flow into it. Note the key to the colours that represent different categories of changes in surface water. (Credit: Global Surface Water Explorer)

Many of the lakes in the northern Tibetan Plateau have grown in size during the last 32 years, mainly due to increased precipitation and snow melt. (Credit: Global Surface Water Explorer)

Many of the lakes in the northern Tibetan Plateau have grown in size during the last 32 years, mainly due to increased precipitation and snow melt. (Credit: Global Surface Water Explorer)

There are limitation to the accuracy of the various categories of change, one being the persistence of cloud cover in humid climates, another being the sometimes haphazard scheduling of Landsat Data capture (in some case that has depended on US Government interest in different areas of the world).

More detail on using remote sensing in exploration for and evaluation of water resources can be found here.

When did the Greenland ice cap last melt?

The record preserved in cores through the thickest part of the Greenland ice cap goes back only to a little more than 120 thousand years ago, unlike in Antarctica where data are available for 800 ka and potentially further back still. One possible reason for this difference is that a great deal more snow falls on Greenland so the ice builds up more quickly than in Antarctica. Because ice flows under pressure this might imply that older ice on Greenland long flowed to the margins and either melted or calved off as icebergs. So, although it is certain that the Antarctic ice cap has not melted away, at least in the last million years or so, we cannot tell if Greenlandic glaciers did so over the same period of time. Knowing whether or not Greenland might have shed its carapace of ice is important, because if ever does in future the meltwater will add about 7 metres to global sea level: a nightmare scenario for coastal cities, low-lying islands and insurance companies.

Margin of the Greenland ice sheet (view from p...

Edge of the Greenland ice sheet with a large glacier flowing into a fjiord at the East Greenland coas  (Photo credit: Wikipedia)

One means of judging when Greenland was last free of ice, or at least substantially so, is based on more than a ice few metres thick being opaque to cosmic ‘rays’. Minerals, such as quartz, in rocks bared at the surface to ultra-high energy, cosmogenic neutrons accumulate short-lived isotopes of beryllium and aluminium – 10Be and 26Al with half-lives of 1.4 and 0.7 Ma. Once rocks are buried beneath ice or sediment, the two isotopes decay away and it is possible to estimate the duration of burial from the proportions of the remaining isotopes. After about 5 Ma the cosmogenic isotopes will have decreased to amounts that cannot be measured. Conversely, if the ice had melted away at any time in the past 5 Ma and then returned it should be possible to estimate the timing and duration of exposure of the surface to cosmic ‘rays’. Two groups of researchers have applied cosmogenic-isotope analysis to Greenland. One group (Schaefer, J.G. et al. 2016. Greenland was nearly ice-free for extended periods during the Pleistocene. Nature, v. 540, p. 252-255) focused on bedrock, currently buried beneath 3 km of ice, that drilling for the ice core finally penetrated. The other systematically analysed the cosmogenic isotope content of mineral grains at different depths in North Atlantic seafloor sediment cores, largely supplied from East Greenland since 7.5 Ma ago (Bierman, P.R. et al. 2016. A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years Nature, v. 540, p. 256-260). As their titles suggest, the two studies had conflicting results.

The glacigenic sediment grains contained no more than 1 atom of 10Be per gram compared with the 5000 to 6000 in grains deposited and exposed to cosmic rays along the shores of Greenland since the end of the last ice age. These results challenge the possibility of any significant deglaciation and exposure of bedrock in the source of seafloor sediment since the Pliocene.  The bedrock from the base of Greenland’s existing ice cap, however, contains up to 25 times more cosmogenic isotopes. The conclusion in that case is that there must have been a protracted, >280 ka, exposure of the rock surface in what is now the deepest ice cover at 1.1 Ma ago at most. Allowing for the likelihood of some persistent glacial cover in what would have been mountainous areas in an otherwise substantially deglaciated Greenland, the results are consistent with about 90% melting suggested by glaciological modelling.

Clearly, some head scratching is going to be needed to reconcile the two approaches. Ironically, the ocean-floor cores were cut directly offshore of the most likely places where patches of residual ice cap may have remained. Glaciers there would have transported rock debris that had remained masked from cosmic rays until shortly before calved icebergs or the glacial fronts melted and supplied sediment to the North Atlantic floor. If indeed the bulk of Greenland became ice free around a million years ago, under purely natural climatic fluctuations, the 2° C estimate for global warming by 2100 could well result in a 75% glacial melt and about 5-6 m rise in global sea level.

Read more about glaciation here and here.

Tree-climbing australopithecines

We know that Lucy, the famous Australopithecus afarensis, could climb trees because her many bone fractures show that she fell out of a tree to her death. But that does not mean her species was an habitual tree-climber: plenty of modern humans fall to their deaths from trees, cliffs and the like. But the issue seems to have been resolved by using X-ray tomography of Lucy’s limb bones (Ruff, C.B. et al. 2016. Limb bone structural proportions and locomotor behaviour in A.L. 288-1 (“Lucy”).  PLOS ONE v. 11, e0166095. doi:10.1371/journal.pone.0166095) during the skeleton’s triumphal series of exhibits in the US.

The authors, including two of those who showed that Lucy died after a fall using similar data, compared the digital 3-D models of her surviving arm- and leg bones with those of other hominins and living primates, estimating their relative strengths at different positions. Lucy was probably stronger in the arm than in the leg, but not to the same degree as chimpanzees. This is a feature that would significantlyassist climbing , but her bipedal locomotion on the ground would have been only slightly different from that of later Homo species. If anything, her strength relative to size would have been greater than ours, perhaps reflecting less reliance on tools for getting food and defending herself. But almost certainly Australopithecus afarensis habitually spend more time in trees, perhaps foraging and as a defence against predation, especially at night.

The new data for Lucy allows palaeoanthropologists to better judge the capabilities of other hominins. Interestingly Homo habilis, the earliest of our genus, may have had similar habits. But later species, beginning with H. erectus/ergaster, were as Earth-bound as we are. This suggests a shift in hominin ecology from an early and probably long history of semi-arboreal behavior until humans became masters of their terrain about 1.9 Ma ago, probably through their invention of better tools and the controlled use of fire.

Read more about human evolution here and here

A challenge to the concept of species

Comparison of DNA from ancient hominin fossils with that obtained from a broad spectrum of living people showed that on the road out of Africa in the last 130 thousand years some anatomically modern humans successfully interbred with Neanderthals and Denisovans to produce fertile offspring. All non-African people contain a trace of those liaisons and include fertile hybrids in their ancestry, whereas Africans do not. Using quick, low-cost and sensitive genomic analyses these discoveries made similar searches for hybridisation among other supposedly distinct species a popular and fruitful line of research (Pennisi, E. 2016. Shaking up the tree of life. Science, v. 354, p. 817-820; doi: 10.1126/science.354.6314.817). They also challenge the long-held view that individual species are incapable of fertile interbreeding with others. Yet fertile hybrids have long been known among plants and butterflies without recourse to genomics. Now that it is a basic tool, it has been shown that up to 10% of known plant species have arisen from hybrids and examples are quickly being found among birds, insects, fish and mammals, including the famous Galapagos finches. Hybridisation introduces genetic variation more quickly than does mutation, potentially a major advantage in adaptive radiation.

Page from Darwin's notebooks around July 1837 ...

Page from Darwin’s notebooks around July 1837 showing his first sketch of an evolutionary tree. (credit: Wikipedia)

Of course, the concept of ‘species’ is arbitrarily based on biological ‘form and function’, and in the same fashion as discoveries about epigenetics have shown genetic determinism to have an air of dogma, so hybridisation suggests that a ‘web’ is more apt as shorthand for the progress of evolution than is Darwin’s ‘tree’ or even a tangled ‘bush’. Another welcome outcome spurred by the pioneers of hominin comparative genetics is a powerful challenge to the dominant philosophies of reductionism and dualism among scientists; legacies of René Descartes bound up with the ‘scientific method’ – especially among physical scientists – and ideas such as ‘nature versus nurture’. A major revolution is in progress, from which the seekers for a Theory of Everything, from quantum mechanists through particle physicists to cosmologists need to draw some sharp and perhaps embarrassing lessons.

It is appropriate that the driving agency lies within anthropology, and thrilling too, for everyone can quickly learn a new way of approaching the world by contemplating their own origins. They would be hard-pressed to do that by pondering on the early nanoseconds of the cosmos …

K-T (K-Pg) boundary impact probed

One of the most eagerly followed ocean-floor drilling projects has just released some results. Its target is 46 km radially away from the centre of the geophysical anomaly associated with the Chixculub impact structure just to the north of Mexico’s Yucatan Peninsula. In the case of large lunar impact craters the centre is often surrounded by a ring of peaks. Modelling suggests such features are produced by the deep penetration of immense seismic shock waves. In the first minute these excavate and fling out debris to leave a cavity penetrating deep into the crust. Within three minutes the cavity walls collapse inwards creating a rebound superficially similar to the drop flung upwards after an object is dropped in liquid. This, in turn, collapses outwards to emplace smashed and partially melted deep crustal material on top of what were once surface materials, creating a crustal inversion beneath a mountainous ring of Himalayan dimensions that surrounds a by-now shallow crater. That is the story modelled from what is known about well-studied, big craters on the Moon and Mercury. Chixculub is different because the impact was into the sea and involved debris-charged tsunamis that finally plastered the actual impact scar with sediments. The drilling was funded for several reasons, some palaeontological others relating to the testing of theories of impact processes and their products. Chixculub is probably the only intact impact crater on Earth, and the first reports of findings are in the second category (Morgan, J.V. and 37 others 2016. The formation of peak rings in large impact craters. Science, v. 354, p. 878-882; doi: 10.1126/science.aah6561).

English: K/T extinction event theory. An artis...

Artist’s depiction of the Chicxulub impact 65 million years ago that many scientists say is the most direct cause of the dinosaurs’ disappearance (credit: Wikipedia)

The drill core, reaching down to about 1.3 km below the sea floor penetrates post-impact Cenozoic sediments into a 100 m thick zone of breccias containing fragments of impact melt rock, probably the infill of the central crater immediately following the first few minutes of impact. Beneath that are coarse grained granites representing the middle continental crust from original depths around 10 km. The granite is intensely fractured and riven by dykes and pods of impact melt, and contains intensely shocked grains that typify impacts that produce a transient pressure of ~60 GPa – around six hundred thousand times atmospheric pressure. From seismic reflection surveys this crustal material overlies as yet un-drilled Mesozoic sedimentary rocks. Its density is significantly less than that of unshocked granite – averaging 2.4 compared with 2.6 g cm3. So it is probably filled with microfractures and sufficiently permeable for water to have penetrated once the impact site had cooled. This poses the question, yet to be addressed in print, of whether or not this near-surface layer became colonised by microorganisms in the aftermath (Barton, P. 2016. Revealing the dynamics of a large impact. Science, v. 354, p. 836-837). That is, was the surrounding ocean sterilised at the time of the K-T (K-Pg) mass extinction?; an issue whose resolution is awaited with bated breath by the palaeobiology audience. OK; so theory about the physical process of cratering has been validated to some extent, but will later results be more interesting, outside the planetary sciences community?

Read more about impacts here and mass extinctions here .