Ganymede’s water volcanism

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Jupiter’s giant moon Ganymede is an icy world, as are many satellites of the Outer Planets.  But is also one of the few showing signs of some kind of tectonics.  Its surface is made up of dark, cratered material, presumably an ancient mixture of rocky debris and ice, riven by swaths of lighter surface.  The latter, which covers two-thirds, has little cratering and is a later feature of the moon’s surface.  Somehow, Ganymede underwent a resurfacing, perhaps in a similar manner to neighbouring Europa – a simple ice ball – but not so all-consuming.

The event probably stemmed from the coming together of Jupiter’s largest moons into orbital resonance that generated sufficient gravitational energy to cause internal melting.  Precisely how this achieved the intricacies of Ganymede’s surface is something of a mystery.

Images from Voyager and Galileo missions form stereoscopic pairs from which the moon’s topography can be derived with useful precision (Schenk, P.M. et al.  2001.  Flooding of Ganymede’s’ bright terrains by low-viscosity water-ice lavas.  Nature, v. 410, p.57-60).  Using digital elevation data with high-resolution Galileo images, Schenk et al. have been able to subdivide the light swaths into three kinds of surface, reticulate, grooved and smooth at different elevations from highest to lowest.  Large elevation differences of the order of 2 km are involved.  That in itself is evidence that ice at the prevailing temperature behaves more like rock than glacial ice.

The greater surprise is that the lowest, smooth unit shows evidence of having formed by processes akin to volcanism, with calderas and features that engulf earlier structures.  However, even the fine resolution of the latest images does not reveal “lava” flows.  Some rifting mechanism seems to have encouraged emergence of water-ice “magma” to form the low smooth terrains.  All very counter-intuitive for terrestrial volcanologists, because water “magma” must be more dense than the solidified flows forming from it, unlike silicate liquids or those rich in sulphur on Io.  That makes the formation of high volcanoes impossible.

Presumably, the much higher grooved and reticulate terrains started in the same manner, as linear troughs, then to be deformed and thickened by “water tectonics”.

Bacterial sulphides from the Archaean

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Most of the sulphide mineralization involved in base-metal ore bodies formed by reaction between metal ions and those of sulphur released by bacteria that reduce sulphate ions in water.  They do that while oxidizing organic matter or hydrogen in their metabolism, under completely anaerobic conditions.  Like other biological processes, sulphide production at the cell level fractionates the isotopes of sulphur so that it becomes possible to chart sulphate-reducing bacteria through time.  Depletion of 34S in sedimentary sulphides relative to that in co-existing sulphates (such as baryte) was previously known with certainty back to 2.7 Ga.  Danish and Australian bio-geochemists have now pushed this particular bacterial metabolism back by 750 Ma (Shen, Y.  et al. 2001.  Isotopic evidence for microbial sulphate reduction in the early Archaean era.  Nature, v. 410, p. 77-81).

The data from the Pilbara Craton of Western Australia helps calibrate the evolutionary bush of the prokaryotes, which is based on comparisons between RNA in different living organisms.  The trouble is, sulphate-reducing species with very primitive genetics and similar lifestyles (hyperthermophilic) occur among both the Bacteria and Archaea.  Shen et al. go for the Bacteria Thermodesulfobacterium as the most likely organism responsible.  Their argument is that the mineralization replaces originally sedimentary gypsum, formed at low temperatures, and probably represents hydrothermal processes in which thermophilic organisms could have thrived.  Bacteria that reduce sulphate ions at low temperatures – gram-positive and purple bacteria – are genetically more advanced than their candidate.

See also:  Slime to the rescue Earth Pages December 2000

“Piltdown” bird

Fragmentary remains of vertebrates in particular are notoriously prone to misguided reconstruction – Gideon Mantell placed the Iguanodon’s thumb on its nose, thereby obscuring evidence for the first hitchhiking dinosaur for many decades.  The forger of fossils has two possible motives – spite in the case of Piltdown Man, or profit.  The skilled forgeries of Silurian trilobites by quarrymen from Dudley in Britain’s West Midlands are now more valuable because they were made for rapacious Victorian antiquaries, than bona fide Calymene specimens.  Missing links sought by professional palaeontologists and archaeo-biologists are in a field of their own.  It has long been suspected that birds evolved from small carnivorous dinosaurs, and the early Cretaceous of China has provided spectacular transitional fossils.  Archaeoraptor was announced as the final missing link in 1999.  Within a year it was denounced as a forgery that combines very skilfully the bones of a primitive bird with those of a non-flying dromeosaurid dinosaur.  How it was assembled has finally been revealed using X-ray tomography, which shows that as many as 5 different specimens were “cut and pasted” together (Rowe, T. et al. 2001.  The Archaeoraptor forgery.  Nature, v. 410, p. 539-40).

Cretaceous water lilies

Readers of Earth Pages will be delighted to learn that fossil flowers of Nymphaeales (water lilies) have been found in the Lower Cretaceous of Portugal.  (Friis, E.M. et al. 2001.  Fossil evidence of water lilies (Nympaeales) in the Early Cretaceous.  Nature, v. 410, p. 357-360).

When modern corals emerged

Fossil corals  fall into three taxonomic groups or Orders: tabulate, rugose and scleractinian.  Only the last group is alive today.  Scleractinian corals have been central to the “carbonate factories” that have drawn down CO2 from the atmosphere throughout the Mesozoic and Cainozoic Eras to form reef limestones.  They are major regulators of long-term climate fluctuation.  However, there is something very odd about their appearance in the fossil record, as discussed recently by George Stanley and Daphne Fautin (Stanley, G.D. and Fautin, D.G. 2001.  The origins of modern corals.  Science, v. 291, p. 1913-1914).

The rugose and tabulate corals were exclusively Palaeozoic colonial, carbonate-secreting organism.  Their record ends abruptly with the end-Permian mass extinction.  No examples of scleractinian corals have been found in rocks older than Triassic.  The oddity is a 14 Ma gap in known coral fossils in the earliest Triassic.  Scleractinians secrete calcium carbonate as aragonite, whereas rugose corals formed from calcite; an important difference in processes at the cellular level.  It is hard to avoid the conclusion that the ancestors of scleractinians did not secrete carbonate and were entirely soft-bodied taxa during the Palaeozoic Era.  If Permian Rugosa and Tabulata happily secreted carbonate, while proto-Scleractinia did not, there ought to be a biochemical or geochemical explanation for the last taking on a reef building role in Mesozoic times.

Molecular evidence suggests that scleractinian ancestry goes back to the Late Carboniferous, and that there is a complex “lawn” (as opposed to tree or bush) of genetic relationships between modern hard corals and soft-bodied organisms that are closely related.  The puzzle can potentially be resolved if modern corals and their ancestral lines lost and regained skeleton building several times in the Mesozoic and Cainozoic.  Exploring that requires more understanding of how carbonate is secreted at the cell level, and the geochemical conditions in seawater that underpin the need for secretion.

Following the greatest ever mass extinction at the end of the Permian, early Triassic oceans were almost sterile and anoxic.  Global CO2 levels were high, yet little carbonate was deposited in the marine environment.  That would have increased the amount of calcium and bicarbonate ions in sea water.  Many corals harbour algal symbionts that are involved in calcification.  As calcium carbonate saturation drops so too does carbonate secretion, and vice versa.  Calcium is a two-edged sword in cell metabolism.  On the one hand it is vital in “information” transfer, yet above a threshold it combines with CO2 to form crystalline carbonate within the cell wall, that spells cell death.  In Palaeozoic oceans rugose and tabulate corals, as well as a host of other carbonate secreting animals, would have buffered calcium concentrations below levels tolerable by other, soft-bodied animals.  Their sudden demise 251 Ma ago, along with most everything else, would have left calcium to build up in the early Triassic “Strangelove” ocean.  Survivors of the holocaust would have had a fierce task coping with potential calcium toxicity, and the scleractinians may well have adopted calcification as a survival mechanism.  Thereafter, oceans restocked with reef building organisms would have had tolerable calcium concentrations for most organisms, those now able to secrete carbonate having the benefit of armour against predation and a solid substrate for colony building.

Skulduggery, migration and confusion

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March was a fertile month for news concerning human origins and evolution.  The good news is that the palaeoanthropologists are at each other’s throats again!  I think it is good news because many of them have an air of smugness and triumph, and they get far more money than other Earth scientists (with the exception of those bent on finding a banth on Mars).  Tangling with hominins in Kenya is a sure route to trouble, as the finders of “Millenium Man” (Ororrin tugenensis) – Martin Pickford and Brigitte Senut now discover (Butler, D.  2001.  The battle of Tugen Hills.  Nature, v. 410, p. 508-509).  Not only is their claim that the 6 Ma old fossil is the oldest on the route to humanity hotly disputed (Aiello, L.C. and Collard, M.  2001.  Our newest oldest ancestor?  Nature, v. 410, p. 526-527), but has ended with their taking suit against Richard Leakey and the Kenyan National Museums for unlawful arrest, false imprisonment and malicious harassment over claims that they poached the site where Orrorin was found.  Never an easy atmosphere in which to work, human evolution is now one posing considerable dangers, so much so that some specialists will comment only anonymously.

Books in the field always sell like hot cakes, as much for the intrigue and the chutzpa as for the science that they convey.  Reviewers become drawn into the hype, despite their best intentions (White, T.D. 2001.  Adventures in the Bone Trade: the Race to Discover Human Ancestors in Ethiopia’s Afar Depression, by Jon Kalb.  Nature, v. 410, p. 517-518).  Areas in Afar and Danakil are physically dangerous because of current hostilities between Eritrea and Ethiopia, and dissatisfaction among the local people.  But they have enormous potential for hominin discoveries following those of “Lucy” and Ardepithecus.  On a recent visit to Eritrea I heard rumours of what might amount to claim jumping and attempts to acquire material clandestinely from new and potentially productive sites, hopefully without foundation.

Confusion is washing over hominin cladistics as ever more variants of accepted species, and fossils that seem to warrant new species and genera turn up.  This is particularly rife for early remains that predate the first stone tools (Lieberman, D.E. 2001.  Another face in our family tree.  Nature, v. 410, p. 419-20; Balter, M. 2001.  Fossil tangles the roots of human family tree.  Science, v. 291, p. 2289-2290; ).  Of course, much of the confusion stems from every new find seeming to bear different cranial and dental hallmarks, combined with dogged attempts to chart the path of our descent through the remains and a tendency to change genus and species names (Homo habilis is now sometimes assigned to Australopithecus, despite a probable association with primitive stone tools).  The latest bush figured by Lieberman is notable for every supposed cladistic link being marked by a query.  One gets the impression of rather too much shuffling around of anatomy, and too little consideration of the unseeable, but inevitably vital distinction between the human line and other fossils.  There are still very few hominin fossils!

Tools demand consciousness, and probably social links far stronger than those of other apes.  Only stone tools survive, from around 2.5 Ma ago, but must represent an advanced culture that arose from earlier beginnings.  Abstracting usefulness from surrounding nature and social organisation confer such advantages to its inventors that they set them apart from other animals in relation to natural selection.  Fitness no longer applies to the individual organism, but increasingly to its culture shared with others.  The formerly unfit becomes fit, and that can play havoc with physiological diversity and thereby the cladists’ shuffling.

Culture confers something equally powerful by enabling its carriers to diffuse beyond their geographic range.  The 2 March 2001 issue of Science devotes 33 pages to human migrations (Culotta, E, Sugden, A and Hanson, B. (eds)  2001.  Humans on the move.  Science, v. 291, p. 1721-1753).  For me, this is the most powerful and informative contribution to our self-knowledge in many years.  Eight articles cover the earliest Europeans, the relations between modern humans and Neanderthals, the first colonisers of the Americas, the roles of genetics in teasing out our origins and how tools track physiological change.  Appearing in the midst of tedious and self-regarding squabbles among the “bone people”, it surely marks a proper line of march in this abidingly gripping branch of Earth science.

Human genome “snips” and our evolution

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February 2001 saw the public release of the human genome, with entire issues of both Nature and Science substantially devoted to discussion of its implications, educational CDs and wall charts.  That is if the huge wadge of adverts capitalising on the genome’s release is discounted  Pundits have latched onto the fact that humans seem to possess not that many more genes (around 30 000) than grass, a worm or a fruit fly, making comments about how humbling that is.  Vastly outnumbering protein-coding genes are “snips” (single nucleotide polymorphisms – SNPs), and humans have around 1.4 million of these and possibly far more in the 3 billion sequences of four nucleotides.

The huge variability of “Snips” holds excellent prospects for deeper understanding of human origins and evolution, previously (and unsatisfactorily) addressed by using DNA in mitochondria and the Y chromosome.  Previous means of establishing molecular  “distance” to indicate relatedness and the times of divergence from last common ancestors rely on DNA that occurs only once in each cell, and does not undergo division and recombination during sexual reproduction, so that it is passed on in the female or male line of descent.  Whereas such haploid material is relatively easy to analyse and interpret, it behaves like a single gene.  Differences arise through natural selection or chance events that affect only one item.  That makes it possible only to address the history of one variable, rather than that of a whole species or a population – a single thread rather than the multitude that must constitute the signal of real events.

“Snips” potentially can help resolve the out-of-Africa and multiregional hypotheses for the origin and spread of fully modern humans, and even whether we do carry vestiges of other groups of the genus Homo, such as the Neanderthals or various groups of more archaic beings who began to leave Africa for the rest of the Old World around 1.8 Ma ago.  In a review of the possibilities, Mark Stoneking of the Max Planck Institute in Leipzig (Stoneking, M. 2001.  From the evolutionary past…  Nature, v. 409, p 821-822) cautions that much remains to be done before SNPs can really give believable information.

The earliest ecosystems

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Reconstructing an environment devoid of multicellular life requires some stretch of the imagination.  Before the appearance of the first metazoans in the Proterozoic ecology might seem to have been somewhat tedious.  However, discovery from molecular biology of the antiquity of living prokaryotes and detailed analysis of their highly diverse metabolism makes such a venture fascinating.  In a review of early life and habitats, Euan Nisbet and Norman Sleep (Nisbet, E.G. & Sleep, N.H. 2001.  The habitat and nature of early life.  Nature Insight, v.  409, p. 1083-1091) weave an intricate fabric of biology, geochemistry and tectonics that serves to enthuse undergraduates and professional Earth scientists alike.  The review is understandably speculative, dealing as it does with proxy evidence for Archaean life forms themselves and their possible precursors.  But it presents a useful logic for seeing the Archaean Aeon as having a highly diverse biosphere, albeit one that is probably as alien as any that humans are likely to find…. even if they get to Mars!

Buckyballs and the end-Palaeozoic extinction

The largest mass extinction in the 600 Ma history of multicellular life took place 251 Ma ago, at the close of the Palaeozoic Era.  The end-Permian event wiped out up to 90 percent of marine animals and 70 per cent of land vertebrates.  Spanning the Permian-Triassic boundary are the vast Siberian Traps (continental flood basalts), that have been the most widely suspected trigger for the extinction.  Their emissions of sulphur dioxide may have created acid rain and stratospheric aerosols that cooled conditions through the event.  The boundary shows up in ocean-floor sediments incorporated in a Japanese ophiolite, which suggest less than 100 000 years saw the massive die off.  The popular notion of an impact cause for mass extinctions seemed to be a non-starter for the P-Tr boundary until late February.  There had been sporadic reports of iridium anomalies from the boundary, but not so believable as that at the K-T boundary.

Another tell-tale sign of extraterrestrial causes for extinctions is the presence of peculiar molecules in which more than 60 carbon atoms are bonded in a structure similar to a geodesic dome, called fullerenes after the creator of  this architectural structure, Buckminster Fuller (they are nicknamed “buckyballs”).  Fullerenes are thought to be created in the aftermath of supernovae, and therefore likely to occur in comets from the outer Solar System, where the most primitive material resides.  Their structure allows them to act as immensely strong and impermeable “cages” for gases around at the time of their formation.

In 1996 US geochemists discovered fullerenes in rocks formed in a huge impact crater near Sudbury, Ontario that must have come from space nearly two billion years ago and arrived on Earth intact. Last year the same team showed that even more complex carbon molecules, with as many as 200 atoms, had survived an impact from space at the same time as an impact wiped out the dinosaurs at the K/T boundary.  Sensitive measurements of isotopes of helium ad argon locked within the carbon cages reveal that their proportions are uncharacteristic of more common Solar System materials and must have been formed by nucleosynthesis far off in space.

Samples from the Permian-Triassic boundary in China, Japan, and Hungary contain fullerenes with these unusual combinations of helium and argon isotopes (REF).  This is incontrovertible evidence for an impact influence.  As yet, no candidate crater has been found, though with 70 percent of the Earth being occupied by recyclable ocean floor, it may have vanished down a subduction zone (the oldest sea floor now is late Triassic).  However, the coincidence of impact, massive flood basalt eruptions and a mass extinction is familiar.  The long-running debate about the K-T event is fuelled by such a triple coincidence – the death of the dinosaurs and much else, the Deccan Traps and the Chicxulub structure in the Gulf of Mexico.

Some authorities believe that extinctions big enough to be adopted as the principal boundaries in the stratigraphic column may need a “double whammy” to occur.  But there is also evidence that links the timing of flood basalt events to other extinctions that have yet to reveal a correlation with impacts.  Undoubtedly an outcome of mantle upwelling in superplumes that might start from the core-mantle boundary, the seeming regularity of flood basalt events (around every 30 Ma) poses a conundrum.  Linking two major basalt floods with impacts raises the possibility that superplumes might be triggered by major impacts.  One idea is that seismic energy released by major impact travels to the core, to trigger dislodgement of core-mantle boundary material into a rising superplume at the opposite side of the planet.  The Decccan Traps are at almost the exact antipode of the Chicxulub structure.  Using this logic, the place to look for the P-Tr culprit would be at the antipode of Siberia, when it was part of Pangaea.  That conveniently places the possible site in the huge ocean that encompassed Pangaea at the end of the Permian – it would ultimately be subducted as Pangaea broke up and continents began their latest round of drifting.

BSE in reverse?

Some say that we are witnessing and are even the source of the latest mass extinction.  If so, it is not entirely a product of modern society.  The late-Pleistocene of Australia and the Americas saw massive losses among large marsupial and more advanced mammal species from around 70 thousand years ago, i.e. since the first arrival of humans.  A widely accepted view is that this selective extinction was because the vanished species were eaten because they were naïve and easy prey.  Certainly the disappearances were sudden.  A huge diversity of large American mammals, including several elephants, camels, giant sloths and sabre-toothed cats (about 30 species), was decimated from around 11 to 9 thousand years ago, as humans spread quickly southwards through two continents when climate emerged from the last Ice Age.  Gluttony on such a scale is difficult to comprehend.

In an article in the February issue of Scientific American, Ross McPhee of the American Museum of Natural History in New York introduces an alternative hypothesis, that the extinctions resulted from infectious diseases crossing species barriers.  His idea stems not from evidence for epidemics, but the lack of it for massive butchery in the form of cut marks on bones of these extinct beasts.  Isolated for millions of years from both humans and the diseases that evolved in the Old World, American and Australian mammal populations would have had no immunity to viruses or pathogenic bacteria brought in by the human colonisers.  The crash in population of native Americans following European colonization was mainly due to epidemics.  The fact that several human diseases originally evolved in other species – poxes among cattle, ‘flu in birds and AIDS in African apes, for example – points to mutations possibly occurring in the opposite direction.  Add to that the fact that human immigrants would have been accompanied by dogs and almost certainly rats and infesting insects, and the idea become plausible.

(Source:  McKie, R. Man’s germs wiped out mammoths.  The Observer, 28 January 2001)

Cretaceous owl?

Earth Pages has charted over the last 9 months a tendency for publication in the “journals of record” of fossil arcana.  February 2001 adds yet another.  Early Cretaceous sequences of Cuenca in Spain include lagerstätten (horizons of exquisite preservation).  One provided a near perfect example of a regurgitated pellet, similar to those coughed up by owls (Sanz, J.L. et al. 2001.  An early Cretaceous pellet.  Nature, v. 409, p. 998-999).  In it are the remains of four chicks, including evidence of feathers, of different bird species, whose bones show clear microscopic evidence of having been partially digested.  Being 23 cm2 in flattened form, the pellet is presumably from some predator approximating the dimensions of a modern owl.  That does not necessarily call for Cretaceous owls, for any small predator, such as a pterosaur or small theropod dinosaur may well have encountered difficulty passing bony debris to dung, and resorted to regurgitation.

Loss of Martian atmosphere

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Mars seem quite massive enough to have held a substantial atmosphere, as have Earth and Venus.  That it has barely any is a major puzzle.  One possible reason is that Mars has a tiny magnetic field.  A strong magnetic field on Earth serves to deflect the solar wind, a stream of charged particles emitted by the outer part of the Sun.  Undeflected in this way, the solar wind would gradually strip off an atmosphere.  Currently, Mars has so little atmosphere that photosynthetic life that combines water and carbon dioxide to build carbohydrate is impossible, despite the fact that most of what little air there is  comprises CO2.

In the great chattering about prospects for Martian life at some time in the planet’s past, a central issue is the timing of atmospheric loss.  It is inconceivable that Mars never had an atmosphere, because it possesses the largest volcanoes in the Solar System which must have vented mantle gases.  If its magnetic field slowly dwindled, that gives ample time for life to have emerged.

Unsurprisingly, one of the tasks of NASA’s Mars Global Surveyor Mission has, for the last two years, been a global survey of the Martian ionosphere.   That is a proxy for regional variation in magnetic field strength.  A recent meeting of the Mars Global Surveyor team revealed the maps and their implications to the public.  The oldest terrains – those showing the greatest density of impact structures, as in the Lunar Highlands – show evidence of remanent magnetism.  Those affected by the youngest major impacts – analogous to the 4 billion-year old lunar maria – do not.  This suggests that Mars lost its magnetic field some time in its first half billion years, and thereby any substantial atmosphere.  One possible reason for this loss is that Mars has long been a geologically sluggish planet.  It is turbulent motion in the Earth’s liquid outer core that generates a magnetic field.  That turbulence is probably kept in motion by convective heat transfer in the mantle – it is a companion of terrestrial plate tectonics or any kind of regular mantle overturn.  Mars’ mantle does not do that, either by tectonics or through plume activity (unlike Venus), so its core may well be devoid of motion.

Exactly when magnetism stopped, with the attendant effect of the solar wind on any atmosphere, is crucial for estimates of how long life might have had to appear and begin evolving.  The results certainly rule out evolution beyond the most primitive life forms.  However, establishing that date must await future Mars landers, either staffed or robotic, on which the most important experiments will aim at detecting signs of former of extant life.  The magnetic data are not encouraging for exobiologists.

(Source:  Samuel, E.  2001.  The day the dynamo died.  New Scientist, 10 February 2001 issue.)

And now, Martian glaciers

Readers will have seen scornful comments in Earth Pages, regarding the desperate search for evidence of liquid water on modern Mars.  That water once was there seemed cut and dried from the giant valleys scoured across the Red Planet’s surface.  It was said that vast volumes of deep-seated ice catastrophically melted to flood from large impact sites.  Like the supposed evidence for active watery emissions in recent time, that for past flooding which cut the large valley systems rested on interpretation of the landforms themselves.  Re-examination of the valleys shows that they almost exactly mimic features revealed by sonar sounding on the sea floor surrounding the Antarctic ice sheet.  The Antarctic features probably formed during increased flow regimes when sea level stood at its lowest during glacial maxima.  Such surges can flow uphill, and sure enough the valley systems on Mars do have uphill tracts.

Baerbel Luchita of the US Geological Survey applied work on structure beneath the Ross Ice Shelf to Mars, suggesting that impact-melted water froze on emergence at the surface to flow in a more or less glacial fashion.  Undoubtedly, ice flow is far more capable of large-scale excavation than an equal volume of water, but to form the 1000 km long systems on Mars implies a considerable head.  Also its branching nature forces the assumption of many coalescing glaciers over a very large area.  That meets problems in imagining a widely distributed source of energy that caused the melting.  Impacts are at points, so perhaps yet another mechanism, such as seismicity, will need to be invoked.

(Source:  Hecht, J.  Sliced by ice.  New Scientist, 27 January 2001 issue)

Out of Africa hypothesis confounded?

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Living humans are anatomically the most diverse animals of a single species on the planet.  The differences extend from limb bones to skull characteristics, including the bony underpinnings of our faces.  That shows up plainly in any crowded market, whether that be in Addis Ababa, Bombay or Birmingham. Yet our genetic make up is extremely narrow, and chimps from separate troupes in West African jungle show greater diversity than that of humans across the world.  When physical anthropologists’ only tool was empirical comparisons between the physiognomies of people from different populations, their findings helped serve a political agenda. Statistical groupings drawn from that diversity slaked racists’ thirst for “proof” of their ethnic group’s wished-for “superiority”.  Such furtive longings are as alive today as they ever were in the 1930s: a mischief based on rubbished pseudoscience and ignorance.  We are physically diverse, but genetically distinguishable only by the most exquisitely precise analyses of DNA and other heritable material.

The minute genetic differences between peoples, like those more obviously separating the languages that they speak, result from migrations across the planet that took place before about 10 thousand years ago.  The migrants lived as hunter-gatherers under the climatically adverse condition of the last ice age.  Before the invention in widely separate centres of animal husbandry and agriculture that allowed human populations to explode – no earlier than 10 thousand years ago – our forebears’ total numbers would have barely exceeded the attendance on a Saturday afternoon at English Premier League soccer matches.  Tiny population densities, coupled with groups living in isolation and the random effect of mutations, with time create genetic differences between these groups, and so too for language and culture.  The narrowness of modern peoples’ genetic diversity points strongly to their last common ancestor living not so long ago in geological terms.  Whereas the earliest anatomical evidence for modern humans – a skull from Ethiopia with the chin that sets us apart from other extinct human species – is 450 thousand years old, differences in DNA from mitochondria indicate that divergence of the female half of our make up was about 140 thousand years ago.  Evidence from living men’s Y chromosomes (see November 2000 Earth Pages Eve never met Adam) suggests an even more recent stem, about 70 thousand years ago.  Both analyses point strongly to Africa for the focus of later divergence, that no other lines of descent survived to the present, and that no DNA from different groups, such as Neanderthals or Homo erectus, was involved in living peoples’ ancestors since 140 thousand years ago.  These observations form the core of the “Out of Africa” hypothesis.

There are, however, physical anthropologists who still set great store by statistical analysis of anatomical features, specifically that of skulls from extant humans and fossil ones.  They hold a view that it is possible that modern human’s physical diversity arose by evolution from much older populations of earlier migrants to different regions from Africa – the “Multi-regional” hypothesis explored by Milford Wolpoff of the University of Michigan.  In the case of Asian and Australasians that might have been from H. erectus that arrived in China as long ago as 1.8 million years back – recent dating of sediments in which erects’ remains have been found in Indonesia shows that they survived until as recently as 20 thousand years ago.  Alternatively it could have been from more advanced humans who arrived in Asia less than half a million years ago; the Mapas whose remains resemble those of Neanderthals.  For Europe, the putative ancestors would be Neanderthals, who arrived there at least 350 thousand years ago.  Africans, say the multi-regionalists, evolved continuously from the earliest tool-using humans since 2.5 million years ago.

Wolpoff’s group has used the same statistical technique employed in DNA studies to analyse skull morphologies from 25 individual modern humans from the fossil records of Europe and Australia, and compared the results with those for well-accepted, earlier humans and modern ones from Africa.  They claim (Wolpoff, M.H. et al. 2001.  Modern human ancestry at the peripheries: a test of the replacement theory.  Science, v. 291, p. 293-297) a better statistical fit between data for pairings of modern-human and earlier inhabitants of Australia and Indonesia, and of Europe than between modern-human remains from different regions.  “Out of Africa” proponents question the validity of the method, particularly selection of parameters – facial characters are omitted – and actual fossils.  Statistics is always a problem in studying human fossils, because they are so rare and widely separated in time – the study by Wolpoff’s group used material ranging from 60- to 14 thousand years old, and a total of only 25 specimens.

Even rarer are data for genetic material separated from fossils.  Three years ago, palaeoanthropologists at the Max Planck Institute in Munich reported the first partial DNA sequence from Neanderthal remains, later confirmed by another extraction.  They showed how unlikely it is that conjugation of Neanderthals and contemporary modern humans resulted in any signature surviving in the genes of living people.  Likewise, the data seemed to rule out any relatedness between the two groups since possibly several hundred million years ago; bad news for the multi-regionalists.  Astonishingly, scientists at the Australian National University have recovered useful DNA from 10 fossil humans between that range from 2 to 60 thousand years old.  The oldest not only represents the earliest Australian yet found, but turned out to be very different from that of later inhabitants (Adcock, G.L. et al. 2001.  Mitochondrial DNA sequences in ancient Australians: Implications for modern human origins.  Proceedings of the National Academy of Sciences, v. 98, p. 537-542).  One intriguing aspect is that a sequence in the mitochondrial DNA of “Mungo Man” exists as a remnant “insert” in modern DNA from chromosome 11, long suspected of being old mtDNA that has transferred to that in the cell nucleus.

Although no-one claims “Mungo Man” was an ancestor of living native Australians, there is many a spin that can be placed on the discovery.  The spanner in the works is that he is physically modern, beyond a shadow of doubt for comparative anatomists, but genetically archaic.  One possibility, espoused by the multi-regionalists, is that he evolved from pre-modern human migrants into Asia, either H. erectus or Mapas.  But that runs against the discovery of morphologically erect fossils from Indonesia that are much younger.  Perhaps he descended from interbreeding between early modern human migrants with earlier Asians, his DNA failing to be passed on to the present.  It is also possible that 60 thousand years ago, humans had a much greater range of genetic diversity, and that was filtered to today’s narrowness by a “bottleneck” due either to a disastrous fall in global population or to a cultural innovation that favoured only those who used it in the lottery of evolutionary fitness.  Though grist to the multi-regionalist mill, one DNA datum does not knock the “Out of Africa” hypothesis from its basis on thousands of results from living people.  Humans in one shape or other trekked from Africa to Asia at least three times since 1.8 million years ago, surviving in the case of the erects until quite recently.  It is what tool-equipped, socially conscious beings do, because they are sheltered from environmental pressures by what they do as much as by who they are.  That also surely means that all manner of changes in their genes and their morphology, which in mere beasts might snuff them out, can survive to confound the pure anatomist and the molecular biologist.  As the demise of the Neanderthals shows, when cultures are pitted in environments that offer limited resources, one gives way to another better suited.  Sadly, lifestyles and outlook, that we know to have driven human history for 6 000 years or so, leave little fossil record save stone tools and art, often inexplicable.  Accepting what makes humans unique has somehow to figure in all the empiricism around which centre current ideas on our origins.

(See also: Pennisi, E.  2001.  Skull study targets Africa-only origins.  Science, v. 291, p.231.  Dayton, L.  2001.  The man from down under.  New Scientist, 13 January 2001 issue, p. 6.  Holden, C.  2001.  Oldest human DNA reveals Aussie oddity.  Science, v. 291, p. 230-231)

Siberian role in climate change?

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Climate researchers at MIT in Cambridge, Massachusetts have analysed Northern Hemisphere climate data from 1972 to 1999, in the search for correlations that might help improve long-term weather forecasting.  The most striking match to emerge is that of winter climate with the extent of autumn snow cover in Siberia.  Snow reflects back to space a far greater proportion of incoming solar energy than any other kind of surface, with the exception of salt.  More snow results in less warming in the area.  Although Siberia is at the heart of the Asian continent, and therefore pretty dry, it has cold winters, so that when snow falls it covers large areas and tends to remain.  It is the focus for an enormous mid-continent high-pressure area in winter, appropriately named the Siberian High, which is one of three systems that dominate northern climate.

High-pressure areas do two things: air spills from them into surrounding areas; they isolate the area beneath them from warming, moist winds blowing from the oceans.  In winter the second creates cooling so intense that temperatures can steadily drop to -50°C or below , further building pressure because of the increase in air density.  Siberia sheds cold air westwards into Europe and over the North Pole into North America.  The MIT study bears out the obvious prediction based on this tendency.  However, it may also add the Siberian High to the range of large-scale terrestrial processes – shifts in air pressure over oceans, such as the El-Niño of the tropical Pacific and the North Atlantic Oscillation, and thermohaline controls over Atlantic surface currents – that make ice-age climate patterns so complex.

Cooling of northern Europe and the Canadian Shield does not have to be very extreme to lower the topographic elevation at which snow remains permanently, the glaciation limit – at present that level is only a couple of hundred metres above the tops of Britain’s highest mountains.  Should permanent snow cover return to the highest areas around the North Atlantic, that would amplify the present effect of Siberian autumnal snow and expand the high-pressure area.  That is a positive feedback driving climate towards increased frigidity, and larger winter highs would hold back maritime warming influences.

Computer modelling of the air-flow patterns over Asia shows that the primary influence is the Himalaya and Tibetan Plateau.  In particular, they dry out air passing over them during the South Asian Monsoon, and hinder its influence further into central Asia.  The two huge massifs seem to have risen rapidly and recently, beginning about 8 million years ago, despite the fact that India collided with Asia about 50 million years ago.  Together with other roughly E-W high mountain ranges in central Asia, they also channel Siberian cold air to spill westwards and eastwards, and over the pole.  Behaviour of the Siberian High almost certainly dates from the uplift of the Himalaya and Tibetan Plateau.

Adding another controlling factor to long-term northern climate has an intrinsic potential in refining academic studies of Pleistocene climate.  However, there is an immediacy to the observations.  For snow to cause cooling by reflecting away solar heat it does not have to be thick; a few centimetres will suffice.  The critical factor is the area covered by it.  Siberia is so cold in autumn and winter that it will snow there, provided moist air can enter.  Should more get in then more snow will cover a greater area, to feed the positive feedback to cooling.  Perversely, the more the climate warms globally, the more moisture evaporates from tropical and mid-latitude oceans to move polewards and towards continental interiors……

Mismatches from north to south proven

Whether or not climate changes, especially those of shorter duration than the full glacial-interglacial cycle, occur at the same time everywhere is something that vexes all climatologists.  It encapsulates all the problems of causation: orbital forcing, thermohaline circulation, shifts in the Polar Front and Intertropical Convergence Zone, etcetera.  The problem mainly stems from uncertainties in the correlation of  time series that show proxies for climate change.  This is particularly bad for ocean-floor sediment cores, which depend upon radiometric dates for calibration from depth to time sequences and an assumption of constant rates of sedimentation between dated samples.  Imprecision often means that correlations are not believable, except at a very general level.  Many analyses end up by correlating the patterns shown by the proxies, which defeats the object of assessing the degree of global synchronicity of climate changes.

Cores taken through ice sheets offer a way out, for annual layers of ice are there to be counted, but only in the upper parts.  For deeper parts, converting depth to time relies on models of how ice compacts and how it thins by glacial flow.  Another seeming advantage of ice-core records is that a great deal more ice accumulates than does ocean-floor sediment over a particular time.  That means that the resolution of ice core records can be finer – potentially at the level of decades compared with hundreds of years for sediment cores.  A seeming key to correlation between ice cores lies in the way that ice traps air.  Being rapidly mixed, the atmosphere should have the same composition everywhere.  This is particularly so for methane, partly because it soon becomes oxidised to carbon dioxide, and partly because its level is highly variable from emissions by rotting vegetation and unstable gas hydrate on the shallow ocean floor.  Thomas Blunier and Edward Brook of Princeton University and the University of Berne used the methane records of Greenland and Antarctic ice to correlate the other proxies therein over the last 90 thousand years (Blunier, T. and Brook, E.J. 2001.  Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period. Science, v. 291, p. 109-112).  They show a consistent mismatch between rapid warmings of the air over the two polar ice sheets, where Antarctic changes precede those over Greenland by 1500 to 3000 years.  Interestingly, when frigidity gave way to comparative warmth in a matter of a few decades over Greenland, the Antarctic was shifting from warm to cool conditions.

Commenting on the paper in Science’s Compass, Nicholas Shackleton of Cambridge University shows yet more emerging oddities (Shackleton, N. 2001.  Climate change across the hemispheres.  Science, v. 291, p. 58-59).  In the North Atlantic Ocean, surface water temperatures apparently changed according to Greenland’s pace, while those for deep water match that of the Antarctic.  To add to the complexity of climate change through the last glacial period – until a few years ago it was all supposed to link to the astronomical forcing of solar heating at high northern latitudes – the oxygen isotope changes in the same deep water of the North Atlantic match those of ice volume around the north pole.

Whereas Blunier and Brook have proved that air-temperature changes above ice sheets at high northern and southern latitudes are not synchronous, this still leaves problems in correlating between ice and sediment cores, and between the oceanic record at the many sites world-wide, especially those at low latitudes.  With a growing number of hypotheses for climate changes of the order of a few thousand years – driven by changes associated with northern ice sheets, Antarctica and the tropics – onlookers await with interest the development of a means of precise correlation among all the time series.

Hands-on planetology

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up with playing Solitaire or Hearts between those moments of productive inspiration?  NASA Ames Research Center has set up a cottage industry (unpaid) to help Mars specialists there build a catalogue of impact craters on the Martian surface.  As those flyers tucked under your windscreen wipers say, “No experience needed”.

Probably the most important scientific breakthrough from studies of the Moon since the 1960s has been the discovery that its pocked surface resulted from impacts by chunks of interplanetary debris.  The rate of impact and the size of the colliding bodies, and therefore the energy that they delivered, has varied since the Moon formed.  The lunar cratering record, backed up by accurate dates of its products, is a detailed chronology of how impacts influenced Earth’s evolution – vital, since signs of impacts rapidly become masked by our planet’s vitality.

Mars, on which NASA scientists and many more besides focus their undivided attention, is also cratered as a result of the same kind of process.  Counting craters, measuring their diameters (a proxy for the energy involved in their formation) and looking for their age relative to one another and other features of the Martian scene is an excellent means of assessing aspects of the Red Planet’s evolution.  But Mars is a great deal bigger than the Moon, and the sheer tedium of doing the work has become a burden.  Those geologists who compiled the lunar record have moved on, and few relish the task as a profession, hence Ames’ appeal for public participation.

The idea is that the basic information on crater occurrence, size and relative age – that’s based on relations between overlapping craters and degradation by Mars’ “weather” – can easily be gathered by interested, but untrained people.  The statistical work can then be done much more quickly.  If you fancy being a NASA “Clickworker”, then connect to http://clickworkers.arc.nasa.gov/top

Since inception on November 17, 2000, all clickworkers combined have contributed 340,070 crater-marking and 93,891 crater-classification entries.  It seems better by far than simply running the SETI distributed software to analyse radio frequencies for possible signs of intelligence out there.  You get to look at some magnificent high resolution images too.

China’s fossil treasure house

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For small, shelly faunas that just preceded the Cambrian Explosion, outcrops that span mass extinction events, the evolution of vertebrates and much else besides, the huge diversity of Chinese geology has become a hive of palaeontological activity.  Perhaps this is due to an astonishing run of good fortune through the Phanerozoic as regards excellence of preservation, or the patience, ingenuity and skill of Chinese fossil experts.  The embarras de richesse is probably a blend of both with the fact that for decades following the Cultural Revolution little work was possible for political reasons.  Pent-up enthusiasm and curiosity is a marvellous driving force in research when released.

Such is the degree of interest that the 12 January 2001 issue of Science devotes 10 pages (Stokstad, E., Normile, D. and Lei, X. 2001.  Paleontology in China.  Science v.  291, p. 232-241) to a summary of discoveries so far, how Chinese palaeontologists are organising and funding their work, the in-fighting that goes on (not so different from anywhere else!) and the dangers of unique material being looted in the manner of rare works of art.  One difference in fossil hunting between developed and poor countries that are geologically well-endowed, is that in the former most of it is by professionals or well-heeled amateurs seeking entertainment.  In China it is a potential source of extra income for rural people, in the same manner as artisanal gold working, widespread in Africa.  That is double edged: while leaving no stone left unturned where fossils crop up in soil, it is the source of semi-legal international trade in treasures like dinosaur eggs containing embryos, and untutored fossickers make no records of stratigraphy.

The most important issue discussed in the revue concerns how essential overseas resources focus on scientific potential in less well-heeled countries.  There is a tendency, which has tempted most scientists with access to funds to pay lip-service to transnational collaboration, merely to add names to proposals and publications of individuals who for various reasons have not played a full, or sometimes any role at all.  That is a device to attract funds with an air of philanthropy, and to get official access to material.  It has no benefit for transfer of knowledge, skills and technology.  Most Chinese palaeontologists now rightly demand to participate fully in order to boost and widen expertise in their community.

The Chinese experience offers plenty of lessons for Earth scientists in other poor countries.  For one thing, it has focussed the government’s attention on reversing the previous drain of excellence by earmarking affordable funds for research.  Another is that it shows how curiosity and plain hard work can open up entirely new knowledge from the previously overlooked.  There is no reason why their application in other poorly-known geological scenarios shouldn’t uncover crucial threads for many other problems of the Earth’s evolution – about 75% of the continental surface still remains to be mapped at scales better than 1:1 million.

Oh Dear, another weird dinosaur!

China isn’t the only new frontier for palaeontologists.  It looks as though Madagascar is on the fossil map, because of fine preservation in late-Cretaceous, terrestrial sediments there.  The latest find there is a somewhat diminutive (~1.8 m long), but nontheless strange abelisaurid theropod – the group best known for having T. rex as a member (Sampson, S.D. et al. 2001.  A bizarre predatory dinosaur from the late Cretaceous of Madagascar.  Nature, v. 409, p. 504-506).

Masiakasaurus knopfleri  (the expedition crew included the few surviving fans of Dire Straits) had nimble teeth; in fact a whole gob-full of them.  Not a beast on whose snout to place a little kiss, for lots of pointy and serrated fangs protrude in a most alarming manner.  “It shows there’s still more to theropod lifestyles than we thought”, observed Tom Holtz of the University of Maryland; something with which we can all agree.  But upon what victims did it prey?  There are similarly equipped fossil crocodiles, and M. knopfleri certainly seems well-equipped to snaffle the odd passing trout.  However, the late Cretaceous greenhouse world had an atmosphere with high oxygen levels due to much greater rates of photosynthesis than now.  It probably teemed with large flying insects, because oxygen levels determine the maximum size compatible with the high metabolism needed for flight.  The discoverers plump for an insectivorous lifestyle.

But just what constitutes “weirdness”, the adjective “bizarre”?  To me, they are appropriately applied to living beetles that boil formic acid and spit it on a predator, giant squid whose sexuality involves males injecting packets of sperm under high pressure into the tentacles of females, who, at their leisure, rip off the skin that heals the wounds to impregnate themselves, and, of course, the recently discovered phyllum that lives exclusively on the lips of lobsters.