For ease of access to annual developments within the general topics that Earth-logs covers I have now compiled all the Earth-logs posts from 2020 and 2021 into the categories: Geohazards; Geomorphology; Human Evolution; Magmatism; Palaeobiology; Palaeoclimatology; Physical Resources; Planetary Science; Remote Sensing; Sediments and Stratigraphy, and Tectonics. You can download them by ‘hovering’ over the Annual logs pull-down in the main menu and clicking on a category, whose index page will appear. Then scroll down to the 2020 or 2021 entry and click on the link to the PDF.
I hope that readers find this option useful in showing how each general topic has developed over the 21st century so far. Of course, it is based on my personal view of what constitute important developments published in international journals
Two decades ago the world of palaeoanthropologists was in turmoil with the publication of an account of a new find in Chad (see: Bonanza time for Bonzo; July 2002). A fossil cranium, dubbed Sahelanthropus tchadensis (nicknamed Toumaï or ‘hope of life’ in the Goran language), appeared like a cross between a chimpanzee and an australopithecine. The turmoil erupted partly because of its age: Upper Miocene, around 7 Ma old. Such an antiquity was difficult to reconcile with the then accepted ~5 Ma estimate for the evolutionary split between humans and chimpanzees, based on applying a ‘molecular clock’ approach to the difference between their mtDNA. The other point of contention was the size of Sahelanthropus’s canine teeth: far too large for australopithecines and humans, but more appropriate for a gorilla or chimp.
Cast of the reconstructed skull of Sahelanthropus tchadensis. (Credit: Didier Descouens, University of Toulouse)
In the absence of pelvic- and foot bones, or signs of the foramen magnum where the spinal cord enters the skull – crucial in distinguishing habitual bipedalism or being an obligate quadruped – encouraged the finders of a 6.1 to 5.7 Ma-old Kenyan hominin Orrorin tugenensis to insist that its skeletal remains – several teeth, fragments of a lower jaw, a thigh bone, an upper arm and of a finger and thumb but no cranial bones – were of ‘the earliest human ancestor’. In Orrorin’s favour were smaller canine teeth than those of later australopithecines. At the time of the dispute, centred mainly on absence of crucial evidence, doyen of hominin fossils Bernard Wood of George Washington University and an advocate of ‘untidy’ evolution, suggested that both early species may well have been evolutionary ‘dead ends’ (see: A considered view; October 2002). And there the ‘muddle’ has rested for 20 years.
In 2002 not only a cranium of Sahelanthropus had been unearthed. Three lower jaw bones and a collection of teeth suggested that as many as 5 individuals had been fossilised. A partial leg bone (femur) and three from forearms (ulna) cannot definitely be ascribed to Sahelanthropus but, in the absence of evidence of any other putative hominin species, they may well be. It has taken two decades for these remains to be analysed to a standard acceptable to peer review (Daver, G. et al. 2022. Postcranial evidence of late Miocene hominin bipedalism in Chad. Nature v. 608, published online; DOI: 10.1038/s41586-022-04901-z). The authors present convoluted anatomical evidence that Toumaï’s femur, which had been gnawed by a porcupine and lacks joints at both ends, suggesting that it was indeed suited to upright walking. Yet the arm bones hint that it may have been equally comfortable in tree canopies. Yet it does look very like an ape rather than a hominin.
Much the same conclusion has been applied to Australopithecus afarensis, indeed its celebrated representative ‘Lucy’ met her end through falling out of a large tree ~3.2 Ma ago (see: Lucy: the australopithecine who fell to Earth?; September 2016). So, dual habitats may have been adopted by hominins long after they emerged. Yet Au afarensis was capable of trudging through mud as witnessed by the famous footprints at Laetoli in Tanzania. Only around 3 Ma has reasonably convincing evidence for upright walking similar to ours been discovered in Au africanus. The full package of signs from pelvis and foot for habitual bipedalism dates to 2 Ma ago in Au sediba. Even this latest known australopithecine seems to have had a gait oddly different from that of members of the genus Homo.
So, in many respects the benefits of full freeing of the hands to develop manipulation of objects, as first suggested by Freidrich Engels, may have had to await the appearance of early humans. Earlier hominins almost certainly did make tools of a kind, but the revolutionary breakthrough associated with humanity was more than 5 million years in the making.
Charles Darwin’s ideas on the evolution of species through natural selection became imprinted by his participation in the second survey expedition of HMS Beagle (1831-1836), commanded by Captain Robert Fitzroy. The voyage aimed at comprehensive surveys along its circumnavigation, Darwin having been engaged to provide geological expertise. At that time he would have been best described as a ‘natural historian’ and his only qualification was that he had an ordinary degree (BA) from Cambridge and had read widely in natural science: had it not been for joining the Beagle he may have become a country parson.
The voyage was a maritime venture typical of British and other European imperialism and colonisation during the early 19th century – a survey not only of geodesy, geography and natural science but also of the economic potential of the places that it visited. European science benefitted immensely from such voyages and overland expeditions. Today, research in the natural sciences is still dominated by academics from the better-off nations. Significantly, the charting of the ocean floor during the 20th and 21st centuries has been conducted almost exclusively by those nations with a global reach: plate tectonics is a science for the very wealthy. It is only in the last 60 years that geological mapping of the bulk of the continental surface has been relinquished by former colonial powers to local surveys. In the majority of cases the geological surveys of these now independent countries are grossly underfunded and they still largely depend on maps produced more than half a century ago by their former rulers.
In the 19th century global palaeontology, botany and zoology, which lie at the roots of evolutionary studies, shipped specimens to the museums and universities of the colonising powers. Their scientists today still retain a near monopoly of access to those old collections. Now it is economic power that enables continued collection by researchers mainly from the former colonising countries and their institutions. There are a few exceptions, such as the rapid rise of Chinese natural science in a mere three to four decades, which has become a major ‘player’ in early and Mesozoic evolution. Gradually, hominin palaeontology has drawn in local scientists from countries well-endowed with productive sites, such as Kenya, Tanzania and Ethiopia, yet funding remains largely external. Nussaïbah Raja at Friedrich-Alexander University in Erlagen, Germany and colleagues from Britain, South Africa, Brazil and India (Raja, N.B. et al. 2021. Colonial history and global economics distort our understanding of deep-time biodiversity. Nature Ecology & Evolution, v. 6, p. 1-10 ; DOI: 10.1038/s41559-021-01608-8) have used the vast Paleobiology Database (PBDB) to assess which countries are the main influence over global fossil collection.
Proportion of publications on national fossil data with a local lead author, for regions of the world. (Credit: Raja et al., Extended Data Fig 9)
Their findings are unsurprising. The 29 thousand papers referenced by PBDB that give fossil-occurrence data from the last 30 years involved 97% of authors who were resident in high- and upper-middle-income countries: more than a third from the US and the rest of the top ten from, in order, Germany, Britain, France, Canada, Russia, China, Australia, Italy and Spain: and 92% of the publications were published in English. Interestingly, it appears that old colonial ties still exert an influence on palaeontology research in former colonies: a quarter of that conducted in Morocco, Tunisia and Algeria was done by scientists based in France; 10% of work in South Africa and Egypt was authored by UK-based researchers; and 17% of Namibian palaeontology was conducted by scientists from Germany. When it comes to first authors of papers about fossils, local scientists get increasingly short shrift as the overall wealth of their homelands decreases. The authors of the PBDB study devised an index of what they call ‘parachute science’, based on the proportion of a country’s fossil data that was contributed by foreign teams that lacked any local co-authors.
The ‘Parachute Index’ for the ten countries most exploited by external palaeontological researchers. (Credit: Raja et al., Fig 3b)
This lack of engagement with and assistance for local scientists ‘hinders local scientists and domestic scientific development, by favouring foreign input and exacerbating power imbalances between those from foreign countries and those located ‘on the ground’. Furthermore, this can also lead to mistrust by local scientists towards foreign researchers, affecting future collaborations’. Scientific ‘colonialism’ is still pervasive for much of the world, and is a major force in imposing opinions on evolution in particular. Raja and colleagues rightly call for external economic and ‘intellectual’ power over research to be replaced by ‘equitable, ethical and sustainable collaboration’. Without that, scientific expertise will advance at a very slow pace in less well-endowed regions, with the same-old, same-old beneficiaries getting the benefits.
A follower of Earth-logs has brought to my attention a wide range of concerns regarding the veracity of the paper by Bunch et al in Nature Scientific Reports, which Earth-logs covered on 8 October 2021. The reactions are summarised by the Retraction Watch website (Criticism engulfs paper claiming an asteroid destroyed Biblical Sodom and GomorrahRetraction Watch 1 October 2021). It seems that the Chief Editor of Scientific Reports is considering the issues that have been raised. Anyone who has downloaded and read the paper by Bunch et al will have noted the very large amount of data that it cites. It is alleged that there are flaws in the evidence, and that some of the figures may have been falsified. Some of the authors also contributed to the ‘airburst’ hypothesis for onset of the Younger Dryas, covered in Earth-pages several times, which uses similar data. More information can be accessed through Paul Braterman’s comments on the Sodom post
Full-frontal skull of ‘Sue’, the best-preserved and among the largest specimens of T. rex (Credit: Scott Robert Anselmo, Wikimedia Commons)
Long-term followers of Earth-logs and its predecessor Earth-pages News will have observed my general detachment from the dinosaur hullabaloo, which just runs and runs. That is, except for real hold-the-front-page items. One popped up in the 16 April 2021 issue of Science (Marshall, C.R. et al. 2021. Absolute abundance and preservation rate of Tyrannosaurus rex. Science, v. 372, p. 284-287; DOI:10.1126/science.abc8300). For over two million years in the Late Cretaceous, just before all dinosaurs – except for birds – literally bit the dust, the authors estimated a lot of the dinosaurian poster-childTyrannosaurus rex lurking in North America. I write ‘lurking’ because ‘tyrant lizard the king’ when fully grown was so big that if it ran and fell over, it would have been unable to get up! Tangible evidence from trackways suggests that it ambled from place to place. The leg bones of a 7-tonner would probably have shattered at speeds above 18 km per hour, and accelerating to the speed of a human jogger would, anyhow, have exhausted its energy reserves, But it was agile enough to be an ambush predator; it could even pirouette! And it could crush bones so well that it was able to consume prey entirely. It has been suggested that T. rex may have been a scavenger, at least in old age. Whatever, how is it possible to estimate numbers of any extinct species, let alone dinosaurs?
The stumbling block to getting a result that is better than guesswork is the fossil record of a species. First, it is incomplete, secondly the chance of finding a fossil varies from area to area, depending on all kinds of factors. These include the degree of exposure of sedimentary rock formed by the environment in which they thrived, as well as the vagaries of preservation due to post-mortem scavenging, erosion and water transport. In life the population density of a particularspecies varies between different ecosystems and from species to species. For instance, more lions can thrive in open rangeland than in wooded environments, whereas the opposite holds for tigers: probably because of different hunting strategies. Many factors such as these conspire to thwart realistic estimates of ancient populations. Studies of living species, however, suggest that population density of an animal species is inversely related to the average body mass of individuals. Take British herbivores: there are many more rabbits than there are deer. On the grasslands of East Africa hyenas and wild dogs outnumber lions. This mass-population relationship (Damuth’s Law) outlined by US ecologist John Damuth also depends on where a species exists in the food chain (its trophic level) as well as its physiology. Yet for living species, populations of flesh-eating mammals of similar mass show a 150-fold variation; a scatter that results from their different habits and habitats and also their energy requirements. Because they are warm-blooded (endothermic), small carnivorous mammals need a greater energy intake than do similar sized, cold-blooded reptiles, which need to eat far less. But not all living reptiles are ectothermic, especially the bigger ones. The Komodo dragon is mesothermic, midway between the two, and uses about a fifth of the energy needed by a similar-sized mammal carnivore. Population densities of dragons in the Lesser Sunda Islands are more than twice those of physiologically comparable mammalian predators.
A number of features suggest that the metabolism of carnivorous dinosaurs lay midway between those of large predatory mammals and big lizards like the Komodo dragon. This is the basic assumption for the analysis by Charles Marshall and colleagues. They did not focus on the biggest T. rex specimens, but on the average, estimated body mass of adults. There are numerous smaller specimens of the beast, but clearly some of these would have been sexually immature. It has been estimated that adulthood would have been achieved by around 15 years. The size data seem to show that achieving sexual maturity was accompanied by a 4 to 5 year growth spurt from the 2 to 3 tonnes of the largest juveniles to reach >7 t in the largest known adults which may have lived into their early 30s. The authors used this range to estimate a mean adult mass of 5.2 t. Taking this parameter and much more intricate factors into account, using intricate Monte Carlo simulations Marshall et al. came up with an estimate of 20 thousand T. rex adults across North America at any one time: but with an uncertainty of between 1,300 to 328,000. Spread over the 2.3 million km2 area of Late Cretaceous North America that lay above sea level their best-estimated population density would have been about 1 individual for every 100 square kilometres. An area the size of California could have had about 3800 adult Tyrannosaurus rex, while there may well have been two in Washington DC. Lest one’s imagination gets overly excited, were tigers and lions living wild today in North America under similar ecological conditions there would have been 12 and 28 respectively in the US capital. Yet those two adult Washingtonian T. rexs would have been unable to catch anything capable of a sustained jog, without keeling over. The juveniles weighing in at up to 3 tonnes would probably have been the real top predators; the smaller, the swifter and thus most fierce. Which leaves me to wonder, “Did the early teenagers catch the prey for their massive parents to chow-down on?”
I have now compiled all the Earth-logs posts from 2019 into PDFs for the categories: Geohazards; Geomorphology; Human Evolution; Miscellaneous; Palaeobiology; Palaeoclimatology; Physical Resources; Planetary Science; Sediments and Stratigraphy, and Tectonics. They are available for download through the Annual logs pull-down in the main menu – just select a category, then scroll down to the2019 list of contents and click on the link.
I hope this format is useful for reference purposes.
Understandably, the nature of what lies at the centre of the Earth is as much the subject of speculation as tangible evidence. That there must be something very dense within the planet emerged once the Earth’s bulk density was calculated. Because a high proportion of meteorites are dominated by an alloy of the metals iron and nickel, geoscientists adopted that combination as plausible core material. Study of the arrival times around the globe of seismic waves from earthquakes then revealed the actual size of the Earth’s core. Iron-nickel alloy fitted the bill quite nicely. It also fits geochemical evidence, such as the crust and mantle’s depletion in some trace elements that theoretically have an affinity for iron. The fact that seismology showed also that the outer core was molten and able to flow, together with metals’ high electrical conductivity, gave rise to the current concept of the geomagnetic field being generated by a dynamo effect in the core. However the density of Fe-Ni is not ‘quite right’ because the core is somewhat lighter than predicted for the pure alloy under stupendous pressure: it must contain a substantial amount – up to 13% – of lower density materials. Silicon, sulfur and oxygen have been suggested as candidates, with evidence from a variety of minor minerals in metallic meteorites.
A recent model for core formation (credit Crystal Y. Shi et al 2013; DOI: 10.1038/NGEO1956 Fig. 5)
The world is currently awash with models that attempt to throw light on the course of the Covid-19 pandemic. Many are based on highly uncertain data, leading to suggestions by some people that they have become tools for political elites and a means of helping ambitious scientists into the limelight: a sort of fuel for hubris. In the midst of this unprecedented turmoil there has appeared a suggestion (from modelling) that the core also contains abundant hydrogen (Li, Y. et al. 2020. The Earth’s core as a reservoir of water. Nature Geoscience, v. 13, published online; DOI: 10.1038/s41561-020-0578-1). Yunguo Li and colleagues, from University College London, the Chinese Academy of Science and the University of Oslo, explore the idea that the dominant hydrogen of the pre-planetary Solar nebula, which accreted to form the Earth, may have joined iron during core formation. This had been predicted from the thermodynamics of chemical reactions between water and iron. The team takes this further through the geochemical theory that elements and compounds tend to enter other materials preferentially. For example, during partial melting of the crust alkali metals (Na, K etc) are more likely to enter the granitic melt than to remain in the solid residue. Li et al. have used thermodynamics to predict the partitioning of hydrogen between iron and silicate melts under the very high temperature and pressure conditions at the boundary between the core and mantle.
Their calculations suggest that hydrogen then behaves in much the same manner as, say gold and platinum: it becomes ‘iron-loving’ or siderophile and prefers the molten core, as would H2O. The amount that gets in depends on the water content of the molten silicate that eventually becomes the mantle. If the water now making up Earth’s ocean was ‘degassed’ from the mantle during core formation then the original silicate melt would have been ‘wetter’ than it is now. The implication of such early degassing is that the core may contain 5 ‘oceans worth’ of water! The alternative scenario for Earth’s becoming a watery world is the later accretion of, for instance, cometary material. In that case, the early core would have been drier. Yet, the continual subduction of hydrated oceanic lithosphere into the deep mantle during billions of years of plate tectonics would steadily have added water to the core, in the form of iron oxides and hydrogen. So, the core might, in either case, contain several ‘oceans’ of the components of water. One line of indirect evidence is the deficiency in Earth’s actual water of the heavier isotope of hydrogen (deuterium) relative to the D/H ratio of chondritic meteorites. Theory suggests that D has slightly more affinity for joining iron than does H. Substantial water in the core does help explain the core’s apparent low density, but that notion requires as much faith as politicians seem to have in ‘following the Science’ during the current pandemic …
Well, surely we ought to know, 52 years after W. Jason Morgan proposed that the Earth’s surface consists of 12 rigid plates that move relative to each other. But that is not completely true, although most of its mechanisms expressed by external and internal Earth processes are known in great detail. It is still a ‘chicken and egg’ issue: do convective motions in the mantle drive the superficial plates around by dragging at the base of the lithosphere or is it the subduction of plates and slab-pull force that result in overturn of the mantle? Nicolas Coltice of the University of Paris and colleagues from those of Grenoble, Rome and Texas consider that posing plate tectonics in such a manner is an abstraction; rather like the plot for a novel that is yet to be written (Coltice, N. et al. 2019. What drives tectonic plates?Science Advances, v. 5, online eaax4295; DOI: 10.1126/sciadv.aax4295). Instead, all the solid Earth’s vagaries and motions have to be considered as an indivisible whole rather than the traditional piecemeal approach of focussing on the forces that act on the interfaces between plates.
Their approach is to model a combination of mechanisms throughout the Earth as a single, evolving three-dimensional system without the constraint of perfectly rigid plates, which of course they are not. The physical parameters boil down to those involved in relative buoyancy, viscosity, and gradients of temperature, pressure and gravitational potential energy within a spherical planet. Designing the algorithms and running the model on a supercomputer took 9 months to reconstruct the evolution of the planet over 1.5 billion years.
Still from a movie of simulated breakup of a supercontinent, in bland blue-grey, showing what happens at the surface (left) and, at the same time, in the mantle (right): note the influence of rising plumes (credit: Nicolas Coltice)
The result is a remarkable series of unfolding scenarios. In them, 2/3 of the planet’s surface moves faster than does the underlying mantle, suggesting that the surface is dragging the interior. For the remainder, mantle motions exceed those of the surface. Continents are dragged by the underlying mantle to aggregate in supercontinents, which in turn are torn apart by the sinking of cold oceanic slabs. The model takes on a highly visual form, showing in 3-D, for instance: ocean closure and supercontinent assembly; and example of continental breakup; how subduction is initiated.
It will be fascinating to see the reaction of the authors’ peers to their venture, and the extent to which the technicalities of the paper are translated into a form that is suitable for teaching. My suspicion is that most Earth scientists will be happy to stay with the old conceptions until the latter is achieved, and laptops are able to run the model(!)
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