More time for modern humans to have mingled with Neanderthals

When anatomically modern humans (AMH) became established in Europe the days of the Neanderthals were numbered. Yet, genomic evidence is mounting for many instances of interbreeding between the two groups (see Human evolution links). The longer they were in contact the chances of meeting and having sex were likewise increased. So, for how long were the two groups able to make contact? Neanderthals declined and eventually disappeared between 41 and 39 ka, except for a possible refuge for a tiny number in southern Spain until 37 ka and maybe in the northern Urals where there are disputed Mousterian stone tools as young as 34 to 31 ka. Undoubtedly, the appearance of AMH somehow contributed to the demise of our close relatives, but there are many possible reasons why. Until recently, the earliest European entry of AMH had been placed at around 41 ka, based on dating of H. sapiens remains in Romania (but note: a single 210 ka possible AMH skull from Greece). This is now exceeded by data from a Bulgarian cave.

Bacho Kiro cave in Bulgaria (credit: Getty images)

The Bacho Kiro site was first excavated in the 1970s, and revealed stone tools that represent the earliest Upper Palaeolithic culture, known as the Bachokirian. Mitochondrial DNA from excavated bone fragments is clearly of AMH origin (Hublin, J.-J. and 31 others 2020. Initial Upper Palaeolithic Homo sapiens from Bacho Kiro Cave, Bulgaria. Nature, v. 581, online; DOI: 10.1038/s41586-020-2259-z). Dating the Bacho Kiro cave sediments has been difficult, but new analytical and statistical approaches using the radiocarbon (14C) method have yielded ages between 46 to 44 ka and perhaps as far back at 47ka (Fewlass, H. and 20 others 2020. A 14C chronology for the Middle to Upper Palaeolithic transition at Bacho Kiro Cave, Bulgaria. Nature Ecology and Evolution, v. 4, online; DOI: 10.1038/s41559-020-1136-3). This is the earliest unequivocal, direct evidence of our species in Europe and its association with the initial Upper Palaeolithic culture. Among the finds are perforated animal teeth and ivory beads that probably formed pendants, which resemble those found elsewhere in association with late Neanderthals: the Chatelperronian culture that seems to have been shared between AMH and Neanderthals.

The new data add up to 6 thousand years to the period of AMH-Neanderthal co-occupation of Europe, or about 400 generations. Plenty of time to ‘get to know one another’, and perhaps to assimilate genetically

See also: Rincon, P. 2020. Longer overlap for modern humans and Neanderthals. (BBC News 11 May 2020); Metcalfe, T. 2020. A tooth offers evidence modern humans reached Europe earlier than previously thought. (NBC News 11 May 2020)

Changing conditions of metamorphism since the Archaean

Metamorphic petrologists have known since their branch of geology emerged that the intensity or ‘grade’ of metamorphism varies with position in an orogenic belt. This is easily visualised by the sequence mudstone-shale-slate-phyllite-schist-gneiss that results from a clay-rich starting material as metamorphic grade increases. Very roughly speaking, the sequence reflects burial, heat and pressure, and must have been controlled by temperature increasing with depth and pressure: the geothermal gradient. In turn, that depends on internal heat production, geothermal heat flow and the way in which heat is transferred through the deep crust: by thermal conduction or mechanical convection. A particular rock composition gives rise to different metamorphic mineral assemblages under different temperature and pressure conditions.

George Barrow was the first to recognise this in the Southern Highlands of Scotland as a series of zones marked by different index minerals. For instance, in once clay-rich sediments he recognised a succession of new minerals in the sequence chlorite; biotite; garnet; staurolite; kyanite; sillimanite in rocks of progressively higher metamorphic grade. Barrow found that once basaltic lavas interleaved with the sediments displayed zones with different characteristic minerals. Other metamorphic terrains, however, revealed different index minerals. Experimental mineralogy eventually showed that Barrow’s zones and others reflected a wide range of chemical reactions between minerals that reach equilibrium over different combinations of pressure and temperature. This enabled geologists to distinguish between metamorphism that had occurred under conditions of high-pressure and low-temperature, low-P and high-T and intermediate conditions (see diagram). This suggested that metamorphic rocks can form in areas with different heat flow and geothermal gradients. Geochemical means of assessing the actual temperatures and pressures at which particular rocks had reached mineralogical equilibrium, known as ‘thermobarometry’, now enable such variations to be assessed quantitatively.

The latest division in pressure-temperature space of different styles of metamorphism (colours) and the main mineral equilibria (dashed lines) that define them

It has long been suspected that the average T/P conditions revealed by metamorphic rocks have varied over geological time, as well as from place to place at any one time. A recent paper has analysed thermobarometric data from the earliest Archaean to recent times (Brown, M. et al. 2020, Evolution of geodynamics since the Archean: Significant change at the dawn of the Phanerozoic: Geology, v. 48, p. 488–492; DOI: 10.1130/G47417.1) They conclude that from the Archaean to the start of the Neoproterozoic the average P/T ratio was more than twice as high as it was in the following billion years. At about 2 Ga they suggests a relatively sudden decrease that correlates with what they regard as the first major assembly of continental crust: the Columbia (Nuna) supercontinent. The Mesoproterozoic Era, occupied by the disassembly of Columbia and the eventual creation of the Rodinia supercontinent, retained a high mean T/P. That began to decline with the break-up of Rodinia and a succession of tectonic cycles of ocean opening and closing during the Neoproterozoic and the Phanerozoic. This phase of truly modern plate textonics saw first the assembly of Gondwana and then the all-encompassing Pangaea, followed by its break up as we witness today. There are other correlations with the T/P variations, but they need not detain us.

The raw metamorphic data (564 points spanning 3.5 Ga) are by no means evenly spaced in time, and four dense clusters of points show a very wide spread of T/P – up to 2 orders of magnitude. Yet the authors have used locally weighted scat­terplot smoothing (LOWESS) to reduce this to a smoothed curve with a zone of uncertainty that is a great deal narrower than the actual spread of data. Frankly, I do not believe the impression of systematic change that this approach has produced, though I am not a statistician. To a lesser extent than me, it seems that neither does Peter Cawood, who comments on the paper in the same issue of Geology: more clearly than do the authors themselves.

Peter Cawood’s ‘take’ on the relationship between tectonic development and other important variables in the Earth-system with the estimate by Brown et al. of the mean metamorphic T/P (‘thermobaric’) variation through Earth history

Cawood’s view is that it was all due to a steady fall in mantle temperature and related broad changes in tectonic processes. But metamorphic rocks form in only the outermost 100 km of the Earth. The post-800 Ma examples include a much greater proportion of those formed under high- and ultrahigh pressures – blueschists and various kinds of eclogite – than do the earlier metamorphic belts. This weights the post-800 Ma record to lower mean T/P. Such rocks form in subduction zones and their high density might seem to doom them to complete resorption into the deep mantle. Yet large chunks now end up embedded in continents, interleaved with less extreme materials. Cawood suggests, as do others, that cooling of the mantle has enabled deeper break-off of subducted slabs to meet their end at the core-mantle boundary. The retained low T/P lithosphere since 800 Ma may have been sliced into the continents by increased underthrusting during continent-continent collisions that dominate the more modern orogenic-metamorphic belts.

See also:  Cawood, P.A. 2020 Earth Matters: A tempo to our planet’s evolution: Geology, v. 48, p. 525–526; DOI: 10.1130/focus052020.1

Genetic material from a baby dinosaur

A clutch of Massospondylus carinatus eggs from the Jurassic of South Africa (credit: Brett Eloff)

Recently, a lot of publicity focussed on stunning CT scans of embryos preserved in fossilised eggs of a Jurassic sauropodomorph dinosaur, which were obtained using very high energy X-rays generated by a synchrotron in France (Chapelle, K.E.J. et al. 2020. Conserved in-ovo cranial ossification sequences of extant saurians allow estimation of embryonic dinosaur developmental stages. Nature Scientific Reports, v. 10, article 4224; doi: 10.1038/s41598-020-60292-z). The images suggest that the embryos’ skulls developed in much the same way as do those of living reptiles. Within a week there emerged an even more compelling dinosaurian scoop: a fossil nestling of a duck-billed dinosaur (hadrosaur) from the Upper Cretaceous of Montana is reported to have yielded evidence for a broad spectrum of cellular materials (Bailleul, A.M. et al. 2020. Evidence of proteins, chromosomes and chemical markers of DNA in exceptionally preserved dinosaur cartilage. National Science Review, v. 7, advance publication NWZ206; DOI: 10.1093/nsr/nwz206).

Alida Bailleul, who works at the Chinese Academy of Sciences in Beijing, and fellow molecular palaeontologists from Canada, the US and Sweden, examined material from the nestling’s skull that was suspected to contain traces of cartilage. Their methods involved microscopic studies of thin sections together with staining and fluorochemical analysis of cellular material extracted by dissolving away bone tissue in acid. The same methodologies were also applied to similar material from modern emu chicks as a means of validating the results from the fossil. Staining used the same chemical that previously had revealed blood proteins in a specimen of Tyrannosaurus rex (see: Blood of the dinosaurs  in Palaeobiology, January 2011). The fluorescence approach dosed the dinosaur cartilage with antibodies against bird collagen, and revealed an immune reaction (green fluorescence) in both fossil material and that from the baby emus.

The researchers also isolated cartilage cells (chondrocytes) from the dinosaur preparations. Two stains (PI and DAPI, for short) that show up DNA were applied, giving positive responses. The PI (propidium iodide) stain is useful as it does not respond to DNA in living material, bit only to that in dead cells, thereby helping to rule out contamination with modern material. Apparently, the double-staining experiments support the presence of double-stranded material that involves at least six base pairs (of ACTG amino acids). This does not prove the existence of dinosaur DNA, but does demonstrate that the hadrosaur’s cell nuclei are preserved.

Does that suggest that the hunt is on for a dinosaur genome, with all its connotations? OK, a complete genome has been extracted from a frozen Siberian mammoth a few tens of thousand years old, which encourages ‘re-wilding’ aficionados, but that animal preserved intact cells of many kinds. A 70 Ma old dinosaur fossil, however exquisitely preserved, is mostly ‘rock’, in that preservation is through mineralisation of bone and tissue, and even cells … Moreover, it is possible that what the team found may even be material from post-mortem bacterial colonisation of any age younger than 70 Ma.

See also: De Lazaro, E. 2020. Scientists Use X-rays to Peer inside Fossilized Dinosaur Eggs Sci News, 10 April 2020; Black, R. 2020. Possible dinosaur DNA has been found. Scientific American, 17 April 2020

Earliest direct evidence of plate motions

There are two ways that we recognise the movement of tectonic plates. Since the latter half of the Mesozoic Era, following break up of the Pangaea supercontinent, it bests manifests itself in the magnetic ‘stripes’ on the ocean floor. They result from alternating polarisation of the geomagnetic field as new oceanic lithosphere is generated at constructive plate boundaries to drive sea-floor spreading. The oldest remaining stripes date back to the early Jurassic. For earlier times geologists have to turn to the continental crust.  Lavas and some sedimentary rocks undergo magnetisation at the time of their formation and retained that imprint. Such remanent, palaeomagnetism reveals the original latitude at which it was imprinted, together with the subsequent rotation of a drifting continent relative to an assumed N to S axis joining the opposed magnetic poles. The apparent ‘wandering’ of the pole through time when successive ancient pole positions of different ages are plotted in relation to the present position of a continent is a good guide to its history of drifting as a result of plate tectonics. Comparing the polar-wander paths of two continents allows the time when they were formerly united to be estimated. So palaeomagnetic pole data makes it possible to reconstruct not just Pangaea but a whole series of earlier supercontinents, ancient magnetic data being supplemented by other geological evidence such as reconnecting the trends on different continents of ancient mountain belts.

Apparent polar wander paths for two continents for a period when they were united then split and were separated by sea-floor spreading, eventually to collide and reunite

The further back in time the fewer palaeomagnetic pole positions have been estimated, and the more uncertain are the apparent polar wander paths and the more complex each continent’s accumulated geological history. One of the reasons for such uncertainty is that episodes of metamorphism can reset a rock’s remanent magnetisation, hundreds of million years after it originally formed. Thus, the harder it becomes to be certain about early supercontinents that have been suggested, of which there are quite a few. The earliest that has been proposed is Vaalbara, albeit on grounds of geological similarity, that supposedly united the Kaapvaal and Pilbara Cratons of southern Africa and Western Australia, respectively. Its duration is suggested to have been between 3.6 to 2.8 Ga (billion years ago). The oldest supercontinents with sound palaeomagnetic records date from the end of the Archaean Eon (2.5 Ga). It is the lack or uncertainty of earlier palaeomagnetic evidence that makes the start of plate tectonics the subject of so much debate.

However, geophysicists continually strive to improve the detection of ancient magnetisation, and advances have been made recently to unravel original magnetisation signals from those that have been superimposed later. The fruits of these developments are borne out by a study of a sequence of mafic lavas from the Pilbara Craton that formed about 3.2 Ga ago (Brenner, A.R. et al. 2020. Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga. Science Advances, v. 6, article eaaz8670; DOI: 10.1126/sciadv.aaz8670). Alec Brenner and colleagues from several US universities measured palaeomagnetism in more than 200 diamond drill cores from two localities in this sequence and combined their data with others from the Pilbara to cover a roughly 600 Ma period between 3.35 to 2.77 Ga. The palaeopoles form a polar wander path that spans roughly 50 degrees of palaeolatitude. From this they have been able to estimate, in considerable detail, the rate at which the Pilbara Craton had moved in Mesoarchaean. In the first 170 Ma the average horizontal motion was about 2.5 cm per year, falling rapidly to 0.4 cm per year over the following 410 Ma. The earlier speed is comparable with the average of modern plate motions. Data from the later period suggests relative stagnation. Motions over the entire ~600 Ma could be due to episodic operation of plate tectonics on the global scale, or a local slowing in the rate of plate growth.

Pterosaur corner

I recall an anecdote related by David Attenborough about a celebrity reception that he once attended one evening after he had been filming for a sequence on the aerodynamics of pterodactyls. A venerable and obviously well connected lady engaged him in conversation, and asked him what he had been doing recently. “Actually, today I was flying a pterodactyl”. To which the old lady retorted, “Yes, they are so graceful, aren’t they”. They do have a large following, perhaps second only to dinosaurs, and three interesting items came to my attention in the last couple of weeks.

One of the known pterosaur groups is the Tapejaridae, comprising small to medium-sized pterosaurs with wingspans up to 4 m. They are quite spectacular in appearance, having large crests relative to their overall size. Their fossils have turned up in Cretaceous sediments in South America, Europe and China, and a new find in Morocco (Afrotapejara zouhrii) extends their range to Africa (Martill, D.M. et al. 2020. A new tapejarid (Pterosauria, Azhdarchoidea) from the mid-Cretaceous Kem Kem beds of Takmout, southern Morocco. Cretaceous Research. V. 112: onlin, 104424; DOI: 10.1016/j.cretres.2020.104424). See also: De Lazaro, E. 2020. New species of pterosaur discovered in Morocco (Sci News, 6 April)

Also reported in Cretaceous Research are three new species of toothed, fish-eating pterosaurs of the ornithocheirid group. They too come from the Cretacous Kem Kem beds of Morocco, and again adding Africa to the range of the genera to which they belong. Even the largest flying animals known to science have emerged from the same strata. These are the azhdarchid pterosaurs, the largest of which had a wing span of more than 9 metres and stood at the height of a giraffe when on the ground.

See: Anderson, N, 2020. New pterosaur fossils unearthed in Morocco (Sci News, 26 March)

Being so widely spread, these pterosaur group’s mode of flight must have been extremely efficient, perhaps even matching that of today’s albatrosses, which use turbulence over ocean waves to glide effortlessly, indeed the epitome of graceful travel. How they achieved such vast ranges is partly due to their extremely light-weight bones that were paper thin but strong because they contained vesicles filled with gas, much like the expanded polystyrene used in model pterosaurs of the kind flown by ‘Whispering Dave’ as Sir David Attenborough is fondly known. Their bone structures are similar, in this respect, to those of modern birds.

launch of Hatzegopteryx
Reconstruction of the giant pterosaur Hatzegopteryx launching into the air, just after the forelimbs have left the ground (credit: Mark Witton)

So, how did these graceful beasts fly? Like those of bats, pterosaurs’ wings were membranes, but rather than being supported by five elongated digits, as in bats, those of pterosaurs extended from their bodies to a single elongated ‘finger’ or digit: hence their old name pterodactyl, translated from the Greek as ‘wing finger’. For a long while, it was believed that pterosaurs had to live on high ground, even cliffs, in order to launch themselves in the manner of a hang glider. Reconstructions of their gait on the ground generally look extremely ungainly: they walked on their ‘wrists’ and the other three, small ‘fingers’ of their forelimbs.. How they probably launched themselves emerges from a detailed paper linking natural flight modes of birds, bats and pterosaurs to conceivable developments in aeronautics inspired by them (Martin-Silverstone, E. et al. 2020. Volant fossil vertebrates: potential for bioinspired flight technology. Trends in Ecology and Evolution, v. 35, in press 9 April 2020; DOI: 10.1016/j.tree.2020.03.005). The authors point to the great strength of the membrane structure itself, conferred by its three-layered structure, and to the aerodynamic properties of the wing. They conclude that, whereas pterosaurs were probably incapable of high-speed flight, they were extremely efficient at low speeds, ideal for soaring and for low-speed landing that would not endanger their fragile bodies. Simply by springing into the air using all four limbs they could attain sustained flight, although the largest of them were close to the limit. The necessary muscles actually made up about 40% of their body mass. See a reconstruction of the launch of the largest pterosaur, Quetzalcoatlus from the Late Cretaceous of North America

See also: Fossil Flyers Hold Secrets to Better Flight Technologies (Sci News, 18 April)

How did monkeys get to South America?

This is one of the great mysteries of palaeontology. There are plenty of monkey species in South and Central America and in Mexico. They are members of five families, collectively known as platyrrhine (‘flat-nosed’) primates, all having wide-spaced nostrils compared with the primates of the ‘Old World’. They are the catarrhines (‘hook-nosed). There are other differences, such as the unique prehensile tails of many ‘New World’ monkeys. The two monkey groups are genetically related, but their last common ancestor is estimated, using the ‘molecular clock’ approach, to have lived at least 31 Ma ago, in the Oligocene. The earliest platyrrhine primates of the Americas date to around the Eocene-Oligocene boundary (34 Ma). Interestingly, they are predated by the earliest rodent remains by only a few million years (41 Ma). Both primates and rodents had been inhabiting other continents long before this, so it is certain that, somehow, members of the two groups must have migrated to become isolated in the Americas. The problem lies with palaeogeography. By the late-Eocene the Americas were completely separated from Eurasia and Africa by the actively spreading Atlantic Ocean, then between 1500 to 2000 km wide. Complete isolation of the Americas dates from around 60 Ma ago, when the northernmost part of the North Atlantic began to open. The South Atlantic had become a wide ocean long before that, beginning in the far south during the early Cretaceous Period (138 Ma), with the mid-Atlantic Ridge steadily propagating northwards thereafter.

35 Ma
World palaeogeography at the Eocene-Oligocene boundary. The site of a recent fossil primate discovery in eastern Peru is marked by the yellow dot.

Since 60 Ma years ago it would have been impossible for the ancestors of ‘New World’ rodents and primates simply to have walked there. In any case the earliest known primate fossils from China are just 55 Ma old. Island hopping across the far northern, narrowest part of the North Atlantic during the Eocene may have been possible, although many islands there could have been subject to intense volcanic activity, as is Iceland today. The only alternative is a sea trip across the mighty Atlantic. Unless, that is, there is a hitherto undiscovered land bridge. The Walvis-Rio Grande Rise – a hotspot track – that spans the South Atlantic Ocean floor from Namibia to São Paulo in Brazil, has been the subject of some speculation since it is dotted with sea mounts and in places has micro-continental fragments. But it is too deep to have emerged as a result of falls in sea level. To suggest that the > 1500 km migration to the Americas of ancestral platyrrhine primates, or rodents for that matter, involved their being carried on drifting vegetation rafts obviously invites scepticism. For starters, why only two groups of animals? Or, could that imply a one-off event carrying only ancestral rodents and monkeys? It would need to be a special kind of raft: large enough to provide security against storm waves; immune to waterlogging, and carrying substantial food. On the plus side, there are powerful east-to-west currents in the equatorial Atlantic and trade winds going in the same direction, thanks to the Coriolis effect and ultimately Earth’s rotation. Islands as ‘way-points’ or temporary refuges are less convincing, for they would have to be heavily vegetated themselves to provide onward rafts. Apparently, in the absence of anything more plausible, Sherlock Holmes’s principle points to trans-Atlantic rafting.

This issue recently became ‘live’ again, with a fossil discovery in Peru, in an upper Amazon river bank close to at the Andean watershed but around 4000 km from the east coast of South America (Seiffert, E.R.  et al. 2020. A parapithecid stem anthropoid of African origin in the Paleogene of South America. Science, v. 368, p. 194-197; DOI: 10.1126/science.aba1135). The site had previously yielded both playrrhine monkey and rodent remains. To these have been added teeth with distinct similarities to those of fossils previously known only from Egypt, Libya and Tanzania: parapithecid anthropoids whose teeth are sufficiently different from those of platyrrhines to warrant a separate suborder, which includes baboons and primates. This is the only trace of parapithecids in South America and it may be assumed that, although they were possibly fellow-travellers with New World monkey ancestors, they were unable to compete and became extinct.

However, there is another possibility. Albeit with a sparse record of fossils resembling primates, North America does have at least one. George Gaylord Simpson (1902-1984), once the doyen of US palaeontologists, found a marmoset-like fossil in the early-Eocene of Wyoming, which he named Teilhardinia after the French Jesuit philosopher and palaeontologist Teihard de Chardin. It is about 56 Ma old and the size of a mouse. So was this diminutive the pioneer New World primate that crossed the northern North Atlantic? If so it would have had an equally perilous journey to reach South America, because the Isthmus of Panama was also open sea until around 4.5 Ma ago. With Teilhardinia, the plot thickens for there are several known species: in the US T. brandti from Wyoming and T. magnoliana from Mississippi; in Asia and Europe T. asiatica and T. belgica respectively. An embarrassment of riches that may well ignite: it has been suggested that North American Teilhardinia may have been the first of all primates and spread across the Eocene forests of North America, Europe and Asia. That hypothesis sort of implies that the entry of monkeys into South America may well have started with the tiny continent hopper who passed on its proclivities to its descendants in Africa

See also: Godinot, M. 2020. Rafting on a wide and wild ocean. Science, v. 368, p. 136-137; DOI: 10.1126/science.abb4107; Ancient teeth from Peru hint now-extinct monkeys crossed Atlantic from Africa. Science Daily, 9 April 2020. Oldest-known ancestor of modern primates may have come from North America, not Asia. Science Daily, 29 November 2018

Human evolution links

Time and energy permit me to summarise only one or two research developments each week. Yet there is a continual flow of other publications in fields which interest me, and hopefully most readers of Earth-logs. I come across them during my weekly search for suitable inspiration, so have decided occasionally to provide links to informative summaries in other blogs.

Last week, Science Daily reported on a paper in the journal Genetics that evaluates new genetic evidence that interbreeding between anatomically modern humans (AMH) occurred more often than previously suggested, when the two groups were in contact in Eurasia (New research adds to growing evidence that our ancestors interbred with Neanderthals at multiple times in history. Science Daily 1 April 2020). Other Neanderthals also left signs that around 40 ka ago they wove cordage from woody cellulose (in Scientific Reports): they were clearly as technologically adept as contemporary AMH (40,000 year old evidence that Neanderthal’s wove string. Science Daily 9 April 2020).

The early-April issue of Science also published dating of a key site in South Africa to show that around 2 Ma ago the earliest known Homo erectus co-inhabited the surrounding area with Australopithecus naledi and the earliest known Paranthropus. One of the highlights is that this rules out A. naledi as a direct human ancestor, as previously claimed by some. (When three species of human ancestor walked the Earth. Science Daily 2 April 2020).

In its last March issue Science carried a paper suggesting that Neanderthals in Portugal were avid consumers of seafood (Neanderthals ate mussels, fish, and seals too. Science Daily 26 March 2020).

Alternative explanation for interglacial climate instabilities; and a warning

For the past two and a half million years there has been no such thing as a stable climate on our home world. The major fluctuations that have given rise to glacial and interglacial episodes and the times that separate them are most familiar, as is their connection with the periodicity of gravitational effects on the Earth’s orbital and rotational behaviour. There are mysteries, such as the dominance of a ~100 ka cyclicity with the least effect on solar heating since a million years ago and the shift that took place then from dominant ~40 ka cycles that preceded it. But over shorter time scales there are more irregular climatic perturbations that can not be attributed to gravity variations in the Inner Solar System. In the run-up to maximal glacial conditions in the Northern Hemisphere are changes in the isotopic records that reveal increases and decreases in the mass of ice on continental masses. Known as Dansgaard-Oeschger events they occurred on a (very) roughly 10 ka basis and lasted between 1000 and 2000 years. They resulted in rapid temperature changes spanning up to 15°C over the Greenland ice cap and have been explained by changes in surface- and deep-water circulation within the North Atlantic. Effectively, the Gulf Stream and the thermohaline circulation that drives it were periodically shut down and turned on. Even more irregular in occurrence are sudden global coolings in the midst of general warming into interglacial episodes. The most spectacular of these was the Younger Dryas cooling to almost full-glacial conditions between 12.8 and 11.5 ka, at a time when the Earth had achieved a mean surface temperature almost as high as that which has prevailed over the last 11,000 years.  There have been lesser cold ‘snaps’ during the Holocene, and in every one of the earlier interglacials for which there are data. Their occurrence seems unpredictable, even chaotic.

In 2006 the Younger Dryas was explained as the result of massive amounts of freshwater flooding into the Arctic Ocean from huge, ice-dammed lakes in North America. Decreased density of the high-latitude surface water resulted in its failure to sink and thus drive thermohaline circulation (see The Younger Dryas and the Flood June 2006). This hypothesis has subsequently been applied to other such sudden climatic events, such as the cooling episode around 8.2 ka during the Holocene. A recent study set out to test this notion from ocean-floor records of the last half-million years (Galaasen, E.V. and 9 others 2020. Interglacial instability of North Atlantic Deep Water ventilation. Science, v. 367, p. 1485-1489; DOI: 10.1126/science.aay6381). The data are from a seafloor sediment core in a trough south of Greenland, where cold, salty and dense bottom water flows southward from the Arctic to drag warmer surface water northwards in the Gulf Stream to replace it. That warm surface water has a high salinity because of evaporation in the tropics, so once it cools it sinks, thereby maintaining thermohaline circulation.

Modelled circulation rate of Atlantic circulation during 10 ka of the last interglacial before the Holocene (credit: Thomas Stocker, 2020 Science)

Eirik Galaasen of the University of Bergen and colleagues from several countries flanking the North Atlantic found large, abrupt changes in the mass flow of water through the trough – based on studies of carbon isotopes in bottom-living foraminifera – during each of the four interglacials that preceded the current one. The higher the δ13C in the forams the more vigorous the deep flow, whereas low values suggest weak flow or stagnation, due to waning of thermohaline circulation. Transition between the two states is rapid and each state lingered for several centuries. While the Holocene records only one such perturbation of note, that at 8.2 ka, previous interglacials reveal dozens of them. One possibility is that the thermohaline circulation system of the North Atlantic behaved in a chaotic fashion during previous interglacial episodes, producing similarly erratic shifts in climate. Seemingly, the Holocene bucks the trend, which may have added an element of luck to the establishment of human agricultural economies throughout that Epoch. All the signs are that current, anthropogenic global warming will slow down the water circulation in the North Atlantic. Might that set-off what seems to have been the norm of chaotic interglacial climate shifts for the best part of that half-million years? Hard to tell, without more studies …

See also: Stocker, T.F. 2020. Surprises for climate stability. Science, v. 367, p. 1425-1426; DOI: 10.1126/science.abb3569; How stable is deep ocean circulation in warmer climate? (Science Daily)

Early days of the dog

Wolves and dogs are interfertile and the mating of a domestic dog with a wolf results in fertile offspring, unlike the case with hybrids of horse and donkey, lion with tiger etc. This suggests that both canids are so closely related that domestication of wolves led to the entire range of dog breeds shown at Crufts every year. The question is, “When did humans first domesticate wolves”? Provided the instinctive ‘rules’ of wolves are followed by a human a wolf pup can become a pet, if it is taken from its mother between 14 and 21 days after birth. But, not only are they expensive to feed on raw meat, they may well attack a stranger as they would in the wild go for a wolf from another pack. They are often loyal and playful towards whoever raised them, but are strictly ‘one-person’ animals, and difficult to train because they easily become bored. Taming wolf puppies and deliberate selection is one route to domestication and the first dogs, another being ‘self-domestication’ when wolves become dependent on humans for a share in food.

pet wolf
Raven the wolf greets a visitor to the Mission: Wolf sanctuary in Colorado USA (credit: Wikipedia)

Comparison of wolf (Canis lupus) and domestic dog (Canis familiaris) genomes suggest an age of divergence for the two populations may have occurred between 20 to 60 thousand years ago. Indeed the DNA of wolf remains from Siberia showed it to belong to a wolf population whose descendants contributed to domestication of sledge dogs, such as Greenlandic huskies and Alaskan malemutes. Yet this approach is difficult and the results uncertain. Discovery of canid skulls associated with the remains of humans and mammoths at a 28.5 ka old site in the Czech Republic seems to have resolved both a minimum age for domestication and how it was achieved (Prassack, K.A. et al. 2020.  Dental microwear as a behavioral proxy for distinguishing between canids at the Upper Paleolithic (Gravettian) site of Předmostí, Czech Republic. Journal of Archaeological Science, v. 115, published online; DOI: 10.1016/j.jas.2020.105092).paleolithic dog

The Předmostí canids show two skull shapes: one with long jaws like wolves, the other with shorter, more dog-like jaws. Kari Prassack of the US National Park Service and colleagues from the USA, the Czech Republic and Belgium, turned to dental micro-wear patterns to resolve differences between the two groups as regards diet. Teeth from the more wolf-like group showed wear patterns consistent with a diet dominated by raw flesh, whereas the short-jawed canids ate mainly hard, brittle foods, probably bones. A truly remarkable find at the site was a near-complete canid skull of the short-jawed type, with a bone between its front teeth. Could this be a sign of a carefully buried pet ‘proto-dog’?

Earlier studies of the Předmostí canids included isotopic analyses of their bones, and those of associated humans. Interestingly, the more wolf-like group and the humans had diets dominated by mammoth flesh. The possible proto-dogs had focused on reindeer and other prey, as had the lions whose bones also occur at the site. This further complicates interpretation. Did both wolves and proto-dogs accompany the humans, the first being fed with mammoth meat that they helped bring down, while the second were fed scraps from smaller, more commonly killed prey? Perhaps the early dogs developed over a long period as scavengers on the kills of lions, and then became associates of humans. Yet neither canid would find a mammoth easy prey, even hunting in packs. So did the ice-age hunters have two companion animals, perhaps one to help in hunting mammoth, the other for more day-to-day hunting, which became more domesticated and even kept as pets? As the authors conclude; more data are needed.

See also: Dog domestication during ice age (Science Daily)

A lowly worm from the Ediacaran?

Humans are more or less symmetrical, our left and right sides closely resembling each other. That is not so comprehensive for our innards, except for testes and ovaries, kidneys, lungs, arteries and veins, lymph and nervous systems. We have front- and rear ends, top and bottom, input and output orifices. All that we share with almost all other animals from mammals to worms, particularly at the earliest, embryonic stage of development. We are bilaterians, whereas sponges, ctenophores, placozoans and cnidarians are not – having either no symmetry at all, or just a bottom and a top – and are in a minority.  Fossil collections from Cambrian times also reveal bilaterians in the majority, at least insofar as preservation allows us to tell. Before 541 Ma ago, in the Precambrian, there are few signs of such symmetry and faunas are dominated by the flaccid, bag like creatures that form much of the Ediacaran Fauna, although there are traces of creatures that could move and graze, and had a rudimentary sense of direction (see: Burrowers: knowing front from back, July 2012 and Something large moved 2 billion years ago). Unsurprisingly, palaeobiologists would like to know when ‘our lot’ arose. One route is via comparative genetics among living animals, using DNA differences and the ‘molecular clock’ approach to estimate the age of evolutionary separation between ‘us’ and ‘them’. But the spread of estimated ages is so broad as to render them almost meaningless. And the better constrained ages of very old trace fossils rely on accepting an assumption that they were, indeed, formed by bilaterians. Yet ingenuity may have revealed an actual early bilaterian from such traces.lowly worm

Palaeobiologists from the US and Australia have scoured the famous Ediacara Hills of South Australia for traces of burrowing and signs of the animal that did it (Evans, S.D. et al. 2020. Discovery of the oldest bilaterian from the Ediacaran of South Australia. Proceedings of the National Academy of Sciences, v. 117, online; DOI: 10.1073/pnas.2001045117). One Ediacaran trace fossil, known as Helminthoidichnites is preserved as horizontal trails on the tops and bottoms of thin, discontinuous sand bodies. Luckily, these are sometimes accompanied by elongate ovoids, like large grains of rice. From numerous laser scans of these suspected burrowers, and the traces that they left the authors have reconstructed them as stubby, possibly segmented, worm-like animals that they have called Ikaria wariootia, which may have grazed on algal mats. This name is derived from the local Adnyamathanha people’s word (Ikara  or ‘meeting place’) for the locality, a prominent landmark, near Warioota Creek. The age of the sedimentary sequence is between 551 to 560 Ma, and perhaps a little earlier. They could be the earliest-known bilaterians, but the sandy nature of the rocks in which they occur precludes preservation of the necessary detail to be absolutely sure: that would require silt- or. clay-sized granularity

See also: Fossil worm shows us our evolutionary beginnings (BBC, Science and Environment)