Humans gorged on giant mole rats during Ethiopian glaciation

Until recently it was believed that humans only adapted to life at high elevations, such as those of the Tibetan Plateau, during the Holocene. Then it turned out that the DNA of modern Tibetans contains a mutated gene (EPAS1) that boosts haemoglobin production that underpins their comfortably living at above 4000 m. In quick succession it was discovered that modern humans were living in Tibet as early as 30 to 40 ka, the same gene was found in Denisovan DNA and then a jawbone of that earlier human emerged from a Tibetan cave. It has been estimated that ancestral Tibetans inherited the DNA segment from Denisovans at around 40 ka. The ancestral African homeland of our genus Homo has large highland tracts that rise above 4000 m, most notably Mount Kilimanjaro (5895 m, Tanzania), Mount Kenya (5199 m Kenya) and Mount Stanley (5109 m, Rwenzori, Uganda). Those three retain glaciers, albeit small ones. But during the last glacial maximum permanent ice fields also capped highland areas in Morocco, Ethiopia and South Africa. Today there are permanent or seasonal habitations above 4000 m in all these African settings because of warmer conditions, but DNA analyses of the inhabitants have yet to be tested for the EPAS1 genetic mutation.

erratic Bale
Glacial erratic in the Bale Mountains National Park, Ethiopia (credit: James Steamer)

Understandably, research into the former glaciation of highland areas in tropical Africa is a hot topic. One of the largest areas of glacial till and moraine in Africa lies on the >4000 m high Sanetti Plateau in the Bale Mountains of south-eastern Ethiopia. These mountains are the dissected remnants of a Miocene shield volcano and host a rich ecosystem; in fact the largest reserve of Afro-alpine flora and fauna. Like many mountains in tropical Africa, Bale helps rising moist air to condense as mists. The resulting rich ecology makes such mountain systems high-elevation ‘oases’ surrounded by semi-arid to arid savannah and desert. Because this was likely to have been equally true during the more arid conditions of the last glacial period areas such as Bale may have been refuges for humans during those times, despite the risk of altitude sickness (hypoxia). Aarchaeologist Götz Ossendorf of the University of Cologne, together with a large team from Germany, France, Ethiopia, Switzerland the USA, set out to test this hypothesis ( Ossendorf, G. and 21 others 2019. Middle Stone Age foragers resided in high elevations of the glaciated Bale Mountains, Ethiopia. Science, v. 365, p. 583–587; DOI: 10.1126/science.aaw8942).

Their main target was to excavate a rock shelter at around 3500 m, but outcrops of volcanic glass (obsidian) at 4200 m had clearly attracted human interest  as they are scattered with flaked tools and debitage from their manufacture. The upper sediment layers in the rock shelter yielded ashes, charcoal, a few pottery shards and a glass bead, together with evidence for herbivore droppings. Dates fall in the last 800 years; hardly surprising as the Bale Plateau is seasonally visited by local herders who use rock shelters as corrals for livestock. The lower levels, however, contain artefacts of the Middle Stone Age (MSA); the African terminology roughly equivalent to the Upper Palaeolithic in Eurasia. The MSA layer also contain coprolites, some of hyena in its upper parts but also massive amounts likely to be human that extend to the base of the cave sediments. Dated at 47 to 31 ka, the sediments bracket the age of maximum glacier extent.

Alert giant mole rat in Ethiopia’s Bale Mountains (credit: M. Watson)

The lower cave sediments contain abundant animal bones and signs of several hearths. Some of the bones show signs of cooking from burn marks. Although several prey species occur, more than 90% of the bones are those of giant mole rats (Tachyoryctes macrocephalus). It is not difficult to conclude that the human population’s meat consumption was almost entirely of roasted mole rat. That is not surprising because the thin soils of the Bale Mountains support at least 29 mole rats per hectare, each adult weighing around a kilogram. Like the guinea pig (Cavia porcellus), which forms a major source of protein for people living today in the high Andes of Peru and Bolivia – an estimated 65 million being eaten annually by Peruvians, mole rats are extremely easy to catch; an attractive proposition for consumers surviving under the stress of hypoxia. They also reproduce at a phenomenal rate Today, Andean people domesticate guinea pigs for the table. Until other sites of human habitation during the Bale ‘ice age’ whether the MSA people lived permanently at high elevation or migrated there seasonally, to gorge on mole rats, cannot be resolved.

Metamorphic evidence of plate tectonic evolution

The essence of plate tectonics that dominates the Earth system today is the existence of subduction zones that carry old, cold oceanic lithosphere to great depths where they become denser by the conversion of the mineralogy of hydrated basalt to near-anhydrous eclogite. Such gravitational sinking imparts slab-pull force that is the largest contributor to surface plate motions. Unequivocally demonstrating the action of past plate tectonics is achieved from the striped magnetic patterns above yet-to-be-subducted oceanic lithosphere, the oldest being above the Jurassic remnant of the West Pacific. Beyond that geoscientists depend on a wide range of secondary evidence that suggest the drifting and collision of continents and island arcs, backed up by palaeomagnetic pole positions for various terranes that give some idea of the directions and magnitudes of horizontal motions.

Occasionally – the more so further back in time – metamorphic rocks (eclogites and blueschists) are found in linear belts at the surface, which show clear signs of low-temperature, high pressure metamorphism that created the density contrast necessary for subduction. Where such low T/P belts are paired with those in which the effects of high T/P metamorphism occurred they suggest distinctly different geothermal conditions: low T/P associated with the site of subduction of cold rock; high T/P with a zone of magmagenesis – at island- or continental arcs – induced by crustal thickening and flux of volatiles above deeper subduction. Such evidence of geothermal polarity suggests a destructive plate margin and also the direction of relative plate motions. The oldest known eclogites (~2.1 Ga) occur in the Democratic Republic of the Congo, but do they indicate the start of modern-style plate tectonics?

Interestingly, ‘data mining’ and the use of statistic may provide another approach to this question. Determination of the temperatures and pressures at which metamorphic rocks formed using the mineral assemblages in them and the partitioning of elements between various mineral pairs has built up a large database that spans the last 4 billion years of Earth history. Plotting each sample’s recorded pressure against temperature shows the T/P conditions relative to the thermal gradients under which their metamorphism took place. Robert Holder of Johns Hopkins University and colleagues from the USA, Australia and China used 564 such points to investigate the duration of paired metamorphism (Holder, R.M. et al. 2019. Metamorphism and the evolution of plate tectonics. Nature, v. 572, p. 378–381; DOI: 10.1038/s41586-019-1462-2).

The 109 samples from Jurassic and younger metamorphosed terranes that demonstrably formed in arc- and subduction settings form a benchmark against which samples from times devoid of primary evidence for tectonic style can be judged. The post-200 Ma data show a clear bimodal distribution in a histogram plot of frequency against thermal gradient, with peaks either side of a thermal gradient of 500°C GPa-1 (~17°C km-1); what one would expect for paired metamorphic belts. A simple bell-shaped or Gaussian distribution of temperatures would be expected from metamorphism under a similar geothermal gradient irrespective of tectonic setting.

Metc PvT
Pressure-temperature data from Jurassic and younger metamorphic rocks (a) pressure vs temperature plot; (b) Frequency distribution vs log thermal gradient. (Credit: Holder et al. 2019, Fig. 1)

Applying this approach to metamorphic rocks dated between 200 to 850 Ma; 850 to 1400 Ma; 1400 to 2200 Ma, and those older than 2200 Ma, Holder and colleagues found that the degree of bimodality decreased with age. Before 2200 Ma barely any samples fell outside a Gaussian distribution. Also, the average T/P of metamorphism decreased from the Palaeoproterozoic to the present. They interpret the trend towards increased bimodality and decreasing average T/P as an indicator that the Earth’s modern plate-tectonic regime has developed gradually since the end of the Archaean Eon (2500 Ma). Their findings also tally with the 2.1 Ga age of the oldest eclogites in the DRC.

Plate tectonics is primarily defined as the interaction between slabs of lithosphere that are rigid and brittle and move laterally above the ductile asthenosphere. Their motion rests metaphorically on the principle that ‘what comes up’ – mantle-derived magma – ‘must go down’ in the form of displaced older material that the mantle resorbs. That is more likely to be oceanic lithosphere whose bulk density is greater than that supporting the thick, low-density continental crust. Without the steeper subduction and slab pull conferred by the transformation of hydrated basalt to much denser eclogite, subduction would not result in low T/P metamorphism paired with that resulting from high T/P conditions in magmatic arcs. But, while ever lithosphere was rigid and brittle, plate tectonics would operate, albeit in forms different from that which formned terranes younger than the Jurassic

Ediacaran glaciated surface in China

It is easy to think that firm evidence for past glaciations lies in sedimentary strata that contain an unusually wide range of grain size, a jumble of different rock types – including some from far-off outcrops – and a dominance of angular fragments: similar to the boulder clay or till on which modern glaciers sit. In fact such evidence, in the absence of other signs, could have formed by a variety of other means. To main a semblance of hesitancy, rocks of that kind are now generally referred to as diamictites in the absence of other evidence that ice masses were involved in their deposition. Among the best is the discovery that diamictites rest on a surface that has been scored by the passage of rock-armoured ice – a striated pavement and, best of all, that the diamictites contain fragments that bear flat surfaces that are also scratched. The Carboniferous to Permian glaciation of the southern continents and India that helped Alfred Wegener to reconstruct the Pangaea supercontinent was proved by the abundant presence of striated pavements. Indeed, it was the striations themselves that helped clinch his revolutionising concept. On the reconstruction they formed a clear radiating pattern away from what was later to be shown by palaeomagnetic data to be the South Pole of those times.

29 Ma old striated pavement beneath the Dwyka Tillite in South Africa (credit: M.J Hambrey)

The multiple glacial epochs of the Precambrian that extended to the Equator during Snowball Earth conditions were identified from diamictites that are globally, roughly coeval, along with other evidence for frigid climates. Some of them contain dropstones that puncture the bedding as a result of having fallen through water, which reinforces a glacial origin. However, Archaean and Neoproterozoic striated pavements are almost vanishingly rare. Most of those that have been found are on a scale of only a few square metres. Diamictites have been reported from the latest Neoproterozoic Ediacaran Period, but are thin and not found in all sequences of that age. They are thought to indicate sudden climate changes linked to the hesitant rise of animal life in the run-up to the Cambrian Explosion. One occurrence, for which palaeomagnetic date suggest tropical latitude, is near Pingdingshan in central China above a local unconformity that is exposed on a series of small plateaus (Le Heron, D.P. and 9 others 2019. Bird’s-eye view of an Ediacaran subglacial landscape. Geology, v. 47, p. 705-709; DOI: 10.1130/G46285.1). To get a synoptic view the authors deployed a camera-carrying drone. The images show an irregular surface rather than one that is flat. It is littered with striations and other sub-glacial structures, such as faceting and fluting, together with other features that indicate plastic deformation of the underling sandstone. The structures suggest basal ice abrasion in the presence of subglacial melt water, beneath a southward flowing ice sheet

Ecological hazards of ocean-floor mining

Spiralling prices for metals on the world market, especially those that are rare and involved in still-evolving technologies, together with depletion of onshore, high-grade reserves are beginning to make the opportunity of mining deep, ocean-floor resources attractive. By early 2018, fifteen companies had begun detailed economic assessment of one of the most remote swathes of the Pacific abyssal plains. In April 2018 (How rich are deep-sea resources?) I outlined the financial attractions and the ecological hazards of such ventures: both are substantial, to say the least. In Japan’s Exclusive Economic Zone (EEZ) off Okinawa the potential economic bonanza has begun, with extraction from deep-water sulfide deposits of zinc equivalent to Japan’s annual demand for that metal, together with copper, gold and lead. One of the most economically attractive areas lies far from EEZs, beneath the East Pacific Ocean between the Clarion and Clipperton transform faults. It is a huge field littered by polymetallic nodules, formerly known as manganese nodules because Mn is the most abundant in them. A recent article spelled out the potential environmental hazards which exploiting the resources of this region might bring (Hefferman, O. 2019. Seabed mining is coming – bringing mineral riches and fears of epic extinctions. Nature, v. 571, p. 465-468; DOI: 10.1038/d41586-019-02242-y).

ocean floor resources
The distribution of potential ocean-floor metal-rich resources (Credit: Hefferman 2019)

Recording of the ecosystem on the 4 km deep floor of the Clarion-Clipperton Zone (CCZ) began in the 1970s. It is extraordinarily diverse for such a seemingly hostile environment. Despite its being dark, cold and with little oxygen, it supports a rich and unique diversity of more than 1000 species of worms, echinoderms, crustaceans, sponges, soft corals and a poorly known but probably huge variety of smaller animals and microbes inhabiting the mud itself. In 1989, marine scientists simulated the effect on the ecosystem of mining by using an 8-metre-wide plough harrow to break up the surface of a small plot. A plume of fine sediment rained down to smother the inhabitants of the plot and most of the 11 km2 surrounding it. Four subsequent visits up to 2015 revealed that recolonisation by its characteristic fauna has been so slow that the area has not recovered from the disturbance after three decades.

The International Seabed Authority (ISA), with reps from 169 maritime member-states, was created in 1994 by the United Nations to encourage and regulate ocean-floor mining; i.e. its function seems to be ‘both poacher and gamekeeper’. In 25 years, the ISA has approved only exploration activities and has yet to agree on an environmental protection code, such is the diversity of diplomatic interests and the lack of ecological data on which to base it. Of the 29 approved exploration licences, 16 are in the CCZ and span about 20% of it, one involving British companies has an area of 55,000 km2. ISA still has no plans to test the impact of the giant harvesting vehicles needed for commercial mining, and its stated intent is to keep only 30% of the CCZ free of mining ‘to protect biodiversity’. The worry among oceanographers and conservationists is that ISA will create a regulatory system without addressing the hazards properly. Commercial and technological planning is well advanced but stalled by the lack of a regulatory system as well as wariness because of the huge start-up costs in an entirely new economic venture.

The obvious concern for marine ecosystems is the extent of disturbance and ecosystem impact, both over time and as regards scale. The main problem lies in the particles that make up ocean-floor sediments, which are dominated by clay-size particles. The size of sedimentary particles considered to be clays ranges between 2.0 and 0.06 μm. According to Stokes Law, a clay particle at the high end of the clay-size range with a diameter of 2 μm  has a settling speed in water of 2 μm s-1. The settling speed for the smallest clays is 1,000 time slower. So, even the largest clay particles injected only 100 m above the ocean floor would take 1.6 years to settle back to the ocean floor – if the water column was absolutely still. But even the 4,000 m deep abyssal plains are not at all stil, because of the ocean-water ‘conveyor belt’ driven by thermohaline circulation. An upward component of this flow would extend the time during which disturbed ocean-floor mud remains in suspension – if that component was a mere >2 μm s-1, even the largest clay particles would remain suspended indefinitely. Deepwater currents, albeit slow, would also disperse the plume of fines over much larger areas than those being mined. Moreover such turbidity pollution is likely to occur at the ocean surface as well, if the mining vessels processed the ore materials by washing nodules free of attached clay. Plumes from shipboard processing would be dispersed much further because of the greater speed of shallow currents. This would impact the upper and middling depths of the oceans that support even more diverse and, in the case of mid-depths poorly known, ecosystems Such plumes may settle only after decades or even centuries, if at all.

Processing on land, obviously, presents the same risk for near-shore waters. It may be said that such pollution could be controlled easily by settling ponds, as used in most conventional mines on land. But the ‘fines’ produced by milling hard ores are mainly silt-sized particles (2.0 to 60 μm) of waste minerals, such as quartz, whose settling speeds are proportional to the square of their diameter; thus a doubling in particle size results in four-times faster settling. The mainly clay-sized fines in deep-ocean ores would settle far more slowly, even in shallow ponds, than the rate at which they are added by ongoing ore processing; chances are, they would eventually be released either accidentally or deliberately

A mining code is expected in 2020, in which operating licences are likely to be for 30 years. Unlike the enforced allowance of environmental restoration once a land-based mining operation is approved, the sheer scale, longevity and mobility of fine-sediment plumes seem unlikely to be resolvable, however strong such environmental-protection clauses are for mining the ocean floor.

A dinosaur nesting colony

Imagine visiting a colony of nesting seagulls on an exposed sandbar. Their nests are roughly equally spaced, out of pecking range. As well as incubating individuals on their nests the air is full of screaming birds swooping towards you, and even pecking or buffeting your head. Only a relative few bird species nest in colonies. Some bury their eggs communally in warm sand or compost abandoning them for solar energy to hatch. The last approach is also that of many reptiles, notably turtles and crocodiles, but some crocodiles do behave like gulls, females guarding their buried clutches, so why not dinosaurs? Brooding in colonies has been suspected of dinosaurs, although most fossil eggs had been buried.

Upper Cretaceous sedimentary rocks in Mongolia have yielded more dinosaur eggs than most other places, especially in the northern Gobi Desert’s largely unvegetated outcrops. It is from there that exquisitely preserved, firm evidence has emerged of dinosaurs nesting communally (Kanaka, K. and 9 others 2019. Exceptional preservation of a Late Cretaceous dinosaur nesting site from Mongolia reveals colonial nesting behavior in a non-avian theropod. Geology, v. 47, p. 1-5; DOI: 10 .1130 /G46328.1). The site exposes 15 clutches about 1.5 m apart that, together, contain more than 50 spherical eggs 10 to 15 cm in diameter. Modern erosion has dissected the occurrences, and it is estimated that up to 32 clutches may have been laid in an area of ~286 m2. That the eggs had been laid on the surface, covered – possibly with organic matter – and then incubated is clearly evidenced by all of them resting in pockets on an erosion surface covered by the same thin, continuous layer of bright red sand. About 60% of them seem to have hatched successfully. Each eggshell contains the same doubled-layered infill of fine sediment made of surrounding sediment and broken shell fragments.

dino nest
Clutch of near-spherical dinosaur eggs from Mongolia: scale bar = 10 cm. (Credit: Kanaka et al. 2019; Fig. 2A)

The detail of the nests suggests that they were created on an exposed surface during a single dry season and after hatching, when their infills formed, they were gently flooded as stream levels rose to deposit the thin, red covering layer. Whether or not the eggs were brooded or merely protected cannot be assessed, despite the excellence of preservation. But the high hatching success suggests that adults fended off predators during incubation. Egg shape and size point to their having been laid by a single species of theropod dinosaur; probably not ancestral to birds, but a group that includes velociraptors and tyrannosaurs. Yet nest-tending has clear parallels among later birds.

Out of Africa: The earliest modern human to leave

The 2017 discovery in Morocco of fossilised, anatomically modern humans (AMH) dated at 286 ka (see: Origin of anatomically modern humans, June 2017) pushed back the origin of our species by at least 100 ka. Indeed, the same site yielded flint tools around 315 ka old. Aside from indicating our antiquity, the Jebel Irhoud discovery expanded the time span during which AMH might have wandered into Eurasia, as a whole variety of earlier hominins had managed since about 1.8 Ma ago. Sure enough, the widely accepted earliest modern human migrants from Skhul and Qafzeh caves in Israel (90 to 120 ka) were superseded in 2018 by AMH fossils at Misliya Cave, also in Israel, in association with 177 ka stone artefacts (see Earliest departure of modern humans from Africa, January 2018). Such early dates helped make more sense of very old ages for unaccompanied stone tools in the Arabian Peninsula as tracers for early migration routes. Unlike today, Arabia was a fertile place during a series of monsoon-related cycles extending back to about 160 ka (see: Arabia : staging post for human migrations? September 2014; Wet spells in Arabia and human migration, March 2015). The ‘record’ has now shifted to Greece.

hominin sites
Key ages of early H. sapiens, Neanderthals and Denisovans (credit: Delson, 2019; Fig. 1)

Fossil human remains unearthed decades ago often undergo revised assessment as more precise dating methods and anatomical ideas become available. Such is the case for two partial human skulls found in the Apidima Cave complex of southern Greece during the late 1970s. Now, using the uranium-series method, one has been dated at 170 ka, the other being at least 210 ka old (Harvati, K. and 11 others 2019. Apidima Cave fossils provide earliest evidence of Homo sapiens in Eurasia. Nature, v. 571 online; DOI: 10.1038/s41586-019-1376-z). These are well within the age range of European Neanderthals. Indeed, the younger one does have the characteristic Neanderthal brow ridges and elongated shape. Albeit damaged, the older skull is more rounded and lacks the Neanderthals’ ‘bun’-like bulge at the back; it is an early member of Homo sapiens. In fact 170 ka older than any other early European AMH, and a clear contemporary of the long-lived Neanderthal population of Eurasia; in fact the age relations could indicate that Neanderthals replaced these early AMH migrants.

Given suitable climatic conditions in the Levant and Arabia, those areas are the closest to Africa to which they are linked by an ‘easy’, overland route. To reach Greece is not only a longer haul from the Red Sea isthmus but involves the significant barrier of the Dardanelles strait, or it requires navigation across the Mediterranean Sea. Such is the ‘specky’ occurrence of hominin fossils in both space and time that a new geographic outlier such as Apidima doesn’t help much in understanding how migration happened. Until – and if – DNA can be extracted it is impossible to tell if AMH-Neanderthal hybridisation occurred at such an early date and if the 210 ka population in Greece vanished without a trace or left a sign in the genomics of living humans. Yet, both time and place being so unexpected, the discovery raises optimism of further discoveries to come

See also: Delson, E. 2019. An early modern human outside Africa. Nature, v. 571 online; DOI: 10.1038/d41586-019-02075-9

Ancient proteins: keys to early human evolution?

A jawbone discovered in a Tibetan cave turned out to be that of a Denisovan who had lived and died there about 160,000 years ago (see: Denisovan on top of the world; 6 May, 2019). That discovery owed nothing to ancient DNA, because the fossil proved to contain none that could be sequenced. But the dentine in one of two molar teeth embedded in the partial jaw did yield protein. The teeth are extremely large and have three roots, rather than the four more common in modern, non-Asian humans, as are Denisovan teeth from in the Siberian Denisova Cave. Fortunately, those teeth also yielded proteins. In an analogous way to the genomic sequencing of nucleotides (adenine, thymine, guanine and cytosine) in DNA, the sequence of amino acids from which proteins are built can also be analysed. Such a proteomic sequence can be compared with others in a similar manner to genetic sequences in DNA. The Tibetan and Siberian dentine proteins are statistically almost the same.

Triple helix structure of collagen, colour-coded to represent different amino acids (credit: Wikipedia)

At present the most ancient human DNA that has been recovered – from an early Neanderthal in the Sima de los Huesos in Spain – is 430,000 years old (see: Mitochondrial DNA from 400 thousand year old humans; December 2013). Yet it is proving difficult to go beyond that time, even in the cool climates that slow down the degradation of DNA. The oldest known genome of any animal is that of mtDNA from a 560–780 thousand year old horse, a leg bone of which was extracted from permafrost in the Yukon Territory, Canada. The technologies on which sequencing of ancient DNA depends may advance, but, until then, tracing the human evolutionary journey back beyond Neanderthals and Denisovans seems dependent on proteomic approaches (Warren, M. 2019. Move over, DNA: ancient proteins are starting to reveal humanity’s history. Nature, v. 570, p. 433-436; DOI: 10.1038/d41586-019-01986-x). Are the earlier Homo heidelbergensis and H. erectus within reach?

It seems that they may be, as might even earlier hominins. The 1.8 Ma Dmanisi site in Georgia, now famous for fossils of the earliest humans known to have left Africa, also yielded an extinct rhinoceros (Stephanorhinus). Proteins have been extracted from it, which show that Stephanorhinus was closely related to the later woolly rhinoceros (Coelodonta antiquitatis). Collagen protein sequences from a 3.4 Ma camel preserved in the Arctic and even from a Tanzanian 3.8 Ma ostrich egg shell show the huge potential of ancient proteomics. Most exciting is that last example, not only because it extends the potential age range to that of Australopithecus afarensis but into tropical regions where DNA is at its most fragile. Matthew Warren points out potential difficulties, such as the limit of a few thousand amino acids in protein sequences compared with 3 million variants in DNA, and the fact that the most commonly found fossil proteins – collagens –  may have evolved very little. On the positive side, proteins have been detected in a 195 Ma old fossil dinosaur. But some earlier reports of intact diosaur proteins have been questioned recently (Saitta, E.T. et al. 2019. Cretaceous dinosaur bone contains recent organic material and provides an environment conducive to microbial communities. eLife, 8:e46205; DOI: 10.7554/eLife.46205)


Multiple invention of stone tools

Steadily, the record of stone tools has progressed further back in time as archaeological surveys have expanded, especially in East Africa (Stone tools go even further back, May 2015). The earliest known tools – now termed Lomekwian – are 3.3 million years old, from deposits in north-western Kenya, as are cut-marked bone fragments from Ethiopia’s Afar region. There is no direct link to their makers, but at least six species of Australopithecus occupied Africa during the Middle Pliocene. Similarly, there are various options for who made Oldowan tools in the period between 2.6 and 2.0 Ma, the only known direct association being with Homo habilis in 2.0 Ma old sediments from Tanzania’s Olduvai Gorge; the type locality for the Oldowan.

The shapes of stone tools and the manufacturing techniques required to make them and other artefacts, are among the best, if not the only, means of assessing the cognitive abilities of their makers. A new, detailed study of the shapes of 327 Oldowan tools from a 2.6 Ma old site in Afar, Ethiopia has revealed a major shift in hominin working methods (Braun, D.R. and 17 others 2019. Earliest known Oldowan artifacts at >2.58 Ma from Ledi-Geraru, Ethiopia, highlight early technological diversity. Proceedings of the National Academy, v. 116, p. 11712-11717; DOI: 10.1073/pnas.1820177116). The sharp-edged tools were made by more complex methods than the Lomekwian. Analysis suggests that they were probably made by striking two lumps of rock together, i.e. by a deliberate two-handed technique. On the other hand, Lomekwian tools derived simply by repeatedly bashing one rock against a hard surface, not much different from the way some living primates make rudimentary tools. But the morphology of the Ledi-Geraru tools also falls into several distinct types, each suggesting systematic removal of only 2 or 3 flakes to make a sharp edge. The variations in technique suggest that several different groups with different traditions used the once lake-side site.

Various 2.6 Ma old Oldowan stone tools from Ledi-Geraru, Ethiopia (credit: Braun et al., 2019)

Ledi-Geraru lies about 5 km from another site dated about 200 ka earlier than the tools, which yielded a hominin jawbone, likely to be from the earliest known member of the genus Homo. A key feature that suggested a human affinity is the nature of the teeth that differ markedly from those of contemporary and earlier australopithecines. It appears that the tools are of early human manufacture. The ecosystem suggested by bones of other animals, such as antelope and giraffe was probably open grassland – a more difficult environment for hominin subsistence. The time of the Lomekwian tools was one of significantly denser vegetation, with more opportunities for gathering plant foods. Perhaps this environmental shift was instrumental in driving hominins to increased scavenging of meat, the selection pressure acting on culture to demand tools sharp enough to remove meat from the prey of other animals quickly, and on physiology and cognitive power to achieve that.

See also: Solly, M. 2019. Humans may have been crafting stone tools for 2.6 million years (Smithsonian Magazine)

Geochemical background to the Ediacaran explosion

The first clear and abundant signs of multicelled organisms appear in the geological record during the 635 to 541 Ma Ediacaran Period of the Neoproterozoic, named from the Ediacara Hills of South Australia where they were first discovered in the late 19th century. But it wasn’t until 1956, when schoolchildren fossicking in Charnwood Forest north of Leicester in Britain found similar body impressions in rocks that were clearly Precambrian age that it was realised the organism predated the Cambrian Explosion of life. Subsequently they have turned-up on all continents that preserve rocks of that age (see: Larging the Ediacaran, March 2011). The oldest of them, in the form of small discs, date back to about 610 Ma, while suspected embryos of multicelled eukaryotes are as old as the very start of the Edicaran (see; Precambrian bonanza for palaeoembryologists, August 2006).

Artist’s impression of the Ediacaran Fauna (credit: Science)

The Ediacaran fauna appeared soon after the Marinoan Snowball Earth glaciogenic sediments that lies at the top of the preceding Cryogenian Period (650-635 Ma), which began with far longer Sturtian glaciation (715-680 Ma). A lesser climatic event – the 580 Ma old Gaskiers glaciation – just preceded the full blooming of the Ediacaran fauna. Geologists have to go back 400 million years to find an earlier glacial epoch at the outset of the Palaeoproterozoic. Each of those Snowball Earth events was broadly associated with increased availability of molecular oxygen in seawater and the atmosphere. Of course, eukaryote life depends on oxygen. So, is there a connection between prolonged, severe climatic events and leaps in the history of life? It does look that way, but begs the question of how Snowball Earth events were themselves triggered. Continue reading “Geochemical background to the Ediacaran explosion”

Soluble iron, black smokers and climate


Phytoplankton bloom in the Channel off SW England (Landsat image)

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant food chain. The key factor that is missing is dissolved divalent iron that acts as a minor, but vital, nutrient for phytoplankton. Much of the soluble iron that does help stimulate plankton ‘blooms’ emanates from the land surface in wind blown dust (Palaeoclimatology September 2011) or dissolved in river water. A large potential source is from hydrothermal vents on the ocean floor, which emit seawater that has circulated through the basalts of the oceanic crust. Such fluids hydrate the iron-rich mafic minerals olivine and pyroxene, which makes iron available for transport. The fluids originate from water held in the muddy, organic-rich sediments that coat the ocean floor, and have lost any oxygen present in ocean-bottom water. Their chemistry is highly reducing and thereby retains soluble iron liberated by crustal alteration to emanate from hydrothermal vents. Because cold ocean-bottom waters are oxygenated by virtue of having sunk from the surface as part of thermohaline circulation, it does seem that ferrous iron should quickly be oxidised and precipitated as trivalent ferric compounds soon after hydrothermal fluids emerge. However, if some was able to rise to the surface it could fertilise shallow ocean water and participate in phytoplankton blooms, the sinking of dead organic matter then effectively burying carbon beneath the ocean floor; a ‘biological pump’ in the carbon cycle with a direct influence on climate. Until recently this hypothesis had little observational support. Continue reading “Soluble iron, black smokers and climate”