Who invented stone tools? A great surprise from Kenya

Up to now the earliest stone tools are objects dated to about 3.3 Ma (Late Pliocene) found in the Turkana basin of Kenya in 2015. They are sharp-edged pieces of rock that seem to have been made simply by striking two lumps of rock together (see: Stone tools go even further back; May 2015). These Lomekwian artefacts are similar to the basic tools made today by some chimpanzees in parts of Africa. Their age matches that for the earliest known animal bones that show signs of having meat cut from them, which were unearthed in Dikika, Ethiopia (see: Another big surprise; September 2010) which, like the Lomekwian tools, are not accompanied by tools or hominin remains. The earliest tools associated with members of the genus Homo are significantly more sophisticated. They were found in close association with H. habilis at what seems to have been a well-used butchering site, dated at 2.0 Ma, in Tanzania’s Olduvai Gorge, hence their designation as the Oldowan ‘industry’. The Oldowan ‘tool kit’ includes choppers and blades deliberately shaped to be wielded by hand and made by striking large cobbles with distinctive hammer stones. Earlier tools with this level of deliberate crafting come from the 2.6 Ma Ledi-Geraru site in the Afar Depression of NE Ethiopia but with no sign of their makers.

Oldowan tools used for pounding and cutting from Nyayanga, Kenya (Credit: Thomas Plummer, James Oliver and Emma Finestone/Homa Peninsula Paleoanthropology Project/SWNS)

The presence of Oldowan tools has now been pushed further back, by about 400 ka, thanks to excavations in Late Pliocene sediments at Nyayanga on the shore of Lake Victoria in western Kenya by Thomas Plummer of Queens College in New York State, USA, and his numerous collaborators from the US, Germany, the UK, China, Italy, Australia, Kenya, South Africa and Poland (Plummer, T.W. and 31 others 2023. Expanded geographic distribution and dietary strategies of the earliest Oldowan hominins and Paranthropus. Science, v. 379, p. 561-566; DOI: 10.1126/science.abo7452). Their work also expands the range of Oldowan culture by about 1300 km. The Nyayanga site yielded over 300 artefacts that closely resemble the previously known range of Oldowan tool shapes. Their makers struck flakes from suitable corestones – made of rhyolite, quartz and quartzite – and trimmed them by more intricate means. They seem to have been used to cut up mainly hippo and buffalo, bones of which bear clear cut marks, but had other uses. Analysis of the wear on tool surfaces not only show signs of butchery, but also processing of plant tissue by pounding; the latter resulted in pitting and polishing of tools that seem to have been used many times. Stable-isotope analysis of the bones and animal teeth suggests that in the Pliocene Nyayanga was a grassy and partly wooded savannah close to a substantial water body needed by hippos.

Reconstruction of a Paranthropus head (Credit: Jerry Humphrey, Pinterest)

The ‘great surprise’ is that the only hominin remains associated with the site are two damaged molar teeth. They are so large that their most likely source was a species of Paranthropus.Paranthropoids have long been considered to be a gorilla-like, ‘robust’ branch of australopithecines. Their large cranial crests anchoring jaw muscles and enormous teeth were reckoned to indicate a diet of tough vegetation – the discoverer of the first specimen of P. boisei dubbed it ‘Nutcracker Man’ – although the wear on individual teeth suggests otherwise. But there is no reason to suppose that they could not eat meat. They survived australopithecines by more than a million years to cohabit the East African savannahs with H. ergaster until about 1 Ma ago.

Lead author Thomas Plummer wonders if paranthropoids would have needed tools because they had the largest jaws and teeth of any hominin. But had his team found close association with smaller H.habilis teeth would he have held a similarly negative view? There is evidence from younger sites in South Africa that paranthropoids used a wide diversity of bone tools and may even have been among the earliest fire users. So why the negativity about stone tools? To paraphrase Ali G, ‘Is it because they is ugly?’

See also: Devlin, H, Discovery of 3m-year-old stone tools sparks prehistoric whodunit. The Guardian, 9 February 2023

Is erosion paced by Milankovich cycles?

Both physical and chemical weathering reflects climatic controls. Erosion is effectively climate in continuous action on the Earth’s solid surface through water, air and bodies of ice moving under the influence of gravity. These two major processes on the land surface are immensely complicated. Being the surface part of the rock cycle, they interact with biological processes in the continents’ web of climate-controlled ecosystems. It is self-evident that climate exerts a powerful influence on all terrestrial landforms. But at any place on the Earth’s surface climate changes on a whole spectrum of rates and time scales as reflected by palaeoclimatology. With little room for doubt, so too do weathering and erosion. Yet other forces are at play in the development of landforms. ‘Wearing-down’ of elevated areas removes part of the load that the lithosphere bears, so that the surface rises in deeply eroded terrains. Solids removed as sediments depress the lithosphere where they are deposited in great sedimentary basins. In both cases the lithosphere rises and falls to maintain isostatic balance. On the grandest of scales, plate tectonics operates continuously as well. Its lateral motions force up mountain belts and volcanic chains, and drag apart the lithosphere, events that in themselves change climate at regional levels. Tectonics thereby creates ‘blips’ in long term global climate change. So evidence for links between landform evolution and palaeoclimate is notoriously difficult to pin down, let alone analyse.

The evidence for climate change over the last few million years is astonishingly detailed; so much so that it is possible to detect major global events that took as little as a few decades, such as the Younger Dryas, especially using data from ice cores. The record from ocean-floor sediments is good for changes over hundreds to thousands of years. The triumph of palaeoclimatology is that the last 2.5 Ma of Earth’s history has been proved to have been largely paced by variations in the Earth’s orbit and in the angle of tilt and wobbles of its rotational axis: a topic that Earth-logs has tracked since the start of the 21st century. The record also hints at processes influencing global climate that stem from various processes in the Earth system itself, at irregular but roughly millennial scales. The same cannot be said for the geological record of erosion, for a variety of reasons, foremost being that erosion and sediment transport are rarely continuous in any one place and it is more difficult to date the sedimentary products of erosion than ice cores and laminations in ocean-floor sediments. Nonetheless, a team from the US, Germany, the Netherlands , France and Argentina have tackled this thorny issue on the eastern side of the Andes in Argentina (Fisher, G.B. and 11 others 2023. Milankovitch-paced erosion in the southern Central Andes. Nature Communications, v. 14, 424-439; DOI: 10.1038/s41467-023-36022-0.

Burch Fisher (University of Texas at Austin, USA) and colleagues studied sediments derived from a catchment that drains the Puna Plateau that together with the Altiplano forms the axis of the Central Andes. In the late 19th century the upper reaches of the Rio Iruya were rerouted, which has resulted in its cutting a 100 m deep canyon through Pliocene to Early Pleistocene (6.0 to 1.8 Ma) sediments. The section includes six volcanic ash beds (dated precisely using the zircon U-Pb method) and records nine palaeomagnetic reversals, which together helped to calibrate more closely spaced dating. Their detailed survey used the decay of radioactive isotopes of beryllium and aluminium (10Be and 26Al) in quartz grains that form in the mineral when exposed at the surface to cosmic-ray bombardment. Such cosmogenic radionuclide dating thus records the last time different sediment levels were at the surface, presumably when the sediment was buried, and thus the variation in the rate of sediment supply from erosion of the Rio Iruya catchment since 6 Ma ago.

Measured concentrations (low to high values downwards) of cosmogenic 10Be (turquoise) and 26Al (red) in samples from the Rio Iruya sediment sequence. The higher the value, the longer the layer had resided at the surface; i.e. the slower the erosion rate. (Credit: Fisher et al. Fig 4)

The data from 10Be suggest that erosion rates were consistently high from 6 to 4 Ma, but four times during the later Pliocene and the earliest Pleistocene they slowed dramatically. Each of these episodes occupies downturns in solar warming forced by the 400 ka cycle of orbital eccentricity. The 26Al record confirms this trend. The most likely reason for the slowing of erosion is long-term reductions in rainfall, which Fisher et al have modelled based on Milankovich cycles. However the modelled fluctuations are subtle, suggesting that in the Central Andes at least erosion rates were highly sensitive to climatic fluctuations. Yet the last 400 ka cycle in the record shows no apparent correlation with climate change.  Despite that, astronomical forcing while early Pleistocene oscillations between cooling and warming ramped up does seem to have affected erosion rates based on the cosmogenic dating. The authors attribute this loss of the 400 ka pattern to a kind of swamping effect of dramatically increased erosion rates as the regional climate became more erratic. Whether or not data of this kind will emerge for the more climatically drastic 100 ka cyclicity of the last million years remains to be seen … Anyone who has walked over terrains covered in glacial tills and glaciofluvial gravel beds nearer to the former Late Pleistocene ice sheets can judge the difficulty of such a task.

Arctic climate in the run-up to the Great Ice Age

Around 3.6 Ma ago a large extraterrestrial projectile slammed into the far north-east of Siberia forming crater 16 km across. The depression soon filled with water to form Lake El’gygytgyn, on whose bed sediments have accumulated up to the present. A major impact close to the supposed start of Northern Hemisphere glacial conditions was a tempting target for coring: possibly two birds with one stone as the lowest sediments would probably be impact debris and boreal lake sediments of this age are as rare as hens’ teeth. The sedimentary record of Lake El’gygytgyn has proved to be a climate-change treasure trove (Brigham-Grette, J and 15 others 2013. Pliocene warmth, polar amplification, and stepped Pleistocene cooling recorded in NE Arctic Russia. Science, v. 340, p. 1421-1426).

El'gygytgyn, Russia, is a impact crater with a...
Lake El’gygytgyn impact crater. (credit: Wikipedia)

The team of US, Russian, German and Swedish scientists discovered that the sedimentary record was complete over a depth of 318 m and so promised a high resolution climate record. The striking feature of the sediments is that they show cyclical variation between five different facies, four of which are laminated and so preserve intricate records of varying weathering and sediment delivery to the lake. The sediments also contain pollens and diatom fossils, and yield good magnetic polarity data. The last show up periods of reversed geomagnetic polarity, which provide age calibration independent of relative correlation with marine isotope records.

A host of climate-related proxies, including pollen from diverse tree and shrub genera, variations in silica due to changes in diatom populations and organic carbon content in the cyclically  changing sedimentary facies are correlated with global climate records based on marine-sediment stable isotope. These records reveal intricate oscillations between cool mixed forest, cool coniferous forest, taiga  and cold deciduous forest, with occasional frigid tundra conditions through the mid- to late Pliocene. Compared with modern conditions NE Siberia was much warmer and wetter at the start of the record. Around the start of the Pleistocene sudden declines to cooler and drier conditions appear, although until 2.2 Ma ago average summer conditions seem to have been higher that at present, despite evidence from marine proxies of the onset of glacial-interglacial cycles in the Northern Hemisphere.

In detail, Lake El’gygytgyn revealed some surprises including rapid onset of a lengthy cold-dry spell of tundra conditions between 3.31 to 3.28 Ma. The first signs that the lake was perennially frozen appear around 2.6 Ma, well before evidence for the first continental glaciation in North America, presaged by signs around 2.7 Ma that winters consistently became colder than present ones. Overall the lake record presents a picture of a stepped shift in climate in the run-up to the Great Ice Age. Lake El’gygytgyn seems set to become the standard against which other, more patchy records around the Arctic Ocean are matched and correlated. Indeed it is the longest and most detailed record of climate for the Earth’s land surface, compared with 120 and 800 ka for the Greenland and Antarctic ice-caps.

Modelling their findings against likely atmospheric CO2 levels the authors provide grist to the media mill which focuses on how the late Pliocene may be a model for a future warm Earth if emissions are not curtailed, with visions of dense polar forests