Saturday 30 September 2023

Evidence for the construction of wooden structures in the Middle Pleistocene of Zambia.

The first evidence of woodworking is associated with the Early Pleistocene of Oldowan Culture of East Africa, with a number of stone tools showing wear traces and residues left when they were used to work wood. The earliest known wooden tools come from the Middle Pleistocene of Southern Africa, with waterlogged wooden items from several sites, associated with Middle Stone Age and Acheulean stone tools. One such site is at Kalambo Falls in northern Zambia, where a series of wooden artefacts were recovered in the 1950s and 1960s, from horizons which also produced Acheulean tools. These included a wood chip, and three objects with transverse notches interpreted to have been made by intentional shaping of the wood. The Amanzi Springs site in Eastern Cape Province, South Africa, a stick with a possible chop mark was discovered in a waterlogged deposit which also produced Acheulean tools in the 1960s. These deposits were radiometrically dated to between 404 000 and 390 000 years before the present, from unmodified wood within the same strata. A wooden object with cut marks and fine striations from the Florisbad Spring Deposit in Free State, South Africa, was found alongside Middle Stone Age stone tools and remains of the Hominin Homo helmei; this is considered to be the oldest known clearly modified wooden object, but has not been dated beyond Middle Pleistocene.

In a paper published in the journal Nature on 20 September 2023, Larry Barham of the Department of Archaeology, Classics & Egyptology at the University of Liverpool, Geoff Duller of the Department of Geography and Earth Sciences at Aberystwyth University, Ian Candy of the Department of Geography at Royal Holloway, University of London, Christopher Scott, also of the Department of Archaeology, Classics & Egyptology at the University of Liverpool, Caroline Cartwright of the Department of Scientific Research at the British Museum, John Peterson again of the Department of Archaeology, Classics & Egyptology at the University of Liverpool, Ceren Kabukcu, again of the Department of Archaeology, Classics & Egyptology at the University of Liverpool, and of the Interdisciplinary Center for Archaeology and Evolution of Human Behaviour at the University of Algarve, Melisa Chapot, also of the Department of Geography and Earth Sciences at Aberystwyth University, F Melia, again of the Department of Archaeology, Classics & Egyptology at the University of Liverpool, Veerle Rots of the Prehistory TraceoLab  at the University of Liège, Nikki George, again of the Department of Archaeology, Classics & Egyptology at the University of Liverpool, Noora Taipale, also of the Prehistory TraceoLab at the University of Liège, Peter Gethin, once again of the  Department of Archaeology, Classics & Egyptology at the University of Liverpool, and Perrice Nkombwe of the Moto Moto Museum describe recently described evidence for wood being used tob build a structure in the Middle Pleistocene at Kalambo Falls.

New excavations at Kalambo Falls in 2019 recovered five modified wooden artefacts from four different areas, at different layers within the sediments, both above and below the current level of the river. A sixth wooden object showed no signs of intentional modification, but was associated with one of the modified wooden items and some Acheulian stone tools, in a layer below the current river level. One of the other modified wooden objects was also found (separately) associated with Acheulian stone tools, in a layer below the current river level. The remaining modified wooden objects were found without associated stone tools in layers above the current river level, two of them together and one on its own. 

Location of Kalambo Falls archaeological site and excavated areas. (a) Site location in south-central Africa. (b) Course of the Kalambo River (in outline) from around 1956 to 2006 in relation to previous excavations at sites A, B, C, D and C North. Site BLB (2019) and excavation units BLB1, BLB2, BLB3, BLB4 and BLB5 are located along the current main channel (blue). (c) Cross-section of the 2019 excavation units showing the location of 16 luminescence dating samples (KF01–KF17, dark blue circles, uncertainties (±) shown at 1 − σ) by unit. Unit BLB1 is a geological section of the full cliff exposure from ground surface to below water level. The three colour bands indicate clusters of pIR IRSL ages grouped by mean ages and standard error of the mean. The earliest wood objects (BLB5, BLB3) are in the lower green band with a mean age of 476 ± 23 000 years. The blue band has a mean age of 390 ± 25 000 years and incorporates the wood object in BLB2. The overlying yellow band has a mean age of 324 ± 15 000 years and incorporates wood objects in BLB4. Red diamonds indicate modified wood objects. Barham et al. (2023).

At Kalambo Falls a nine-metre-deep exposure of fluvial deposits, interpreted as having been laid down by a laterally migrating river with moderate-to-high energy flows, comprises mostly gravels and course sands, with occasional non-contiguous beds of fine sand, silt, and clay. This deposit has an elevated water table, which has preserved wooden objects and plant matter within the bottom two metres of the sequence. Wooden items are interpreted as either having been deliberately deposited by Hominins or carried to its current location by the flow of the river. 

Modified wood tools from site BLB, Kalambo Falls, 2019. (a) BLB5 structural element (object 1033). (b) BLB3 ‘wedge’ (object 660). (c) BLB2 ‘digging stick’ (object 219). (d) BLB4 cut log. (e) BLB4, tapered piece with single chop-mark. Scale bars are 10 cm. Barham et al. (2023).

Sixteen dates were obtained from this sequence using a combination of optically stimulated luminescence dating of single quartz grains and postinfrared infrared stimulated luminescence dating of potassium-rich feldspars. These dates form a stratigraphic sequence (i.e. the older dates are below the newer dates in the succession), with one exception, and fall into three distinct age clusters. The oldest cluster, centred on 476 000 years ago (and extending approximately 23 000 years in either direction of this date) includes all of the samples from below the current river level, including two modified wooden objects, BLB3, which was found with a piece of unmodified wood and a collection of Acheulian tools, and BLB5, which was found with another group of Acheulian tools. The next cluster, centred on 390 000 years ago and extending 25 000 years on either side of that date, contained a single wooden tool, BLB2, found on its own, above the level of the current river, while the final cluster, centred on 324 000 years ago and extending 15 000 years on either side of that date, includes a pair of wooden artefacts, BLB4, found together.

It was possible to identify the wood from which the artefacts were made, but attempts at carbon dating them produced 'infinite' results, indicating they are more than about 50 000 years old. Infrared spectroscopy of these specimens showed that they had begun to mineralize, with silica beginning to replace the original wood. The tools were interpreted by comparison to later wooden tools from Holocene archaelogical sites in Zambia and the UK, as well as those made by modern Human populations.

The oldest tool, object 1033 from layer BLB5, is a Large-fruited Bushwillow, Combretum zeyheri, log 141.3 cm in length and 25.6 cm in width, which was found overlying another log at an angle of 75°. Importantly, this overlying log has a notch, 13.2 cm long and 11.4 cm wide, intentionally cut into it where it rests on the underlying log, which also shows signs of modification. Bark is still present on both logs, bur is absent around the notch, emphasizing that this was intentionally cut, although this is not in doubt, due to the visible chop and scrape marks on the exposed wood within the notch. 

Structural unit formed by two overlapping logs in BLB5. The underlying log passes through a central notch cut into the upper log (object 1033) and extends into the section. Plan view of the unit (left) and during excavation (right). The numbers refer to the distance in centimetres. Barham et al. (2023).

The notch shows several areas of scrape marks, overlapping in the centre. These are 'v'-shaped in profile, up to 24 mm long and 3.2 mm wide. The end of the log is tapered, and also appears to have been modified.

Annotated images of the BLB5 upper log (object 1033) showing areas of intentional modification. From left to right, the location of the central notch in profile, shaping marks in and on the margins of the notch (a)–(k), the notch in profile from the opposite side. The image on the right shows the upper surface of the log, and the three parts of the log (1)–(3) separated by cracks. White arrows indicate locations of shaping facets on the sides and upper surface of the log. Barham et al. (2023).

The underlying log also shows signs of modification, with a series of small striations running across the grain of the wood at its mid-point, again with 'v'-shaped profiles indicative of scraping, and the end of the log which passes through the notch in the upper log showing signs of having been intentionally thinned by the scrapping of a series of notches along the grain of the wood.

Shaping marks on the upper surfaces of object 1033 and on the underlying treetrunk. Clockwise, from left upper left; chop marks on Part 2; cluster of small convex hewing marks on Part 1, near Part 2; cutmark (upper arrow) and small facets (lower arrows) on Part 1 near Part 3; intercutting chop marks on the upper right edge of the Part 3 taper; underlying log midsection, intersecting cutmarks transverse to the grain (bold arrow, upper left, indicating direction of grain). Marks on underlying treetrunk interpreted as result of scraping, perhaps from debarking. Barham et al. (2023).

Infrared spectroscopy suggests that the notch has also been modified by fire-shaping, something which was reported on another log of similar size (165 cm) previously found with Acheulian artefacts at Kalambo Falls, but not dated, which also had a wide, deep groove cut into it and tapering ends. This wood was presumed to have been part of a structure, and thought to have been modified by the actions of Early Humans. The new discovery of two logs interlocking on a similar notch, strongly supports the idea that the function was constructional in purpose, although the age of the strata from which the wood was recovered considerably pre-dates the oldest estimates for the emergence of Modern Humans.

Shaping marks on the treetrunk underlying object 1033 notch, BLB5. Surface modifications on the treetrunk beneath and just beyond the notch. Images described counter clockwise: white arrows highlight direction of striations, some V-shaped in profile, cutting across the grain from the edge toward the middle of the surface; a set of parallel transverse striations/cutmarks, with V-shaped profiles (white arrows) along ridge descending to a flat tapered surface; inset detail of the striations on the ridge crest; overview of the set of striations along the ridge; profile view of the flattened surface below the ridge; profile view of the ridge and break of slope onto the flat tapering surface which extends beyond the notch; plan view of the flat tapering end with transverse striations and cutmarks with V-shaped profiles (white arrows) interpreted as shaping marks. Barham et al. (2023).

The second collection of material below the modern river level, BLB3, comprises another collection of Acheulian tools, including flake tools, cleavers, handaxes and core axes, as well as two wooden objects, a 'v'-shaped piece of Figwwod, Ficcus sp., showing no signs of intentional modification, and a flattened branch of Sausage Tree, Kigelia africana, 36.2 cm in length and with one end carved to a point.

Shaping and possible use marks on object 660, BLB3. Surface modifications on ‘wedge’ shown clockwise: white arrows indicating location of modifications on base and either side of the tip; basal crack (top arrow) and side split, facet ‘a’ intercepts a striation, possible use damage, and ‘b’ indicates a set of faint transverse striations; ‘c’ and ‘d’ intersecting facets above tip, ‘e’ and ‘f’ are sets of parallel faint transvers striations; tip face with convex step terminations, possibly from use. Barham et al. (2023).

Barham et al. believe that this artefact has been intentionally shaped by repeated high-impact blows, in order to form a wedge. Similar wedges have been recovered from the Mesolithic Starr Car site in the UK, and are known to have been used for a variety of purposes by Aboriginal Australians, but nothing similar has ever been discovered in a Middle Pleistocene deposit.

The BLB2 horizon, dated to about 390 000 years ago produced a single wooden item, broken into two pieces which fitted together; one of these was found still embedded in the sediment, the other in the river directly below, where it had apparently fallen. The combined wooded object is about 62.4 cm in length, made of African Sausagewood, stripped of all bark and intentionally fashioned into a point, being 11.9 cm wide at the base, 6.1 cm wide in the middle, and 1.3 cm wide at the tip. The object has striations cutting across the grain of the wood in several places, these being less than 10 mm wide and convex in profile. The tip of the object is slightly rounded, which might be a sign of wear from use. or derive from its time in the river. The object is interpreted as a digging stick similar to those used historically by foragers in the Kalahari. Similar artefacts are known from the Middle Holocene of Zambia, but again the Middle Pleistocene occurrence is unprecedented. 

Shaping marks on object 219, BLB2. Upper image shows all views of the object with white arrows indicating location of marks on either side of the tip and above the tip. Both sides of the tip (two images on left) have a faint set of transverse striations and two or more small facets away from the tip. (Scale bar is 1 cm.) The vertical image to the right shows four groups of faint parallel transverse striations in an area with two knots, no scale. The object is interpreted as a digging-stick. Barham et al. (2023).

The BLB4 horizon, dated to about 324 000 years ago, produced two wooden artefacts and no stone items. The uppermost of these is a rectangular piece of Large-fruited Bushwillow wood measuring 59.24 cm by 29.34 cm by 7.7 cm, with traces of bark and sapwood still present. This shows signs of having been flattened, but this may have been due to the weight of the sediment above it. It also has a large chop mark running across its obverse surface. Barham et al. interpret this item as a piece of tree trunk which has been intentionally cut to size, indicating a capacity to process large pieces of wood.

Chop marks on ‘cut log’, BLB4. Upper image shows all views of the object and location of modifications (white arrows) on the ends, ‘a’ and ‘b’, two marks on the underlying surface and the preservation of bark on both surfaces. Image below is ‘a’ with its distinctive stepped chop marks (black arrows), and underlying grey image (denatured RTI) shows the width of the step between the first and second chop-mark expanding, left to right, from 13 to 25 mm. The deep, slightly convex cut of the first chop is visible. The underly step expands left to right from 6 mm to 11 mm. A third step starts with a width of 7 mm. Bottom image is end ‘b’ with black arrows indicating location of chop marks. Barham et al. (2023).

The second item is a Sausagewood branch, with a side branch 37.9 cm in length, tapering from a 12.3 cm wide base to a 2.1 cm wide tip. The wood has been split centrally, and has a transverse chop mark above the tip. The purpose of this artefact is unclear.

Images of split branch, BLB4 (‘notched stick’). Above, close-up of the facet/chop-mark with arrow indicating direction of initiation. Below, all views of split branch with chop (white arrow) and close-up of the entry facet. The object was found cracked in situ. Barham et al. (2023).

The Kalambo Falls site has yielded a number of exceptionally well-preserved, intentionally modified wooded objects, with the most recent discoveries confidently dated to the Middle Pleistocene (and previous, undated discoveries likely to be of similar vintage). This has significant implications for our understanding of the technological capabilities of earlier members of the genus Homo, which have previously only been known from stone tools. The implications are clearly that these people were capable of felling large trees, and constructing objects such as platforms, which would have required considerable skills in selecting trees, felling them, and processing the wood obtained. Since this appears to have been done with stone tools, it shows that stone- and woodworking evolved together, and helps to explain some of the larger stone tools produced as part of the Acheulean technology. 

Between about 470 000 and about 274 000 years ago the Kalambo Falls site would have had an extensive forest cover and a high water table, leading to periodic flooding of the river plain. In such an environment the ability to produce raised platforms, wooden walkways, or even structures with solid foundations would have been highly advantageous.

The invention of hafted tools (tools with wooden handles) is thought to have occurred between about 500 000 and about 200 000 years ago, and would have required a good understanding of the properties of wood, and how to shape it by chopping and scraping. The interlocking logs at the lowest dated level at Kalambo Falls show a similar level of technological cognition, as well as a previously unanticipated ability to create the first built environments.

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