Vegetation during UMBI and deposition of Tuff IF at Olduvai Gorge, Tanzania (ca. 1.8 Ma) based on phytoliths and plant remains

As part of ongoing research at Olduvai Gorge, Tanzania, to determine the detailed paleoenvironmental setting during Bed I and Bed II times and occupation of the basin by early hominins, we present the results of phytolith analyses of Tuff IF which is the uppermost unit of Bed I. Phytoliths were identified in most of the levels and localities on the eastern paleolake margin, but there are not always sufficient numbers of identifiable morphologies to infer the specific type of vegetation due to dissolution. Some surge surfaces and reworked tuff surfaces were vegetated between successive ash falls, as indicated by root-markings and the presence of a variety of phytolith morphotypes. Dicotyledonous wood/bark types were dominant except at the FLK N site just above Tuff IF when monocots are dominant and for the palm-dominated sample from the reworked channel cutting down into Tuff IF at FLK N. The area between the two fault scarps bounding the HWK Compartment, approximately 1 km wide, was vegetated at various time intervals between some of the surges and during the reworking of the Tuff. By lowermost Bed II times the eastern margin was fully vegetated again. Climate and tectonic activity probably controlled the fluctuating lake levels but locally the paleorelief and drainage were probably the controlling factors for the vegetation changes. These data support a scenario of small groups of hominins making brief visits to the paleolake during uppermost Bed I times, followed by a more desirable vegetative environment during lowermost Bed II times.     

(40)Ar/(39)Ar dating of Bed I, Olduvai Gorge, Tanzania, and the chronology of early Pleistocene climate change

⁴⁰Ar/³⁹Ar dating of tuffs and lavas of the late Pleistocene volcanic and sedimentary sequence of Olduvai Gorge, north-central Tanzania, provides the basis for a revision of Bed I chronostratigraphy. Bed I extends from immediately above the Naabi Ignimbrite at 2.038 ± 0.005 Ma to Tuff IF at 1.803 ± 0.002 Ma. Tuff IB, a prominent widespread marker tuff in the basin and a key to understanding hominin evolutionary chronologies and paleoclimate histories, has an age of 1.848 ± 0.003 Ma. The largest lake expansion event in the closed Olduvai lake basin during Bed I times encompassed the episode of eruption and emplacement of this tuff. This lake event is nearly coincident with the maximum precessional insolation peak of the entire Bed I/Lower Bed II interval, calculated from an astronomical model of the boreal summer orbital insolation time-series. The succeeding precessional peak also apparently coincides with the next youngest expansion of paleo-Lake Olduvai. The extreme wet/dry climate shifts seen in the upper part of Bed I occur during an Earth-orbital eccentricity maximum, similar to episodic lake expansions documented elsewhere in the East African Rift during the Neogene.     

Fingerprinting facies of the Tuff IF marker,with implications for early hominin palaeoecology,Olduvai Gorge,Tanzania

The top of Tuff IF marks the upper boundary of the Plio-/Pleistocene Bed I succession exposed in Olduvai Gorge, NE Tanzania, a tephrostratigraphic interval that is well known for remarkable Oldowan archaeological and vertebrate fossil assemblages, including early hominins. Geochemically, Tuff IF is characterized by a relatively consistent silica-undersaturated trachytic to phonolitic composition that relates both in terms of numerical ages (1.79 Ma) and compositional constraints to Olmoti volcano, located 22–38 km E of Olduvai Gorge.Measured Tuff IF sections are characterized by a thick unwelded pyroclastic flow in proximal settings and a succession of surges and ashfalls that interfingers with fluvio-lacustrine deposits of the Olduvai Basin in the medial to distal settings. Only in those areas with a relatively low topographic gradient, in distal reaches and beyond the toe of an Olmoti-sourced volcaniclastic fan, did the Tuff IF marker develop a typical threefold subdivision comprising: (1) primary surges and minor fallout, (2) reworked pumice units as lateral correlatives of proximally emplaced pyroclastic flows and (3) a succession of mass flows in conjunction with aeolian and fluvially reworked units.The pyroclastic marker unit is thus highly heterogeneous with regard to its vertical and lateral facies architecture and this has immediate effects on its preservation potential and in situ burial of fossils and stone artifacts. Even though the volcanically-related environmental perturbations were probably more severe at the eastern lake margin, the evidence for at least temporary freshwater sources and trees suggests environments conducive to hominin activities, but not during emplacement of Tuff IF pyroclastic flows and surges. This inference is supported by the presence of rich Oldowan stone artifact assemblages immediately preceding and following the deposition of Tuff IF, but only extremely sparse archaeological traces from one site in Tuff IF, restricted to the upper, fluvially reworked portion of Tuff IF.Tuff IF facies record the maximum of a regressive (drying) cycle that correlates with regional marine arid indicators. An ecological crisis covering ～ 2000–3500 a of time during Tuff IF deposition resulted not only from the lethal effects of explosive volcanism but also from its coincidence with a pronounced period of climate induced drought. These combined effects appear to have made at least the eastern basin uninhabitable by hominins and other vertebrates for most of the time during which Tuff IF accumulated.     

Unraveling hominin behavior at another anthropogenic site from Olduvai Gorge (Tanzania): new archaeological and taphonomic research at BK, Upper Bed II

New archaeological excavations and research at BK, Upper Bed II (Olduvai Gorge, Tanzania) have yielded a rich and unbiased collection of fossil bones. These new excavations show that BK is a stratified deposit formed in a riverine setting close to an alluvial plain. The present taphonomic study reveals the second-largest collection of hominin-modified bones from Olduvai, with abundant cut marks found on most of the anatomical areas preserved. Meat and marrow exploitation is reconstructed using the taphonomic signatures left on the bones by hominins. Highly cut-marked long limb shafts, especially those of upper limb bones, suggest that hominins at BK were actively engaged in acquiring small and middle-sized animals using strategies other than passive scavenging. The exploitation of large-sized game (Pelorovis) by Lower Pleistocene hominins, as suggested by previous researchers, is supported by the present study.     

Fossil sedges, macroplants, and roots from Olduvai Gorge, Tanzania

A variety of macroplants has been recorded and collected from the eastern paleolake margin of Olduvai Gorge, Tanzania, from Upper Bed I and Lower Bed II, dated at ∼1.7-1.85 Ma. The plant groups represented are sedges, grasses, and woody and herbaceous dicotyledons. Most of these plants are fragmented, but the roots are in situ. The modes and quality of preservation, however, are very variable. Silicification is the dominant type of preservation; it ranges from high quality faithful replacement of cells resulting in silicified wood and sedge culms that are identifiable on the basis of their internal anatomy, to poor quality biotubes lacking internal anatomy or external features that prevent assignment to a specific plant or invertebrate origin. In between this range are silicified roots and grass culms identified by their external anatomy, and leaf and stem impressions. Interpretation of the paleoecology is limited by the quality of preservation. The in situ root horizons are useful for recognizing paleo-surfaces. The best quality preservation where internal anatomy is preserved occurs at HWK E and MCK, localities that are in the middle of the fault compartments so the vegetation can be reconstructed for these sites. Some sedge culms are described, illustrated, and identified as possible species of Cyperus, Fuirena, and Schoenoplectus.     

A revised stratigraphic framework for Olduvai Gorge Bed I based on tuff geochemistry

Olduvai Gorge, Tanzania has rich records of Plio-Pleistocene fauna and flora, hominin fossils, and stone artifacts preserved between well-dated tephra layers (tuffs). Accurate correlation between sites in the two million year section is complicated by faulting, erosion, change in physical appearance of the tuffs, and quality of preservation. Traditional tuff geochemical techniques using glass cannot be applied because of poor glass preservation, and previous physical mapping has led to miscorrelations in Bed I. A new approach, using a combination of glass and mineral compositions (feldspar, augite, hornblende, and oxides) produced successful geochemical fingerprints for all ten major Bed I (∼2.03-1.79 Ma) tuffs. These fingerprints make available a reliable means for correlating specific tuffs between the well-dated "Junction" sites, such as FLK and HWK, and less well-dated sites at the basin margins. The new correlations provide a high-resolution stratigraphic framework for Bed I and correct previous miscorrelations in the west of the basin. Olduvai Hominin 65, from western Olduvai, was recovered from a level between Tuff IC and the Ng’eju Tuff, and therefore dates to 1.79-1.84 Ma.     

Late Pliocene grassland from Olduvai Gorge,Tanzania

The Olduvai fossil plants documented by us in this paper are the first direct evidence for open grassland in the late Neogene of Africa based on macroplant remains. Silicified remains of herbaceous ground cover are exceptionally well preserved in situ within Late Pliocene sediments below the initial pyroclastic ash surge unit of Tuff IF in the uppermost part of Bed I, Olduvai Gorge, northern Tanzania. Published radiometric and palaeomagnetic dates place this grass layer between 1.839 + 0.005 Ma and 1.785 + 0.01 Ma. Exposed at localities on the south side of the Gorge this herbaceous ground cover grew on a floodplain developed on a dried out lake bed, following pronounced lake retreat of saline–alkaline palaeo-Lake Olduvai during a developing dry climatic phase. Sheathed basal culms, rhizomes and roots are interpreted as those of one or more small mat-forming grasses or less likely, sedges. Small dicotyledonous herbs were probably also present. The proximity of adjacent plants indicates a relatively dense ground cover. Roots extended at least 8 cm below the ground surface. Aerial parts of the plants were absent or were not preserved when the weathered basal culms were covered by a thin layer of brown waxy clay, followed by fallout of pyroclastic ash. Both units were mostly eroded away prior to emplacement of a wet, cool pyroclastic surge which then buried and preserved in situ remnants of the herbaceous ground cover. Preservation of the semi-woody rhizomes implies well-drained soils, otherwise the plant material would have quickly rotted. Collections from discontinuous exposures indicate the grassland covered an area of at least a few hectares. This open grassland would have provided grazing for the Late Pliocene fauna.     

A new species of cormorant (Aves: Phalacrocoracidae) from the Pleistocene of Olduvai Gorge,Tanzania

Phalacrocorax owrei nov. sp. is a small cormorant from the Lower Pleistocene of Olduvai Gorge. Osteological proportions are established for the four subgenera of Phalacrocorax, and P. owrei is assigned to the subgenus Stictocarbo. The species was the most abundant bird in the Bed I deposits and is also represented by a few specimens in the middle part of Bed II. Its last known appearance coincides with a change in the local environment when the climate became more arid and the Olduvai Lake became more saline and more alkaline. At other localities in Bed II the extinct P. tanzanice occurs, as well as P. africanus and P. corbo, which breed on the African lakes and seacoast today.     

Examining time trends in the Oldowan technology at Beds I and II,Olduvai Gorge

The lithic analysis of the Bed I and II assemblages from Olduvai Gorge reveals both static and dynamic time trends in early hominids' technology from 1.8 to 1.2 m.y.a. The Bed I Oldowan (1.87-1.75 m.y.a.) is characterized by the least effort strategy in terms of raw material exploitation and tool production. The inclusion of new raw material, chert, for toolmaking in the following Developed Oldowan A (DOA, 1.65-1.53 m.y.a.) facilitated more distinctive and variable flaking strategies depending on the kind of raw materials. The unique characters of DOA are explainable by this raw material factor, rather than technological development of hominids. The disappearance of chert in the subsequent Developed Oldowan B and Acheulian (1.53-1.2 m.y.a.) necessitated a shift in tool production strategy more similar to that of Bed I Oldowan than DOA. However, the evidence suggests that Bed II hominids might have been more skillful toolmakers, intensive tool-users, and engaged in more active transport of stone tools than the Bed I predecessors. Koobi Fora hominids maintained a more static tool-using behavior than their Olduvai counterparts due mainly to a stable supply of raw materials. They differed from Olduvai hominids in terms of less battering of cores, consistent transport behavior, and few productions of side-struck flakes, indicating a regional variation of toolmaking and using practice. However, they shared with Olduvai hominids a temporal trend toward the production of larger flakes from larger cores after 1.6 m.y.a. Increased intake of animal resources and the expansion of ranging area of Homo ergaster would have led to the development of technological organization. Technological changes in the Oldowan industry are attested at Olduvai Gorge, Koobi Fora, and Sterkfontein, suggesting that it was a pan-African synchronous phenomenon, beginning at 1.5 m.y.a.     

Analysis of orientation patterns in Olduvai Bed I assemblages using GIS techniques: implications for site formation processes

Mary Leakey’s excavations at Olduvai Beds I and II provided an unparalleled wealth of data on the archaeology of the early Pleistocene. We have been able to obtain axial orientations of the Bed I bone and stone tools by applying GIS methods to the site plans contained in the Olduvai Volume 3 monograph (Leakey, 1971). Our analysis indicates that the Bed I assemblages show preferred orientations, probably caused by natural agents such as water disturbance. These results, based on new GIS techniques applied to paleoanthropological studies, have important implications for the understanding of the formative agents of Olduvai sites and the behavioral meaning of the bone and lithic accumulations in Bed I.     

Revised stratigraphy of Area 123, Koobi Fora, Kenya, and new age estimates of its fossil mammals, including hominins

Recent geologic study shows that all hominins and nearly all other published mammalian fossils from Paleontological Collection Area 123, Koobi Fora, Kenya, derive from levels between the KBS Tuff (1.87+/-0.02 Ma) and the Lower Ileret Tuff (1.53+/-0.01 Ma). More specifically, the fossils derive from 53 m of section below the Lower Ileret Tuff, an interval in which beds vary markedly laterally, especially those units containing molluscs and algal stromatolites. The upper Burgi Member (approximately 2.00-1.87 Ma) crops out only in the southwestern part of Area 123. Adjacent Area 110 contains larger exposures of the member, and there the KBS Tuff is preserved as an airfall ash in lacustrine deposits and also as a fluvially redeposited ash. We observed no mammalian fossils in situ in this member in Area 123, but surface specimens have been documented in some monographic treatments. Fossil hominins from Area 123 were attributed to strata above the KBS Tuff in the 1970s, but later they were assigned to strata below the KBS Tuff (now called the upper Burgi Member). This study definitively places the Area 123 hominins in the KBS Member. Most of these hominins are between 1.60 and 1.65 myr in age, but the youngest may date to only 1.53 Ma, and the oldest, to 1.75 Ma. All are 0.15-0.30 myr younger than previously estimated. The new age estimates, in conjunction with published taxonomic attributions of fossils, suggest that at least two species of Homo coexisted in the region along with A. boisei until at least 1.65 Ma. Comparison of crania KNM-ER 1813 and KNM-ER 1470, which were believed to be of comparable age, is at the focus of the debate over whether Homo habilis sensu lato is in fact composed of two species: Homo habilis and Homo rudolfensis. These two crania are separated in time by approximately 0.25 myr, and therefore, arguments for their conspecificity no longer need to confront the issue of unusually high contemporaneous variation within a single species.     

Carbonate horizons, paleosols, and lake flooding cycles: Beds I and II of Olduvai Gorge, Tanzania

This study documents the petrology and stable isotope geochemistry of carbonates from six horizons from Beds I and II of Olduvai Gorge, Tanzania. The studied succession, immediately below and above Tuff IF, consists of interbedded waxy and earthy claystones with discrete carbonate horizons and thin sandstones. The succession was deposited in response to repeated flooding and withdrawal of a saline-alkaline lake. The carbonates and their overlying disconformities are important because they help define the surfaces on which hominin activity took place and allow very high-resolution correlation of geographically separated levels of hominin exploitation.The range of different carbonates includes unambiguous land-surface and pedogenic features including calcified rootmat horizons, rhizocretions, and micritic nodules, together with less determinate sparry calcite nodules. Stellate nodules are interpreted as pseudomorphs after sulfate-roses. The carbonate nodules are synsedimentary features, truncated by fluvial and other erosional surfaces. The isotopic composition of the carbonates is variable with δ¹⁸O ranging from −7.0‰ to −4.3‰, and δ¹³C from −8.5‰ to −1.6‰. A covariant increase in δ¹³C and δ¹⁸O repeats in each carbonate horizon and in individual nodules (inner to outer layers): it reflects the evolution of synsedimentary groundwaters. At times of low lake level, the carbonates started to precipitate from meteoric waters with low isotopic values and continued to form as lake levels rose and the waters became increasingly saline. Some of the samples have a last-stage cement of strontium rich dolomite, which supports late-stage flooding by the saline-alkaline lake. Previous studies of carbonate horizons from Olduvai have interpreted carbon isotope values in terms of changes in C₃ and C₄ plants that colonized the land surface. This study demonstrates that in some instances the isotope values from carbonates deposited in these lake marginal settings reflect changes in hydrology rather than vegetation.     

Taphonomic perspectives on hominid site use and foraging strategies during Bed II times at Olduvai Gorge, Tanzania

The faunal assemblages excavated by Mary Leakey in Bed II of Olduvai Gorge, Tanzania, have, like the more well-known Bed I assemblages, traditionally been interpreted as the result of hominid butchering activities in the lake margin and riverine settings of the paleo-Olduvai Basin. A reexamination of all of Leakey's Bed I sites has shown that hominids played little or no role in the formation of all but one of those faunal assemblages, a finding that prompted the reanalysis of the Bed II sites presented here. We expand upon a previous taphonomic study that provided systematic data for HWK East Levels 1–2, MNK Main, and BK. In addition to these assemblages, we provide data on HWK East Levels 3–5, FC West, TK, and SHK. Our data contradict previous interpretations of MNK Main as a hominid accumulation but uphold the contention that BK represents a primarily hominid accumulation reflecting early access to carcasses. The small and poorly preserved assemblages from FC West and TK are difficult to link unambiguously to either hominids or carnivores. Site MNK Main and HWK East Levels 3–5 appear to be death arenas where carcasses accumulated via natural deaths and/or serial predation. Site SHK is severely biased by selective retention and therefore little can be said of its formational history. Nevertheless, no hominid modifications were documented in this assemblage. Comparisons with other Olduvai sites indicate a more conspicuous hyena taphonomic signal during Bed II times than Bed I times, which appears to mirror the changing configuration of the large carnivore guild. These findings also beg the question of what activities were being carried out by hominids with the stone tools discarded at these sites. Although it seems clear that hominids were utilizing stone tools to carry out subsistence activities unrelated to carcass butchery, more excavation and techniques such as phytolith analysis should be employed to explore alternative explanations.     

Landscape distribution of Oldowan stone artifact assemblages across the fault compartments of the eastern Olduvai Lake Basin during early lowermost Bed II times

The density and composition of Oldowan stone artifact assemblages deposited during the first ca. 20,000 years of lowermost Bed II times show a recurrent pattern of variation across recognized synsedimentary faults that compartmentalized landscapes of the eastern Olduvai Lake Basin. When active, the faults created minor topographic relief. The upthrown fault footwalls accumulated assemblages with relatively high densities of artifacts, including types retaining potential usefulness, particularly volcanic flaked pieces, manuports, pounded pieces, and split cobbles. Values for these assemblage characteristics decline toward the lower-lying hangingwall of the fault compartments, accompanied by an increase in the proportionate weight of artifact assemblages comprising quartzite, particularly flaking shatter and potentially useful detached pieces. Values reverse once again at faults, either on the downthrown, hangingwall side or on the upthrown side. The patterns are stronger for the volcanic components of the artifact assemblages than for the quartzite components, reflecting the additional influence of distance from the local source on quartzite assemblage characteristics reported previously. The landscape distributions of artifact assemblages are consistent with a landscape-fault model in which minor fault-induced topographic relief at times created a mosaic of vegetation environments repeated within each of the three fault compartments of the lake margin and distal alluvial fan. The fault-compartmentalized landscape model is currently supported only by sediment thickness and facies changes across synsedimentary faults, but it provides predictions for spatial variation in the cover abundance of trees, freshwater reservoirs and associated distributions of resources and hazards associated with stone artifact use and discard that can be tested if sample sizes of key paleoenvironmental indicators are increased.     

A Homo habilis maxilla and other newly-discovered hominid fossils from Olduvai Gorge, Tanzania

In 1995, a 1.8 million year old hominid maxilla with complete dentition (OH 65) was excavated from Bed I in the western part of Olduvai Gorge. The molar crowns are small relative to the long flaring roots, and the root of the canine is very long and straight. The broad maxilla with wide U-shaped palate and the form of the tooth roots closely match those of KNM-ER 1470 which, in its parietal size and morphology, matches the type specimen of Homo habilis, OH 7. Thus, OH 65 and KNM-ER 1470 group with OH 7 as representatives of H. habilis while some other Olduvai specimens, such as OH 13 and OH 24, have more in common in terms of morphology and brain size with Australopithecus africanus. Between 1995 and 2007, the OLAPP team has recovered teeth of eight other hominid individuals from various parts of Olduvai Gorge. These have been identified as belonging to H. habilis, Paranthropus boisei, and Australopithecus cf. africanus.     

Sapropels and the age of hominins Omo I and II, Kibish, Ethiopia

The provenance and age of two Homo sapiens fossils (Omo I and Omo II) from the Kibish Formation in southern Ethiopia have been much debated. Here we confirm that Omo I and the somewhat more primitive-looking Omo II calvariae are from similar stratigraphic levels in Member I of the Kibish Formation. Based on (40)Ar/(39)Ar age measurements on alkali feldspar crystals from pumice clasts in the Nakaa'kire Tuff, a tuffaceous bed in Member I just below the hominin levels, we place an older limit of 198+/-14ka (weighted mean age=196+/-2ka) for the hominins. A younger limit of 104+/-7ka (weighted mean age=104+/-1ka) is provided by feldspars separated from pumice clasts in the Aliyo Tuff in Member III. Geological evidence indicates rapid deposition of each member of the Kibish Formation, concurrent with deposition of sapropels in the Mediterranean Sea. The (40)Ar/(39)Ar age measurements, together with correlations with sapropels, indicate that the hominin fossils are close in age to the older limit. Our preferred estimate of the age of the hominins is 195+/-5ka, making them the earliest well-dated anatomically modern humans yet described.     

Correlation of the KHS Tuff of the Kibish Formation to volcanic ash layers at other sites, and the age of early Homo sapiens (Omo I and Omo II)

Hominin specimens Omo I and Omo II from Member I of the Kibish Formation, Ethiopia are attributed to early Homo sapiens, and an age near 196 ka has been suggested for them. The KHS Tuff, within Member II of the Kibish Formation has not been directly dated at the site, but it is believed to have been deposited at or near the time of formation of sapropel S6 in the Mediterranean Sea. Electron microprobe analyses suggest that the KHS Tuff correlates with the WAVT (Waidedo Vitric Tuff) at Herto, Gona, and Konso (sample TA-55), and with Unit D at Kulkuletti in the Ethiopian Rift Valley. Konso sample TA-55 is older than 154 ka, and Unit D at Kulkuletti is dated at 183 ka. These correlations and ages provide strong support for the age originally suggested for the hominin remains Omo I and Omo II, and for correlation of times of deposition in the Kibish region with formation of sapropels in the Mediterranean Sea. The Aliyo Tuff in Member III of the Kibish Formation is dated at 104 ka, and correlates with Gademotta Unit 15 in the Ethiopian Rift Valley.     

Soricidae (Mammalia) from the Early Pleistocene of Olduvai Gorge, Tanzania

Soricid remains collected from Bed I of Olduvai Gorge are described. The great majority of the specimens are mandibles. A survey of the mandibles of living African species revealed many differences in characters of the lower teeth and jaw that can be used for identification. On the basis of these characters, nine species are distinguished in the Olduvai collection, of which six are well enough preserved to permit a discussion of their relationships to living species. Three new species and one new subspecies are described. All the Olduvai shrews differ in some respects from their nearest living relatives; three species are close to species from Makapansgat, Swartkrans and Sterkfontein, RSA, though there appear to be slight differences. A change in the soricid fauna takes place within Bed I, interpreted as due to increasing aridity.     

Long-distance carcass transport at Olduvai Gorge? A quantitative examination of Bed I skeletal element abundances

Relative abundances of skeletal elements at Plio-Pleistocene archaeological sites have long been interpreted to represent selective transport of portions of large prey. Models from optimal foraging theory suggest that the degree of carcass transport selectivity reflects transport constraints, particularly transport distance. A quantitative analysis of skeletal element abundances in five bone assemblages from Bed I, Olduvai Gorge, indicates that within the subset of elements most likely to resist attritional processes, there is no evidence for preferential transport of small or large mammals. The results suggest relatively low carcass transport costs and are most consistent with site formation models favoring short-distance carcass transport. The data are also consistent with the possibility that hominins were not responsible for transporting bones at some sites. Several Bed I assemblages, with the exception of FLK-Zinjanthropus, lack evidence of a functional relationship between flaked stone artifacts and the faunal remains, such as cut-marks or percussion-marks on bone. In conjunction with the skeletal part data, this suggests that hominin involvement with the bone assemblages was minimal at all sites but FLK-Zinjanthropus. The patterning at Bed I contrasts strongly with Middle Stone Age and Middle Paleolithic assemblages, which provide clear evidence for selective transport, suggesting higher transport costs and longer transport distances.