2019: Anthony Tan, Kerlyn Fu, Ryan Lim
- 1. Introduction – Stone Tools in Hominin Evolution
- 2. The Technological Origins of Language
- 3. Evidence from neuroanatomy
- 4 Experimental Approaches
- 5 Shared social contexts
- 6. References
The first evidence for tool production dates from about 2.5 million years ago (mya), though controversial examples dating back to even further back are aplenty. The onset of obligatory stone tool use after 0.3 mya may be linked to the evolution of spoken language (Shea, 2017). Stone tool-making is a deliberate act; each resulting tool is thus a snapshot of hominin behavior and an important material phenomenon we can use to contextualize claims of cognitive change against. Adopting stone tool technology thus made a difference to our survival prospects as large populations were not only capable of sustaining innovations, but using them to expand and adapt to new environments (Davidson, 2016).
Complex tool use and language are unique characteristics of the human species and contains widespread evidence of human behavior (Shea, 2017). Archaeological records demonstrate that the first recognisable tools were made from organic material decompose e.g. stone. A scheme of tool classification was devised by Grahame Clark (1969), that distinguishes the five different modes. Modes cannot be chronologically divided per se, rather, they reflect the groupings of tools that share distinctive morphological characteristics (Foley & Lahr, 2003). As such, the implementation of one mode at a certain point in time is not a universal affair – an older mode might have been replaced in one region, but continue to be used in another.
As the last two modes are only well attested in the final stages of the Upper Paleolithic (50kya onwards), the earliest three modes are the most pertinent for our focus on language emergence.
Mode 1 represents the simplest mode of production, which is Oldowan stone tools. Tools are made by using a hammerstone to strike flakes off a core – if successfully flaked, the sharp edge of a flake becomes a cutting tool. The number of flakes could vary, but what held this system of production together was the simple platforms and lack of preparation involved. This mode resulted in relatively little diversity of toolform and ease of preparation of the striking platforms.
Oldowan tool-making demands little in terms of cognition as stone tool producers are only required to visually identify ideal platforms and remove flakes until the core is exhausted. Oldowan tool manufacture requires the coordination of visual attention and motor control to successfully remove simple flakes. Experimental findings reveal that ape-like cognition was the minimum to produce Oldowan tools 1.8mya, whereas human-like cognition in modern knappers is not fully recruited in knapping (Shea, 2013).
Acheulean tool production spanned over 1.5my and produced some of the most significant artefacts, including the characteristic handaxe. (Beyene et al., 2013). The Acheulean handaxes were produced by the bifacial reduction of a block or large flake blank around a single, long axis. Acheulean handaxes are thought to have been produced by two extinct hominin species, Homo erectus and Homo heidelbergensis (Corbey, 2016).
Cognitively, the brain areas activated in Acheulean handaxe production overlaps those used in speech and language (Putt, 2017). Previous studies on Acheulean stone tool use by Putt (2017) suggest that the neural architecture for spoken language might have been in place around 1.7 Ma.
Acheulean tool-making is a process that incorporates many intermediate goals (Section 3.3) requiring the cooperation of higher-order motor planning, working memory, and auditory feedback mechanisms to attend to information from multiple modalities (Putt et al., 2017). In order to produce the thin bifaces, Archulean knapping employs techniques such as platform faceting and raising the plane of the intersection, taking small strikes at a steep angle on the surface, followed by a large strike at a shallow angle on the opposite surface (Shipton, 2018). Information from multiple modes must be considered as well. For example, knappers must distinguish between different sounds produced during knapping and relate such sounds to the different stages of making a handaxe.
Interestingly, the Acheulian technology coincides with the evolution of a derived middle ear anatomy in Homo that was more attuned to human speech frequencies (Quam et al., 2013). Acheulian knapping may have contributed to the evolution of this auditory function, and perhaps facilitated the evolution of neural connections involved in speech perception. Putt et al., (2017) also suggested that the working memory might have played a role in early Acheulean tool-making, which is also important for the learning of other complex behaviours.
The Mode 3 stone technology represents a major shift in the production of lithic stone tools, although it shares with some elements of tool production with Mode 2. The earliest Mode 3 tools began appearing 500kya. The key difference is that in the production of Mode 3 tools, the core is prepared prior to the striking off of a major flake so as to have greater control over the flake’s thickness and shape. The Levallois technique (Figure 5) is one well-attested technique, where a core is treated and prepared in a much more sophisticated manner than its Acheulean predecessor. Although the cognitive basis for Mode 3 tool-making shares much in common with the production of Mode 2 handaxes, products of Mode 3 tool-making are much more diverse and specialized in function, with a greater emphasis on smaller items.
The technological hypothesis (TH) does not refer to a single specific or empirically testable regarding the evolution of language, rather, it refers to a loose collection of explanations which identify technology development as the critical spark for the linguistic faculties of our hominin ancestors.
Since language emerged parallel to, if not as a consequence of, tool development, TH rejects propositions that our language faculty began as an inherently complex phenomenon. For example, TH is incompatible with the single random mutation put forth by Chomsky & Berwick (2016). One direct consequence is that complex linguistic structures can only be posited to have emerged after Oldowan technology. Oldowan technology was characterized by an extended period of stagnation, which persisted over 700ky and until the emergence of Acheulean tools 1.7mya (Lepre et al. 2011); hence no technological trigger for complex forms of communication should be found in the Oldowan era.
In evolutionary theory, an exaptation is a trait which originally evolved to serve one particular function, but has since then evolved to serve another distinctly unrelated function.
A frequently cited example of an exaptation is the evolution of bird feathers: the discovery of bird feather structures in dinosaurs, which were incapable of flight, prompted biologists to propose that bird feathers were originally designed for attracting mates or for temperature regulation. Somewhere along the way and after the extinction of dinosaurs, they were co-opted by current species for flight.
TH similarly incorporates notions of exaptation by proposing that technologically skilled hominins were selected for certain capabilities, which were then repurposed for communicative, and ultimately linguistic functions. In this sense, language co-opted or even ‘hijacked’ the initial tool-making purposes of these technological capabilities.
While most TH proponents identify our language capabilities as an exaptation of stone tool capabilities, there is less consensus on the kind of language that developed. Corballis (1992a, 1992b) suggests that one possibility for the nature of “proto-language” is that it was fundamentally gestural in nature. According to Bickerton (1995), the compositional view of protolanguage hypothesizes that Homo erectus communicated by a protolanguage in which a few words were strung together without syntactic structure. It has long been argued that the origins of language lie in gesture, and it is clear from modern sign languages that speech is merely one particular manifestation of the linguistic capacity (Donald, 1991; Goldwin-Meadow, 1993; Kendon, 1993). Hence, proto-language may have involved all the elements of spoken language but was gestured rather than spoken.
Corballis (1992b) describes a possible gesture-to-speech scenario, arguing that such a transition would have freed the hands. Therefore, this laid the foundations for the eventual technical and artistic developments during the Upper Palaeolithic. While this idea appears to be unfairly dismissive of the technological achievements of the Middle Palaeolithic, Lieberman (1991, 1992) has argued that Neanderthals could only produce a very restrictive range of vocal sounds due to the relative positions of the larynx and windpipe.
Others suggest that gesture may not have emerged before speech, but perhaps in parallel (Steklis & Raleigh, 1979b). Shared neural structures in the left hemisphere (Section 3) provide strong evidence for tool use preceding and giving rise to human speech. However, speech may have evolved not out of an intermediate gestural phase, but in response to rapidly changing environmental pressures. These environmental pressures, in turn, would have led to increased development of these neural structures, laying the foundations for the control needed to articulate speech.
One means of moving the gesture versus speech debate forward is the use of experimental approaches (Section 4). These are particularly useful to infer the exact means our Paleolithic ancestors needed to transmit stone tool technology.
Since the study of tool-making fundamentally treats non-linguistic phenomena as primary evidence, some criteria are necessary to determine the validity of links drawn between these non-linguistic phenomena and the specific properties of language.
A key notion in evolutionary research concerns the distinction between homology and analogy. Consider the case where a property observed in tool-making seems to parallel a specific feature of human language. The challenge is then deciding if these two similar traits can be attributed to a common ancestral or phylogenetic origin (homology), or alternatively, that they lack a common origin, but parallel features have emerged as a result of similar responses to another shared circumstance (analogy).
Evidence demonstrating homologous rather than analogous links between technology and language is of much greater utility because it strengthens the plausibility of TH’s scenario where language and technology co-evolved. Even if particular traits within tool-making and language reveal a similar organization, this only amounts to an analogy. Greenfield (1991) was one of the first to introduce criteria for determining homology, suggesting that a language-technology homology requires: (i) a common neuroanatomical base, and (ii) that this base is specific to language. Subsequently, the homology-analogy distinction is a useful framework to assess the robustness of the various technology-language relationships detailed below:
In 1876, Friedrich Engels pointed out that bipedalism and language were two out of three features most important in human evolution. Much like language, learning to use our hands for fine manipulation is critical in accounting for hominin’s exceptional evolutionary trajectory, since bipedalism would have been nowhere as advantageous otherwise. As anybody who has ever attempted to pick up a pencil with their toes (rather than fingers) will attest to, it is a much easier affair controlling our hands over anywhere else. In fact, one finger roughly corresponds to an entire foot in terms of the motor precision we possess. This specialization of the hands is known as manual praxis, and it has been identified as the ability we rely on when using a pair of scissors, threading needles (Guiard 1987) or completing other non-object manipulation tasks as in all communicative gesture.
Despite the stone tools’ simple appearances, successful stone tool manufacture is a sophisticated affair for which manual praxis is critical. Knapping even the oldest Oldowan tools always involves a transformation from sensory information about the object into how the hand can best grasp the object (Frey et al. 2005) – this is a highly coordinated effort which mobilizes both hands (Ruck 2014):
- The non-dominant hand is used to manipulate the core by looking for a suitable edge angle and thus prepare it for the knapping action.
- The dominant hand makes a blow that is aimed accurately and at a minimum speed, while the non-dominant hand maintains its precise grip on the core.
Within the brain, the inferior frontal gyrus (IFG) is one productive area of study, revealing many documented overlaps between language and tool use. The linguistic significance of the IFG stems from a much older tradition beginning with Broca’s seminal identification of Broca’s area (Figure 9). The study of aphasic patients, who suffer lesions in Broca’s area, has long associated damage in Broca’s area with agrammatic speech. However, recent evidence has also been steadily connecting this with non-linguistic effects, including the inability to perform simple actions involving tool manipulation as well as inability to pantomime tool use (Kroliczak & Frey 2009). The IFG is thus being re-conceived as a ‘supramodal processor’ (Stout & Chaminade 2012) involved in translating abstract sequences into specific motor actions, be it the articulation of speech or the control of a stone core.
The IFG differs from Broca’s area as it extends to regions BA 47 (pars orbitalis) and BA 6 (ventral premotor cortex) of the frontal lobe. These regions are significant because it has been used to characterize gradient functioning within the IFG. The division of work across posterior and anterior IFG reflects increased linguistic complexity (Putt 2016) from phonological to syntactic and semantic domains respectively. A similar gradient has been identified in manual actions, with increased activation of BA 45 and BA 47 (most anterior) during more motorically complex manual actions (Molnar-Szakacs et al. 2006).
The IFG’s connection to tool-making becomes apparent here, because as demonstrated earlier, the evolution of tool-making also reflects increasingly complex degrees of manual praxis. What distinguishes the very first deliberate Oldowan tool manufacture from simple percussion on rocks (striking) then becomes this feature of manual praxis. Stout & Chaminade (2012) reported, in a positron emission tomography study conducted on Oldowan tool knappers, that Oldowan tool manufacture heavily demanded grasp coordination. In turn, areas such as the ventral premotor cortex (BA 6) were activated; this area had also been previously identified by Hagoort (2005) as a plausible member of the left frontal language network. Crucially, the anterior IFG lying on the opposite, abstract end of the gradient, was not recruited. Oldowan tool-making is manipulatively complex and its execution involves finely calibrated and executed motor actions. However, its structural organization is nowhere as complex that the anterior, higher-order functions are mobilized.
Such studies have been productive in isolating the manual praxis of tool manufacture within specific neural substrates. However, conflicting explanations exist as to the linguistic counterpart, broadly diverging along the vocal versus gesture-centric interpretations of TH described earlier. For example, Stout & Chaminade (2012) suggest that above study’s results favour a vocal scenario where ‘the elaboration of a praxic system… supported the enhanced articulatory control required for speech’, might indicate that it is the precursor to the fine distinctions in human language’s feature-based phonological systems.
However, the same neural evidence of manual praxis in tool-making may also support a gestural origin of language. Fine motor control developed for tool manufacture may have been repurposed for intentional gestural communication, perhaps beginning with transmission through imitation.
Both interpretations converge in establishing tool-making’s motor demands as the prerequisites for intentional communication, be it vocal or gestural. The neural foundations Oldowan tool-making laid are fundamentally compatible with vocal or gestural outcomes; the question becomes whether voice or gesture came first (or perhaps simultaneously) (Vaesen 2012). One way to further advance this debate involves new experimental setups, which study transmission specifically aim to investigate if one mode may be more critical for passing on Oldowan technology over the other (see Section 4).
One of the first attempts to explicitly connect linguistic hierarchies with the general hierarchies behind object manipulation and combination was made by Greenfield (1991). Observing infants’ assembly of Russian dolls, she identified subassembly as a complex manipulative strategy where two objects are first combined before the combined object is subsequently placed into a larger one.
Subassembly, accessible by only older children, then demonstrates increasing hierarchical complexity in ontogenetic development, which is in turn reflective of manual intelligence. Likewise, the first phrases formed by children (want [more cracker]) demonstrate elementary nesting and a greater complexity over the telegraphic speech (more cracker) observed earlier. Citing Grossman (1980)’s study of aphasics and their ability to replicate hierarchically organized tree structures (Figure 10) Greenfield further located a common neural basis, first asserting that these two uses of hierarchical processing do not modularly originate from a region like Broca’s area per se. Rather, they reflect the ontogenetic development of neural circuits specialized for manual sequencing and grammar respectively.
While researchers today are more wary of accepting infant speech as a suitable parallel to the first languages (Bickerton 2003), both the hierarchical ordering of language and the goal-driven, subordinated nature of object manipulation remain widely accepted. Moreover, hierarchical ordering is not only a matter of syntax; phonological rules for combining sounds also constitute linguistic hierarchies, for example syllable structure subordinates combinations of vowels and consonants (Lieberman 1984). Such assumptions are the bases which research into tool-making, cognition and language have used to present scenarios for TH.
Research into the neurological nature of late Acheulean tool-making is particularly promising – the emergence of Acheulean tools mark the next major cognitive shift related to lithic technology, some 700ky after the beginnings of Oldowan tool manufacture (Rusk 2014). Late Acheulean technology can be distinguished from its Oldowan predecessors in the following aspects (Stout et al. 2008):
- Increased motor skills and awareness of stone morphology
- Elaborate planning and consideration of long-term objectives
- Greater diversity of implements designed to achieve specific knapping steps
For example, creating the thin, symmetrical bifacial handaxes begins with the removal of thin-flakes (bifacial edging). Before these, Late Acheulean toolmakers would prepare the striking surface through small-scale chippings (elementary percussion). What is important here is to coordinate multiple lower-layer actions, in advance preparation for the specific goal of bifacial edging. To proceed with bifacial edging right after a single preparation of the striking surface, as in linear sequences, runs the risk of making a tool with an inconsistent (and sometimes unusable) thickness. Longer-range dependencies between isolated actions thus reveal a nested organization, from which we can posit neural structures facilitating this.
In a study by Stout et al. (2011), late Acheulean tool-making produced a different set of neural activations from its Oldowan counterpart. Additional areas of the prefrontal cortex, including the pars triangularis (BA 45), participated in late Acheulean tool-making but not in Oldowan knapping conditions. Generally, increased prefrontal activation was attributed to the specific division of Acheulean tool-making labour in a layered manner. These therefore suggest the separation of tool-making sequences into preparatory actions, intentions and outcomes might demand more neural processing, which in turn overlap with layered language processing.
While classical thinking has long associated language and tool use with the left hemisphere, recent research is paying more attention to the right hemisphere’s contributions. Discourse-level processing, prosody and connotative meaning (Bookheimer 2002) are such instances of language-related tasks dominated by right hemisphere processes. Alongside this, new research is now uncovering the role of the right hemisphere in sequentially developing and coordinating of action sequences which are important for object manipulation.
Lateralization of function is a term used to describe work (linguistic and non-linguistic) being distributed across both hemispheres. One intriguing application of lateralization is the bimanual (i.e. use of both hands) nature of stone knapping discussed earlier. The non-dominant hand (typically the left) tends to not receive scholarly focus; nevertheless its role carries direct implications for the right hemisphere (which delivers input for the left hand). Particularly, significant correlations between non-dominant hand participation and the level of knapping expertise have been reported (Stout et al. 2008). While novice knappers are more preoccupied with the rapid striking actions on the right hand, expert knappers with substantial practice factor in precise and forceful left-hand grips and thus are able to capitalize on the RH-left hand system.
In Late Acheulean tool-making, achieving goal-directed nature of intermediate steps (Section 3.3) also relies on functions found in the right hemisphere. To achieve the thin handaxe shape, knappers must reject immediately desirable options such as continuing to thin one handaxe edge (at the expense of the other edge). Neurally, an inhibiting mechanism would motivate these, something which has been identified in the right ventral premotor frontal cortex (Aron et al. 2004). The highly segmented process of Late Acheulean handaxe manufacture thus mobilizes both left and right hemispheres.
Implications for Research
As a whole, discovering lateralized functions provides two possibilities to further investigate TH’s language-technology co-evolution scenario. The first concerns the emergence of functional descriptions of the human language faculty, in general linguistic theory.
Functionalist linguistics originates from the work of Halliday’s Systemic Functional Linguistics, and broadly assert that language is a means of interpersonal interaction rather than mere representation (Painter 2004). Accordingly, neurological emphasis should be placed beyond the grammar-constructing facilities typical of the left hemisphere, and on the interactional, social and affective dimensions of language usually processed by the right hemisphere (Rose 2006). These experimental archaeology results, by suggesting tool-making’s dependence on lateralized functions, thus provide a homologue to the same structures modern language uses to achieve its functional aims. Thus, it contributes to the possibly technological origins behind language evolution.
Additionally, unearthing a relationship between lateralization and level of expertise makes ethnographic approaches particularly attractive, as such approaches examine the specific process of teaching and transmission. Teaching and transmission are significant because they are a plausible means with which hominids took full advantage of and elaborated upon tool-making’s neural correlates. In other words, teaching is the potentially critical scaffold supporting the exaptation of technology’s neural functions by language. Section 5.3 introduces one such case study by Stout (2002), which makes use of ethnography to infer possible social behaviors governing stone tool production.
According to TH, cultural transmission of tool making skills might have been involved in the manufacture of simple Oldowan tools but there was an unclear relation between speech evolution and stone tool making (Cataldo et al., 2018). Based on the premise that tool making is a hands-on activity, instruction by gesture would seem to be a preferred mode of teaching aid over verbal instructions.
The study by Cataldo (2018) premised on the hypothesis whether verbal instruction is most effective in skills transmission. The experiment investigates the effectiveness of speech as a teaching aid during experimental transmission of the Oldowan-style tool making by comparing the flintknapping performance of subjects. The transmission efficiency of tool-making skills was compared under the four conditions: gesture group, speech group, full language group as well as a no instruction group. The full language group (gesture plus speech) would represent the current mode of human communication and a no-instruction group was included to serve as a control. Flake measurements were recorded and statistical analyses were performed.
The experiment revealed significant differences across the flintknapping performance in the four communication groups. The speech group (purple) performed significantly worse than the full language group and no difference from the control group that received no instruction. Despite the richness of the linguistic resource, speech appeared to be a poorer instruction aid in comparison to gesture. The results did not support the hypothesis, and in fact, suggest the otherwise. Speech was not an effective form of cultural transmission and lithic production might not be the triggering factor that caused speech evolution.
The experiment revealed that speech was not the most effective way in cultural transmission of Oldowan-style tool-making skills as no distinction was found between the performance of subjects in the non-instruction group and the group that received verbal instructions. This might suggest that the evolution of gesture preceded speech as speech could not have evolved in earlier tool-making process given its ineffectiveness.
Questionnaires revealed that the participants in the speech group (purple) ranked the lowest amongst the other instructional groups – gesture (orange) and full language (red). Speech was rated as a less satisfactory than gesture as a teaching aid, indicating that speech is not the most desirable form of instruction due to its inefficiency in the transmission of skills.
Therefore, judging the satisfaction of the full language group, the addition of verbal instruction with gestures did not increase the flintknapping performance or satisfaction (in comparison to the gesture group). Hence, an alternate view can be formed against TH. The experiment revealed that gestures could have possibly preceded speech to facilitate tool making in early hominins, while speech may have emerged in parallel with gesture at a later stage. This might be the explanation to the possible distinct communicative functions and how the full language evolved to be the default mode of communication in Homo sapiens.
Under the view that language and stone tool making are related by their shared social contexts, such a relationship implies that language and tool-use capabilities provided a complementary context for coevolution (Stout, 2010). Scientists, such as Kevin Laland (2017), who take on such a view argue that linguistic behaviour and Palaeolithic technologies (such as stone tools) should depend on similar cultural learning mechanisms for their reproduction. For Laland (2017), such a mechanism primarily involves teaching.
To evaluate between competing theories of the original function of language, work by Szamado & Szathmary (2006) and Bickerton (2009) have established six criteria to judge these theories and their merits relative to each other. Laland (2017) adds one further criterion of cultural niche construction, to produce seven benchmarks to evaluate alternative explanations for the original adaptive advantage of early language.
These seven constraints are (Laland 2017):
- The theory must account for the honesty of early language.
- The theory should account for the cooperativeness of early language.
- The theory should explain how early languages could have been adaptive from the outset.
- The theory should explain symbol grounding (i.e., the means by which early words acquired their meanings).
- The theory should explain the generality of language
- The theory should account for the uniqueness of human language
- The theory should explain why communication needed to be learned
How teaching satisfies the seven constraints
According to Laland (2017), teaching satisfies these seven criteria. First, if language evolved to teach relatives, then we would expect it to accurately represent information such that it is honest, since inaccurate instruction would be a waste of the tutor’s time and effort. For example, if words are easy and cost-free to produce (i.e. talk is “cheap”), then anyone is unlikely to believe what others say and there would be no incentive to learn from others if one cannot be confident that the words of others convey an accurate message. Second, if language evolved to teach solutions to problems faced by our ancestors, then language had emerged in contexts that required cooperation. Third, if cooperative traits are adaptive from the outset, then so is teaching, provided that the traits being taught are adaptive and require cooperation to function. Subsequently, teaching could be extended to support other cooperative (and adaptive) processes, such as trade and hunting. Fourth, language’s beginnings in a teaching context requires the ability to make and convey meaning. Simple attention-grabbing commands, such as pointing and gesturing, have been proven to help facilitate social learning (Tomasello, 1999). Fifth, teaching can explain language’s power to generalize information since teaching can be applied to many skills that are difficult to learn, such as foraging procedures, food-processing methods, and hunting skills (Stringer and Andrews, 2005). Sixth, the uniqueness criterion is also met because no nonhuman animals evolved language to the point that it can be used in extensive teaching—only in humans did language, teaching and cumulative culture coevolve. Last but not least, not only do cultural practices—like tool-making—needed to be learned by the next generation, it is also subject to variation. In the case of stone tools, we are talking about guided variation which entails improvements to the stone tools (e.g. reducing costs and increasing its benefits).
Nearly 10,000 years ago, all human societies had made and used stone tools in one form or another, but in modern times the advance of more complex technologies has left few remnants of such stone tools. However, anthropological work by Toth et al (1992) and Stout (2002) on contemporary Stone Age craftsmen—particularly on the stone axe makers of the village of Langda in Indonesian Irian Jaya—suggest that stone axe production is a social phenomenon, defined by mentor-apprenticeship relations, social norms, and mythic significances.
Unlike Mode 1 and 2 stone tools, these stone axes produced by the people of Irian Jaya are more complex, and can be considered, under Clark’s (1969) to be Mode 5 stone tools, which are marked by the integration of other materials (such as wood or bone) into the stone tool. Further, the stone axe makers of Irian Jaya requires communication which is involved not only in naming the stone axe, but in teaching and instruction. For instance, Stout (2002) noted that the Irian Jaya craftsmen have subdivided the stone axe heads into many distinct parts (Figure 14). Each stone axe head also receives an ancestor name and a name to signify its “place of origin” (Stout 2002). This means that stone axe making in Langda embodies a large number of terminology and other technical concepts to learn, and so the stone axe production in Langda requires that experienced stone axe specialists pass on their knowledge to their apprentices. Hence, the manufacture of stone axes is considered a social phenomenon where the social structure, rules, and norms provide a framework for the process of learning the art of stone tool making (Toth et al 1992; Stout 2002).
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