(adapted from section 2, "Respiratory, postural and spatio-kinetic motor stabilization, internal models, top-down timed motor coordination and expanded cerebello-cerebral circuitry: a review".)
Paleoanthropologically knapping dates back to 2.5 mya (Semaw, Renne et al. 1997), though the example examined below that allows the reconstruction of the manufacture of such stone tools is young at ca. 2.34 mya (Delagnesa and Roche 2005). In knapping, an individual holds a core stone in the nondominant hand that they strike with a hammerstone held in their dominant one to produce flakes and a modified core (fig. 1.). Reconstruction of surviving flakes and cores shows (Delagnesa and Roche 2005) that this was done in terms of an overall knapping action plan (see fig. 2.). There has also been recent studies upon contemporary people as to the motor and the neurological processes involved (Marzke, Toth et al. 1998; Stout, Toth et al. 2000; Marzke 2006; Roux and David 2006; Stout, Toth et al. 2008).
Knapping is illustrated using
the reconstructed knapping stones found at
Lokalalei 2C (
Need for impact stabilization
The success of knapping, first, has been found to depend crucially upon the hammerstone hitting an appropriately stabilized core. Electromyography research shows that there needs to be a millisecond timed scheduled stiffness adjustment to stabilize the hand holding the core that prepares the hand for the otherwise perturbing force of the hammerstone’s impact (Marzke, Toth et al. 1998) (Fig 2 A and B). At the moment of impact, a coordinated loosening of hand grasp is also needed to protect the hand that “is brief enough that it does not allow displacement of the tool [hammerstone]” (Marzke 2006: p. 248). If this timed anticipatory stiffness is prepared/relaxed incorrectly, not only could the hand be injured but also the core will not be stabilized when hit, and as a result—a defective flake or none—will be produced.
Need for overall manufacture plan
Second, knapping critically depends upon the executive formulation of an overall manufacture plan (Roux and David 2006) and so abstract projectuality (Amati and Shallice 2007) (see fig. 2). That is (i) having before commencement, a notion of the final product, (ii) an ability to treat each knap as intermediate stage to such a product, and (iii) capacity to modify the force, position and manner of each knap in terms of the changing debitage of the core to achieve the intended product (the knaps in fig. 2 A and C). Since cores can need secondary modification to enable knapping, and knaps can fracture the core in nonintended ways, it also requires, (iv) skill to identify errors and adjust new knaps to correct for them within the overall product making plan (as in fig. 2 B). Thus, the motor faculty of knapping requires cognitive specializations such as sustained attention, error detection, short- and long-term memory, action set shifting, and the hierarchization of ultimate and substage motor goals. For instance, the sequence illustrated in fig. 2 upon the refitted group core stone 2 found at Lokalalei 2C (Delagnesa and Roche 2005) could not have been done—as they were—2.34 myr by a brain that lacked the ability to organize its motor actions by such executive-type functions.
is illustrated using the reconstructed knapping stones found at
Lokalalei 2C (
Fig 2. The refitting group core stone 2 was knapped into nine flakes. This happened in three stages. (A) five flakes, 1, 2, 3, 4 and 5 were knapped. (B) The knapper reshaped the core by removing a protrusion on the other side of the stone, ( 6 and 7 flakes) so creating an edge that was used (C) to make two more knaps, 8 and 9 (Delagnesa and Roche 2005).
Stone tool making in nonhuman primates
No animal apart from Homo can anticipatorily adapt in a time accurate manner the stiffness in the core holding hand in regard to the hammerstone’s impact force, nor engage in the executive function-like modification of motor control guided by the use of an action plan. Attempts to tutor the bonobos, to knap, for example, have been without success (Schick, Toth et al. 1999). This is not because they cannot use motor control to modify stones, nor because they lack the idea to make tools. Bonobos, such as Kanzi and Panbanisha are able to grasp the idea of stone modification—but they are only able to engage in this (even when shown how to properly knap) through (i) an initial uncontrolled “thrust percussive” method of throwing and braking stones on the ground, and (ii) later with more experience by a stabilization using the ground (Davidson and McGrew 2005, see fig. 3 on page 800 in which Panbanisha holds a stone against the ground with her left hand). The former method of stone modification is also found in wild chimpanzees (Boesch and Boesch 1990; Mercader, Barton et al. 2007). Both types of “thrust percussive” tool making is characterized by not needing the anticipatory coordination of the two hands, nor highly complex action planning.
Byrne (2005, p. 166) has noted “it may be very well that no living great ape is capable of learning the motor skill involved in aiming a powerful and accurate blow at an object held in the other hand: it is the combination that may be beyond them, because there is no doubt that living apes have both great limb power and delicate precision, in separate contexts”. Here this inability is identified as the lack of a capacity to coordinate energetic force in one hand, while the other engages in accurately timed adjusted stabilization against its quick forceful impact.
Reasons for human uniqueness
What has limited nonhuman primates from possessing the above noted motor dexterity needed to knap? One explanation might be human specific hand anatomy: (i) human fingers have optimal length to work with the human thumb (Napier 1960), (ii) a stable and extended pulp region exists on the human thumb for aiding firm precision pinches and object holding (Susman 1994), and (iii) human wrist bones allow for a power grip (Marzke, Wullstein et al. 1992). These musculoskeletal adaptations while optimizing the human hand for knapping, however, are not responsible for creating the motor capacity to anticipatorily stiffen effectively the core holding hand. In addition, human hand anatomy contrary to “long standing” assumptions is not completely unique to the human species as: “each of the features forming the human morphological pattern appears variably in at least one or more nonhuman primate species” (Shrewsbury, Marzke et al. 2003: p. 41). Nor is refined finger control peculiar to humans: young chimpanzees, for example, appear to have more individual control over their fingers than humans (Landsmeer 1993: p. 330).
Novelty of human motor control and knapping
The human specific motor control that underlies knapping is critically dependent upon its capacity to be organized within accurate time-based internal models.
˜ First, these internal models allow the refined time prediction of the kinematics and kinetics of the musculoskeletal system of the upper body so that motor control can in a non-stereotypical timed manner stabilize “stiffness” in one motor element (the core holding arm) in regard to another independent motor element (the striking hammerstone) (Marzke, Toth et al. 1998).
˜ Second, such internal models allow the predictions needed to modify each temporally successive knap/stabilization in terms of the requirements of a complex action plan.
˜ Third, a factor not discussed in depth due to lack of space is aiding the generation of the intersegmental dynamic coordination of forces in the dominant hammerstone holding arm (Sainburg and Kalakanis 2000; Sainburg 2002; Wang and Sainburg 2007) that is needed to accurately target the hammerstone and its strike impact on the core stone. Though the emphasize in this paper is upon stabilization, internal models also play a significant role in enhancing the voluntary capacities of musculoskeletal action.
In regard to motor ability, hand anatomy is secondary to the primacy of such human specific motor competence.
Nonhuman specific motor stabilization and its limits
Innovations to provide this motor control are needed if humans are to knap since from a biological perspective, the submovement stabilization of the core holding hand cannot be timed in a task-determined manner by the already existing stabling processes found in nonhuman vertebrates such as preflexes and spinal adjustment reflexes.
Preflexes and spinal adjustment reflexes embedded locally in the body’s musculature underlie the motor stabilization of movements in nonhuman animals. Thet are unlike the anticipatory motor adjustment of proficiency since these can only be acquired through prolonged practice when mastering motor expertise in a particular task. Instead, preflexes and spinal adjustment reflexes are evolutionarily precustomized to provide specific stabilizations for (i) frequently encountered, or (ii) survival critical perturbations, that are faced during the execution of (iii) an evolutionarily limited repertoire of highly adapted stereotypical motor movements. This makes such musculoskeletal embedded stabilization mechanisms functional only for a few highly evolved motor employments of the musculoskeletal system, not task-determined motor skills that require prolonged mastery.
Such lower-level musculoskeletal stabilization mechanisms are not able to offer the constantly modified time-based feedforward motor stabilization needed for knapping (and more generally, other practice-dependent forms of expert dexterity). First, when the two stones strike, it requires a task specific (not stereotypic) timed anticipatory coordination between the two hands that takes many years of practice and learning to master. Second, this anticipatory and timed adjustment must be situationally highly tailored as it is successively modified for each knap in terms of action goals (given the core’s changing shape and intended core debitage). Third, knapping illustrates only one of the many different kinds of hand stabilizations that humans acquired through prolonged practice as they make and use diverse artifacts. Other kinds of motor stabilization with different task-determined time accurate coordination, for example, were needed by early Homo for the expert usage and making of tools that due to perishable materials—wood, bone, skin—are not preserved.
Knapping and internal models
The anticipatory stabilization needed for knapping reflects the existence in humans of the evolution of an expanded neurobiological capacity for (1) making time-organized internal predictive models of the kinetics, kinematics and action organization, and (2) the integration of them needed to engage in complex and practiced tasks. The increased ability in Homo to practice and refine such models allows Homo to engage in motor control that can for each occasion be exact and proficient in its anticipatory adjustments. As a result, motor control can stabilize effectively its actions to produce constant motor effects in spite of changing circumstances such as when a hammerstone hits a core stone of changing weight and shape. This top-down motor timed adaptation based upon internal models that characterizes proficient human motor control underlies not only for the expert and dexterous use of the hands (as in knapping) but also human bipedality and human song/speech vocalization.
Although not discussed in a paleoanthropological context, internal model processes are well researched, especially in regard to their implementation in the circuits between the cerebellum and the cerebral cortex (parietal, motor, premotor and prefrontal areas) (Kawato and Gomi 1992; Wolpert and Kawato 1998; Doya 1999; Wolpert, Doya et al. 2003; Ito 2006; Ito 2008). Such top-down internal model based motor adaptation is paleoanthropologically important because these circuits in humans are uniquely expanded compared to those in nonhuman primates (MacLeod, Zilles et al. 2003), particularly in regard to the prefrontal cortex (Ramnani 2006; Ramnani, Behrens et al. 2006), and in preliminary evidence that H. sapiens sapiens under went specific cerebellum enlargement at the cost of overall cerebral hemisphere size (Weaver 2005). They are, moreover, known from studies upon cerebellar agenesis to be involved in dexterity (Nowak, Timmann et al. 2007), and from neuroimaging to be activated during knapping (Stout, Toth et al. 2000; Stout, Toth et al. 2008).
Further, humans have a prolonged nonadult stage in development, “adolescence” in which the relationship between gray matter and its connective white matter in the brain changes (Lenroot and Giedd 2006). These changes can be specified linked to hand motor skill mastery. Learning to play the violin to an expert level thickens the connections between the two cerebral hemispheres (Schlaug, Jancke et al. 1995), and acts to increase nearly threefold the area devoted to the left hand (Elbert, Pantev et al. 1995). In keyboard players, there is an enlargement of the motor cortex that is proportional to the number of years that they have been practicing their instrument (Amunts, Schlaug et al. 1997). Musicians also have slightly expanded cerebellums—a difference that increases with the number of years that they have been practicing (Hutchinson, Lee et al. 2003). Consistent with these motor skill mastery changes in the brain linking to enhanced internal models, there is increased accuracy during adolescent in the internal models of hand movements, as well as improved fine-motor skills (Choudhury, Charman et al. 2007).
Knapping, human uniqueness, and motor control summary
Knapping illustrates four things.
First, that humans can proficiently make actions that other animals cannot.
Second, that though anatomy helps (the human hand is optimized for a power grip), this is not the explanation: what is critical is the timed adjustment and coordination of different movement elements made possible by time-exact internal models, a form of motor stabilization that cannot be done by the locally embedded preflexes and spinal adjustment reflexes that underlie the motor control of nonhuman animals. These models, moreover, provide the basis for the temporal and sequential goal hierarchization that allows for different body movements to be organized as part of a task action plan.
Third, that the opportunity to base motor control upon such internal models depends upon something unique to Homo—expanded cerebello-cerebral cortex circuits including those with prefrontal areas, and also the existence of a prolonged nonadult maturation stage needed to refine and integrate them together as expert motor skills (Skoyles 2008).
Fourth, knapping, it can be added, is of great adaptive advantage (it provides sharp cutting implements essential for the extraction of high-energy foods such as bone marrow) (Dominguez-Rodrigo, Pickering et al. 2005). This makes the origin of the internal model based motor stabilization central to any understanding of human origins. This is particularly so—if, as proposed below—that such internal models also underlie the biological distinctive character and utility of human bipedality and human vocalization
Knapping, thus, shows that there is an evolutionary link between changes in motor stabilization mechanisms (the shift from preflexes and spinal adjustment reflexes to accurately task timed anticipatory adjustments), and the evolution of what is peculiar to humans. Further, that this shift, identifies processes that are scientifically already well experimentally investigated.
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