Nests created by long-tailed tits ( Aegithalos caudatus ), Great Britain. Long- tailed tits build their nests from thousands of pieces of lichen, moss, and spiderweb, as well as numerous feathers. The nests are thought to be a result of innate instructions and individual learning. Photograph A shows a nest under construction, while photograph B shows a completed nest. (Photograph A: Alan Shearman/Wikimedia Commons; Photograph B: nottsexminer/Wikimedia Commons). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] 

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... of language, tool-making, population-level functional lateraliza- tion and association cortex expansion in human evolution.” 99 We think this line of enquiry is promising with respect to a possible genetic component in handaxe design, but suspect that the capacities in question may well turn out to be much more specific than Stout et al. suggest. Rele- vant points of comparison between tool-making and speech are manual control and articulatory control, action sequences and syntax, as well as mental templates and semantics, with bird song as a promising third relevant field. 100 An obvious concern about the genetic transmission hypothesis is whether the production of an object as complex as an Acheulean handaxe could really be under genetic control. We contend that the idea is not implausible if handaxe production is compared to some complex bird behaviors. We begin by discussing bird song and bird tool-use as “soft” options that involve a combination of genetic influence and social learning. We then discuss a “hard” option — structure building by birds, in which strong genetic determination interacts with the availability of raw material and local environmental conditions but not with much, if any, cultural learning. Bird song, a well-studied behavior, is strongly influenced by genetic predispositions and implemented by dis- crete, well-defined neural circuits. 101 Songbirds, hummingbirds, and par- rots have fixed, species-wide basic songs from which individuals and regional subpopulations develop their own varieties. Auditory-guided vocal motor learning is important, for when members of these taxa are raised in isolation their songs remain simple. The genetically controlled early song of zebra finches ( Taenio- pygia guttata ), for instance, is plastic. The young need several months of social learning to perfect their individual, adult song. 102 It appears that in some bird species, tool use also combines fixed species-wide basic motor behaviors with cultural learning. It has been found, for example, that hand-raised Caledonian crows ( Corvus monedu- loides ) reliably develop leaf-tool manufacture without ever having observed it in others, but never develop the sophisticated behaviors found in wild populations where individuals are exposed to models and competitors. A study by Kenward and coworkers 103 illustrates this. By hand-rearing several crows in isolation, they showed that the tool behavior of isolated individuals was so similar to that in wild ones that it was difficult to avoid the con- clusion that the behavior is innate. They argue against “the extreme pos- sibilities that tool-use depends entirely on social inputs (i.e., is sus- tained exclusively by cultural transmission and thus does not reflect a dedicated evolved adaptation), and that it has a purely individual, insight-based origin.” 103:1340 If that were the case, Kenward and colleagues 103:1340 contend, we would not see “inherited action patterns that must have evolved through selection and that are crucial in sus- taining tool-oriented behavior in adult crows.” The same point has been made with respect to the tool- use behavior of hyacinth macaws, Egyptian vultures, and woodpecker finches, the only other bird species that is known to habitually use stick tools in the wild. 104–106 Thus, if modeled on bird song and bird tool-use, the production of Acheulean handaxes would have involved both genetic transmission and social learning. Raw material selection, the manufacturing process, and basic design principles would have been under genetic control, but fine-tuned through social learning. The latter would have been depend- ent on the presence of role models during sensitive periods, but would not necessarily have involved cultural group-specific templates or explicit instruction. The combination of genetic transmission and social learning is predicted to produce uni- formity of overall design (due to the fixed component) and slight local variance (due to the process of socialization), which is the pattern observed with Acheulean handaxes. It appears that one important reason why archeologists tend to think that the behaviors involved in the production of handaxes were socially learned is that the handaxe chaine op eratoire required multiple decisions at multiple stages, from the selection of the size, shape, and quality of raw material to the final retouch. 15 Again, this argument seems reasonable on its face. However, it is not as secure as it first appears. The reason for this is that the construction of nests and other struc- tures by birds often involves long hier- archical sequences even though, as is generally agreed, the required behaviors do not involve much, if any, cultural learning. Eurasian long-tailed tits ( Aegithalos caudatus ) exemplify the potential complexity of nest building behavior. These birds create intricate spherical nests from thousands of pieces of lichen, moss, and spiderweb, as well as numerous feathers (Fig. 3). The final product is accomplished through a combination of innate instructions and individual learning. 107,108 The former specifies materials and methods and a limited repertoire of repetitive, stereotyped actions. There is a chain of stimuli and responses that presup- poses construction rules and local insight, but not necessarily complex, overall planning, conscious decision- making, or a mental image of the ulti- mate goal. The nest is an emergent property of the stimulus-response chain. The bower-birds (Ptilonorhynchi- dae) of New Guinea provide another excellent set of examples. These birds create hut-like shelters and decorate their interiors and the area immedi- ately in front of them with brightly colored objects (Fig. 4). These “bowers” are individually and ecologi- cally variable as a result of individual learning and, possibly, some cultural learning. It is thought, however, that the stability of their basic form indicates that genes underlie their design and construction. 109 As with handaxes, the bowers show a mix of variation and uni- formity. Individual bowers display variation, but share a general form within a species. The same can be said for handaxes: Beneath their variability, general themes are present in all of them. Structure building by birds is also heuristically relevant when trying to clarify exactly what was under genetic control in handaxe manufacture. It probably was not just a simple target form, but rather a predisposition toward the basic behavioral routines involved, such as invasive bifacial reduction while realizing cutting edges in the secant plane, working from the tip down, and keeping symmetry. These routines would have operated in combination with causal understanding, manipulative skill, and intuitive (“folk”) physics. 110 Although Gowlett 111:217 thinks of handaxes as cultural objects, our proposal is consistent with his approach when he argues against the notion of a template as being “hard and fast” and instead talks of “instruction sets [from which] a computer could gener- ate the form of a biface mindlessly.” Unfortunately, genetic evolution usually happens too slowly for researchers to adequately study changes in genetically controlled behaviors such as nest building by birds. The time needed to observe changes in evolutionary trends extends far beyond the current length of study of such phenomena. Direct comparisons between the handaxe and animal behaviors such as birds’ tool use or structure building are therefore difficult. Nevertheless, mounting evidence points to a central role for genes in maintaining aspects of animal technology. We suggest that the same should be considered for Acheulean handaxes. It is important, under the genetic transmission hypothesis, to clarify the nature of the other large cutting tools in the Acheulean industry and the handaxe-like bifaces in the industries that come after the Acheulean. Were these tools also partly genetically determined or were they solely cultural phenomena? McElreath’s 52 discovery that in his study-groups different mechanisms underpinned attitudes toward friends versus kin, respect for elders, and belief in witchcraft clearly indicates that there is no reason to expect all Acheulean tools to involve the same transmission mechanisms. Nevertheless, we are of the opinion that the applicability of the genetic transmission hypothesis should be investigated in connection with the other large cutting tools of the Acheulean, among them cleavers, picks, trihedrals, and unifaces. Cleavers appear to be a particularly good candidate for another large cutting tool that was under at least partial genetic control. Cleavers are bifacially reduced pieces with a straight cutting edge perpendicular to the long axis of the piece. They were mostly made on flake blanks struck from bifacially prepared, usually more or less tortoise-shaped cores. The clea- ver has been argued to be a subcate- gory of the Acheulean handaxe. If this is the case, then parsimony suggests they could have been under partial genetic control as well. We suspect the situation is different with respect to the handaxe-like tools in the industries that came after the Acheulean. As we explained earlier, in Europe the Acheulean was replaced by the industries of the Middle Paleolithic between 300 and 200 Ka. Several European Late Middle Paleolithic industries contain bifacial tools, including the French Mousterian of Acheulean Tradition and the Middle European Keilmessergruppen, also called the Mico- quian. 112,113 The tools in question resemble Acheulean handaxes, but we think it is unlikely that they were under genetic control. There are two reasons for this. First, Late Middle Paleolithic bifaces are actually only superficially similar to Acheulean handaxes. A recent morphometric comparison of Acheulean and Late Middle Palaeolithic handaxes from Western Europe done by Iovita and McPherron 12:69 demonstrated that Acheulean handaxes show an ...