'Snakes and ladders' in paleoanthropology: From cognitive surprise to skillfulness a million years ago.

A paradigmatic account may suffice to explain behavioral evolution in early Homo. We propose a parsimonious account that (1) could explain a particular, frequently-encountered, archeological outcome of behavior in early Homo - namely, the fashioning of a Paleolithic stone 'handaxe' - from a biological theoretic perspective informed by the free energy principle (FEP); and that (2) regards instances of the outcome as postdictive or retrodictive, circumstantial corroboration. Our proposal considers humankind evolving as a self-organizing biological ecosystem at a geological time-scale. We offer a narrative treatment of this self-organization in terms of the FEP. Specifically, we indicate how 'cognitive surprises' could underwrite an evolving propensity in early Homo to express sporadic unorthodox or anomalous behavior. This co-evolutionary propensity has left us a legacy of Paleolithic artifacts that is reminiscent of a 'snakes and ladders' board game of appearances, disappearances, and reappearances of particular archeological traces of Paleolithic behavior. When detected in the Early and Middle Pleistocene record, anthropologists and archeologists often imagine evidence of unusual or novel behavior in terms of early humankind ascending the rungs of a figurative phylogenetic 'ladder' - as if these corresponded to progressive evolution of cognitive abilities that enabled incremental achievements of increasingly innovative technical prowess, culminating in the cognitive ascendancy of Homo sapiens. The conjecture overlooks a plausible likelihood that behavior by an individual who was atypical among her conspecifics could have been disregarded in a community of Hominina (for definition see Appendix 1) that failed to recognize, imagine, or articulate potential advantages of adopting hitherto unorthodox behavior. Such failure, as well as diverse fortuitous demographic accidents, would cause exceptional personal behavior to be ignored and hence unremembered. It could disappear by a pitfall, down a 'snake', as it were, in the figurative evolutionary board game; thereby causing a discontinuity in the evolution of human behavior that presents like an evolutionary puzzle. The puzzle discomforts some paleoanthropologists trained in the natural and life sciences. They often dismiss it, explaining it away with such self-justifying conjectures as that, maybe, separate paleospecies of Homo differentially possessed different cognitive abilities, which, supposedly, could account for the presence or absence in the Pleistocene archeological record of traces of this or that behavioral outcome or skill. We argue that an alternative perspective - that inherits from the FEP and an individual's 'active inference' about its surroundings and of its own responses - affords a prosaic, deflationary, and parsimonious way to account for appearances, disappearances, and reappearances of particular behavioral outcomes and skills of early humankind.

Vertical string-pulling in green jays (Cyanocorax yncas).

The cognition of green jays (Cyanocorax yncas), a non Corvus corvid species, was investigated by using the string-pulling paradigm. Five adult green jays performed a vertical string-pulling task in which they had to retrieve a worm attached to the end of a vertical hanging string while sitting on their perch. In the first experiment, three of the subjects managed to retrieve the worm by pulling on the string with their beaks and stepping on the resulting loop, and thereafter repeating this sequence until the worm was accessible. When subjects were given a choice between two strings in subsequent experiments 2-4, they chose at random between the string connected to the worm and the one connected to a slice of a wooden dowel. In experiment 5, subjects that had failed the previous discrimination series were able, nevertheless, to solve a more stringent vertical string array in which they had to pull up the whole length of the string without any visual access to the worm at the end. We discuss green jays' performance in comparison with other corvid species in which cognition has been more extensively investigated.

Tool choice on the basis of rigidity in capuchin monkeys.

Wild capuchin monkeys select stone tools to crack open nuts on the basis of their weight and friability, two non-visual functional properties. Here, we investigated whether they would select new stick-like tools on the basis of their rigidity. In Experiment 1, subjects faced an out-of-reach reward and a choice of three unfamiliar tools differing in color, diameter, material, and rigidity. In order to retrieve the reward, capuchins needed to select the rigid tool exemplar. Capuchins gathered information regarding tools' pliability either by (1) manipulating the tools themselves (manipulation condition), (2) observing a human demonstrator repeatedly bending the tools (observation condition), or (3) seeing the tools placed on a platform without any manipulation taking place (visual static condition). Subjects selected the rigid tool above chance levels in both the manipulation and observation conditions, but not in the visual static condition. In Experiment 2, subjects needed to select and use a flexible tool to access a liquid reward (as opposed to the rigid tool, as in previous experiment). Again, capuchins selected above chance levels the appropriate tool (i.e., flexible), thus demonstrating a good appreciation of the relation between the tool properties and the task requirements.

Spontaneous use of tools as straws in great apes.

Great apes can use multiple tools to extract food embedded in substrates and can invent new ways to exploit those resources. We tested five bonobos, five chimpanzees, and six orangutans in a task in which they had to use (and modify) a tool as a straw to drink the juice located inside a container. Experiment 1 showed that four orangutans and one chimpanzee invented the use of a piece of electric cable to get the juice. Experiment 2 investigated whether subjects could transform a non-functional hose into a functional one by removing blockages that impeded the free flow of juice. Orangutans outperformed chimpanzees and bonobos by differentially removing those blockages that prevented the flow of juice, often doing so before attempting to extract the juice. In Experiment 3, we presented chimpanzees and orangutans with four 3-tool sets (each tool set contained a single straw-like tool) and allowed them to select one tool. Unlike chimpanzees, orangutans succeeded in selecting the straw-like tool above chance levels without having to physically manipulate it. We suggest that orangutans' superior performance is related to their greater reliance on mouth actions during foraging. Experiment 4 investigated whether orangutans were also capable of selecting the suitable tool not by its appearance, but by the effects that it produced. After witnessing the experimenter blow bubbles or absorb liquid with a functional tool but fail to accomplish the same thing with the non-functional tool, orangutans failed to select the functional tool above chance levels.

Great apes select tools on the basis of their rigidity.

Wild chimpanzees select tools according to their rigidity. However, little is known about whether choices are solely based on familiarity with the materials or knowledge about tool properties. Furthermore, it is unclear whether tool manipulation is required prior to selection or whether observation alone can suffice. We investigated whether chimpanzees (Pan troglodytes) (n = 9), bonobos (Pan paniscus) (n = 4), orangutans (Pongo pygmaeus) (n = 6), and gorillas (Gorilla gorilla) (n = 2) selected new tools on the basis of their rigidity. Subjects faced an out-of-reach reward and a choice of three tools differing in color, diameter, material, and rigidity. We used 10 different 3-tool sets (1 rigid, 2 flexible). Subjects were unfamiliar with the tools and needed to select and use the rigid tool to retrieve the reward. Experiment 1 showed that subjects chose the rigid tool from the first trial with a 90% success rate. Experiments 2a and 2b addressed the role of manipulation and observation in tool selection. Subjects performed equally well in conditions in which they could manipulate the tools themselves or saw the experimenter manipulate the tools but decreased their performance if they could only visually inspect the tools. Experiment 3 showed that subjects could select flexible tools (as opposed to rigid ones) to meet new task demands. We conclude that great apes spontaneously selected unfamiliar rigid or flexible tools even after gathering minimal observational information.