The role of terrestriality in promoting primate technology.

''Complex technology'' has often been considered a hallmark of human evolu- tion. However, recent findings show that wild monkeys are also capable of habit- ual tool use. Here we suggest that terrestriality may have been of crucial importance for the innovation, acquisition, and maintenance of ''complex'' technological skills in primates. Here we define complex technological skills as tool-use variants that include at least two tool elements (for example, hammer and anvil), flexibility in manufacture or use (that is, tool properties are adjusted to the task at hand), and that skills are acquired in part by social learning. Four lines of evidence provide support for the terrestriality effect. First, the only monkey populations exhibiting habitual tool use seem to be particularly terrestrial. Second, semi-terrestrial chimpanzees have more complex tool variants in their repertoire than does their arboreal Asian relative, the orangutan. Third, tool variants of chimpanzees used in a terrestrial setting tend to be more complex than those used exclusively in arboreal contexts. Fourth, the higher frequency in tool use among captive versus wild primates of the same species may be attributed in part to a terrestriality effect. We conclude that whereas extractive foraging, intelligence, and social tolerance are necessary for the emergence of habitual tool use, terrestriality seems to be crucial for acquiring and maintaining complex tool variants, par- ticularly expressions of cumulative technology, within a population. Hence, comparative evidence among primates supports the hypothesis that the terrestriality premium may have been a major pacemaker of hominin technological evolution.