Are we chemically aposematic? Revisiting L. S. B. Leakey's hypothesis on human body odour

Aposematism entails a mutually beneficial avoidance of signal emitters by heterospecific signal receivers. The celebrated palaeoanthropologist L. S. B. Leakey hypothesized that humans are chemically aposematic, suggesting that our body odour repels large carnivores because they associate our 'bad smell' with what he alleged is our unpalatable flesh. Unpalatability, however, is one of many unprofitable traits potentially denoted by aposematic signals. Moreover, aposematism may arise in interactions with offenders other than predators. Here, I propose that the body odour of humans and, historically, of hominins denotes chemical emitters who exhibit formidable defensive traits, including large body size, agility, vigilance and the capabilities of deploying projectiles and other weapons and/or marshalling group defences. This hypothesis maintains that selection acts against (1) offenders, including carnivores, that fail to avoid chemicals from hominins, and (2) hominins who fail to emit distinguishing chemicals, thereby giving rise to a chemically mediated avoidance that is mutually beneficial, i.e. chemical aposematism. This hypothesis is examined in light of information on free-ranging New and Old World carnivores that avoid humans and on non-domesticated mammals suspected or confirmed to avoid human scent.