The navigational nose: a new hypothesis for the function of the human external pyramid

Abstract

One of the outstanding questions in evolution is why Homo erectus became the first primate species to evolve the external pyramid, i.e. an external nose. The accepted hypothesis for this trait has been its role in respiration, to warm and humidify air as it is inspired. However, new studies testing the key assumptions of the conditioning hypothesis, such as the importance of turbulence to enhance heat and moisture exchange, have called this hypothesis into question. The human nose has two functions, however, respiration and olfaction. It is thus also possible that the external nose evolved in response to selection for olfaction. The genus Homo had many adaptations for long-distance locomotion, which allowed Homo erectus to greatly expand its species range, from Africa to Asia. Long-distance navigation in birds and other species is often accomplished by orientation to environmental odors. Such olfactory navigation, in turn, is enhanced by stereo olfaction, made possible by the separation of the olfactory sensors. By these principles, the human external nose could have evolved to separate olfactory inputs to enhance stereo olfaction. This could also explain why nose shape later became so variable: as humans became more sedentary in the Neolithic, a decreasing need for long-distance movements could have been replaced by selection for other olfactory functions, such as detecting disease, that would have been critical to survival in newly dense human settlements.

How the evolution of air breathing shaped hippocampal function

“…A corollary of the olfactory spatial hypothesis is that if a species is orienting to odours, this should predict related morphological adaptations [24]. Across the animal kingdom, invertebrates have evolved paired structures, such as antennae and tentacles, to enhance their accuracy of spatial orientation using stereo olfaction [28,65,66].…”

“…Events in human history have also contributed to human olfaction being underestimated and understudied [20,21]. Naturalistic spatial orientation to odours has been addressed in even fewer studies [22][23][24].…”

Radial symmetry molding skull roundness: a human fetal MR study

“…A narrower human nose could have allowed an increased sensitivity to odorants, improved the accuracy of directional orientation, with greater survival possibilities in the wild environment. The existence of two symmetrical nostrils suggests that humans might use bilateral sampling to extract information about the location of a speci c odorant source 32,33 . On the other side, the development of the visual perception allowed humans to gather precious information from the environment concerning threat and safety, successful foraging and sexual opportunity 34 .…”

The PROUST hypothesis: the embodiment of olfactory cognition

“…Spatial orientation to odors may arguably have been the class of associative learning that was the impetus for the radiation of animal phyla in the Cambrian (Ginsburg and Jablonka 2010 ; Jacobs 2012 ). It is even possible that selection for olfactory navigation in the genus Homo may have led to the evolution of the nasal pyramid, and hence shaped the evolution of our own species (Jacobs 2019 ).…”

“…Yet, the majority of research in olfactory cognition, whether in humans or other species, is designed to study how the brain identifies and assigns valency to an odor, what could be called diagnostic olfaction . But this downplays or ignores a critical function of olfactory cognition, its role in spatial orientation, i.e., directional olfaction (Jacobs 2019 ). This is an important distinction because many paradoxes of olfactory anatomy and psychophysics can only be explained in terms of directional olfaction functions (Jacobs 2012 , 2022 ; Marin et al 2021 ).…”