Olfaction

Olfaction #

Noses #

Why noses? #

3 hypotheses:

• evolutionary artifact: result of neutral evolution, genetic drift
• respiration: nose evolved to warm, humidify air (proposed in 1913, refuted 2016)
• stereo olfaction: evolved to support enhanced, adapted movement + navigation
  • leads to greater space use → larger brains

Using Olfaction #

• Odors are objects
  • elements (single), configurations (mix)
  • source of directional information
• odor mapping prevalent in highly visual species, incl. humans
• olfaction before brain: olfactory system/bulb evolved before neocortex
• hippocampus related to olfaction
  • dolphins don’t have either

Stereo Olfaction #

• more accurate mapping w/ two nostrils
• related to antennae (wide to determine location accurately)
  • also seen in hammerhead sharks, snakes’ bifurcated tongues
  • Procellariiformes (tube nose birds) → navigation, prey detection

The Human Nose #

• no other ape has a wing/pyramid-shape nose
  • first appears in homo erectus
• evolved in part from climate change:
  • environment becomes drier, more open
  • rise of cooperative hunting carnivores w/ olfactory pursuit
  • endurance pursuit allowed humans to compete bipedally
  • nose widened/narrow based on geographic location → increase/decrease in space use
    • (ex. Arctic noses very narrow, low need for space)
  • agriculture → sedentary life → low space use
• evolved as response to agriculture: increased pathogens, decreased space use
  • 2 modes: navigational (1.5 mya), diagnostic (15k)

The Scent of Disease #

• early physicians trained to detect odors correlated w/ disease
• dogs can detect tuberculosis, cancer, low blood sugar, covid…

Human: