Final answer:
The human sense of smell is less powerful than that of dogs or rats due to fewer olfactory receptors and genes coding for them. Primates, including humans, have evolved a lesser reliance on smell, instead developing more complex visual systems. While humans can detect a vast number of smells, the difference in receptor quantity and brain processing areas makes our olfactory sense less acute than in many other mammals.
Step-by-step explanation:
Humans possess a less powerful sense of smell compared to many other mammals such as dogs or rats. This is evident from the number of functional genes for olfactory receptors, where dogs have between 800 and 1200, while humans have fewer than 400. The number of olfactory receptors in humans is about 12 million, which is significantly less compared to animals like dogs, who have up to 1 billion, and bloodhounds with about 4 billion. Furthermore, bloodhounds, which are bred for their sense of smell, have even larger and more complex olfactory epithelia.
Another contributing factor is the evolution of primates, which have a reduced reliance on their sense of smell compared to other mammals. Primates have a smaller area in the brain for processing olfactory information and instead have developed more sophisticated visual systems featuring color vision and stereoscopic sight. The olfactory system computes a mixture of volatiles to create a mental representation of the olfactory environment; thus, the lesser number of receptors in humans may limit this computation ability. Notably, Bushdid et al. (2014) identified that humans can distinguish a vast number of unique odorant molecules, estimating about 1.72 trillion different detectible smells. Despite having fewer olfactory receptors, the combination of receptor subtypes allows humans to sense a wide array of odors.
Odorants enter the nose and dissolve in the olfactory epithelium, triggering specialized neurons that send impulses to the brain's olfactory bulb. The size and quantity of these receptors are key to the olfactory capabilities of different species. The reduction in the number of olfactory receptors in humans reflects a divergence from the ancestral reliance on smell as the primary sensory system for navigating the environment.