Wikipedia and the scientific community
Human beings have brains three times larger than chimps (mostly due to our much larger association areas), but some parts of our brain are smaller than in chimps. For example, our olfactory bulb is only 40% the size of theirs which means we lack the ability to process as many smells in our brain. Although all primates have a reduced sense of smell compared to many terrestrial species — a dog for instance has a sense of smell between 10,000 - 100,00 times as acute as ours — humans also have a very poor sense of smell comparative to all other apes .
The evolutionary advantages of a good sense of smell, out on the plains, is fairly obvious. If you are a hunter, you can locate your prey, determine which way it is travelling, how big the herd is, whether there are any injured individuals or other predators to compete with, etc. If you are the hunted, you can sniff your hunter (as long as he remains upwind), detect dangers, smell water and food, changes in weather patterns, etc.
In the forest, a sense of smell is useful, but to an animal that spends most of its time in the trees, perhaps not quite as essential, which may explain why primates are not as concerned with smells as wolves. But there is only one environment where a sense of smell would not be of much use: in water. (The exception is for predatory hunters like sharks and polar bears which have a very powerful olfactory sense and are able to detect their prey, or the smell of blood, in water or on ice).
For a shallow-diving ancestral homo species, predominantly vegetarian and already possessing good 3-dimensional vision, the most important senses in locating slow-moving or sessile foods under water, would be superior underwater vision and more sensitive touch, both of which appear to have become adapted in humans.
Humans also have a universal aversion to foul odours, with particularly 'offensive' smells universally agreed to be the smells of human excrement, rotting fish and bad eggs. Retaining enough of a sense of smell to recognise something that is good to eat and something that may be harmful, such as bad fish or eggs, is obviously significant.
Other species which have profoundly reduced or completely lost their sense of smell are the cetacea (see below), and genomic and fossil data suggests that this happened relatively early on in the animals' evolution and return to the sea from land.
Aquatic adaptation and the evolution of smell and taste in whales
Takushi Kishida, JGM Thewissen, Takashi Hayakawa, Hiroo Imai & Kiyokazu
Human Uniqueness Compared to "Great Apes": Absolute Difference
The olfactory receptor (OR) gene family contains approximately 1000 genes for olfaction, and humans have experienced varying lineage-specific alterations in these genes. Of the ~1000 genes within the family, over 60% are pseudogenized in humans, a percentage twice as high as other non-human primates, and many OR genes have undergone lineage-specific conversion events affecting odorant binding sites. Additionally, humans have accumulated mutations four times faster in these genes than other primates. One example of this phenomenon is the OR gene 912-93, which in humans has experienced a nonsense mutation rendering it nonfunctional. In contrast to this general pattern of loss/pseudogenization, a DNA segment containing multiple OR genes has expanded from 1-2 copies in chimpanzee and gorilla to 7-11 copies in humans.
James Sikela, Veronica Searles 
Sense of Smell
Human Uniqueness Compared to "Great Apes": Likely Difference
The sense of smell plays a significant and often essential role in social and sexual behavior, learning and memory, identification of food, and detection of hazards. Olfactory receptors, which detect scents from the environment, belong to the largest gene superfamily in mammals, highlighting their apparant importance across species. However, the human sense of smell is commonly believed to be relatively poor compared to that of other mammals, including great apes.
Genetic evidence indicates humans have lost the function of all but 388 of over 1,000 olfactory receptors encoded in the genome. These non-functional genes, called pseudogenes, appear to have accumulated faster and be in higher abundance in the human genome compared to their putative orthologs in chimpanzee, gorilla, orangutan, and rhesus macaque genomes. This loss in the repertoire and function of olfactory componentry indicates reduced evolutionary selection pressure and may reflect reliance on other sensory systems. In another study, a comparison of the human genome to olfactory receptor gene orthologs in the chimpanzee genome indicates the number of intact olfactory receptor genes in both humans and chimpanzees is shrinking, indicating a diminished importance in both species from relaxed positive selection constraints.
Loss of olfactory receptors is linked to the evolutionary appearance of a heightened trichromatic visual system in humans; similarly, primates with trichromatic color vision have a larger fraction of pseudogenes than other mammals, suggesting olfaction is less important in species with trichromatic vision.
Comparative behavioral studies on the function of olfactory sense in humans and other hominids is lacking, particularly for utility in their appropriate environments. Based on genomic comparisons of pseudogenes, great apes should benefit from a larger repertoire of functional receptors. However, behavioral studies indicate human smell perception is equal, or in fact greater, to that of other mammals and great apes, suggesting human sense of smell may, in fact, be relatively good despite a larger proportion of pseudogenes.
Rachel Zarndt 
Aversion to Foul Odors
Human Uniqueness Compared to "Great Apes": Likely Difference
Humans across all cultures show an aversion to foul odors. While the identity of some of those odors varies across cultures, there are universally avoided foul odors, such as that of rotting meat or excrement. Great apes do not appear to show the same aversion to foul odors and, in fact, sometimes will manipulate and eat their own excrement in captivity. 
|Website: F. Mansfield, 2015|
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