Olfactory genes are one of the largest gene families in mammals. A stonking 1% of the genome.

Our last commonly conserved olfactory receptors were with fish called lancelets 700 million years ago. Aquatic creatures as a rule have a lower number (100 ) versus land mammals (500).

They undergo many gene rearrangements to produce receptor variety. They cluster near the end of chromosomes by telomeres. Telomere proximity is a marker of high gene activity and re arrangement.

Something distinct to mammalian evolution is actually an official nose! Over time not only did a complex turbinate system evolve to increase olfactory/nasal epithelium, but the number of olfactory neurons jumped.

Some mammals even developed a vomeronasal organ (which is vestigial in humans and non existent in some higher primates). It appears olfaction regressed for humans as other senses became more evolutionarily important such as tricolour vision.

Olfaction differs in prevalence between primates with accordance to importance of social aromas and pheromones for mate selection.

There have been mixed studies about whether humans can still detect fear aromas, although it has been shown menstrual synchronisation in households of women is a subconscious olfactory process.

The olfactory bulb has many projections to the amygdala and hippocampus where it plays a significant role in memory and is extremely provocative as it can re-evoke even fear memories which have been made extinct.

This has been hypothesised to be due to the need to avoid predators.