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Origins of Olfaction

"You smell with your body, not just your nose"

Sense of smell and scenting

The sense of smell, or olfaction is the ability to detect and identify chemicals and molecules in the environment through an olfactory system. This sensory capability allows organisms to perceive odors, which are essentially chemical compounds in the air. At its core, smelling is scenting and interpreting the chemical composition of the surroundings. Scenting is fundamentally about detecting and responding to environmental stimuli through our sensory system. The verb "to scent" is derived from the Old French "sentir" and the Latin "sentire" which means "to feel" or "to perceive". The evolution of the sense of smell is a series of adaptations and changes over millions of years, from chemoreceptors to the olfactory system and the sense of smell. 
 

Chemosynthesis

It is hypothesized that the earliest forms of life may have emerged in extreme environments, such as near hydrothermal vents, around 4 billion years ago, shortly after the formation of Earth's oceans. In these environments, iron and sulfide compounds precipitated on the sea floor, potentially providing the necessary conditions for life's emergence.

These early organisms likely relied on chemosynthesis. Chemosynthesis is the process by which certain organisms produce energy from chemical reactions involving inorganic molecules like hydrogen sulfide or ferrous iron. These primitive life forms depended on chemoreceptors to identify crucial elements like iron and sulfur minerals, which they used as catalysts for metabolic processes in anaerobic environments.

 

One example of these organisms is sulfur bacteria, which use chemosynthesis to metabolize sulfur compounds such as hydrogen sulfide for anaerobic respiration and energy generation. Some of these microorganisms and biochemical systems are still found today in environments such as hydrothermal vents and even within organisms like us.

Olfactory receptors

As organisms evolved and developed egg cells, chemoreceptors diverged and evolved to form more sophisticated sensory functions such as olfactory receptors. These specialized olfactory receptors emerged, enabling single cells, such as the human spermatozoa to detect chemicals in their surroundings as they navigate towards ligands released by the egg cell. This adaptation allowed sperm cells to sense and respond to hormones lignads, released by the egg cell, guiding them towards the egg cell. As simple nervous systems emerged, early fish or arthropods developed specialized olfactory organs or antennae to detect and interpret chemical signals. And over time, olfactory systems evolved to become more specialized and sophisticated, as animals diversified into various ecological niches.

 

Olfactory system

Animals developed specific organs like the olfactory epithelium in mammals' nasal cavities, and specialized receptor cells evolved to detect a wide range of odour molecules. The brain adapted by developing an olfactory bulb and cortex to receive and process signals from these receptors, enabling organisms to perceive and distinguish between different odours. In the case of humans, babies begin to form olfactory neurons, establishing their olfactory system in the womb. This early development allows them to detect and become familiar with nutrients from their mother's diet, facilitating recognition of both their mother and her breast milk through their sense of smell after birth.

 

While the principles of chemoreception remain consistent across organisms, the evolution of specialized sensory organs and neural circuits led to the development of our complex sense of smell. The distinction between a sense of smell and other forms of chemical detection can be subtle, the nuances lie in the complexity of the sensory mechanisms involved, making chemoreceptors the precursors of the sense of smell. If we consider the five classic senses, the sense of smell ought to be one of the oldest, because the earliest forms of life were likely using chemosensors from at least 3.5 billion years ago.

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