Why Do Humans Have Weak Sense of Smell

Compared to nearly all other animals, humans have the weakest sense of smell. You would think that humans and our closest evolutionary cousins (chimpanzees, apes, etc) would have similar sense of smell, but humans still have a much weaker olfactory sense.

Why might that be?

Well, research from Karolinska Institutet have discovered that humans are the ONLY mammals whereby no new neurons are formed in the olfactory bulb after birth. Or if there is any, it is VERY little.

In all other mammals including apes, new neurons are constantly being formed in the olfactory bulb which might explain why all mammals with the exception of humans have a superb sense of smell.

How did scientists figure out the absence of neuron growth in humans?

They looked at Carbon-14 levels within the DNA of the olfactory bulb. Carbon-14 levels in the atmosphere increased significantly due to nuclear bomb testing during the Cold War.

As such, IF new neurons were being formed, one would expect Carbon-14 levels which gets incorporated into developing DNA, to be similar to current atmospheric levels. Or at least find a certain turn-over percentage of both "old" and "new" neurons reflective of the Carbon-14 atmospheric levels at time of neuron development.

However, what researchers have found was Carbon 14 concentrations that corresponded to the atmospheric levels at the time of birth of the individuals establishing that there is very limited, if any, postnatal neuron development in the human olfactory bulb.

Reference:
The age of olfactory bulb neurons in humans. Neuron. May 24, 2012, 10.1016/j.neuron.2012.03.030

Another Reason to Stop Smoking... Decreased Sense of Taste

Greek researchers published a study in which they described a significant decrease in taste ability in smokers compared to non-smokers. In the study titled "Evaluation of young smokers and non-smokers with electrogustometry and contact endoscopy," 62 male Greek soldiers were evaluated for smoking and taste by using two different instruments called electrogustometry (measures taste thresholds) and contact endoscopy (wich evaluated the morphology and density of papillae on the tongue's tip).

This study could explain why smokers not only complain of decreased sense of taste, but also change in the way things taste (dysgeusia).

Click here to read the study abstract.