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

Spit Analysis Can Reveal Person's Age

There are certain compounds in a person's spit that can reveal their age to within 5 years. No kidding. There are certain things we know about DNA. We know that DNA gets methylated (like ornaments on a Christmas tree). We also know that the methylation pattern on the DNA changes with age. So the hypothesis was whether looking at the methylation pattern of the DNA can reveal a person's age? And guess what... it does according to a recently published study.

In this study published on PLoS ONE, researchers studied the saliva of identical twins to figure out what sites appeared to correlate best with age. They narrowed it down to 88 DNA methylation sites that seemed to correlate with age. Based on these 88 sites, they than verified it in 60 men and women, ages 18 to 70, and found that two genes in particular had the strongest age correlation to within 5 years. Amazing!

This finding can certainly help with crime-fighting, but also has potential for spit being a marker for all sorts of other biological problems that have yet to be discovered (perhaps even replace painful needle sticks for blood draws). After all, if a person's spit can reveal their age, who knows what else it can reveal about the person!

Reference:
Epigenetic Predictor of Age. PLoS ONE 6(6): e14821. doi:10.1371/journal.pone.0014821

For $349, Test Your DNA for 100+ Hereditary Diseases

A company called Counsyl claims to be able to test an individual's DNA and determine the risk of passing on over 100 hereditary diseases to future offspring for $349 (or free with insurance). Such hereditary diseases they are able to test for include the commonly known cystic fibrosis to more rare conditions like Nijmegen Breakage Syndrome. The full list can be viewed here.

Though many of the diseases listed are associated with ear, nose, throat problems, the list unfortunately does not include hereditary hearing loss conditions.

To do the testing, all you need to do is get the kit mailed to your home, spit into their test tube, and send the kit back to the company.

Read more about this in the New York Times here.

Though this all sounds great, there are some potentially unforeseen implications one needs to consider prior to obtaining this test beyond the issues the NYT article brought up. If the testing comes back positive, one runs the risk of:

1) Having your health insurance dropped or premiums increased... not just for you, but for your offspring (pre-existing condition)
2) Businesses may not want to hire you as you would "cost" them more (missed days from work due to illness, etc)

Though insurance may cover the testing... not sure if you actually want the insurance company to pay and have them potentially find out the results, especially if positive.

Image from wikipedia.