The rule in gene survival is "use it or lose it".
When a gene ceases to be useful, mutations are likely to accumulate as they are passed on down the generations. These broken genes are called pseudogenes and they provide powerful clues to our evolutionary past.
Compared with other animals our ability to detect odours is poor, but if evolution is correct there was a time in our distant past when we relied on our sense of smell much more than we do today.
Our olfactory receptors are coded for by OR genes. Geneticists Linda Black and Richard Axel won the Nobel Prize in 2004 for their work in this field. They discovered that humans have 800 such genes but fully half of these are inactive relics of our past. As our brain combines signals from a number of receptors simultaneously that means our sense of smell is only a fraction of what it once was.
What though of our closest relatives? Well not surprisingly there was found to be a direct correlation between the closeness of our evolutionary cousins and the number of active and inactive OR genes. We carry this genetic baggage because it was needed in our distant ancestors who relied on a keen sense of smell for survival.
Clearly a mechanism that enables us to detect airborne odours is not going to work the same under water. In fish we find one kind of OR genes and in amphibians and mammals there is another. So what about aquatic mammals like dolphins? If they really were once land animals as evolution claims, there should still be evidence in their genome that they once had an acute sense of smell.
Not surprisingly the evidence is irrefutable. 80% of the OR genes in a dolphin are inactivated; hundreds of them remain in their genome as silent testimony to their evolutionary past. Dolphins have the instructions in their genes to construct the tools for detecting thousands of airborne smells. This makes no sense if dolphins were specially created.
The most primitive fish still alive today is a jawless fish called Lamprey. Fossils of these creatures 320 million years old bear a very close resemblance to it’s modern cousin. When the DNA of the lamprey is studied it turns out their OR genes are neither air nor water specific; they combine features of both. These creatures arose before smelling genes split into two types.
Like all our other genes our OR genes tell a story of our species’ past. They are very similar to primates, less similar to other mammals, less similar still to reptiles, amphibians and fish in that order..