A team of scientists has shown how our sense of smell has evolved and has even reconstructed how a long-extinct human relative would have been able to smell.
Humans have about 4 million smell cells in their noses, divided into about 400 different types. There is tremendous genetic variability within and between populations for our ability to detect odours. Each smell cell carries just one type of receptor or "lock" on it, the smell floats through the air, fits into the "lock" and then activates the cell.
Most receptors can detect more than one smell, but one, called OR7D4, enables us to detect a very specific smell called androstenone, which is produced by pigs and is found in boar meat.
The University of Manchester's Matthew Cobb and the other researchers studied the DNA that codes for OR7D4 from over 2,200 people from 43 populations around the world, many of them from indigenous groups. They found that different populations tend to have different gene sequences and therefore differ in their ability to smell this compound.
For example, they found that populations from Africa, where humans come from, tend to be able to smell it, while those from the northern hemisphere tend not to. This shows that when humans first evolved in Africa, they would have been able to detect this odour.
Statistical analysis of the frequencies of the different forms of the OR7D4 gene from around the world suggested that the different forms of this gene might have been subject to natural selection.
One possible explanation of this selection is that the inability to smell androstenone was involved in the domestication of pigs by our ancestors - andostroneone makes pork from uncastrated boars taste unpleasant to people who can smell it. Pigs were initially domesticated in Asia, where genes leading to a reduced sensitivity to androstenone have a high frequency.
The group also found that Neanderthal OR7D4 DNA was like our own - they would have been able to smell androstenone. The Denisovans' DNA showed a unique mutation, not seen in humans or Neanderthals, which changed the structure of the OR7D4 receptor.