If you walk through the woods and pass a beehive, you may catch the sweet scent of honey in the wind and suddenly be flooded with memories: having tea with grandma or eating warm biscuits on a Sunday morning.
If you were taking that walk 300,000 years ago with a Denisovan – a now-extinct hominid closely related to Homo sapiens and first discovered in 2010 – by the time you smelled the honey, your companion might have already been climbing the tree for a sugary treat.
That's because Denisovans appear to have been especially sensitive to sweet smells like honey or vanilla, new research published in the journal iScience in January suggests. That may have helped them find food.
Meanwhile, one group of a related species – Neanderthals – developed a mutation that could have spared them the smell of their own body odors.
Humans have a lot of genetic diversity in our olfactory receptors, which govern smell, allowing us to detect a wide array of scents.
Researchers think that helped humans adapt to new environments as they spread across the globe, sniffing out new foods and new predators.
It's a popular idea that humans have a bad sense of smell, as compared with dogs, for instance. But dogs live in the world so differently that the comparison may not mean much.
Understanding our earliest relatives – the other Homo species that migrated out of Africa alongside us – can offer better context for our own sense of smell and give us a sniff of life at our origins.
The researchers Kara Hoover, a biological anthropologist from the University of Alaska at Fairbanks who is now working at the National Science Foundation, and Claire de March, a biochemist at the Université Paris-Saclay, reconstructed odor receptors from the genomes of three Neanderthals, one Denisovan, an ancient human, and a database of modern human genomes. It was an attempt to recreate the noses of our closest ancient relatives.
"We have to really understand ourselves within our own context," rather than comparing humans to dogs or mice, as previous research on smell receptors has done, Hoover said. "When people look at humans, they see us as this bizarre outlier. But in fact, we weren't."
Bringing ancient noses to life in the lab
Hoover compared the genomes of the Neanderthals and the Denisovan with those of humans, targeting 30 olfactory receptors – genes that allow us to perceive odors.
She identified 11 receptors that contained unique DNA variations in the extinct species, variations that didn't appear in humans.
Then de March built those unique receptors in the lab by mutating human receptors to match the amino-acid sequence of the extinct Neanderthal or Denisovan.
She then exposed the extinct receptors to hundreds of odors and measured their responses by how quickly and intensely they lit up with activity.
The sample size in this study was small, since only a few individual Neanderthals and Denisovans have been genetically mapped.
Graham Hughes of University College Dublin, who studies sensory perception in the genomes of mammals and is not affiliated with the study, also noted that DNA degrades over time, which can affect the results of any assessment of ancient genomes.
Still, "the fact that we can now look at the genomes of ancient species and determine their possible sensory spaces and dietary specialties is very exciting for the field of sensory perception," Hughes told Insider in an email.
To Hoover's surprise, the Neanderthals, Denisovan, and humans all appeared to have the same repertoire of smells.
It wasn't that our extinct relatives could smell scents undetectable to humans, or vice versa. Instead, the Denisovan turned out to have a more sensitive nose than humans, while Neanderthals seemed to have weaker noses – especially, in one group, for stinky body odors.
A lucky mutation for cave-bound Neanderthals
One of the Neanderthals had a genetic mutation that diminished its ability to smell androstadienone – a chemical associated with the scents of urine and sweat. That could've been a big help for those living in close proximities with other Neanderthals in caves.
"It is kind of funny that of all the things they would stop smelling, it would be that," Hoover said.
The Neanderthal used in the study represents an entire population of the species that lived at high altitude in Siberia. The other Neanderthal samples, from different parts of the world, didn't have that mutation.
Only two smell-related genes from the Neanderthal genome were different from humans'.
A Denisovan by any other name would smell what's 'sweet'
The Denisovan's propensity for sniffing out sweet scents may have helped them find high-calorie, sugary foods like honey. Its receptors also responded with heightened sensitivity to spicy smells, like cloves or herbs.
Hoover described this as one of the first biological insights we have into Denisovans.
It's difficult to jump from genetic information to the activity of odor receptors and then to the subjective sensory experience of an individual – much less how they might behave in response.
"Each person might perceive things slightly different, and we can never say that what we consider as being 'sweet' smelling is the same as what another species would consider 'sweet-smelling," Graham said.
Still, the study opens a bridge from the DNA to the real-world experience of our extinct relatives. More research like it, with more samples of ancient genomes, could reveal a clearer picture of Neanderthal and Denisovan life.
"Ultimately, what our work showed us is that we're more alike than we are different" when it comes to smell, Hoover said.
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