Today, humans are the only members of our genus — the Homo from Homo sapiens — left alive. Remnants of our closest relatives, including Neanderthals and Denisovans, range from teeth and bones to tools and artwork, not to mention a genetic legacy in many modern humans thanks to interbreeding.
With a new study published in iScience, a group of researchers announced that ancient DNA has helped illuminate another aspect of our fellow humans’ lives: their sense of smell.
This research builds on previous studies, starting in 1997. A Swedish geneticist named Svante Pääbo and his colleagues published a paper, announcing that they had sequenced DNA from the 40,000-year-old arm bone of a Neanderthal.
Ancient Human Genomes
Since then, Pääbo and his colleagues at the Max Planck Institute for Evolutionary Anthropology have gone on to publish entire Neanderthal and Denisovan genomes; he won the 2022 Nobel Prize in Physiology or Medicine.
“Without that lab, and what they've generated, this study would be impossible,” says Kara Hoover, a professor at the University of Alaska Fairbanks and the senior author of the new study about ancient human’s sense of smell.
Read More: How Do You Tell a Neanderthal From a Denisovan?
Denisovan and Neanderthal Genomes
A genome is sort of like a recipe for an organism — a complete set of all the genetic instructions to create the proteins that make up our bodies. The genomes of ancient humans are therefore an invaluable resource for scientists who want to understand how different aspects of their physiology compare with ours.
“As an anthropologist, I'm interested in human evolution, and I have always wondered, ‘How do we adapt to new environments or changing environments?’” says Hoover. “I finally realized that olfaction might be something important to look at in terms of recognizing food in new environments.” Essentially: what we’re able to smell might play a big role in how we interact with the world around us.
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Neanderthal and Denisovan Smell Test
Examining what ancient humans could have smelled was easier said than done, though. First, the researchers had to recreate the ancient DNA. Since Neanderthal DNA is 99.7 percent identical to ours, the researchers could zero in on the sections with genetic instructions for olfactory receptors (sensors on the outer membrane of the nerve cells that send smell stimuli to the brain).
From there, they looked for sections of code where our mostly-identical DNA didn't match up: those spots might be the source of differences in our smelling capabilities.
Recreating Ancient Human Sense of Smell
The researchers then modified samples of human DNA to include the non-matching bits of Neadnerthal and Denisovan genetic code related to smelling. Voilà: lab-grown Neanderthal and Denisovan gene sequences for detecting odors.
Genetic sequence on its own doesn’t do much, so the study’s first author, Claire de March, an assistant professor at CNRS, Paris-Saclay University, injected the DNA into cells. Then, she “injected odorant molecules on those cells, and receptors of Denisovan and Neanderthal were responding to other molecules,” says de March.
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To detect whether the recreated ancient olfactory receptors were picking up the different smelly chemicals she exposed them to, de March treated the cells with the same bioluminescent chemical that makes fireflies glow. If the odorants activated the olfactory receptors, they would emit a tiny glow, imperceptible to the naked eye but readable by lab instruments. That’s exactly what happened.
“I think that was really the moment where I lost my mind in front of my computer, where I saw these odorant receptors from Denisovan and Neanderthal responding to odors — this is something that didn't happen for 30,000 years,” says de March.
Difference Between Neanderthals and Humans
They noted the spots where the ancient DNA differed from ours today. Fred H. Smith, a professor emeritus of anthropology and biological sciences at Illinois State University, who wasn’t involved with the study, says, “I question whether there's really any adaptive difference between these variations. I think what you're looking at is just genetic drift between two lineages.”
Above all, though, Smith says that the study is a testament to the advances in our understanding of human evolution in recent decades.
“If somebody had told me in, let's say, 1990, that in 20 years, we would have essentially a full genome of a Neanderthal, I would have laughed at them. I would have said, this is not possible,” says Smith. The fact that Hoover and de March could use these genomic data to test what Denisovans and Neanderthals could smell, he says, “I found it very fascinating.”
Read More: The Fascinating World of Neanderthal Diet, Language and Other Behaviors
Modern and Ancient Human Similarities
As far as modern and ancient humans go, the team found some similarities in the sensitivities we have to various smells. For instance, “what we saw in Denisovans is that their receptor is more responsive to a compound that is present in honey,” says de March.
But while we vary in how sensitive we are to different scents, the overall breadth of smells that we can detect are similar across humans, modern and ancient.
“The big take-home picture, in terms of human evolution, would be that we're more alike than we are different, and that the other lineages within our genus Homo pretty much smelled the world the same way we do,” says Hoover.
Read More: Why Did Neanderthals Disappear?