How scientists are decoding what the past smelled like
Smells hover just below our conscious awareness, conjuring up emotions and memories that shape how we perceive and navigate the world.
An unexpected whiff of a long-forgotten snack or a dusty book can transport a person to years past — enabling a kind of time travel that makes hazy memories more vivid.
It's puzzling then that smell is a sense that, according to scientists, has been largely — and unfairly — ignored in most attempts to understand the past. A growing number of researchers now want to reconstruct ancient aromas and use them to learn more about how we used to live.
During the Covid-19 pandemic, many people who caught the disease temporarily lost their sense of smell, prompting a newfound appreciation of the importance of odor in their lives. New research projects are underway to understand what the past smelled like and identify what contemporary scents should be preserved for posterity.
"It's a very vital sense. Smell was also very important in the past and it was probably even more important because in the past not everything was so sanitized," said Barbara Huber, a doctoral researcher of archaeology at the Max Planck Institute of Geoanthropology in Jena, Germany.
The challenge of finding past smells is how to capture an ephemeral phenomenon: Archaeologists typically find and study things we can touch, and these are the artifacts we encounter in museums.
Odor compounds are volatile in nature — once their source is gone, they too disappear, evaporating into the air. And most smells stem from biological materials — plants, food, human and animal bodies — that decay rapidly, Huber explained.
Despite all these challenges, Huber said a few new and powerful biomolecular approaches are helping scientists decode ancient scents.
The smell of history
The key to unraveling smells of the past is often invisible to the naked eye.
Scientists can study imperceptible biomolecular residues left on incense burners, perfume bottles, cooking pots and food storage jars using techniques like chromatography, a process for separating components in a mixture, and mass spectrometry, which can detect different compounds by calculating the weight of different molecules.
The most informative biomolecules, according to Huber, include lipids — fats, waxes and oils — that aren't soluble in water. They're often found embedded in porous ceramics, after having been used in items such as lamp fuel or scented ointments people once put on their bodies or on corpses. Lipids are also found in feces.
Huber also studies secondary metabolites, organic compounds produced by plants and left by plant-based products used in the past, including resins, scented woods, herbs, fruits and spices. The compounds can reveal the ingredients, and scent of, incense, drugs and food.
Sequencing of ancient DNA and proteomics, the study of proteins found preserved in things like calcified dental plaque, have detected amino acids that signal conditions like gum disease — associated with bad breath.
But, as Huber's research illustrates, collecting these olfactory clues is often only the beginning.
Recreating smellscapes
In her work, Huber has studied incense burners found in the archaeological site of Tayma, Saudi Arabia's oldest settlement that dates back 5,000 years, in order to try and reconstruct the "olfactory landscape" of the ancient oasis.
She detected secondary metabolites that revealed the use of scented resins containing frankincense, myrrh and pistachio in private buildings, graves and temples, respectively. Huber then worked with a perfumer to try and recreate the scents, revealing what these places might have smelled like thousands of years ago.
"The resins looked really similar ... but when you burn them, they have a totally different smell. So for example, the frankincense was really a rich smell — very balsamic — and you could really feel maybe this was used in order to kind of clean out the houses right to avoid an unliked smell or something like that," Huber explained.
Sean Coughlin, a researcher of ancient and medieval thought at the Czech Academy of Sciences, is attempting to recreate the perfumes Cleopatra herself might have worn, based on recipes recorded in ancient Egyptian texts and from inscriptions on temple walls.
"The problem is a simple one. Normally, when you follow a recipe, you kind of know what you're supposed to get. When you reproduce a historical recipe, you have no target," Coughlin said.
"What we're really trying to do is use organic chemistry to be able to tell us something about the process, because we think that the process was actually what would determine the range of possible scents," he added.
Coughlin likens his experiments to the testing process of the cooking show "America's Test Kitchen." While the results have been hit-and-miss, he said they are making progress.
For example, one perfume recipe Coughlin studied known as Mendesian indicated that ancient perfumers heated oil for 10 days and 10 nights before infusing it with woods like cinnamon and resins like myrrh.
"That was a big mystery to us," he said. "If you've ever cooked oil for 10 days it stinks." But after his team heated oil in test tubes for up to 12 days, Coughlin found that the technique accelerated the natural process of the oil going rancid, removing any smelly compounds and ultimately allowing the perfume to last longer.
"There is also a stage, after heating the oil, but before making the perfume itself, where they added mildly aromatic things like roots, wine and resins. Our hypothesis is that these not only covered up the bad smell (by adding a pleasant scent), but also absorbed the bad odor in the oil," he explained.
Most present-day perfumes use ethanol, a type of alcohol, as a base, Coughlin said, although some delicate natural scents still require the use of oil or fat, which needs to be refined in some way.
But today's chemists still owe a lot to these ancient perfumers, he added. They pioneered many techniques still used in modern science, such as distillation and methods of fractionating liquids.
Similarly, researchers are now taking steps to preserve currently available smells to give future generations a sense of our time and the more recent past.
An archive of smells
The Institute for Sustainable Heritage at UCL, a London university, identified the chemical recipe for old book smell — specifically capturing the scent of the library at St. Paul's Cathedral in London prior to a renovation that started in 2018.
Visitors to the library, which until the renovation had changed little since it was built in 1709, often remarked they found the old book smell appealing.
"In the age of digitization, working with physical records is an increasingly rare practice, and therefore the opportunity to touch and smell the documents is perceived as valuable," a 2017 study on the project noted.
Researchers used information from volatile organic compounds retrieved at the library to reproduce the historic book smell. They also put together an odor wheel — a tool used by perfumers and winemakers and a first step toward documenting and archiving smells of the past.
Cecilia Bembibre, a lecturer at the UCL Institute for Sustainable Heritage, said preserving the scent of the library was important because the smell was an integral part of its identity.
"Since the space has been undergoing important conservation work in the last few years, and the collection was removed, it is reasonable to assume the scent has gone," said Bembibre, who's also a participant in Odeuropa, a European research project that aims to bring historic aromas back to life. "Now the preservation kit we created...(is) now the only existing archive of a lost heritage smell."