Most people catch the common cold at least once a year, making the seasonal sniffles a staple of the human experience. But when in Homo sapiens' history did people first start catching the common cold?
The question is difficult to answer, in part because many viruses cause colds and few of them preserve well in human remains. But it's possible that some of the earliest Homo sapiens were catching colds at least 300,000 years ago, the time the oldest archaeological evidence of our species dates to.
"Common cold" is an umbrella term for a group of respiratory infections that tend to be mild in people with healthy immune systems. Rhinoviruses, coronaviruses and respiratory syncytial virus (RSV) are often to blame. But before these pathogens began to spread between people, humans probably acquired them from other vertebrates.
"Living in close proximity to animals is a surefire way to be exposed to new viruses and to get the repeated exposure that could result in it becoming a human endemic virus," Joel Wertheim, an evolutionary virologist at the University of California, San Diego, told Live Science.
Related: Why do people get sick when the seasons change?
Usually, when an animal virus jumps into humans, it fails to establish an infection because it's not adapted to its new host. However, a virus will occasionally possess the right set of genes to make the leap successfully and even spread among humans. That's how the viruses behind COVID-19 and "swine flu" emerged, for example.
Scientists have different hypotheses as to when cold viruses first took off, placing their onset at vastly different points on the human timeline. Some researchers think the viruses could have started spreading from animals to humans at the dawn of human civilization — approximately 5,000 to 6,000 years ago. Humans were starting to live in close quarters where pathogens could spread easily, and they began farming animals swarming with viruses.
But not all scientists agree with this hypothesis.
François Balloux, a computational biologist at University College London, said hunter-gatherer populations that didn't farm would have been similarly exposed to animal viruses through hunting. Hunter-gatherers have been around since before Homo sapiens — for instance, in extinct human species like Homo erectus that first emerged 2 million years ago. Balloux suggested that different common cold viruses came and went throughout human evolution at various times.
"I think probably one event that led to a significant increase of acquisition of pathogens in humans is the expansion out of Africa," during which people may have picked up new cold viruses, he said. The earliest Homo sapiens found outside Africa date back 210,000 years.
Excavating ancient cold viruses
"Illness due to the common cold doesn't preserve very well," Wertheim said. These viruses normally leave signs of an infection in soft tissues, such as the lungs, which waste away after death, rather than in the hardy bones and teeth that persist.
Viral genomes have been found in the remains of ancient humans — but only for DNA-based viruses, not for ones containing RNA. This genetic cousin of DNA is far more common among cold viruses.
"RNA degrades more quickly than DNA so is much harder to recover," Lucy van Dorp, a geneticist at University College London, told Live Science in an email. "As of yet, no RNA viruses have been recovered from archaeological material," she explained, including most of the usual suspects behind the common cold.
Balloux and van Dorp have searched for ancient viruses in human teeth from an excavation in Siberia. In an unreviewed paper that was posted to bioRxiv, they reported finding two ancient genomes for a DNA virus called human adenovirus C, which can cause cold symptoms. The researchers estimate that the viruses' last common ancestor dated back approximately 700,000 years — likely long before Homo sapiens emerged. They may have jumped from chimps or gorillas into humans, but exactly when they did so remains speculative.
Although scientists haven't found RNA viruses from ancient times, researchers did find a 16th-century coronavirus in dental pulp from human skeletons in France. The RNA differed from known modern-day coronaviruses, suggesting these historic pathogens may have either died out or evolved beyond recognition.
It is possible, however, that this coronavirus continues to circulate in humans but hasn't been sequenced in modern times. "Many of the viruses responsible for the common cold have been poorly sequenced today," including cold coronaviruses that circulate regularly, van Dorp said.
With sparse remains of ancient viruses, Balloux and his colleagues are now focusing on pathogens of the recent past. They're studying chemically preserved human remains collected over the past two centuries and stored in medical facilities.
"We'll probably have in the future a decent sum picture of the viruses that were with us under 200 years ago," Balloux said. This could make it easier for scientists to trace the history of these viruses into the distant past.
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