Tuberculosis Is Newer Than Thought, Study Says

After a remarkable analysis of bacterial DNA from 1,000-year-old mummies, scientists have proposed a new hypothesis for how tuberculosis arose and spread around the world.

The disease originated less than 6,000 years ago in Africa, they say, and took a surprising route to reach the New World: It was carried across the Atlantic by seals.

The new study, published Wednesday in the journal Nature, has already provoked strong reactions from other scientists.

“This is a landmark paper that challenges our previous ideas about the origins of tuberculosis,” said Terry Brown, a professor of biomolecular archaeology at the University of Manchester. “At the moment, I’m still in the astonished stage over this.”

But Helen Donoghue, an expert on ancient DNA at the University College London, rejected the idea that tuberculosis could have emerged so recently. “It just cannot be right,” she said, citing earlier fossil evidence of the disease.

Tuberculosis has long been one of the deadliest diseases. In 2012, 8.6 million people became ill with this infection, and it caused 1.3 million deaths, according to the World Health Organization. The invading bacterium, Mycobacterium tuberculosis, attacks the lungs, where it causes widespread scarring.

To better understand the disease, a number of scientists have worked for decades to reconstruct its history.

In some people who get tuberculosis, the bacteria attack the skeleton, and archaeologists have found signs of tuberculosis damage in bones dating back centuries. Other scientists have examined DNA from different strains of the bacteria, using the mutations in different lineages to draw its family tree.

But archaeological studies and genetic research have often reached different conclusions about the disease’s origins.

Some scientists have argued that tuberculosis spread from cows to humans when the animals were first domesticated 10,000 years ago. Others have argued that the disease is far older, having evolved about 70,000 years ago, and spread from humans to cows and other animals.

The geographical distribution of the disease has also puzzled scientists. Tuberculosis strains found today in the New World are closely related to strains from Europe. That genetic link suggests that colonists brought the bacteria with them.

But archaeologists have found human bones with telltale signs of tuberculosis that date back centuries before Columbus’s arrival.

In 2012, a group of archaeologists and geneticists joined forces, searching for tuberculosis DNA in samples of bones from 68 sets of New World remains.

The scientists failed to find tuberculosis DNA in most of the specimens, but they succeeded in finding genetic material in three 1,000-year-old mummies from the Chiribaya culture of southern Peru.

The scientists retrieved many fragments of DNA from each mummy — so many, in fact, that they could recreate the entire genome of the bacteria in each victim. Before this new study, the oldest tuberculosis genome was reconstructed from a woman who died in Hungary in 1797.

The Peruvian genomes held two surprises.

The scientists expected that the tuberculosis DNA would be most closely related to some particular human strain of the disease. Instead, they found that it was most closely related to animal strains.

When they expanded their search, they found that the most closely related DNA belonged to tuberculosis strains found only in seals.

“We had not thought about seals at all,” said Kirsten I. Bos, a postdoctoral researcher at the University of Tubingen and lead author of the new paper.

The second surprise came when Dr. Bos and her colleagues used the genomes to estimate when tuberculosis first evolved.

Over many generations, bacteria accumulate mutations at a clocklike rate. Understanding that the Chiribaya bacteria were 1,000 years old, the scientists were able to estimate how many mutations the bacteria accumulate in a year, establishing its “molecular clock.”

They used this clock to determine the age of the common ancestor of all the tuberculosis strains in their study.

Dr. Bos and her colleagues ultimately estimated that tuberculosis was less than 6,000 years old — much younger than most scientists had thought.

In the new paper, the team proposes that humans acquired tuberculosis in Africa around 5,000 years ago. The disease spread to people across the Old World along trade routes. Meanwhile, Africans also spread the disease to animals such as cows and goats.

Seals that hauled out onto African beaches to raise their pups became infected. The bacteria then spread through seal populations until reaching South America. Ancient hunters there became infected when they handled contaminated meat.

There is some circumstantial evidence in support of the idea. Seals have given tuberculosis to cows grazing on beaches, and even to zookeepers. And archaeological remains from South America show that pre-Columbian hunters got tapeworms and other pathogens from seals.

Dr. Bos and her colleagues can’t say whether the tuberculosis in the Chiribaya mummies came directly from seals, or even if the bacteria spread from person to person. Nor can the scientists yet say whether the strain spread into North America through human contact.

Dr. Donoghue found the idea of hunters picking up tuberculosis from seals “quite feasible,” but she pointed to fossil evidence suggesting that tuberculosis is far older than 6,000 years.

In Israel, for example, archaeologists have found 9,000-year-old human remains that contain molecules produced by the bacteria. In a Wyoming cave, they have found 17,000-year-old bison bones with similar markers.

“I think they should be rapped over the knuckles for ignoring work that contradicts their conclusions a bit,” Dr. Donoghue said of the new study.