14 DAY TRIAL // Mastodons were an ancient type of elephant related mammals that shallowly resembled mammoths, but were shorter and less hairy, and roamed Africa, Europe, Asia and North America during late Tertiary Age. Unlike the true elephants (mammoths were true elephants), they had more than one molar on a jaw and their heads were longer.
Mastodons entered in North America from Asia about 2 million years ago and thrived until 11,000 years ago, the end of the last Ice Age and around the time humans arrived on the continent, when the last mastodons and mammoths died off.
Bruce Rothschild, a practicing physician and an expert on ancient diseases, and Richard Laub, curator of geology at the Buffalo Museum of Science, studied mastodon bones to find clues about their mysterious disappearance. Rothschild studied 113 mastodon skeletons, from all over North America, of different ages and sizes and found 59 of them (52 %) to have grooved erosions caused by tuberculosis. "Based on the finding, it's likely that virtually every late Ice Age mastodon in North America had tuberculosis," Laub says.
Tuberculosis is a bacterial infection that commonly infects the lungs. After a while, it passes to organs and bones. In humans, a maximum of 7 % of infected people develop bone damage. The fact that more than half of the mastodon skeletons examined had the bone lesions suggests tuberculosis was a "hyperdisease" that afflicted a large percentage of the North American mastodon population.
Tuberculosis infection causes bone beneath cartilage to be carved out ("excavated") Tuberculosis struck the mastodons as early as 34,000-years-ago and persisted till their extinction. "That the disease was widespread and yet persisted for so long in the species suggests it was not immediately lethal," Rothschild said. "Instead, it was probably a chronic disease, one that gradually weakened rather than killed the animals."
"In humans, tuberculosis can lay dormant for several years after initial infection, repressed by the body's own immune system. But it can flare up into full-blown disease during times of stress. A similar flare-up probably happened with the mastodons during times of stress," Rothschild said.
At the end of the last Ice Age, mastodons had reasons to be stressed. Drastic changes brought by rapid climate change combined with the arrival of a new threat: hunting people.
These three factors-disease, climate change and humans-might have been too much for the creatures. Weakened by tuberculosis, the creatures couldn't cope with other diseases, and the crippling bone damage would have affected their ability to walk. "Extinction is usually not a one-phenomenon event," Rothschild told.
Mastodons were largely spread throughout North America from Alaska to Mexico. "All the evidence shows that mastodons were exceedingly adaptive." said Laub.
"The appearance of tuberculosis in an animal as large and adaptive as the mastodon could give scientists insight on how it spread," Laub says. "There's interest in trying to understand why so many species of large North American mammals became extinct at the end of the Ice Age."
Tuberculosis was discovered in a 500,000-year-old buffalo fossil in China. Rothschild found similar tuberculosis-caused bone damage in North American bovids (buffalo is a bovid), a group of animals that included bison, cattle, sheep, goats. Tuberculosis appears to have been first spread in the bovids. Bovids crossed from Asia into North America using the Bering Land Bridge, which connected the two continents (like humans did later), bringing tuberculosis to North America. "Once in North America, the bovids could have spread to mastodons and other species, possibly even humans," Rothschild said.
"These findings also show that it's possible for a disease to work its way rapidly through a species," said Laub.
The hunting stress induced by humans could have had a negative effect on the mastodons' reproductive cycle. Like elephants, mastodons are must have had long gestation periods and produced few offspring in a lifetime. "If they were infected with tuberculosis, that could have had an weakening effect on their physiology," Laub said.
The mass extinction during the end of the Last Ice Age was the most recent to occur. "Extinction is part of evolution, and if we can understand some of the processes that cause these mass extinctions, then we've learned quite a bit about an important mechanism that drives evolution," Laub said.
Researchers can now "really go after the genome of ancient tuberculosis and have the option of comparing it" to modern strains of tuberculosis, Rothschild said. This could help in the search for a cure to modern tuberculosis. "It's given people a better understanding of what the disease's effects are," he said.