The bones of a massive mammoth raised last year from a Michigan farmer’s field may harbor evidence of the earliest traces of humans in the Americas.
Over the decades, the bones of around 30 mammoths and 300 mastodons have been pulled from the state’s farm fields and construction sites. These iconic giants browsed on vast open grasslands and in wooded valleys south of the ice sheets that buried much of Canada and the northern U.S. at the height of the last Ice Age, some 25,000 years ago.
But among those dozens of finds, this recent discovery above all recently captivated the public. On October 1, 2015, scientists from the University of Michigan Museum of Natural History had just one day to carry out the rescue excavation of a mammoth. Dramatic video captured the spectacle of a crane hoisting a massive, remarkably intact mammoth skull complete with tusks from a muddy pit in a farmer’s field near Chelsea, Michigan. The find was dubbed the Bristle mammoth after the name of the farmer, James Bristle.
Renowned mammoth expert Daniel Fisher of the University of Michigan has recently presented fresh evidence that Ice Age hunters were involved in the mammoth kill. Together with a preliminary radiocarbon date, that may put the find among the earliest convincing signs of humans in the Americas.
If the age is confirmed by further tests, the Bristle mammoth could be as much as 15,000 years old. That would put it long before the appearance of the Clovis culture, so named after the distinctive spear point first found at Clovis, New Mexico, in the 1930s, and long considered to be the hallmark of the First Americans. The discovery would join a half-dozen or so other North American sites that add up to strong evidence of a shadowy pre-Clovis human presence, promising to rewrite the earliest chapter in the peopling of our continent.
Among the 55 to 60 nearly complete mammoth bones found at the site, there are several reasons to suspect humans were involved in the animal’s demise, Fisher said. First, many of the skull bones show what he describes as “intentional breakage, targeted toward removal of nutritious tissues that humans might wish to harvest.” These tasty tissues would include the brain, the tusk, and the tusk pulp cavities.
Second, close to the skull were three roughly basketball-sized boulders that shouldn’t have been there. The site consists of fine layers of sediment that were probably the mucky bed of a pond. There were no other rocks around or signs that a fast-flowing current could have deposited the boulders from elsewhere. Moreover, Fisher has seen similar finds before at other mastodon and mammoth kill sites in the upper Midwest and has a theory to explain them: they were the equivalent of Ice Age meat lockers. If hunters were fortunate to bring down a mammoth, they would have a bounty of meat on their hands but faced the problem of saving it from spoilage and the unwelcome attentions of scavengers and predators. The solution was stashing carcasses at the bottom of a pond, weighed down by boulders that were probably lashed together with ropes made of hide or plant fiber.
Fisher has become a legend in anthropology circles by putting his meat locker theory to the test. After butchering a dead draft horse, he stored the remains in a Michigan lake. Then each month he would pull up a joint and cook it to see if the meat was indeed still fresh—an experiment that he evidently survived.
The meat locker theory is reinforced by another sign of human involvement. Some of the Bristle bones were fully articulated and in proper anatomical order, just as when the mammoth was alive. But these intact portions were spread apart from each other in separate piles, such as would happen if the hunters had dropped butchered joints of meat into the pond, one by one, to store them for the future.
The Bristle mammoth originally weighed about nine tons and was a male in its mid-40s when it died, Fisher and his team report. The age assessment came from minute examination of annual growth rings—similar, in principle to tree rings—visible at the base of the mammoth’s massive 11 foot-long-tusks. Modern elephant tusks grow quickly during the spring and summer when food is abundant and more slowly during fall and winter. A single layer of elephant or mammoth ivory corresponds to a single year’s growth. Amazingly, and unlike tree rings, micro-CT scans of the tusks can pick out much finer growth layers corresponding to periods of a couple of weeks or even individual days.
Dubbing the tusks “ivory autobiographies,” Fisher has shown how changes in the rings store a wealth of clues to a mammoth’s health and life history. These include how fast it grew, the age of sexual maturity, the spacing of pregnancies, and the timing of the breeding season in fully grown males. Another timing indicator—the weaning age when calves stops nursing, revealed by distinctive stable isotope ratios in the layers—even throws light on one of anthropology’s fiercest and long-debated questions: whether it was human hunters or the warming climate toward the end of the Ice Age that finally pushed mammoths into extinction.
Based on Fisher’s extensive tusk collection from Siberia, his doctoral student Michael Cherney recently showed that, over the course of 30,000 years leading up to the extinction, mammoth calves weaned at progressively younger ages. Modern elephant studies show that if the animals are under nutritional stress from a changing habitat, weaning is generally delayed. On the other hand, hunting pressure like today’s epidemic of poaching in many areas of Africa has the opposite effect, forcing young elephants to wean at a younger age.
Cherney argues that his finding that Siberian mammoth calves were weaned at younger and younger ages presents “compelling evidence” that hunting played the key part in extinguishing the great beasts. It’s not yet clear whether a similar signal is detectable in North American tusks.
As more finds like the Bristle mammoth accumulate, these impressive fossils are not only a source of abiding fascination for the public but are yielding a trove of secrets about the vanished world of Ice Age giants.