The great strength of the scientific method is that it welcomes new, even contradictory information. True science not only allows for, but even demands a continual updating of pet theories. A recent case in point involves Yukon and Alaskan mastodon bones.
For the past couple of decades, the media have popularized some researchers’ suspicions that it was early humans – not asteroids, viruses, or extreme cold, for instance – that caused the mass extinctions of large ice-age mammals. This, many thought, could have happened as recently as 10,000 years ago, shortly after the Pleistocene ice ages gave way to the more temperate Holocene epoch.
Now, thanks to updated fossil analysis, it appears that humans with spears may actually not have played a significant part in the great extinction of megafauna such the iconic mastodon, at least not in Yukon and Alaska. In the western Arctic and in Beringia, human hunters may have never even crossed paths with these ancient proboscidea. They were both in the right place, but apparently at the wrong time.
As a graduate student in Edmonton in the early 2000s, paleontologist Grant Zazula studied the contents of ancient Arctic ground squirrel nests exhumed from the muck of the Klondike gold fields. Long preserved in ice, the nests contained seeds, leaves and petals that provident rodents had cached during the Pleistocene.
Thanks to those caches, Zazula had a pretty good idea of the vegetation that made up the Beringia ecosystem during the ice ages and the warm spells that intermittently punctuated them. The plant remains indicated the humidity, the temperature and the food available in the days of ancient vegan megafauna. It became more possible to accurately determine which beasts would have thrived where and when.
So when Zazula learned that a senior northern paleontologist had a mastodon bone sample radio-carbon dated to 18,000 years ago, he was nonplussed. He believed that 18,000 years ago cold Beringia conditions favoured grazers like mammoths, not their very-distant cousins, the browsing mastodons.
“I thought there was something fundamentally suspicious about this whole issue,” recalls Zazula.
The most readily apparent difference between a mastodon and a mammoth skeleton can be seen in their molars. Mammoths had roughened, ridged molars, suited for grazing on steppe grasses. Mastodons, however, had large knobs on their molars. These were ideal for crunching the branches and twigs of forests that were more prevalent in the North during the warmer, interglacial periods of the Pleistocene.
Scientists could tell mammoths from mastodons, but something else about the research wasn’t digesting well. Perhaps it was the methodology.
Radio-carbon dating is effective back to about 50,000 years; after that the carbon 14 in a fossil is all but depleted. “Basically, after 50,000 years, C14 radioactivity is so off the chart it’s unreadable,” says Zazula.
In 2006, Zazula moved north to take on the position of territorial paleontologist. Shortly afterwards, Yukon miner Earl Bennett showed up at his office with a gift of mastodon bones for the territorial collection. A grateful Zazula had them dated. Bingo! The forest-browser bones were more than 50,000 years old.
Zazula called for bones, more bones.
“I contacted anyone I knew with mastodon bones and a pattern started emerging,” he says. More and more fossils dated to more than 50,000 years ago – like Bennett’s gift bones. But then bones from the American Museum of Natural History showed up and the figures for their ages began wandering around the late Pleistocene … again.
Could the fossils have become contaminated when the bones were first being readied for storage or display?
Zazula’s suspicions were borne out. “Back in the old days, the first thing they did when they got back to the lab was just shellac and varnish the heck out of these things,” he says. Rather than ensure pristine fossil preservation, the varnish soaked deep into the bone pores.
“We had thought that pre-treatment methods would get rid of the varnish, but some of these specimens were still coming back at 25,000 and 30,000 years old.” The lab technicians would try again and come up with new numbers such as 35,000 and then they’d try again and get 18,000. “It was all over the map. There was no continuity,” says Zazula. Some goblin was playing tricks.
That brings us back to radio-carbon dating. If carbon-14 has been breaking down for 50,000 years, there will be only a few atoms left, says Zazula. These are easily overwhelmed by more recent contamination, say from the varnish, dust or handling.
But this is science, after all, and techniques change and adapt with time, technology and humility. Zazula learned of a researcher at Oxford University who was breaking bones samples down into various amino-acid components. One, called hydroxyproline, is only found in bones, not in varnish. When that acid was separated out from the bone and dated, the Yukon and Alaska mastodon relics came in at over 50,000 years old – far too early to be extirpated by ancient human settlers.
In all good science, as questions are answered other questions pop up. “We end with a mystery,” wrote Zazula and project colleagues in their 2014 paper with the megatitle “American mastodon extirpation in the Arctic and Subarctic predates human colonization and terminal Pleistocene climate change.” At the Pleistocene’s end, warmer, moister conditions were luring forests northward once again. The trees should have drawn the twig-crunching mastodons north along with them, possibly onto the waiting spears of Arctic hunters.
We should be “tripping on their bones,” as we do with mammoths.
“Why then were mastodons stopped in their tracks, failing to go all the way to the Arctic and Subarctic?” ask Zazula and his colleagues in their paper.
Why indeed? Where did they go? Earlier answers may have been too simple. The search for a fatal factor or, more likely, some combination of factors continues. Answers may rest within the bones of other northern megafauna, such as camels, horses and sloths, says Zazula. Stay tuned.
This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at http://www.yukoncollege.yk.ca/research/publications/your-yukon