Rise and Fall of the Beringian Steppe Bison

The widespread extinctions of large mammals at the end of the Pleistocene epoch have often been attributed to the depredations of humans; here we present genetic evidence that questions this assumption. We used ancient DNA and Bayesian techniques to reconstruct a detailed genetic history of bison throughout the late Pleistocene and Holocene epochs. Our analyses depict a large diverse population living throughout Beringia until around 37,000 years before the present, when the population’s genetic diversity began to decline dramatically. The timing of this decline correlates with environmental changes associated with the onset of the last glacial cycle, whereas archaeological evidence does not support the presence of large populations of humans in Eastern Beringia until more than 15,000 years later. Article Summary by Jonathan C. Driver, May 2015 This article explores the movement of North American bison and the special significance of Tse’K’wa in this research, shedding insight into how the first Paleoindian peoples of Canada arrived at the land. One of the fascinating aspects about archaeology today is the speed with which new scientific techniques are developed and applied to archaeological materials. This paper demonstrates why it is so important to keep material safely stored after it has been excavated – because you never know when it will be valuable to a future researcher or when it can yield new information about the past. Quite a few years after the excavations at Tse’K’wa were finished I was approached by Alan Cooper and Beth Shapiro of Oxford University about the possibility of sampling early bison bones from the site, to see if they still had DNA preserved. Beth was writing her doctoral thesis on the evolution of bison, and she was being supervised by Alan, a highly accomplished researcher into ancient DNA. It turned out that the Tse’K’wa bison were well preserved, and Beth was able to extract DNA and include it in the study of hundreds of samples from North America and Asia. As an added bonus, Beth and Alan arranged for radiocarbon dates to be run on every bone they studied.This added to our understanding of the age of the site's earliest material. Beth summarized her doctoral research in this paper, published in the very prestigious journal “Science”. The many contributors to her research were listed as co-authors, recognizing that each of us had contributed in a small way to the research, by sharing samples and ideas. While the paper mainly deals with Beth’s conclusions about the evolutionary history of bison, for those interested in Tse’K’wa and the history of the early peoples of Canada, there was a fascinating tidbit of information contained in the Tse’K’wa bison DNA – they were from two quite distinct populations. To understand the significance of this, we must first consider the impact of the extensive ice sheets that formed across Canada about 20,000 years ago. The ice sheets separated North American bison into two populations. One group of bison herds lived to the south of the ice sheets, in what is today, the continental USA and very southern parts of Saskatchewan, Alberta and B.C. The other population lived in unglaciated regions of, what is today, Yukon, Alaska and northeast Asia. These two populations can be distinguished by minor differences in their DNA. When Beth studied the bison fossils from across the vast region of their ice-age distribution she found that in every location except one the bison were either northern or southern types. The one exception was Tse’K’wa where bison from both the north and south were found. The explanation for this seems fairly straightforward. When the western Canadian ice sheets melted and new pastures were developed, northern bison began to move south through the Yukon and into northern and central BC and Alberta. At the same time, the southern bison began to move north, also following pastures that developed on the recently de-glaciated landcapes. Finally, the northern and southern populations met in the Peace River region. There is no evidence that they bred with each other, and, based on the genetics of modern bison in North America it appears that the southern form survived and evolved into the bison we are familiar with today. The Tse’K’wa data suggest the meeting of bison populations must have occurred around the same time the site was first occupied by humans, as it is very unlikely that two genetically distinct populations of bison could have lived in the same environment without interbreeding or one of them becoming extinct. This means the establishment of a viable migration route for animals (and people) from Alaska through Yukon, B.C., Alberta and into continental USA must have happened after the dates of the earliest human presence in the continental USA and South America. Therefore, the first people to enter North America could not have migrated through an ice-free corridor along the eastern slopes of the Rockies. Assuming they entered the Americas during the last ice age, then the most likely route would be down the coast of Alaska and British Columbia. Interestingly, this hypothesis was proposed by the director of the 1983 Tse’K’wa excavations, Knut Fladmark, in a paper published in 1979! The Tse’K’wa stone artifacts lend support to this scenario. The earliest spear point at the site – the “fluted point” – is stylistically very similar to artifacts found in the south, which date a little earlier than those at Tse’K’wa. This supports the notion that the earliest inhabitants of the site were part of a human population that began to move north (perhaps out of southern Alberta and northern Montana) following the bison herds as they moved north to exploit the newly created grassland environments that formed as ice melted and glacial lakes drained away.