When I started this project to reconstruct a timeline of forests since the glaciers departed, I figured, hey, no problem. Easy peasy. I’ll look at pollen records and then climate data piece together the story from what others have found. Whoa, was I wrong. The story is complicated, there’s an incredible amount of data out there with sometimes conflicting interpretations, and there’s still just so much unknown. And nowhere has this murkiness been so clear as when I tried to dig into how and when humans first arrived here in Vermont.

Beringia Standstill - ice sheets in North America blocked passage on to the continent during the LGM (from National Geographic)

We step briefly back in time from the tundra to just before the Last Glacial Maximum (LGM), roughly 30,000 years ago. There was enough ice trapped up in glaciers across the northern and southern hemispheres that sea levels were 400’ lower than they are today (that is, 400’ lower relative to the land; remember that the continent was also being depressed by all the weight from the glaciers). With sea levels drastically lower, a stretch of land nearly 1,000 km wide, known as the Bering land bridge, became exposed, connecting modern-day Russia to Alaska. While the surrounding areas were enshrouded in a massive sheet of ice, the extreme cold, arid conditions, and low levels of snowfall in Beringia prevented the ice sheet from extending onto the land bridge. Humans, however, were undeterred by the rugged conditions and followed mammoths onto the grassland steppe. At the LGM, the ice sheets in the Yukon and Siberia expanded enough that they bookended the land bridge and trapped humans in Beringia between 30,000 and 14,000 ya (a period called the Beringia Standstill). Passage into the eden of megafauna awaiting in North America was blocked by the Cordilleran ice sheet to the south and east and by the Laurentide ice sheet to the northeast. 

Interestingly, the same climate that trapped humans on the Bering land bridge created cold, arid conditions that prevented Asian and North American tree species from crossing the land bridge during this time (much like climate prevents forests from developing in the far north or above treeline). Thus the flora on either side of Beringia remained distinct. Had Beringia been more hospitable to trees, this guide might include quite a few other species of larch, spruce, and fir.

Paleoindians hunting a woolly mammoth (from Wikimedia)

It is likely that some humans escaped the Beringia Standstill on boats, skirting around the glaciers along a coastal route down into the Americas between 25,000 and 15,000 ya. There is no direct record of these early seafarers, if they existed at all (indirect evidence, e.g. linguistic and genetic analysis of indigenous peoples, points to this first wave of movement onto the continent: source). The earliest widely accepted archaeological evidence of humans on the mainland, however, isn’t until 14,500 ya, and these sites go as far south as the Monte Verde site down in Chile. Perhaps the absence of evidence isn’t evidence of absence as much of the coastal route has since been covered in water by rising sea level.

There is, however, solid evidence for another wave of humans to arrive in North America. Around the time the first parts of Vermont were free of glaciers at the end of the Pleistocene (beginning 15,000 ya), the melting glaciers in the Yukon also opened an overland passage from Beringia down into the Americas. The first people, collectively called Paleoindians, rapidly made their way onto the new continent. They brought with them dogs, sleds, and a new style of stone point, called the Clovis point. These Clovis points, which first appear around 13,000 ya were thin, fluted stone projectiles of various sizes that were hafted onto spears or darts (for atlatls) and used by small bands of humans to pursue whatever large mammals they encountered. And they encountered many new species on the mainland. The Clovis culture thrived for the next 1,000 years, and by 12,000 ya, they had colonized much of North and South America.

Boreal forest on fire

Had things gone another way, Vermont would be a vastly different ecological community today. It’s likely that continued warming and increased precipitation would have brought other now-extinct species like the short-faced bear, giant ground sloth (Megalonyx spp.), and giant beaver (Castoroides ohioensis) into Vermont, just as it eventually would bring moose, white-tailed deer, and black bears. But things did not go that way, and within the first 2,000 years of humans arriving in North America over 35 genera (~50 species) of large mammals went extinct across the continent. The same pattern followed in South America. North America lost lions, cheetahs, camels, horses, and giant versions of everything from skunks to armadillos.

Alfred Russel Wallace wrote of these extinctions: “We are in an altogether exceptional period of the earth’s history. We live in a zoologically impoverished world, from which all the hugest, and fiecest, and strangest forms have recently disappeared…Yet it is surely a marvelous fact…this sudden dying out of so many large mammalia, not in one place only, but over half the surface of the globe” (from The Geographical Distribution of Animals, 1876). A century later, Paul Martin had more data and further concluded that in the absence of keystone megafauna species, the ecosystems of today are in a state of disarray. Their extinctions put a finality to the ecosystem changes that had started when megafaunal populations collapsed 14,800 to 13,700 ya. Trees like Kentucky coffeetree and osage orange (Maclura pomifera) relied on these now extinct megafauna for dispersal and scarification of their seeds (as did avocado with gomphotheres in South America). Absent their mutualists, the range and abundance of these trees has dwindled markedly. Without large carcasses to scavenge, California condors, dire wolves, and a host of land snails died off (a small population of condors survived). Mammoths grazed in the winter by using their tusks to scrape away snow and exposing low lying vegetation, which also exposed the soils to colder winter temperatures and deepened the layer of permafrost. Without large grazers trampling down vegetation and maintaining permafrost, the grasslands of the north were replaced by woody shrubs and conifers. The cold, dry conditions that dominated these boreal forests slowed the decomposition of the woody plant biomass, increasing fuel and resulting in more frequent and severe fires (source). Novel plant communities emerged in this strange land (where ash grew alongside hophornbeam, elm, spruces, and larches: source).

Archaeologists tend to fall into two main camps for the cause of these megafaunal extinctions: overkill (the hypothesis first put forth by the paleobiologist, Paul Martin that suggests overhunting by newly arrived humans caused a critical and irreversible decline in many large mammals) and a series of abrupt and rapid changes in climate. Both likely played a key role in the demise of gomphotheres, glyptodons, and stilt-legged horses (Haringtonhippus spp.), though which played the more dominant role is still up for debate (I tend to lean towards the overkill hypothesis).

When small bands of Clovis people first arrived in Vermont after the cold and dry Younger Dryas period, around 11,000 years ago, they were just in time to see the last vestiges of tundra retreat into Canada and up into our higher elevations. They also witnessed (or precipitated) the extinction of New England’s remaining Pleistocene megafauna and likely hunted whales and seals along the shores of the Champlain Sea which filled much of the Champlain Valley. Their population was small, nomadic, and sparse, likely less than 10 people per 100 square kilometers, and concentrated down in the Champlain Valley and Connecticut River Valley. These are the ancestors of the Abenaki, who have continuously occupied this land for 11,000 years. Their culture would shift and adapt as the climate, forests, and animals changed through the millenia.

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