Editor's note: This is part three of Barry Zellen's look at how the Earth has formed from prehistoric times to now.

Indeed, geological history provides us with some compelling metaphors for understanding the Arctic, its place in our planetary history, and its place in the human story.

SA国际影视传媒�Snowball Earth,SA国际影视传媒� during the aptly-named Cryogenian period 600-700 million years ago, is believed to have been essential to kick-starting evolution beyond microorganisms by toughening up early life forms through shell formation and other adaptations to the eraSA国际影视传媒檚 planet-wide deep freeze. The Paleocene/Eocene boundary (where these two epochs transition, around 56 million years ago), when tropical forests colonized the High Arctic, forming what is now a somewhat counterintuitive oasis of SA国际影视传媒渢ropicalSA国际影视传媒� life that stretches the imaginations of those who perceive the Arctic as perpetually frozen; and the period that followed the LGM, a relatively recent 20,000 years ago, when sea levels dropped, revealing the Beringian land bridge connecting Eurasia to North America SA国际影视传媒� one of the principal, but no longer considered only migration routes for the peopling of the Americas, extending humanity to every continent (with, until our own time, the singular exception of Antarctica).

Near the end of the Proterozoic Eon, which ran from 2.5 billion years ago to around 500 million years ago, Earth had an ice age like no other during Snowball Earth. While not everyone agrees on the details, most agree that all or nearly all of Earth became frozen over, and this frigid period is aptly called the Cryogenic.

The theory is that there was so much oxygen replacing carbon dioxide in the atmosphere that temperatures dropped to freezing even at the equator, with an echo of todaySA国际影视传媒檚 climate change (but whose direction is in reverse). It was in essence the opposite of todaySA国际影视传媒檚 excess atmospheric carbon that is warming the Earth. Some believe that the challenge of surviving Snowball Earth toughened life up, hence the notion of it SA国际影视传媒渒ick-startingSA国际影视传媒� evolution, in a new, robust direction.

After the big thaw that followed some 600 million years ago, life proliferated, much as it would yet again after the last Ice Age, when humanity ascended and globally proliferated. Snowball Earth can thus be understood as a vital mechanism for lifeSA国际影视传媒檚 (and later humanitySA国际影视传媒檚) forward evolutionary journey, suggesting that what we think of as Arctic is indeed central to the story of life and humanity on Earth SA国际影视传媒� that just as SA国际影视传媒渁ll roads lead to RomeSA国际影视传媒� during Pax Romana two millennia ago, the Arctic has been a central crossroads and springboard of life for planet Earth. As todaySA国际影视传媒檚 Arctic thaws, it can continue to serve in this central role, perhaps more visibly and obviously than in the past.

Archaeologists have long known from the ancient fossil record that CanadaSA国际影视传媒檚 Arctic tundra was formerly covered in rich forest during the Paleocene/Eocene boundary, and the few visitors to venture as far north as the Queen Elizabeth Islands in the High Arctic often come across the petrified forests from this time long ago.

As the University of Saskatchewan research office has described in a media release promoting the research of palaeobotanist Christopher West, SA国际影视传媒淭he heady aroma of magnolia blossoms and lotus flowers might have wafted to your nostrils if you had gone for a walk 56 million years ago in the lush green forest which covered CanadaSA国际影视传媒檚 northernmost islands.SA国际影视传媒� 

As West further describes: SA国际影视传媒淚tSA国际影视传媒檚 very surprising how similar these ancient polar forests were to some of our modern forests. I identified fossil plants related to many modern temperate trees: birch, alder, elmsSA国际影视传媒攅ven plants belonging to the grape family. Some of the fossils are related to trees now found only in East Asia SA国际影视传媒� The presence of these forests gives us an idea about what could happen over long periods of time if our modern climate continues to warm, and also how forest ecosystems responded to greenhouse climates in the distant past.SA国际影视传媒�

If we consider the metaphorical fluidity and centrality of SA国际影视传媒淎rcticSA国际影视传媒� to our world, and how at one point in time (long ago, in the Cryogenian), our entire world was a world of ice and snow, not unlike like what Pluto looks like in our outer solar system today, with its famed ice mountains and continent-sized glaciers; while at another point in time (after Snowball Earth underwent its transformative thaw), even the highest of High Arctic territories became veritable SA国际影视传媒渢ropicalSA国际影视传媒� paradises teaming with life, from dense and fertile forests to productive waters with abundant marine life; and then at another point in time, one closer to our time, as the last Ice Age ended, a Eurasian/North America transit corridor opened up between the old and new worlds, globalizing the human story with the peopling of the Americas (whether by land or by sea, or by both).

Each of these snapshots in deep history presents us with a different metaphor for the future Arctic that remain connected to one another by their facilitation of the human journey. A land of perpetual ice and snow, as many had expected the Arctic to remain forever until just over a decade ago, was not a barrier to lifeSA国际影视传媒檚 evolutionary advance, but instead a corridor connecting deep time with our time, and uniting the whole of the world at its top. An increasingly warm and fertile region today, where plant and animal life that evolved far to its south is now taking root and thriving, was once itself a warm and fertile region, perhaps even more so, than we can now glimpse in our future.

And as a strategic transportation corridor uniting the continents, the emergent sea lanes so widely discussed as part of our future (and perhaps a seed for future conflict, as many emergent Arctic cold warriors predict), are not unlike those of yesteryear. Each of these SA国际影视传媒淎ges of the ArcticSA国际影视传媒� starting in deep geological history and continuing to the dawn of humanity and from there to our own time, is intriguing unto itself, but the dynamic range of variation in the SA国际影视传媒淎ges ofSA国际影视传媒� (and even inclusive of StruzikSA国际影视传媒檚 foreseen SA国际影视传媒淓nd ofSA国际影视传媒�) the Arctic, is perhaps the most compelling takeaway.

The Arctic as a dynamic and at times paradoxical state of change, oscillating between its polar extremes while always nudging life (and humanity) further along its road to tomorrow, helps us to understand and contextualize Arctic of today, where change, from the extremes of freeze and thaw, is of itself the common thread that united this disparate ages that span almost a billion years of EarthSA国际影视传媒檚 story.