The Eternal Frontier

As a kid I was transfixed by the remote worlds illustrated in the Peabody Museum’s murals of prehistoric creatures, and learned the skill of getting lost in the Museum of Natural History. With the scientific developments in the decades since those exhibits, I continue to find a well-told natural history an awe-inspiring tale; the stories of evolution, population dynamics, continental drift and climate change play out with accidents, contingencies, and deep patterns.

Tim Flannery’s Eternal Frontier is a big picture ecological history of North America, from the demise of the dinosaurs til yesterday. From a basic following of science news, I’d heard the theory that dinosaur extinction was caused by an asteroid impact. The book assembles a wide swath of evidence to pull together the big picture of massive destruction – the impact caused fire that burned most of North America; probably even more deadly was the dispersal of debris into the atmosphere, disrupting photosynthesis for months, causing ecosystems depending on land plants and plankton to die off. The result was massive extinctions on land and ocean. The stratigraphic evidence around the world shows a layer of sediment containing iridium, an element characteristic of asteroid material. In what is now North Dakota, 80% of all species disappeared above the iridium layer. According the fossil record, more species survived in fresh water, where the ecosystem is more dependent on detritus, than on dry land or ocean, where the ecosystem depends on photosynthesizing plants and plankton.

Many more deciduous plant species survived than evergreens, because they can “shut down” in times of stress, and for ten million years after the impact, decidious trees dominated in areas where evergreens would be otherwise favored by the climate. The anomalies caused by the asteroid impact serve to illustrate the more typical, longer-term patterns in North American ecology.

One of the strengths of the book is the way that Flannery illustrates large-scale patterns that play out over deep evolutionary time. One such pattern is North America’s distinctive sensitivity to climate change. The continent is shaped like wedge shape, with mountain chains running north/south. This geography results in causing in dramatic seasonal changes in temperature during the year than in other parts of the world and also magnifies the effects of global change in temperature. During two periods of global cooling at 50 and 38 million years ago, the deep sea temperature fell 4-5 degrees Celsius overall, but fell by about 9 degrees Celsius on the gulf coast.

Another deep pattern Flannery illustrates is the characteristic constellations of species in ecosystems. The African Seregeti has several major species: elephant, buffalo, rhino, lion. A similar ecosystem in North America was populated by mastodons, and later gomphotheres. The rhino role was played by Aphelops and Teleoceras. The big cat role was played by nimravids, and later on “barbourofelis” (illustration by the amazing natural history illustrator Carl Buell, aka Olduvai George.)

After major disruptions, Flannery shows that the ecosystem tends to repopulate with creatures of similar size, playing similar roles. This leads Flannery to leads to a recommendation (that he has supported for many years in his native Australia as well) to re-introduce species of megafauna such as elephants and camels that are missing in today’s ecosystem.

Speaking of missing species, Flannery reviews the evidence and finds the case compelling that humans caused the extinction of megafauna – sabre toothed tigers, mammoths, camels, sloths that roamed North America before humans arrived 13,000 years ago. The pattern isn’t just found in North America – humans arrived 50,000 years ago in Australia, and 6000 years ago in Cuba, and the megafauna disappeared at the time the humans arrived. Flannery makes that case that the species that flowed into North America after the arrival of humans had behavior that enabled them to survive predation – buffalo lived in large protective herds, and wolves had evolved near humans in Eurasia and had evolutionary time to learn fear. These behaviors worked until humans upgraded from knives to guns.

One of Flannery’s strengths is bringing together the evidence to tell big stories and illustrate big patterns. Two of the biggest patterns Flannery discusses also seems to me to be the most problematic.

The question with which Flannery frames the book is which continent originates the most species. Continents – largely isolated large landmasses – are biologically meaningful units in which evolution proceeds largely in isolation, so; examining the relative direction of population flow reveals interesting patterns. This lens also reveals interesting factoids – squirrels, dogs and camels all originated in North America. I don’t know about you, but I always wondered about species that seemed common to North American and Europe – what originated where? This book answers those questions. In addition to the question about population flow, there is also a real “history of science” question – the early dominance of North American evidence in paleolontology appears to be be a historical accident of caused by early enthusiasm and progress in North America; when you assemble paleontological results from other parts of the world you get a more balanced picture. And yet, aside from the real scientific and social history issues, the book is also replete with metaphorical language speculating about which continent will prove to be the “winner” in the global contest for originating the most species. This competitive framing sounds a bit too suspiciously like human geopolitics for comfort; the continental competition narrative reads like the Olympic television coverage of paleontology.

An even more problematic thesis is that of the frontier. There is a scientific element to it, in that North America has historically drawn influxes of species from Eurasia when the Bering crossing was open, and from South America when migration was possible; North America is a “frontier” into which new species spill and spread. Flannery sees the history of the immigration and diffusion of human cultures into North America in modern times as an instance of the same pattern. But the economic circumstances that have driven human migration to North America seem very weakly analogous to the geographic patterns that drove animal migration; the weakness of the hypothesis can be seen by looking at migrations that have nothing to do with geographical access – African Americans travelling North for manufacturing jobs; workers fleeing the rust belt for other parts of North America when manufacturing jobs move south and overseas. The reasons people move have everything to do with with human culture and financial resources.

Flannery draws his picture of the frontier from Turner – a historian who drew a romantic picture of a rough-and-ready, independent settler whose mindset is shaped by geographical expansion. There have been strong historical critiques of Turner – I’m most familiar William Cronon from his course in the American West and his book on the history of Chicago. Cronon shows how the exploitation of timber, mineral, and other resources were always closely tied to urban cultures and urban financial structures. More than that, the myth of frontier was shaped very early by theater and advertising; that Romantic self-image was heavily colored by fiction. And Turner’s focus on the white, Anglo frontiersman reflects his bias -there were African-Americans, Mexicans, ethnic Europeans; women and men. Turner’s Frontier is an important cultural myth, but a much weaker base for scientific comparison.

As a cultural myth, the Frontier and the death of the Frontier is a compelling narrative to explain the relentless exploitation of natural resources and the terrifying awareness — much later than the crisscrossing of the continent by railroads and telegraphs — that natural resources are limited and humans have the power to destroy our own civilization by mis-utilizing resources. The connection to the the flows of animal populations based on climate and geology is most tenuous. It would be better if Flannery drew a distinction but he doesn’t; the book tries to draw a seamless analogy between the population flows into North America across millions of years, and the cultural mythologies of manifest destiny and environmental exploitation, but the seams show.

Despite the weakness of the title argument, I really liked the book. if you are already deeply familiar with the scientific literature and have been following the topics closely across recent decades, this book may not have much new for you. If you are generally interested in the topic but not as familiar with the details, the book is fascinating. It is a strong entry in a genre of environmental history that weaves together paper-level detail to an accessible big picture story that shows the larger patterns across deep time.