Land Bridges Ancient Environments, Plant Migrations, and New World Connections

Before plate tectonics became an accepted idea in geology, Lyell’s doctrine of uniformitarianism still ruled supreme (see my review of Cataclysms for a short introduction). A corollary was that the continents supposedly had always been where they are now. One observation scholars had to explain away was that the same fossils occur on both sides of the various oceans. Looking at maps, some people noticed the thin strip of land connecting North and South America and concluded that land bridges must have formed and sunk beneath the waves at just the right times in history to enable migrations (see Four Revolutions in the Earth Sciences for more details). As explained in The Tectonic Plates are Moving!, we know better nowadays. Nevertheless, the concept of land bridges is still alive and well today, and palaeobotanist Alan Graham here introduces five of them, exploring their effects on biogeography, climate, and human history.

Graham has spent decades researching the vegetation history of North and Latin America. Next to numerous papers, he has written three books on the topic. The expert will want to have these at hand as background to this book, though the general reader can take them as a given.

Given this research focus, Graham here discusses five New World land bridges that formed or disappeared over the last 100 million years. For each of these, he summarises the current climate, vegetation, and geographic setting, followed by their geologic history and their utilization by organisms, as revealed by (foremost) plant fossils. Where applicable, the role of these land bridges in human prehistory is also discussed. I also kept the recently reviewed Earth History and Palaeogeography at hand as a reference work. Although Graham provides a few maps and links to a useful interactive tool called EarthViewer, I find this book very useful for the current big picture on where all the continents were over time.

Out of the land bridges discussed, three are the archetypal “strips of land between continents”: the currently existing Central American Land Bridge, the recently defunct Bering Land Bridge between Siberia and Alaska, and the long-gone Magellan Land Bridge between southern South America and Antarctica.

The first of these likely formed some 3 million years ago, as evidenced by the so-called Great American Biotic Interchange where flora and fauna started showing up on the other side of the bridge in the fossil record. Similarly, changes in the marine fauna, which became separated in what were now the Atlantic and Pacific oceans, also support this date. Some scientists want to push it back to 10-15 million years ago, but Graham thinks the weight of the evidence is not on their side so far. The Bering Land Bridge has been in place since roughly the Paleogene, some 65 million years ago, but has been submerged on and off as sea levels rose and dropped in tandem with climatic changes. It plays an outsized role in our collective imagination as the likely route by which human beings peopled the Americas (I first mentioned it here in my review of Fishing but see also First Peoples in a New World). On the other side of the planet, the Magellan Land Bridge connected South America and Antarctica until the Drake Passage opened up some 34-29 million years ago.

The other two bridges are less archetypal. The Antillean Land Bridge was never a bridge proper. Although some scientists favour this idea, and some of the Caribbean islands were connected in the past, Graham supports the idea of a “stepping stone” bridge. Most islands are close enough that animals and plants can relatively easily hop, float, or blow across. The last land bridge here considered feels a bit like a stretch (pardon the pun). The North Atlantic Land Bridge is the huge swathe of land that connected Europe with Greenland and Canada before they got interrupted by the formation of the North Atlantic.

This book provides a far more detailed picture of the geological details of these specific areas than in Earth History and Palaeogeography, and I found the details surrounding the various lines of evidence as to when these bridges existed or disappeared really quite interesting. Graham further livens up selected chapters with episodes from history, regaling the reader with tales of explorers such as Vitus Jonassen Bering or Ernest Shackleton.

This is juxtaposed with the hardcore data on palaeobotany. What Graham makes very clear is that the fragmentary fossil record, and the many misidentified species in museum collections and herbaria greatly hamper interpretations. Even so, land bridges emerge as one of several routes by which plants have spread in the past, with dispersal by wind, water, and birds being important additional possibilities. Next to frequent references to his previous three books, a lot of the data is tucked away in an online supplement where Graham lists over 200 pages of additional tables in 25 PDF files. I admit that for a general reader such as myself these sections were less riveting (sorry botany, you were never my greatest love), but for the specialist, this collation of data and literature references will be exceedingly useful.

One thing that hampers this book slightly is the reproduction quality of the images. The paperback copy that I reviewed is printed by Lightning Source in the UK, which means it is digitally rather than offset printed if I’m not mistaken. For the black-and-white photos, this results in a grainy though acceptable reproduction, but the custom-made maps have a fair bit of writing on them. They have here been reproduced in greyscale rather than colour and are quite small which makes it hard to read all the details. The reproduction of maps and illustrations from other publications varies, depending on the quality of the source files, and, with a few exceptions, is decent to good.

Overall then, this book will be of interest to a select audience. Its primary value will be as a reference work for palaeobotanists and (palaeo)biogeographers but it is also attractive to lay readers such as myself who have a strong interest in geology and palaeontology. Graham has done a remarkable job carefully enriching the book with many interesting geological, archaeological and historical details, providing a broader picture of the role of land bridges.