In your imagination, step back and see us living on the crust of a small planet revolving around an average star. All life on Earth lives on this crust. But go out a comparatively short distance from Earth into space, and you can see the crust, but nothing alive on it. Small as the planet is, we amount to little on its thin crust. And as our star has a history, and our planet a history, so our planet's crust has a history. One of the most important continuing discoveries of the past two centuries has been our realization that the great puzzle of the history of life on Earth is bonded to the dynamic history of Earth's crust. Scientists call this bond "biogeography"--and if you're unfamiliar with it, and you have an imagination that allows you to roam in time and space, biogeography will change forever the way you think of yourself.
The field of biogeography is shaped by the merging of two momentous and crucial phenomena: Darwinian evolution and plate tectonics.
"Evolution" is one of the great buzzwords of the modern age, especially in recent decades. These days it seems that everyone has ideas about evolution, ideas that range from calling it a hoax to anointing evolution as directly responsible for everything human, including religion. Since the science behind Darwinian evolution more or less blasts into pieces religious fundamentalist ideas about the Earth and life on the Earth, it's an irony that a current popular fandango is the proposition that it's evolution that directly produced religion in the first place. For me that's a stretch, but maybe (I hope) it's a step forward to a time when most Americans will at least accept Darwinian evolution as a fact of life.
Meanwhile, the aspect of Darwinian evolution of most importance for biogeography, the aspect critical for what we call biodiversity, is the idea that given a variety of individuals in a species, natural forces (environment, physiology, and so on) will make some individuals more likely to reproduce, and their genes, over the long term, more likely to predominate and shape the traits of the species. So the giraffe has a long neck not because stretching the neck to get food changes the genome, but because if slight variations in neck length are present in the species, giraffes with longer necks will be better able to get food and so better able to survive to reproduce the genes that made their necks a bit longer in the first place. The statistical end result for giraffes, after maybe millions of years, is a very long neck. We call it "natural" selection, and it's an idea that explains so much in biology that it seems self-evident now.
The second momentous phenomenon involved in biogeography, plate tectonics, is not so familiar to the public, but the basic idea is not at all complicated.
If you look at a map of the Earth's continents, some of the continents seem to fit together like the pieces of a jigsaw puzzle. Mapmakers already noticed this 400 years ago. In the 19th century, paleontologists also noticed that many fossil plants and fossil animals are remarkably similar from one continent to another, and that some rock formations in distant continents are also similar. What caused the similarities? In the early 20th century, geologists proposed that about 300 million years ago a supercontinent covered nearly all of the Earth's surface, and that eventually the supercontinent broke apart to form various continents and ocean basins. Over millions of years, the continents, essentially parts of a malleable crust, drifted away from each other, and in some cases came together in new arrangements. The process is called "continental drift" -- and it's now the cornerstone of geology.
Continental drift would also explain paleoclimate change, the change of climate through geologic time. As continents drifted through different climate zones, ocean circulation became altered by the changing distribution of land and sea. The interactions of merging and separating land masses provided a mechanism for the origins of mountains, volcanoes, and earthquakes.
The central idea is that the motion of land masses is explained as a consequence of moving "plates" -- large fragments of the Earth's surface layer in which the continents are embedded. The term "tectonics" refers to any process of construction, so in geology, "plate tectonics" refers to the process of the formation and movements of the continents by the movements of the crustal plates.
Another buzzword these days is "biodiversity" -- the variation of biological form, function, and behavior. Much of it is due to geography, which, in the time-scale of evolutionary process, means to the geologic history of land masses and oceans. We need our imagination to roam over geologic time, over the 4.5 billion years of the history of Earth, and over the 2.5 billion years of the history of life on Earth, and over the one million years or so of the history of the life of our current human species on Earth. It's a grand time-and-space voyage of the imagination, the drift of continents, the appearance and rise and fall and extinction of new species, the human story with all its tragedy and complexity. At this Christmas time comes a wonderful little book by the biogeographer Dennis McCarthy. In less than 200 pages you can travel with him through time and space to discover the panorama of geologic change producing biodiversity -- the marvelous story of how life on Earth has always been bonded to the history of Earth's crust. Read this one, a great pleasure, and if geologic time and space in the history of life are new for you, at the end of the book you will be someone different.
Dennis McCarthy. (2009). Here Be Dragons: How the Study of Animal and Plant Distributions Revolutionized Our Views of Life and Earth. New York: Oxford University Press.
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