One last look back at Neil Shubin’s Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body:
On the Rocks
“Every rock sitting on the ground has a story to tell: the story of what the world looked like as that particular rock formed. Inside the rock is evidence of past climates and surroundings often vastly different from those of today. Sometimes, the disconnect between present and past could not be sharper. Take the extreme examples of Mount Everest, near whose top, at an altitude of over five miles, lie rocks from an ancient sea floor. Go to the North Face almost within sight of the famous Hillary Step, and you can find fossilized seashells. Similarly, where we work in the Arctic, temperatures reach minus 40 degrees Fahrenheit in the winter. Yet inside some of the region’s rocks are remnants of an ancient tropical delta, almost like the Amazon: fossilized plants and fish that could have thrived only in warm, humid locales. The presence of warm-adapted species at what today are extreme altitudes and latitudes attests to how much our planet can change: mountains rise and fall, climates warm and cool, and continents move about. Once we come to grips with the vastness of time and the extraordinary ways our planet has changed, we will be in a position to put this information to use in designing new fossil-hunting expeditions.
“If we are interested in understanding the origin of limbed animals, we can now restrict our search to rocks that are roughly 375 million to 380 million years old and that were formed in oceans, lakes, or streams. Rule out volcanic rocks and metamorphic rocks, and our search image for promising sites comes into better focus.
“We are only partly on the way to designing a new expedition, however. It does us no good if our promising sedimentary rocks of the right age are buried deep inside the earth, or if they are covered with grass, or shopping malls, or cities. We’d be digging blindly. As you can imagine, drilling a well hole to find a fossil offers a low probability of success, rather like throwing darts at a dart board hidden behind a closet door.
“The best places to look are those where we can walk for miles over the rock to discover areas where bones are ‘weathering out.’ Fossil bones are often harder than the surrounding rock and so erode at a slightly slower rate and present a raised profile on the rock surface. Consequently, we like to walk over bare bedrock, find a smattering of bones on the surface, then dig in.
“So here is the trick to designing a new fossil expedition: find rocks that are of the right age, of the right type (sedimentary), and well exposed, and we are in business. Ideal fossil-hunting sites have little soil cover and little vegetation, and have been subject to few human disturbances. Is it any surprise that a significant fraction of discoveries happen in desert areas? In the Gobi Desert. In the Sahara. In Utah. In Arctic deserts, such as Greenland.
“This all sounds very logical, but let’s not forget serendipity. In fact, it was serendipity that put our team onto the trail of our inner fish. Our first important discoveries didn’t happen in a desert, but along a roadside in central Pennsylvania where the exposures could hardly have been worse. To top it off, we were looking there only because we did not have much money . . . .”