Introduction
Standing at the edge of the Grand Canyon, it is hard, though not impossible, to imagine that this great landmark was carved by the Colorado River over millennia. It is a testament to the power of time and the slow but steady movement of water. Earth is a dynamic system, constantly changing. This change is sometimes explosive and sudden, but more often it is like the Colorado carving the Grand Canyon – slow and steady. Learning about Earth’s 4.5 billion-year history is a lesson in the slow and steady. It is a lesson that teaches us that given enough time, we can see considerable changes not only in geology but in the life forms that have existed.
In science classrooms across the country, students are shown an image of a clock. This clock represents the entire history of Earth in a 24-hour span. It isn’t until the clock strikes 11:58:43 p.m. that humans make their first appearance. In fact, it is not until approximately 4:00 a.m. on this 24-hour clock that living organisms, single-celled bacteria, come into the picture.1 Much of what we know about this considerable story is gleaned from ice and rock. Our ability to understand the past requires that we understand the clues that are given to us in the form of microscopic molecules to life-size preserved fossils to layers upon layers of sedimentary rock. It could be argued that it is the chapters in which life appears that the story gets really interesting. Unfortunately, our story is marred with holes, especially when it comes to the myriad life forms that have existed on Earth. As stated by evolutionary biologist Jerry Coyne, “true, this is a history book torn and twisted, with remnants of pages scattered about, but it is there, and significant portions are still legible.”2 It is through fossils that we are able to reconstruct these significant portions, and it is through our understanding of the relationship between evolution and environment that a robust narrative of the vast majority of Earth’s history can be written.
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