Introduction
It’s a daunting task to set out to chronicle the history of the world in a single book. And I don’t mean just the history of human beings and our various achievements and downfalls, I mean the entire history of the planet, from its formation in the spinning clouds of gas and dust from which our Sun and solar system came to be some 4.5 billion years ago, to the inevitable destruction of our planet in the eventual death throes of that same benevolent star some 5 billion years from now. Of all the things that have happened on, in, and around our world, which of them warrant consideration as being among the 250 most important milestones in the history of Earth?
I’ve taken a stab at answering that question, based on my own training as a geologist and planetary scientist; my own background and experience in field work, remote sensing, and computational analysis of data; and my own biases. For example, much of my professional teaching and research work focuses on planetary and space science—studies of other planets and solar system bodies, such as Mars and the Moon, using earth science and the study of our own planet as a basis for studying others (and vice versa!). So, I tend to think of the Earth not just as our home and the home of millions of other species, but as a member of a family of planets, moons, asteroids, and comets that orbit the Sun in our cosmic neighborhood. Indeed, studying the Earth from space, and studying other planets to learn about our own, is a major way that we’ve learned much of what we know about our home planet.
When I teach earth science, I make sure to point out that studying the Earth is like studying a series of nested, intertwining spheres. There is the lithosphere, the rocky surface and interior of our planet; the atmosphere, the thin layer of gases that warms the surface and sustains life; the magnetosphere, the magnetic bubble that protects our world from harmful solar radiation; the hydrosphere, a thin surface shell of water mostly in the oceans but also in seas, lakes, rivers, glaciers, and polar caps; and finally the biosphere, the collection of all living things on our planet. Each of these spheres has been critically important to the history of our planet, and they are all interrelated in complex ways that cannot easily be untangled. To truly understand the Earth as a system requires one to understand all these spheres of influence.
Thus, the history of the Earth spans topics in physics, chemistry, biology, astronomy, astrobiology, geology, mineralogy, planetary science, life science, public policy, atmospheric science, climate science, engineering, and many other scientific and social disciplines and subfields. I’ve tried to capture milestone events and discoveries that span all these fields, and to thus hopefully give a sense of the breadth of experience and expertise that goes into figuring out how our world came to be the way it is, and what will happen to it in the future.
Along the way, I’ve singled out about 120 individuals—out of the many thousands of scientists, explorers, inventors, and others who have made it a career to learn about our planet—who have made or contributed significantly to events and discoveries that I have flagged as notable milestones. Some of these people are well known (like Plato, Leonardo da Vinci, Magellan, Newton, Pasteur, Lewis & Clark, Darwin, Cousteau, and Goodall), others are famous within their academic or exploration circles but not so well known to the general public (like Steno, Hutton, Bowen, Wegener, Carrington, Agassiz, Humboldt, Dobson, Amundsen, Peary, and Van Allen). And still others have made critical contributions to the understanding of our world but for some reason have been left relatively obscure in the sometimes-fickle annals of history (like Chladni, Brock, Anning, Nadar, Dokuchaev, Bascom, Griggs, Angel, Norgay, and Lehman, all of whom you will learn more about soon).
As part of my research into these milestones, I was especially impressed by the significant role that many women have played in advancing our understanding of the planet. For most of the history of science, it has been a male-dominated career, with active traditions and other barriers established to keep women out of the club. That began to change—slowly—in the nineteenth and twentieth centuries, although the obstacles were huge early on. Pioneering research and discoveries about the Earth as well as its inhabitants made by women such as Florence Bascom, Dorothy Hill, Inge Lehman, Mary Leakey, Rachel Carson, Dian Fossey, Kathleen Sullivan, Sylvia Earle, and many others attest to the fact that women are just as capable as men in the pursuit of, and success in, science. Still, the fact that women do not yet make up 50 percent of the world’s population of professional scientists means that many gender-based barriers and biases—conscious and unconscious—still exist in earth science and other fields. There is still much work to do.
Another aspect of diversity that I have attempted to capture among these milestones is geographic diversity, both on and within the planet. For example, I point out examples of major mountain belts on all the continents, sampling and celebrating the many different styles of mountain-building (orogenesis) that have occurred throughout our planet’s history. I also point out examples of the major kinds of rocks and minerals that make up our planet, including how they are formed and what role(s) they may also have played in human history. The onion-skin-like layers of the interior of our planet—core, mantle, and crust—also each deserve special attention, for they each play a specific role in the way our planet gives off its internal heat, the way Earth generates a strong magnetic field, and the way the continents and oceans change over time. And you’ll see a major thread run through the book related to the theory of plate tectonics—the way the Earth’s crust is divided into a few dozen major pieces and the ways that they interact with each other to form new continents, ocean basins, and islands, as well as potentially catastrophic earthquakes and volcanic eruptions. Plate tectonic theory provides the foundation of our modern understanding of the way Earth’s surface changes over time.
I also thought it important to mark, as major milestones, the boundaries of Earth’s major periods of geologic time, as reconstructed by modern geologists. These boundaries are part of the internationally accepted Geologic Time Scale, a copy of which is included in an Appendix here for handy reference, courtesy of the Geological Society of America. For example, by far most (almost 90 percent) of the history of our planet was in a single geologic time span called the Precambrian, about which we know relatively little because there are so few rocks and fossils preserved on the surface of our dynamic planet from more than about 550 million years ago. Starting around then, however, in a significant milestone known as the Cambrian explosion, marine organisms began to create hard exoskeletons that remained preserved as fossils after the organisms died and fell to the seafloor. Fossils have provided key milestone markers in Earth’s geologic history ever since, including evidence for at least five episodes of mass extinctions, where huge fractions of all the species on Earth died off, rather quickly. One of these milestones, the disappearance of the ancient dinosaurs and many other species around 65 million years ago, has been linked to the climatic and food chain catastrophe created by the impact of a large asteroid. The other mass extinction milestones remain mysteries, however, with multiple hypotheses like impacts, extensive volcanism, and rapid climate change being explored for their origin in ongoing research and debate.
Yet another daunting task for this research was trying to assign specific chronological dates to many of the discoveries and events chosen here as milestones. When did the Atlantic Ocean form, exactly? When did flowers first appear on the Earth? When will the next ice age begin? In the case of many events in Earth’s history, especially in the deep past (or far future), there is considerable uncertainty or debate about the timing. Thus, in cases where the chronological timing of key events is uncertain or broad or both, I have indicated the best-known approximate date or range of dates with a “c.” (the Latin abbreviation for circa, meaning about) in front.
I’ve also chosen, in some cases, relatively modern milestone dates for past events or features about the Earth that might not be amenable to precise pinning down. For example, the discussion of many of Earth’s biomes (specific ecological zones) as important aspects of the Earth system comes up throughout the book, but when did tundra first appear on the planet? Or the first tropical rainforest? Or for that matter, when was the first hurricane, tornado, wildfire, or landslide? For those kinds of temporally nebulous milestones, I’ve picked important dates that we humans have since related to those specific events or ecological zones, such as the hurricane that devastated Galveston, Texas in 1900, the 1973 creation of the Monteverde Cloud Forest Reserve in Costa Rica, or the United Nations designation as 2011 as the “International Year of Forests.”
Finally, you might notice that many of the milestones that I’ve chosen have to do not just with the Earth, but more specifically with the development of life on our planet. When did life appear? How and when did photosynthesis originate? When did the first mammals appear? How about the first Homo sapiens? All of these and many other highlights in life sciences are milestones worthy of mention, not just for our species, but for our planet. While we now know that there is a short list of other places in our solar system which might be or might have been habitable at one time (places like Mars, Jupiter’s large moon Europa, or Saturn’s small moon Enceladus, for example), Earth is the only place that we know of so far that is not only habitable, but also inhabited.
For all we know, the origin and evolution of life on Earth could be a completely unique occurrence in the entire universe. Or, because we now know that there are likely countless Earth-like worlds in our galaxy and beyond, and that the components and conditions that make a planet like Earth habitable and inhabited are relatively common in the cosmos, perhaps the universe is teeming with life on similar habitable worlds, with each occurrence supremely tuned and adapted to its own unique environment following universal principles like evolution and natural selection. Either way, life on Earth is still special, and understanding the milestones related to life here will help us in the search for life elsewhere. As my mentor and hero Carl Sagan was fond of saying, “We are a way for the universe to know itself.”
It is perhaps more important now than it has ever been in human history for us to understand our home world as a complex, interdependent set of systems, and especially for us to understand the special role that our species plays in that set of systems. We are not the first species on Earth to change the overall climate of our planet (the first were the cyanobacteria—blue-green algae—that developed the remarkable ability to “poison” the early Earth’s atmosphere with massive amounts of oxygen via a brand-new innovation called photosynthesis, starting around 3.4 billion years ago). However, we are the first species with the ability to recognize that we are doing so, and with the power to do something about it. What will we collectively do with that knowledge? How will we collectively wield that power, if at all? These are profound questions that are at the heart of our relationship with our home planet.
Yes, it is indeed daunting to try to sum up everything there is to know about our planet in such a limited space. But just think of what we’ll learn. Read about our planet, follow the Notes and Further Reading links and pointers to learn more, and ponder your role in it all. . . . Enjoy!