How did the world as we know it鈥攆rom the soil beneath our feet to the air we breathe and the life that surrounds us鈥攃ome to be? Geologists have proposed one set of answers while biologists have proposed another. Earth and Life is the first book to reveal why we need to listen to both voices鈥攖he physical and the biological鈥攖o understand how we and our planet became possible.
In this captivating book, Andrew H. Knoll traces how all life is sustained by Earth鈥檚 geological and atmospheric dynamics, and how life itself shapes the physical environment. Taking readers on a thrilling journey across four billion years of Earth history, he shows how Earth and life interact to cycle the very elements of life from rocks, water, and air, and how these and related processes control our climate, regulate our atmosphere, and support the diversification of life-forms great and small.
How 鈥� and why 鈥� did you decide to work at the interface of Earth science and biology?
Andrew H. Knoll: When I entered college, years ago, I had no idea what I might do in life. I was good at math and so enrolled in an engineering program, but I soon came to realize that engineering and I were not destined to have a long relationship. At home, answering the question of my aspirations for the future with the short, but honest response 鈥淚 don鈥檛 know鈥� was not popular, so when I returned to college for my sophomore year, I took five science and math courses in the hope that something might rub off. By some minor miracle, two courses made a deep impression, one in geology and another in biology. Sitting in my room one evening, it dawned on me that these weren鈥檛 necessarily the separate universes they seemed to be by the way they were taught. If I were interested in the history of life, might I not benefit from knowing Earth鈥檚 physical history? And might an understanding of how organisms work inform my understanding of environmental change through time? With that, I saw my future: how have physical and biological processes interacted through time to shape the world we experience? Years of teaching and research have convinced me that we can鈥檛 understand our planet鈥檚 long history without drawing from both disciplines. And equally, we need both to understand the causes and potential consequences of 21st century global change.
What features of Earth may be rare, even unique, within the cosmos?
AHK: Many people would answer this question by pointing out that Earth is unusual and perhaps even unique in supporting life. It is certainly true that, at the present time, Earth is the only planetary body known to harbor organisms. That said, a recent survey found that, overwhelmingly, most scientists believe that life exists throughout the vastness of the universe. The logic is that even if only one out of every million planets or moons sustains life, the universe contains as many as two trillion galaxies, each potentially containing some 100 billion stars. That makes for a lot of potentially inhabited bodies.
Perhaps it is straightforward for life to originate, given the right physical conditions. But equally, habitable environments may disappear in time. As a young planet, Venus may have been habitable, but eventually it became fiendishly hot, the result of a runaway greenhouse. Mars, as well, was once at least transiently (and repeatedly) warm and wet, but now it is prohibitively cold and dry. Perhaps, then, what makes Earth unusual is not that life took root here, but that it has persevered for some four billion years, time enough to evolve technological humans. Just maybe, this can explain Nobel laureate Erico Fermi鈥檚 famous question about life in the universe, 鈥淲here is everybody?鈥�
Why do we need to draw on both Earth science and life science to understand the history of oxygen in the atmosphere and oceans?
AHK: Consensus holds that life could not have begun on a well-oxygenated Earth. Yet, we and many other species could not live in the absence of oxygen gas in the atmosphere and oceans. If both of these statements are true (and they are), then our planet鈥檚 present day surface is not the result of initial conditions established at Earth鈥檚 birth, but rather reflects profound changes through time that shaped and reshaped the world around us. The geological record shows that our planet was two billion years old before even moderate amounts of O2 began to accumulate in the atmosphere and surface oceans. Life, itself, is immediately implicated, because only photosynthesis that generates oxygen can provide the O2 needed to transform our planet. Perhaps, then, the global rise of a permanently oxygenated atmosphere and surface ocean simply reflects the evolution of oxygen-generation by photosynthetic bacteria cyanobacteria. The problem is that both geology and biology strongly suggest that cyanobacteria existed long before Earth became oxygen-rich, so the answer must lie in factors that kept cyanobacterial oxygen formation below the rates at which organisms and physical processes removed O2. The story is still evolving, with no shortage of twists and turns, but the focus is no longer on when photosynthetic bacteria began to generate oxygen, but how much they produced. Rates of oxygen production were controlled by nutrient availability, especially phosphorus, and P input to the oceans reflects continental weathering, and, so, our planet鈥檚 tectonic history. Unequivocally, an improved understanding of Earth鈥檚 story of O, so critical to understanding evolution through time, will require the integration of insights from both biological and physical processes.
Which organisms form minerals, and how does this help us to understand the histories of life and environments recorded in rocks?
AHK: Animal, vegetable or mineral. To Carl Linnaeus, the great 18th century Swedish naturalist, these were the three forms of matter. To most modern readers, however, they recall a children鈥檚 game with simple questions and answers. A clam is animal, grass is vegetable, and a quartz crystal is mineral. Makes sense, but digging a bit deeper, how do we classify the clam鈥檚 shell? Clam shells are made of the calcium carbonate minerals calcite and aragonite, set within a framework of organic molecules: minerals to be sure, but minerals precipitated by organisms. Scientists call these biominerals, and they play a major role in the conversation between Earth and life.
Many groups of animals fashion skeletons of calcium carbonate, from corals and sea urchins to clams and snails. Even some ants have been found to precipitate carbonate minerals within their organic exoskeletons. Some species of red and green algae also form an armor of carbonate minerals, and a few groups of single-celled eukaryotes do the same 鈥� foraminifera and coccolithophorid algae are prolific carbonate precipitators. Today, nearly all of the limestone accumulating on the seafloor consists of biominerals, giving Earth鈥檚 sedimentary record a decidedly biological bent.
Silica (SiO2) is another key biomineral, not much used by animals other than sponges, but abundantly found among unicellular groups, including radiolaria and diatom algae, which pretty much single-handedly remove silica from seawater. As is true for carbonates, silica deposition to form chert primarily reflects the biology of these biomineralizers.
We are biomineralizers, as well, forming our bones from calcium phosphate minerals, and organisms distributed across the Tree of Life form a diverse range of minerals within their cells, serving functions from nutrient storage to navigation. Indeed, by examining the distribution of carbonates, chert and other sedimentary rocks through time, we can reconstruct the evolutionary history of biomineralization 鈥� Earth and life in ever changing conversation.
How does the past inform our understanding of 21st century global change?
AHK: In the 21st century, humans have become important participants in the conversation between Earth and life. Within my lifetime, concentration of the greenhouse gas carbon dioxide has increased by a third, global temperature increase has reached the 1.5 degrees Celsius suggested to be a warming point of no return, glaciers and polar ice have declined noticeably, and global sea level has risen by an average of four inches since 1993, increasing by as much as eight inches in some places.
How does this compare to environmental changes through Earth history? To begin, atmospheric CO2 levels were higher than anything projected for 2100 during most of our planet鈥檚 history, and temperatures were commonly higher, as well. >Sea level has repeatedly risen and fallen by meters, not inches. (One of my grad school professors liked to summarize geologic history as 鈥渢he seas go in, the seas go out.鈥�) Through most of this history, life carried on smoothly.
This may seem reassuring, but two observations give cause for concern. Five times in the past 500 million years, Earth experienced catastrophic extinctions, sufficient to change the course of evolution.< They don鈥檛 correspond to moments of CO2, temperature or sea level maxima, but rather high rates of change.< In the past when environments changed rapidly, biological diversity plummeted, and we live at a time when environmental change is unusually fast. The second observation is obvious, although seldom discussed. The human footprint of cities, farmlands, and more makes us particularly vulnerable. In the past, sea level rise was seldom associated with catastrophe, but now consider a world with Miami, Venice and Bangkok 鈥� it鈥檚 a different story. All in all, the historical record deciphered from rocks provides an important distant mirror on our own time.
Andrew H. Knoll is the Fisher Research Professor of Natural History and Earth and Planetary Sciences, Emeritus, at Harvard University. His books include A Brief History of Earth: Four Billion Years in Eight Chapters and Life on a Young Planet: The First Three Billion Years of Evolution on Earth (快色直播). Recipient of the International Prize for Biology and the Crafoord Prize in Geosciences, he is a member of the National Academy of Sciences.