by Neil Shubin
We, relatively big creatures that we are, barely think of these molecular forces when we go through our day. We swim in water and feel that the water is more viscous than air. But if we were a small creature, say a bug no longer than a quarter of an inch, these forces would dominate our existence, and swimming in water would feel like swimming in Jell-O. The surface of the water would take on new meaning. At our weight, we can dive right through to the bottom of a pool. Since a number of these molecular forces are at work on the surface of fluids, a bug can walk on it.
In 1968, F. W. Went wrote a classic paper, published in American Scientist, that explored the consequences of size for humans. His starting point was the seemingly absurd question: Could an ant begin a workday like a person? Understand that Went was not a crank; the discoverer of a critical plant hormone, member of the august National Academy of Sciences, on the faculty of Caltech, and, later, director of the Missouri Botanical Garden, he was an eminent Renaissance man of science. In the obvious “no” to Went’s ant question lie a number of profound biological truths. Went reasoned that an ant shower, of course, would be out of the question. The droplets of water, being bound by those molecular forces, would hit the ant’s body like cannonballs. A morning cigarette (the essay was written before the Surgeon General’s report had a major impact on our behavior) would also be impossible. The smallest effective size for a controlled fire is about that of the ant’s body. Saying good-bye to the spouse and kids would be different too. The ability to hear deep tones—slow vibrations in air—is possible only at larger size. Also dependent on size is the ability to hold a job in the first place. The development of brain capacity for thinking, forethought, and memory requires a certain body girth. The ant story makes one point abundantly clear: many of our abilities, such as talking, using tools, designing machines, controlling fire, and so on, are possible only because of our size. Size defines the opportunities of our species.
Our ancestors gained new possibilities when they made the shift from van Leeuwenhoek’s microscopic world to the Galilean one over a billion years ago: they left a world dominated by intermolecular forces and entered one more influenced by gravity. This great moment of our past is written inside our cells, deep within the rocks, and in the ways many of us die.
IN THE AIR
Impressions of disks, ribbons, and fronds in slabs of 600-million-year-old rocks are a pretty unremarkable bunch of fossils. But looks are deceiving. These fossils reflect revolutionaries, a new kind of individual that the world had not yet seen. These are the first creatures with bodies composed of many cells.
The advent of bodies changed the planet forever. A single cell is restricted in size because molecules can only diffuse over short distances, a measurement set by the laws of physics. This limitation affects how an animal can feed, respire, even reproduce. Small animals can transport oxygen across their bodies by simple diffusion. Once animals get large, they need new mechanisms to move nutrients and wastes about. How do they deal with this? Large animals have specialized systems to circulate blood, carry wastes, and capture and pump oxygen to their far-flung cells. These kinds of specialized organs are game changers in the world of size. Hearts, gills, and lungs are all inventions necessary in large animals. This complex machinery brings the opportunity to get even bigger and realize a world of new capabilities, as our ant would have experienced when starting its hypothetical workday.
Bodies may make a dramatic appearance in the fossil record, but if the genomes of living creatures hold any clues, changes were under way for a long time. The first 2.5 billion years of the history of the planet were entirely devoid of big creatures; then, by about 1 billion years ago, there were not one but several different species with bodies populating the ancient seas: plant bodies, fungal bodies, and animal bodies, among others. The origin of bodies wasn’t some magical event. The molecular tool kit that makes bodies and their organ systems possible—the proteins, large lipids, and other large molecules that allow cells to stick together and communicate—is not unique to creatures with bodies. Antecedents are present in small single-celled creatures that use versions of these same molecules to feed, to move about, even to communicate with one another. The biological mechanisms needed to build big creatures existed on the planet for billions of years before those creatures ever hit the scene.
What opened the floodgates and turned this potential for big creatures into a tangible reality? Insights, yet again, come from iron-ore-containing rocks.
Standing a wiry five feet six inches tall, Preston Ercelle Cloud Jr. (1912–1991) was one of the most imposing figures in all of paleontology in the postwar decades. Graduating from high school with a lust for travel and the outdoors, he entered the navy for three years, where he became the bantamweight boxing champion of the Pacific Scouting Force. Cloud put himself through school during the Great Depression, eventually rising through the academic ranks to become chief paleontologist for the U.S. Geological Survey. He was a stickler for detail in his fieldwork and commanded respect from his staff. When mapping rocks, he would often crawl on them, putting his eyes inches from the layers. During one of these sessions, he was slithering through juniper thickets in Texas and came face-to-face with a large rattlesnake. As his field colleague at the time said, “Pres was not easily bluffed: after a few minutes of staring at each other, the rattlesnake crawled away.”
Cloud had a talent for using close-up encounters with rock layers to see the global picture. In his eyes, the planet was one big interlinked system: the history of life and the workings of climate, oceans, and continents formed a single unified narrative. If the search for iron led to the discovery of early living creatures, then the iron itself led to understanding their links to the planet.
Preston Cloud. (Illustration Credit 5.4)
Iron-rich layers begin to appear in rocks about 2 billion years old on every continent. No matter whether on Australia, North America, or Africa, they generally form the same series of precisely layered reddish-brown bands. As anyone who has left wet tools in the garage knows, the color is a clue to the iron’s chemistry. Oxygen in the air turns iron reddish brown as it rusts.
This kind of rust is absent from the oldest rocks in the planet. When Earth formed over 4.5 billion years ago, the only major source of atmospheric gases was Earth itself. Volcanoes spew all kinds of molecules but precious little oxygen. We’d have an easier time breathing on the top of Mount Everest than on this ancient Earth. The bands of rust in mor recent rocks reveal the change: a global increase of oxygen in the atmosphere.
The oxygen in the atmosphere exists in a balance between the entities that produce it and those that consume it. Like a bathtub faucet running with a partially open drain, the level in the tub is the outcome of the rates of inflow and outflow.
Clues to the inflow of oxygen in the ancient atmosphere come from ancient life itself. Most of the single-celled creatures found in the oxygen-poor world had one very important thing in common. Judging by their closest living relatives, they used photosynthesis to make energy from the chemicals around them. Photosynthetic creatures use energy from the sun to make usable energy for their bodies. They do not use oxygen; they produce it. The only possible source for the oxygen that changed Earth is one that we see today—photosynthetic creatures. Today that bunch of organisms includes microbes and plants, but billions of years ago there was one main suspect. We see it inside the rocks that Tyler and Barghoorn peered at under their microscopes. With cell walls and distinctive colonies that take a range of forms—small clumps to toadstool-shaped masses—they look much like modern algae. Algae, quietly producing oxygen for hundreds of millions of years, gave breath to life on Earth.
Oxygen exists in a balance between the forces that produce it (algae) and those that consume it (reactions with rock, water, and gases).
If algae were the source of oxygen in the ancient atmosphere, what happened to the sinks that served to remove it? Some molecules in the atmosphere can remove free oxygen from the
air by binding with it to make new compounds. Volcanoes under the sea, for example, belch gases that are derived from the melting of the crust at the ocean bottom. These gases have a special feature: the molecules inside bind to oxygen, removing it from the air. There are a number of ideas to explain the rise of oxygen in the atmosphere. One hypothesis is that around 2 billion years ago, when oxygen became prominent, there was great geographic change: a reduction in the amount of oceans and the number of undersea volcanoes emitting gases that would remove oxygen from the air. With algae pumping oxygen and fewer mechanisms to consume it, the oxygen in the atmosphere increased.
Cloud put the different observations—bands of iron, algae pumping oxygen, and the origin of big creatures—all together. The inspiration for his synthesis lies in the structure of the oxygen atom itself. Oxygen is an atom that is greedy for electrons because it lacks two of them in its outer shell. The powerhouses of our cells—mitochondria—along with aerobic bacteria make use of this fact in their energy processing. Some metabolic reactions, such as respiration, have elaborate cascades that transfer electrons from one molecule to the next, where, at each electron-transfer step, energy is either stored in new forms or released. The more free oxygen there is about, the more fuel there is for living creatures to use.
Cloud also knew that being big costs energy. The proteins, such as collagen, that form much of our bodies take a relatively large amount of energy to build and maintain. The growth and maintenance of a body require a new kind of higher-level energetics.
Clues to the relevance of oxygen levels to animal life came from the observations made as early as 1919 by one of the fathers of the field of physiology. Schack August Steenberg Krogh (1874–1949) was fascinated by physical and chemical laws that govern the physiology of animals. One of them he found by looking at the correlations between the properties of water and the creatures that live in it. In simple marine creatures, ones with no elaborate circulatory or digestive systems, body size is limited by the amount of oxygen in the water. In oxygen-restricted environments, being big is physiologically impossible, so these creatures are restricted to small sizes. With enhanced levels of oxygen comes an increase in the sizes that creatures can attain.
Preston Cloud inferred that rising levels of oxygen—and the new energetic landscape it brought—defined a new realm of possibilities for ancient life. Oxygen lifted the lid on big: it fueled the push from microscopic creatures living in a world dominated by intermolecular forces to a planet with ever-bigger species with new kinds of bodies.
Oxygen also created an entire new world of danger.
Every change is a double-edged sword. The chemistry that makes oxygen so efficient at generating energy can turn it into a poison. A great receptor for electrons from other atoms, oxygen generates energy while it forms new compounds. Unchecked, these molecules can disrupt cells and damage DNA. A number of theories of disease and aging are based on these properties of oxygen. Every time you take an antioxidant like vitamin C, you are trying to fight the effects of these kinds of oxygen-containing molecules.
But there is a deeper challenge to being a big creature living in an oxygen-rich world. A human body is composed of an enormous number of parts—two trillion cells, thirty thousand genes—that function as a whole: organs, tissues, and genes work together to ensure the integrity of each individual. The balance among parts is defined by the ways our different cells attach to each other, communicate, and interact with the molecules that lie between them. When we’re healthy, each organ of our bodies “knows” how to behave; its cells continually divide and die, yet it remains roughly the same size and shape. Each eye is about the same size, as are each thumb and big toe. Our spleen and liver have their right sizes. This intricate harmony of parts is necessary for us to function as many-celled individuals.
Insights into this balance came from odd flies: members of a laboratory at Johns Hopkins noticed some flies in their breeding colony that had eyes way too large for their heads—five times bigger than normal. Being geneticists, they were well prepared for analyzing the genes. They isolated the DNA of the gene, traced its molecular activity, and discovered that it lies in a chain of molecular reactions that tell cells to stop growing. Controlling growth, either through slowing down the division of cells or by managing their death, is the centerpiece of building a harmonious large body.
Using their knowledge of the structure of the fly gene, the Hopkins team was able to trace it to mice and people. Not only is a version of the fly gene present in the genome of mammals, but mutations bring about organs of new sizes. The gene appears to behave in generally similar ways in flies, mice, and people, working as part of a molecular chain reaction to define a balance among parts. In mice, a mutation can bring about a liver five times the normal size.
But mutations of the gene also have another consequence. When cells lose their cues to stop dividing or dying at the right time, they can form deadly malignancies. The genes that define the ways our bodies get big also form the basis to destroy them.
Preston Cloud and others who followed him saw the harmony between living systems and planetary ones. The interplay between living things and their planet led to increasing levels of oxygen in the atmosphere. Oxygen, in turn, changed the world by allowing for the origin of big creatures with many cells. Life changes Earth, Earth changes life, and those of us walking the planet today carry the consequences within.
CHAPTER SIX
CONNECTING THE DOTS
Earth of 530 million years ago was still far from being a place we’d recognize as home. Breathing would have been like summiting Mount Everest without an oxygen tank; atmospheric oxygen levels, while having increased substantially since the early days of the planet, remained only a fraction of those today. Water was the happening place for life: new kinds of soft-bodied animals swam in the oceans. Land, by contrast, was a barren void lacking any sort of plant cover or soil. If you managed a hike on this strange landscape, you could have walked from what is today Boston to Australia without ever seeing ocean.
The modern world came about through changes to rock, air, water, and life. But, as the story of oxygen reveals, these constituents do not exist in isolation from one another. Earth’s history is the product of interlinked changes to the planet and the creatures on it. Learning to see the deep meaning of these grand connections, and the roots for one of the greatest scientific revolutions of all time, begins with one of the most seemingly mundane exercises of all—making maps. Maps tell us where we are and what our world looks like, and, when we use them in the fullness of geological time, expose the links that exist among oceans, mountains, and the organs inside our own bodies.
Children, as every parent knows, are driven to find patterns in the world around them. Take a globe or a map, and show it to kids about seven years old. Ask them to describe what they see, not the names of the regions, but the shapes of various continents, islands, and oceans. Almost invariably, kids will point out that parts look as if they fit together. The east coast of South America and the west coast of Africa seem to match perfectly.
In some quarters, even as recently as forty years ago, this simple observation reflected complete heresy. In junior high school, I had a teacher who lectured class on the pseudoscientific ideas that were in vogue in the late 1960s. One was the notion that the Egyptian pyramids, the Nazca lines of Peru, and the statues on Easter Island were influenced by the work of space aliens who would regularly visit our planet. Like most of the students, I thought the space alien idea great—it was on TV, after all. My teacher’s intent was to show that while these kinds of ideas may have been fun and fantastic stories, they could not be tested in any meaningful way. He lumped these tales of science fiction with another called continental drift, the idea that the continents have moved over time. This lecture transpired in 1972. Little did my teacher know of a great revolution that had its start nearly 120 years before.
In 1856, the brothers William and Henry Blanford arrived in India to w
ork in the famed Talcher Coalfield. This field is still producing today, as part of Mahanadi Coalfields Limited. While studying the layers of coal, the Blanfords, like all good geologists, looked above and below to see geological events surrounding its formation. One of the layers stood out; it was filled with huge, irregular boulders—some even the size of a person. The mystery deepened when they looked closely at the buried boulders themselves: each had slashes and gashes on the surface. If found in the Alps or in the high Arctic, these features would have been taken as evidence of transport by huge glaciers. But here, buried underneath layers of coal in equatorial India?
The boulder layer didn’t stop in India. Soon after the Blanfords’ discovery, geologists working in South Africa discovered the same series of rocks: a coal bed and a layer of irregular and gashed boulders. Were there glaciers in South Africa and India at some point in the distant past? How did ice get to these places?
Eduard Suess (1831–1914) was born in London and, after moving to Vienna, developed a passion to understand the workings of Earth. After school, Suess rose through the ranks to become a leading academic and, later, city councilman for Vienna. There, he put his knowledge of geology to use for the common good by spearheading the development of an aqueduct to bring freshwater into the city from the surrounding mountains. This act is thought to have saved a large number of lives, since typhoid outbreaks from dirty water were common in the city beforehand.
Suess’s view of the importance of rocks was revealed in a speech he gave to an international congress of geology in 1903, when he described the work of a geologist in this way: “The stone which his hammer strikes is but the nearest lying piece of the planet,…the history of this stone is a fragment of the history of the planet, and … the history of the planet itself is only a very small part of the history of the great, wonderful and ever changing Kosmos.” To Suess, each rock, properly interpreted, was a window into a world that stretches across the far reaches of space and time.
