by Neil Shubin
Given that mechanisms of DNA function and cell division are dependent on internal clocks, it should come as no surprise that a number of medicines are most effective at certain times of the day, when our brain anticipates the level of light. Our susceptibility to disease, and our treatment of it, carries the deep signal of a planetary cataclysm that happened over 4.5 billion years ago.
Dotting the landscape of southern Indiana are cemeteries that house the graves of Europeans who settled in the region in the late eighteenth century. This was a hearty crowd whose harsh existence is recorded on their tombstones. Few lived past the age of forty, and judging by the carved dates, the cemeteries were busy places some years. By an accident of geography, these settlers found a near-perfect material for their headstones. The fine grains and hardness of each stone preserve etchings from the early nineteenth century as sharply today as when they were originally carved.
We are so accustomed to looking at the front of tombstones that it is easy to overlook other edges that have stories to tell. The sides of the pioneers’ grave markers are not even; they are composed of a series of ridges with sharp edges and small depressions. The stones were quarried from a pit near Hindostan, Indiana, from an exposure that reveals how the rock was formed. Hundreds of millions of years ago, this part of Indiana lay under the sea. Year after year, sediment settled from the tidal waters, leaving small ripples in the mud. There is a rhythm to these marks, recording the variations of the tides throughout the year. As Earth spun and the moon circled it, the water rose and fell, only to be recorded as ridges in the sediment. The sides of the grave markers reveal the tidal rhythm from a moment when Earth rotated faster and the days were shorter than today. Time is sculpted everywhere on these tombstones, by the work of man and of the planet. The bodies in the graves and the rocks that mark them are united by the history they share with colliding and rotating celestial spheres.
Tombstones from the Hindostan quarry (left) have ridges that correspond to the changing tides (right). (Illustration Credit 4.2)
CHAPTER FIVE
THE ASCENT OF BIG
As the young sun pulled matter into its orbit over 4.6 billion years ago, lumps of rock and ice smashed together and combined. The cataclysm that gave us the moon was only one of many. Judging from the ages of the craters on the moon and meteorites here on Earth, collisions were the order of the day until about 3.9 billion years ago. Then this violent start gave way to a long period of relative quiescence.
In this quiet Earth lay a scientific puzzle that confounded scientists. The very top, or youngest, layers held fossils—the different shells and bones that can be seen in any natural history museum today. Lying below were ancient layers of rock with no evidence of anything living inside: no fossil bones, markings of animals, or plant spores; no evidence of any living thing whatsoever.
This lifeless layer of basement rock wasn’t just a sliver; the barren layers were miles deep. All of human history—and the entire known history of life on the planet itself—were confined to a thin veneer of crust. If the entire history of Earth were scaled to a year, with its formation on New Year’s Day and the present being December 31, Earth was utterly lifeless until mid-November. Translate this relative timescale into years, and we find that about 4 billion years of the history of our planet was devoid of living things. To Charles Darwin, the abrupt appearance of life was an “inexplicable mystery.”
The solution to Darwin’s mystery, along with clues to how our modern world emerged, came from a source no one could have ever predicted.
The steelworks of Gary, Indiana, stand like hulking fossil skeletons of a thriving bygone age. In the 1950s, humming mills fed a burgeoning automotive industry, with plants sprinkled across the Midwest. The need for iron was great, and geologists such as Stanley Tyler worked to feed it by studying the iron-ore-containing rocks in the region.
Because the rocks that contain iron tend to be among the oldest rocks around, ore geologists like Tyler focus mostly on the geological basement. As Tyler knew, these rocks were great for industry, lousy for paleontology.
One afternoon in the mid-1950s, Tyler was studying samples he collected from a deep test pit in northern Michigan. Rock chips from different layers were brought back to his lab in Madison, Wisconsin, where each was ground thin and placed on a slide to observe the fine structure of minerals and grains. Sitting with a checklist and a microscope, Tyler performed the usual scoring and counting of the color, grain size, and mineral content that are the necessary but rote measurements of much geological work.
Elso Barghoorn. (Illustration Credit 5.1)
When Tyler looked at one of the slides under the scope, he saw something familiar yet completely out of place: coal. He knew that coal reflects ancient plant detritus and that most of the coals known at that time were from layers no older than 350 million years, when plant life was abundant. But Tyler also knew the age of his sliver of rock from Michigan. It was almost 2 billion years old. Something was entirely wrong.
Not believing his own eyes, Tyler discreetly passed the rocks to experts. Shuffling from one specialist to the next, the samples eventually ended up in the hands of Elso Barghoorn, a curator of early plants at the Harvard Museum of Comparative Zoology. Scanning the slides under a microscope, Barghoorn immediately confirmed Tyler’s hunch. Tyler had found the earliest life yet known on the planet—coal-forming algae and other microorganisms.
This answer led to a whole new puzzle. The more Barghoorn looked at Tyler’s slides, the more ancient species he saw. Each sample was brimming with algae spores, filaments, and the remains of thousands of single-celled animals. The basement of rock on our planet wasn’t devoid of life but teeming with it.
Now the “inexplicable mystery” lay not in the missing life but in understanding the plethora of living things that were once hidden from our view. When did life get its start on the planet? What did the earliest living things look like? Because there was a world of questions to ask and creatures to find, a new kind of paleontologist emerged—one whose life’s goal was to recover microscopic fossils from rocks billions of years old.
Hunting for fossils in the first 2 billion years of Earth’s history has special challenges. Rocks of the right type to hold super-ancient fossils are likely to have been eroded away, baked by internal heat, or transformed by various movements of Earth’s crust. Let’s say you actually manage to find ideal rocks. How can you tell that a microscopic bleb or filament was once a living thing and not some mineral or inclusion in the rock? The science of early life is one of constructing multiple lines of evidence, from the shape and structure of the putative fossils to the chemistry of the rocks that hold them. The hunt is for creatures that not only look like single-celled forms but also reveal the chemical workings of metabolisms.
With this playbook, Stanley Tyler, Elso Barghoorn, and their scientific descendants exposed the hidden reality inside the rocks: the earliest fossils are now known to be over 3.4 billion years old. Life arose early in the history of our planet and, once off to that start, expanded rapidly to become a menagerie of different kinds of bacteria, algae, and their relatives.
Despite their incredible diversity, the organisms that dominated the first billions of years of the history of our planet share one important thing. They are all single celled and microscopic. Some of them lump together to form colonies, but no individual dwelling on Earth for the first 3 billion years was larger than a grain of rice. Big, in the world of living things, had yet to come about.
THROUGH THE LOOKING GLASS
“That one’s dead,” Thomas Barbour, the director of Harvard’s Museum of Comparative Zoology, shouted while considering a frog lying motionless on the grass in front of him.
On the roof above Barbour stood his colleague Professor Philip Darlington, holding a bucket of frogs with one hand. With the other, he was pitching the frogs one by one onto the lawn five stories below.
Barbour nodded as each frog hit and remained motionless in the grass
below his feet. When Darlington descended with his empty bucket, he asked Barbour how the animals withstood the impact. Seeing the frogs strewn about, Barbour offered, “All dead.”
Darlington was a naturalist of the old school: when he wasn’t teaching courses at Harvard College, he was off in the jungle collecting new species, beetles in particular. Tales of his days in the field are legendary, including the time he was grabbed by a crocodile, pulled to the bottom of a stream, and, as the crocodile began to consume him, kicked himself free. Hiking miles to safety with shredded legs and hands, he wrote to his wife later that night only that he had an “episode with a crocodile.”
In the midst of his explorations in the 1930s, debates were raging about how animals dispersed to new places around the globe. This was the era before plate tectonics, and there were two major ways to explain animal distributions: either there were land bridges between continents—now lost—that allowed animals to walk from place to place, or animals could be blown by wind, water, and storm. Darlington was a firm believer in the latter and his boss, Thomas Barbour, in the former.
The frog “experiment”—something we would obviously never perform today—began as an argument. During coffee at the museum one afternoon, the two got into a tussle about the theories and made a wager. Barbour held that dispersal by wind was impossible because animals would die upon impact. Darlington countered that wind dispersal would work over considerable distances for small animals. The two agreed on the rooftop test of the theory.
And what of Darlington’s dead frogs?
Within minutes after the impact, each frog arose and hopped away. Soon the grass was filled with frogs bounding in different directions. Darlington proved his point.
Of course, there is nothing unique about frogs that allowed them to survive such a fall; this ability is a reflection of their size. Small animals accelerate more slowly during a fall than do large ones, since they experience more air resistance for a given mass. In describing this phenomenon, J. B. S. Haldane, one of the founders of evolutionary genetics, said, “You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away.… A rat is killed, a man is broken, a horse splashes.”
Let’s say you want to predict what an animal can do—how long it lives, how it moves about—and what it looks like without ever having seen it. A number of factors may be influential: the kinds of foods the creature eats, where it lives, where it sits in the food chain, and so forth. People have explored this issue by cataloging measurable traits that creatures possess and hitting the data with a number of different statistical tools to gauge which measurement accounts for the differences we see. In analysis after analysis, one factor reigns supreme in its predictive power—size. Know a creature’s size, and you can make educated guesses about much of its biology, including its resting heartbeats per minute (smaller animals have higher heart rates), its perception of danger (larger animals have less fear), even its life span (in general, larger means longer).
Virtually every part of the world we experience is influenced by our size, even how we visualize size itself. The size and shape of our pupils, eyeballs, and lenses influence our visual acuity just as the shape and structure of the different components of our ears affect the sound frequencies we hear. Because ours is a world tuned to the predators, prey, and other entities of our ancestors’ worlds, we are like a radio that can receive only a small number of channels; vast portions of the world remain hidden to us. Extending our gaze beyond the limitations of our biology has meant seeing our size, and ourselves, in a brand-new light.
Anton van Leeuwenhoek (1632–1723) spent much of his career as a draper and found himself needing to develop magnifying glasses to assess the quality of his fabrics. Becoming fascinated by the properties of glass, he manufactured new kinds of lenses that magnified objects many times beyond the tools common to his trade at the time. He tweaked the shape of the glass again and again, each time seeing smaller things, ultimately magnifying objects two hundred times. With each new lens he crafted, he was exploring a new world.
Van Leeuwenhoek was famously secretive about how he crafted his lenses. For centuries it was thought that he polished the glass into ever-finer slivers. Then, in 1957, a science writer for Scientific American speculated on van Leeuwenhoek’s trade secret: he made his lenses by heating glass rods and pulling them apart. Reheating the broken end made a little ball at the tip. When this little glass bead was separated from the rod, he mounted it in a mechanical contraption that held both the specimen and the bead at a set distance. Peering through the glass bead revealed its magnifying properties, and the bent glass served as a kind of lens.
Everything became fodder for van Leeuwenhoek’s microscope. In one famous experiment he took the plaque from an older gentleman’s mouth and put it in his scope. In it, van Leeuwenhoek found “an unbelievably great company of living animalcules, a-swimming more nimbly than any I had ever seen up to this time.… Moreover, the other animalcules were in such enormous numbers, that all the spittle … seemed to be alive.” This is thought to be one of the earliest known descriptions of bacteria. He looked at pond water and found a carnival of life inside—from algae to microbes—and later described human semen as containing little tadpole-like creatures.
People flocked to see van Leeuwenhoek’s cabinet of wonders in his house in Delft. There, they became the first humans to catch a glimpse of a novel world. For thousands of years all of human knowledge was centered on the universe we can hear, touch, and see with our natural-born senses. By extending beyond our biological inheritance, van Leeuwenhoek revealed we are all big creatures living in a world chock-full of innumerable microscopic ones.
Anton van Leeuwenhoek and his microscope. (Illustration Credit 5.2)
Just a few decades before van Leeuwenhoek’s revelations with a microscope, Galileo Galilei (1564–1642) was doing the exact opposite: grinding glass to make a telescope. With the most powerful telescope of his day, equivalent to a large set of binoculars from an outdoors store today, Galileo was able to see Venus’s phases, that Jupiter had moons rotating around it, and that huge nebulae populate the sky.
Van Leeuwenhoek looked down through a microscope to find a small world. Galileo looked up to the sky and revealed a huge one, with incomprehensibly large planets and vast distances. In van Leeuwenhoek’s world, we are humbled by the diversity of microscopic life beneath our noses and within our bodies; in Galileo’s, by the sheer size of the world around and above us. How did this new humility come into being? By finding new ways to use glass.
Galileo and his etching of the comparison of the leg bones of an elephant and a mouse. (Illustration Credit 5.3)
Around 1633, over twenty years after Galileo constructed his telescope and described the rotation of the bodies of the solar system, he was found guilty of heresy and sentenced to be imprisoned for the remainder of his life. Because he was already seventy years old, he was placed under house arrest, first in Siena and later in his own home in Florence. During this period of confinement, Galileo spent about five years writing about physics. He was forbidden to publish in Italy, so a Dutch printer, Louis Elzevir, secreted his manuscript out of the country.
Galileo’s book—different from any science exposition we are familiar with today—consists of a fictional conversation among three men who set out to explore the fundamental laws of the universe. Their conversation holds the beauty of mathematics applied to the world around us.
On day two of the confab, the three explore the laws that govern the shapes of all objects. What happens to objects when they get bigger? How do small objects differ from big ones? Think of trees: short trees can get by with relatively narrow trunks, but tall trees are an entirely different matter. Assuming the properties of the wood are the same, tall trees will need proportionally wider trunks to protect them from bending and breaking. This simple relation between size and shape defines much of the world around us. A lithograph of upper leg bon
es from Galileo’s book reveals it all. A mouse femur and an elephant femur are similar in many ways, because they have the same joints, and they are composed of similar bones. But the elephant femur is proportionally much wider than that of the mouse. Just as with the tree trunks, larger size necessitates new shapes. This relationship holds for dinosaurs and elephants as much as it does for bridges and buildings. And the reason, as Galileo recognized, is because larger entities have to deal with ever-increasing effects of gravity.
Galileo envisioned that the gravitational pull defining the orbits of celestial bodies also has an effect on animal and plant organs. Bodies are pulled to Earth to a degree that is proportional to their mass. Heavier creatures, being pulled relatively more, need to change their shape to support themselves. This relationship even explains Darlington’s rooftop experiment with frogs. Lighter animals accelerate less during a fall than do big ones for these same reasons. The force of gravity can mean the difference between life and death for large creatures like us.
Gravity is not a significant factor for creatures that dwell in van Leeuwenhoek’s microscopic world. Look no further than at a fly or an ant on a wall. For the fly, the gravitational pull of Earth on its body is negligible; the forces that are really important are the ones that bind molecules together. A fly can stick to a wall because these sticky, and tiny, molecular forces are proportionally more significant than gravity for a light animal. Imagine a hippo on that wall: gravitational pull would far exceed the pull of the molecules on its feet. No amount of molecular Velcro would work to keep the hippo stuck to the side of a room.
