by Neil Shubin
Pretty much every fossil that tells of our history 6 to 2 million years ago comes from some part of this rift system. All of them show that walking on two legs is one of our most ancient human traits, vestiges of which are seen in the creatures from Chad and Kenya. Other finds reveal a number of species with smaller faces, relatively small canine teeth, large molars in back, and a large brain, human features that arise in succession. By 1.9 million years ago, other kinds of human relatives are found outside Africa. Their bones reveal creatures fully capable of walking, even running long distances. By 200,000 years ago, members of our own species, Homo sapiens, were roaming Earth.
Our weather instruments for this time lie inside dust, mud, and bone. Dust trapped in the sediments gives us a hint to the levels of aridity and the direction and speed of wind. Muds from the bottom of the seas tell of the water currents and the amount of water flowing from the Nile into the sea—a measure of rainfall. The sediment at the bottom of the big rift lakes tells how the water levels fluctuated over time. The presence or absence of antelope fossils tells us whether the landscape had forests or grasslands. The presence of hippo fossils tells us whether the area was wet. Even the length of the neck of giraffe fossils tells a tale: the length of the neck can tell us if tall or short trees were around. When you know how to look, almost anything can become a thermometer, a barometer, or even an anemometer.
The level of dust in the sediments of Africa waxed and waned. All the while, antelope roamed wider, while the location of hippos and giraffes became more localized. Closed forest landscapes gave way to ones that were more open and full of grasses as the climate shifted from wet and warm to dry and cold. Despite this directionality to climate change, in any short stretch of time the environment would swing back and forth dramatically.
As any resident of a city near a mountain range knows, elevated land can exert a huge impact on the nearby climate. The development of rifts in Africa heralded a new influence on the local weather: the mountains associated with them soaked up moisture and created rain shadows. Africa’s climate became fragmented between regions that were subject to torrents and those that were colder and drier.
Geographic and climatic transformation in Africa is linked to global changes. Between 2 and 3 million years ago, the ice ages began. Expanding glaciers led to lower sea levels, which, in turn, caused changes to ocean and atmospheric currents. The end result is a gradual transformation of East Africa from a land of forests to an open landscape of grasslands.
This planetary chain reaction ultimately affected our ancestors. The ability to walk on two legs that arose in forested environments in places like Chad, Ethiopia, and Kenya 7 to 4 million years ago now gained importance in the newly open savannas: our ancestors could roam large distances and use newly free hands to make tools.
The rapidly changing climates caused by changing planetary orbits and causes still unknown, characteristic of this time, also meant that any species that could adapt quickly would survive. If there is one thing a big-brained species can do, it is learn and adapt. In this crucible of environmental change—a blink of an eye in geological time—comes evidence of ever-new kinds of stone tools, shellfishing, hunting, painting, burying the dead, fire use, cooking, and Natufian agricultural societies.
CONNECTED
In the early 1980s, Stephen Stigler, a colleague of mine in the statistics department at the University of Chicago, was asked to contribute an essay to a volume celebrating one of the intellectual giants of sociology, Robert Merton. During his long career, Merton made seminal contributions to our understanding of how great ideas and innovations came to be. This effort culminated in 1957, when Merton delivered an address to a major academic society that exposed a curious pattern: as often as not, the idea we most associate with a single person was discovered by somebody else. Moreover, breakthroughs are frequently made simultaneously by different people working in different places. These are “multiples”: the discovery of the same idea or invention by different people working independently.
Charles Darwin, after his trip on the Beagle, described the principle of natural selection for a huge tome he was preparing. Then, as his manuscript was nearing completion, he received word that Alfred Russel Wallace had independently discovered the same concept during a bout of malaria fever suffered in Indonesia a few years before. People had been looking at, drawing, and thinking about our relationship to animals for millennia, yet the discovery of one of the fundamental principles about living things happened in a virtual instant by two people working independently. Gottfried Leibniz and Sir Isaac Newton discovered calculus independently and nearly simultaneously. Elisha Gray and Alexander Graham Bell invented the telephone in the same year. And the list goes on. Many great ideas seem to come to different people at about the same time.
Virtually every graduate student or young scientist lives in fear of this situation. What is the first thought most have when they find something really great? It is not always “Whoopee!” They have promising careers to start, and they need to gain a degree of credit for their work. Their main thought is often “Who else might have already found this?”
With numerous examples in hand, Stigler coined a now famous law. In brief, when referring to a named discovery such as Hooke’s law, Newtonian physics, or Darwinian theory of natural selection, he proclaimed, people should keep in mind that “no scientific discovery is named after its original discoverer.” Steve Stigler named his law “Stigler’s law,” in a fitting tribute to its “discoverer” Merton and his disciples, who built on the work of the original “discoverer,” the father of social science, Francis Bacon. The recognition of the importance of multiples in invention has itself come about many times.
The rich history of innovation is not a linear path from one person to the next but the product of a social milieu with innumerable antecedents and, as a result, multiple inventors. The actual inventor is often less important than the context in which things transpired; rather, something is “in the air” at the time of discovery. Those antecedents include the necessary conditions for inventions to happen and for the opportunities to develop and sustain them. Seeing the importance of antecedents and context, Bacon in the sixteenth century famously said, “Time is the greatest innovator.”
Our bodies and genes are layer after layer of biological inventions integrated with one another over billions of years. Consequently, the biological world, just like the technological one, is loaded with multiples. The ability to breathe air, for example, has come about many times in fish, as have fins capable of moving fish about on mud and land. Lungs, or their equivalents, are seen in a number of freshwater species: some breathe with a saclike lung; others have added blood vessels to other parts of their bodies. Walking, too, is a common behavior in fish and is seen in species as different as frogfish, mudskippers, and epaulette sharks. Some fish even climb trees. Multiples are not just seen in fish; every species, even our own, reveals them in one form or another. Stigler’s law applies to organs, as it does to theorems and devices.
Our own change did not happen in a vacuum: here too time was our greatest innovator. A human being could no more arise during the Devonian era, 375 million years ago, than could an iPad have been invented in the eighteenth century. It took a great number of antecedents for legs, feet, or silicon chips to come about in the first place.
What is “in the air” for the biological inventions is the state of the air itself and its links to rock, water, and life. The bipedalism that was so important to our species was possible only because of changes that happened in fish, worms, and other creatures. The shift from fins to legs happened as our ancestors went from living in water to walking on land. Transport in time to the world of 380 million years ago, when streams and oceans were teeming with life: fish large and small. In the water, it was a real “fish eat fish” world at this time. Big carnivorous fish over fourteen feet long swam in the same streams as numerous smaller armored ones. Land at this time was not a barren ecosystem.
Plants and all sorts of invertebrate animals invaded land first, establishing lush forests, shrublands teeming with life. The earliest fish walking on land hit an environment that was already hospitable: loaded with food and absent any predator. There was a push and pull for our distant fish ancestors to move to land. Any feature that enabled them to escape the large predators in water while capitalizing on the opportunities offered by the life on land would be a distinct advantage.
Plants helped define the geography and environment for our ancestors’ shift to land. Plants have root systems that allow soils to form and thus solid banks for the streams in which our fish ancestors lived. The rise of plants on land, with their photosynthetic metabolisms, also is associated with enhanced levels of oxygen in the atmosphere. Human legs owe their origins as much to the history of trees, shrubs, and flowers as they do to that of fish.
But fish, legs, and plants are only one stop in the story of bodies; the origin of every tissue, cell, and gene is the product of an interrelationship between the planet and living things. Were it not for algae and moving continents, the cellular machinery for legs—or for any of our organs for that matter—would not even exist. That change built on billions of years of history: from the imbalance of matter over antimatter after the big bang to the workings of the solar system to the recycling of Earth’s crust that made our species possible. Our antecedents on Earth are as much our long lineage of animal forebears as they are the planetary and cosmic events that have been intertwined with us, and our history, since the beginning.
The American philosopher William James often said that religious experience emanates from “feeling at home in the universe.” With bodies composed of particles derived from the birth of stellar bodies and containing organs shaped by the workings of planets, eroding rock, and the action of the seas, it is hard not to see home everywhere.
PAST AS PROLOGUE
The team had followed the trail of ancient rift valleys from the east coast of Greenland to the foothills of Morocco’s Atlas Mountains. The layers of rock in northwest Africa took the forms of eroding sandstones and shales we were familiar with in the Arctic: trade the polar bears for goats, glaciers for small villages, and the underlying geological setting felt like home. So were the scientific goals: our success in Greenland gave us the experience and the thirst to find new places to look for ancient mammals and their tiny teeth.
As Farish and I descended to a promising valley of dusty red sandstones, our attention was pulled from the hike by the clatter of donkeys, a cue that local villagers were nearby. Typically, this kind of encounter involved a goatherd or a pack of small children. Their curiosity about us, coupled with their naturally slapstick sense of humor, enlivened many a fruitless day looking for fossils.
The approaching sound gave way to the image of two old men whose bright eyes and wide smiles belied bodies crumpled by age. The donkeys were smaller than their riders, who bore huge toothless grins, faces creased by the sun, and hands and feet gnarled and callused by years of hard labor.
The men had something to tell us, and not speaking a word of Berber, Farish and I employed the usual mix of sign language and facial expressions to convey our thoughts. It was clear that the two were trying to communicate something important, but for the life of us we couldn’t follow them. Finally, in a measure of desperation, one of them took a worn, yellowed piece of paper from his robe and passed it to me. An old picture revealed it was his ID card from his days as a laborer. Then a lightbulb in my head went off: he had worked with the French paleontologists who studied these sites twenty years before, and he was trying to form a connection with us. Farish looked at the document, read the print faded by creases and years of wear, and shook his head. He exhaled softly and said, “I’m older than these guys.” Farish, a fit man in his fifties, looked about forty years younger than our companions.
The impact of the planet was written over the faces, joints, and bodies of our Berber friends. Culture, technology, and economics mediate and referee our interaction with Earth. We don’t need to travel to Morocco to encounter these effects; we can cross a couple of city blocks in Manhattan or Chicago to find vast differences in longevity, literacy, infant mortality, various cancers, diabetes, obesity, and heart disease.
In our past, vagaries of the planet and its geography defined the sharpest distinctions among our ancestors, whether fish, reptile, or human. But the equation is now changed, and the roots of this shift can be traced to something first seen in the rocks in Africa about 3 million years ago.
The first stone tools were used to butcher meat. Since then, we’ve invented devices to perform every function imaginable, from growing food to traveling under the sea. We exchange information in ever-changing ways, from alphabet to voice to digital. Our history has been one of gizmos, medicines, and technologies to make our thoughts real and expand the possibilities of our lives.
With virtually every technology and idea, our species has found new ways to insulate itself from the planet. Ever since the Natufians, stable communities that use agriculture have buffered us from the vagaries of depending on migrating animal populations for food. Clothes insulate us from changes in the weather. Tools and machines allow us to reach beyond the limitations of our physical bodies. We’ve even made objects that enabled us to leave the gravitational pull of the planet itself and extend our senses to other celestial bodies.
Human creativity and biology are like different instruments in an orchestra: they play separately, but together they make one score. The advent of cooking is written inside our guts and in the genes that form them. The origin of agriculture is reflected in the structure of our DNA. Our technological and cultural inventions impact our biological selves. But our biology—so defined by big brains, dexterous hands, and speaking organs—makes these inventions possible in the first place. Biology and culture have been the yin and yang of the human experience on our planet.
Are we in the process of breaking a balance that has been part of us since our beginnings in ancient savannas, forests, and caves? If we take a time machine and return to the planet one thousand years from now—or 1 million for that matter—what will control how fast we run, how long we live, or how much we learn?
Baseball statistics provide clues. We’ve now reached the point where the measurement of human abilities—hitting home runs, for example—needs to be subdivided between those whose performance has been enhanced by technology and those whose has not. Given new technologies that reveal how drugs can impact everything from strength to cognitive abilities, we may find that Nobel Prizes in the distant future will need the same treatment. It has been eleven thousand years since the dawn of human civilization. With the ever-increasing pace of change around us, imagine what humans will be capable of in another eleven thousand.
Seeing the impact of this technology, we can ask: Is Darwin no longer behind the steering wheel? Has the result of millions of years of human evolution been to disconnect us from the planet and from evolution itself?
Biologists use a variety of tools to put numbers on the ideas that sprouted from Darwin. This is not idle fancy; numbers and equations help us do what science often does best: make and test specific predictions. Phrases like “survival of the fittest” simply don’t cut it. To predict evolutionary change, we need real numbers describing the traits creatures possess, how they are passed on to each generation, and the ways they can impact the success of creatures in their environment. In the evolutionary sense, we define these numbers, particularly “success,” as precisely as possible: in the number of viable offspring that a particular creature has over its lifetime in a particular environmental setting. If red birds have more viable offspring than green ones in one place, and color pattern has a strong genetic basis, then, all else being equal, over time natural selection could act to increase the number of red birds. Natural selection never disappears; if certain criteria are met, then it is an inevitable outcome.
Doing this kind of work on humans is complex, and
the numbers we need can be inferred only to an approximation. Ideally, we’d have studies with huge sample sizes that would allow us to follow whole families to see how traits are passed on. The best records of human traits come from large clinical studies designed to follow the health of populations over long periods. The venerable Framingham Heart Study, for example, started in 1948, continues to this day and has followed about fourteen thousand people, recording births, number of children, and a variety of different traits and causes of death. Other studies have looked at arterial disease, reproductive health, and psychological factors. The largest batches of data come from countries with a civil registration system. Denmark, for one, has assembled data on 8 million people, including everything from fertility to family history.
The biologist Stephen Stearns and his colleagues crunched these data and were led to a humbling insight. There is a very big difference among people in different parts of the world. In the developed world, with its access to medical care, food, and technological wizardry, most evolutionary pressure is on aspects of fertility: when we have offspring and how many we have. In the developing world things are very different: passing on one’s genes is about mortality, particularly that of children. In one world, evolutionary success is derived from the age at which people have babies; in the other, such success is derived from survival itself. Socioeconomic, cultural, and technological differences mediate the ways evolution acts in human populations.