The Universe Within (27 page)

Some populations do not change; they die. The end of the last ice age in North America was a double whammy for the mammals that lived there. First, they had to deal with changing climatic conditions. On top of that, they had a new competitor and predator to deal with: people. The change in climate and the arrival of humans from Asia spelled the end for North America’s saber-toothed tigers,
mammoths, and ground sloths.

Still other populations change their way of life altogether.

Dorothy Garrod was known to her colleagues at Cambridge as being “cripplingly shy” and “difficult to know.” Yet she was anything but shy. “My dear Jean,” wrote Garrod to her cousin in 1921, “The last week in France was great fun. It was really almost too moving to be true. You crawl on your stomach for hours … climbing up yawning abysses (lighted only by an acetylene lamp…) and get knocked on the head by stalactites and on the legs by stalagmites, and in the end arrive at all sorts of wonders.” Here was a woman who explored ancient worlds, experienced raw adventures, and had a lot of fun doing it. Discoverer of Neanderthal bones in caves and new archaeological sites around the globe, this “shy” woman became the first female occupant of a chaired professorship at both Oxford and Cambridge.

Digging in
Shukba Cave and the surrounding fields near Jerusalem,
Garrod discovered odd
stone tools shaped like crescents. Nothing like them had been seen before. Then she unearthed a series of mortars, grinding stones, and figurines. The people who lived there had ground wheat and practiced religion.

More digging yielded more discoveries: carefully buried dog skeletons, shelters, bodies in graves with intricate decorations, even elaborate stone sculptures. These people, whom Garrod called Natufians, had the first domesticated dogs, the first sculptures of people having sex, and elaborate burial rituals. The Natufians had settlements with hundreds of people interacting in complex societies that changed over time. Previously, human populations were nomadic: populations adapted to changing climates and food supplies by moving. Natufians exemplify novel strategies: the development of a largely sedentary culture that ranged from mobile camps to semipermanent settlements over
several thousand years—ranging from fifteen thousand to eleven thousand years ago.

No population is insulated from changes to the planet, particularly the kinds of decadal climate shifts recorded in the polar ice. The Natufians lived during a period of rapid climate change about thirteen thousand years ago: a cold plunge brought
glaciers to high latitudes and cold, dry, weather to lower ones. This cold snap meant that traditional grains likely became more scarce. The Natufians and their contemporaties were almost certainly stressed by this shock to the global climate system, let alone to their food supply and way of life. How did they and the cultures that followed manage?

Plump seeds, typical of
domesticated plants, have been found in the remains of Natufian settlements from about eleven thousand years ago. Beginning as rare components in Natufian sites, kernels and grains become common in later human settlements. The seeds are evidence of
agriculture; the mortars and pestles are signals of a society using their crops for food. With these inventions, humans no longer needed to rely on the vagaries of migrating animals for subsistence. With the development of agriculture, and more permanent settlements seen in places such as those with Natufian culture, humans could now establish institutions and cultural practices associated with stable societies.

Just as Dorothy Garrod dug in the earth to discover Natufian culture,
Jonathan Pritchard, my colleague at
Chicago, peers within
DNA to see patterns in its structure and sequence. By comparing the DNA sequences of living humans, he can tell if our differences are due to the vagaries of chance or have been sculpted by the action of
natural selection. If a particular gene offered an advantage in survival or reproduction to the people who possessed it, it should leave a signal in DNA—one that he could see using statistical techniques he developed for just this purpose. All else being equal, if selection has operated on a gene,
it should be more common and less varied in a population than it would be by chance alone.

Jonathan has found stretches of human DNA that carry the signature of natural selection; these are genes that in some way affected the survival or reproduction of our ancestors. This is a kind of holy grail for biologists, because they can tell what biological traits were important. And what do these genes do? Some relate to
color pigment. If the spread of human populations across the globe brought them to areas with different light levels, the genes affecting pigmentation would change, with lighter pigmentations found in populations more distant from the equator.

Other genes reflect changes to the
diet. Genes that became common in some human populations relate to digesting
milk, carbohydrates, and
alcohol. The ability to process these products involves special enzymes that break down the characteristic sugars inside. The genes involved with these functions gained a new importance in the past ten thousand years. The ability to digest milk is evidence of the domestication of cows; processing alcohol relates to fermentation. Both are traits of
agricultural and, to some degree, sedentary human communities.

The effects of rotating planets and past chills are everywhere—from the sand on the beach to exotic boulders in the landscape, even to parts of our own DNA that persist, like the tunnels of Camp Century, as artifacts of changing climates and cultures.

B
y 8 million years ago the shapes of the continents, oceans, and seas would be recognizable to an elementary school class today. The planet looked decidedly modern, except for one important omission: it lacked a big-brained species walking on two legs.

Hints to the r
evolution afoot are first seen inside rocks about 7 million years old from what are today Chad and Kenya. A French team, working at the margin of a lake bed, unearthed a chunk of a skull that has a remarkable mix of traits. With large brow ridges above the eyes and a small cranium, it looks something like a chimpanzee. But the snout and face are far too small for any chimp: these traits are decidedly humanlike. More clues come from slightly younger rocks in Kenya. The portions of femurs and other leg bones that have been found are straight, much like those of a creature that spends time moving about on two legs. Something was happening, as new kinds of apes lived, and perhaps even walked, on the planet.

These creatures certainly didn’t know it, but the ground under their feet was changing. The continent of Africa was beginning to fracture. Upheavals deep within Earth caused the crust to tear, opening up a
rift that began to unzip the continent from north to south. The rip started small and widened to extend about two thousand miles, from Egypt south to Mozambique. As the process continued, these rifts, like those we chased in our own hunt
in Greenland’s 200-million-year-old rocks, caused bulges and depressions in the surface of Earth that formed a series of valleys with mountains.

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.

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.