The Sediments of Time

by Meave Leakey

  Since walking is inherently more stable than running, especially in bipeds, a second aspect of biomechanics significant to our enquiry is stabilization, particularly that of the head and trunk. Trunk stabilization, which stops bipeds from toppling over, is primarily accomplished by contracting the gluteus maximus (the buttock muscles). Head stabilization is more complex but no less important. Everyone has experienced the disorienting sensation of being stationary beside a moving train or other vehicle and believing that the vehicle you are sitting in is the one moving. In the same way, spinning rapidly in a circle makes us dizzy. Our balance relies on the interplay between the balancing structure in the inner ear (the labyrinth) and the eye. When we move, the labyrinth senses the movement and signals to the brain, which then corrects for the moving image on the retina so our gaze seems stable to us. When quadrupeds run, their heads are counterbalanced by the weight of their bodies and their somewhat horizontal necks. As bipedal humans thrust the weight of their bodies forward, there is nothing to counterbalance the head, which is thus subject to rapid pitching. If we didn’t feel this pitching movement through heightened sensitivity in the inner ear, we would not be able to stabilize our head and we would fall over in a dizzy heap.

  Dan and Dennis found that there are indeed some indications of specialization for running in H. erectus. The shape of the pelvis indicates that the areas for the attachment of two muscles critical in trunk stabilization are much larger in early Homo compared to Australopithecus. And complementary research by Fred Spoor, Bernard Wood, and Frans Zonneveld shows even more concrete evidence of features that relate to head stabilization. High-resolution CT scanning techniques show that the semicircular canals in the labyrinth of the inner ear are larger relative to body mass in H. erectus than in Australopithecus and chimps. The presence of such heightened sensitivity is what gives the individual greater balance during running, which involves more pitching than walking or climbing through trees. Taking all this evidence together, Dan and Dennis conclude that it is difficult to think of any human activity other than running that would have selected for increased sensitivity to head pitching.

  We can make an educated guess and say that H. erectus had both the cognition and the physique for endurance running. This could be the solution to the conundrum of how H. erectus could survive and thrive in the opening habitats. It didn’t need sophisticated tools nor did it need to scavenge to avail itself of a whole new feeding niche—a niche that was its ticket to a bigger-brained future.

  15

  The Icehouse

  When Samira was at school learning geography, our fruit basket was filled for a few weeks with oranges with all the earth’s continents drawn lovingly but crudely around their smooth skins—a line circling the girth of the orange to represent the equator, the stem representing the North Pole and an abstract and misshapen Antarctica scrawled around the bottom over the South Pole. “You be the sun, Mummy, stay still!” she would cry as she walked wide circles around me until she got dizzy. But she always maintained the orange stem at the same tilted angle as she orbited me, her sun.

  If the earth were oriented “upright” on a true vertical axis, the equator would always be the closest part of the earth to the sun because of the earth’s curvature, and the north and south poles would be permanent twilight zones. Yet the equator is the closest point to the sun only on days in March and September (the equinoxes). As Samira earnestly explained to me using her orange, there would be no distinct seasons anywhere on earth without this titled axis.

  Samira’s geography teacher naturally did not tell her that it’s a lot more complicated than this. The earth’s seasons, in fact, depend on three different cycles of long-term changes in our orbital pattern around the sun that last different intervals of time and affect the amount of solar radiation beamed to earth. This heavenly variation is what has shaped the massive climatic shifts that have continually reconfigured the landscape over millions of years—and the evolution of all animals, including our ancestors, was shaped ineluctably by this process.

  At the time when Homo erectus appeared in Africa, climatic variations were particularly extreme. Tim Flannery puts it most succinctly in his gripping yet depressing account of our role as perpetrators of climate change, The Weather Makers.

  The environment of these distant ancestors was very different from the one that we inhabit today, for their world was dominated by an icehouse climate, in which the fate of all living things was determined by Milankovitch’s cycles. Whenever they conspired to expand the frozen world of the Poles, all over the planet chill winds blew and temperatures plummeted, lakes shrank or filled, bountiful sea currents flowed or slackened, and vegetation and animals alike undertook continent-long migrations.

  Homo erectus is remarkable for how it was able to spread across the globe and flourish for nearly two million years through many intense climatic upheavals. It is all the more notable that its contemporaries (the other Homo species and Paranthropus) eventually died out between two and one million years ago. We cannot fully comprehend Homo erectus’s dispersal to the far reaches of our planet and its longevity as a successful species without first trying to fathom the world it inhabited—and this entails an abbreviated side trip into the fascinating and immensely complex global climate system. It is a story replete with unexpected twists and turns, and seemingly insurmountable obstacles faced by colourful characters. Each new startling idea was met with a frosty reception, and many different branches of science were involved in eventually finding the evidence to prove as fact what was initially dismissed as fantastic fiction.

  THE FIRST CYCLICAL SHIFT that affects the earth’s seasons relates to the degree to which the earth is tilted. The 23.5-degree angle that Samira was taught at school actually represents the current position, which is midway in a range of 22.1 to 24.5 degrees that cycles approximately every 41,000 years. This three-degree difference in the angle of tilt in the earth’s axis obviously accentuates or moderates the severity of seasonal differences in climate and makes summers and winters much harsher than when the earth’s tilt is at its least severe. Climatologists call this shift in the earth’s tilt “obliquity.” And as we shall see, the dates of the equinoxes are not immutable either.

  There is a second major cyclical change in the earth’s orbit called “eccentricity.” This is determined by the celestial movements of other planets. If the sun and the earth were alone in the universe, the earth would make an even circle around the sun each year. In reality, however, the earth’s orbit is also shaped by gravitational forces exerted by other planets, mainly the heavyweights Jupiter and Saturn and occasionally Venus. The changing proximity of the other orbiting planets affects the strength of gravitational forces pulling on the earth’s orbit and alters the shape of our path around the sun from a rounded to an elongated ellipse in a cycle spanning roughly 100,000 years.

  Since its orbit is elliptical rather than round, the earth is not always equidistant from the sun in its annual journey. It reaches its closest point (the perihelion) around January 3 each year, which is currently a difference of three million miles from its farthest point (the aphelion). But based on the shape of the ellipse at different phases of the eccentricity cycle, the changing distance of the earth from the sun dramatically affects the amount of solar energy reaching earth. During the most elliptical phase of the 100,000-year eccentricity cycle, the earth receives some 30 percent less sunlight at the aphelion than it does at the perihelion, introducing yet another cyclical impact on the degree of the earth’s seasonality.

  There is yet a third cyclical pattern, which is due to a wobble in the earth’s orbit caused by the slight equatorial “bulge” in the girth of the planet and gravitational forces exerted by the moon and the sun. For me, the easiest analogy is a figure skater whose spin is off balance. While her skates are spinning from a fixed position on the ice, her body is not rotating symmetrically around this fixed point, and her extended arms and second leg appear to seesaw as she spins. The e
arth’s wobble is just one rotation of the figure skater’s spin, but it occurs in super slow motion over some 26,000 years. The earth’s wobble also has an impor­tant effect on the earth’s seasons because, combined with the elliptical shape of our orbit around the sun, it affects the position of the four cardinal points—the summer and winter solstices and the two equinoxes—in a slightly shorter cycle lasting around 23,000 years. Today, the North Pole is tilted most directly towards the sun on June 21. But in another half cycle 11,500 years from now, the South Pole will be tilted most directly towards the sun on June 21. This gradual progression of the cardinal points along the orbital path is called the “precession” of the equinoxes

  This 23,000-year precession cycle has left its indelible mark in the sediments at Turkana. When we first began working at Koobi Fora, we constructed our camp out of thatch bandas using the tough reeds that grew along the lakeshore. Our banda met its unfortunate demise when the paraffin freezer started a fire that reduced it to a small pile of ashes. Building a replacement stone structure took some time because we carried a few flat slabs of sandstone back to camp at the end of each working day rather than stopping our fieldwork. Each slab was unique and packed with interesting fossils, mostly fish and molluscs. One particularly attractive piece received pride of place by our front door, and it boasted a large and perfectly preserved hippo anklebone.

  For years, I had pondered these distinctive bands of hard sandstone packed full of snails that we consistently found sandwiched between finer and softer layers. In its undisturbed state, the banding looked to me like a repeating sequence that cropped up all over the Turkana Basin. Frank Brown was the first to look into the reason behind this banding that is most clearly shown in the sediments in the Omo Valley in Ethiopia. With accurate dates from volcanic layers above and below the bands, Frank and his colleagues were able to work out the length of time represented by each repeating layer in the Kibish Formation of the Omo.

  The cyclical patterns of orbital eccentricity, obliquity, and precession influence global climate, which in turn drives environmental changes that shape evolution. These processes have been particularly important for the human story in the last million years.

  Sure enough, the stacked sequence of floodplains seems to have been laid down with astounding regularity as if the swing of a pendulum on an old grandfather clock marked a time period of about 20,000 years to the tune of the precessional cycle. These regular bands of molluscs were laid down close to the shore of a lake that was shrinking and expanding with the variations in climate. In between the mollusc bands and the layers of lacustrine sediments and occasional beds of algal stromatolites marking wet spells, there are softer fluvial deposits (the fine sandy layers laid down on the banks of rivers). These are distinctive for their fossil root casts and other organic matter derived from the ancient riverbanks and floodplains that advanced as the lake receded. Craig Feibel and his students also found the exact same signals at Koobi Fora even though they are less obvious than in the Omo Valley.

  There is another link that ties neatly into the same story. Some of the rainwater feeding the lake and river system in the Turkana Basin drains from the Ethiopian Highlands to the north. These highlands are also the source of tributaries that feed into the Mediterranean and the Gulf of Aden to the east where scientists have extracted long sea cores from the ocean floor. When the fluvial sediments in Turkana mark a dryer spell, these deep-sea cores tell a story of thick clouds of dust that blew off the Sahara and the Arabian deserts, settled over the water, and percolated gently down to the sea floor. Conversely, when the lake in Turkana swelled during the wet part of the precession cycle, the inflow of vast volumes of fresh nutrient-rich water from the Ethiopian Highlands into the Mediterranean was a boon for all manner of tiny organisms. These are preserved in the sea core as thick layers of organic matter known as “sapropels.” Because the sea cores also meticulously preserve layers of volcanic ashes that are accurately dated, we can ascertain that the Mediterranean dust and sapropel layers are the flip side of the same story unfolding in the sediments of Turkana. Of ten sapropels known from the Mediterranean during the Late Pliocene and Early Pleistocene, nine have probable known correlates in the Koobi Fora sediments.

  These three cyclical patterns—obliquity, eccentricity, and precession—are the Milankovitch cycles that drove the wild fluctuations and extremes of the “icehouse” climate of Homo erectus. They are named after the brilliant Serbian scientist who hypothesised in the 1920s that the celestial movements of planets influenced the wax and wane of ice ages. Others before him had also puzzled over the existence of past ice ages. Bernard Friederich Kuhn attributed the erratic placement of boulders in his native Switzerland to ancient and extensive glaciation. The idea that the earth’s surface—and its climate—could change this much was highly controversial for the prevailing worldview in all Christendom was that God had made the world and all its creatures in the form they are found today. Consequently, the subject of past ice ages was long and fiercely debated among geologists before it gained broader acceptance.

  It was James Croll who first correctly deduced the effects of our changing orbit around the sun on global climate. Impoverished and almost entirely self-educated, Croll’s humble and poorly paid position as a janitor at what is now the University of Strathclyde in Glasgow gave him unfettered access to all the great books in the library. Here, in 1864, he turned his attention to finding an explanation for the waxing and waning of ice ages.

  Croll asserted that the changes in eccentricity acting in concert with the precessional cycle would be capable of setting off a response in the global climate system that could result in an ice age. He termed the long interval when eccentricity is large enough to induce glaciation as a “glacial epoch” and the intervals separating them as “interglacial epochs.” The next part of Croll’s argument is key: that the astronomical changes are merely a triggering mechanism whereby a decrease in the amount of winter sunlight favours the accumulation of more snow. Ice is white, of course, and its greater reflective properties are a great coolant because much of the solar energy beamed to the earth is reflected back out to space. Thus the reflective properties of ice and snow magnify and augment the orbital effect. This is what climatologists today call “positive feedback.”

  Croll published Climate and Time in Their Geological Relations in 1875 that summarized his findings and earned him huge recognition. The only problem was that while the length of the precessional cycle had been calculated, methods to determine the exact ages of glacial deposits had not yet been invented. Croll’s ingenious theory could not be tested, and after much intensive debate, it almost fell by the wayside in the absence of conclusive proof. It was fortunate that his ideas were retrieved from obscurity by Milutin Milankovitch many years later.

  Milankovitch, blessed with a formal education, a considerable intellect, and the self-confidence of relative youth, ambitiously set out to “develop a mathematical theory capable of describing the climates of the Earth, Mars and Venus, today and in the past.” This endeavour would take him thirty years to complete, and his labourious manual calculations are the bedrock of modern climate science. When Milankovitch’s epic work was at last complete, geologists worked in earnest to try to find proof of actual glacial and interglacial periods that corresponded to or disproved his mathematical predictions, but limitations in dating techniques hindered the debate raging among the geologists in the first half of the twentieth century. And it was James Croll who had correctly predicted many decades earlier where such evidence might be found: on the sea bottom, where he thought “skeletons, shells and exuviates of creatures that flourished in the seas of these periods” would be preserved. This indeed proved to be true. An 1872 British expedition found that the sea floor was blanketed with sediments with the evocative name of “fine-textured oozes” that were composed almost entirely of the fossilised skeletons and shells of sea-dwelling creatures as predicted by Croll.

  Tiny floating pla
nktonic and bottom-dwelling organisms called foraminifera live within small shells made up of calcium carbonate. Radiolarians, another group of plankton, have skeletons made of more durable silica rather than calcium carbonate. Whenever either group of plankton outgrows these shells and sheds them for the next size up—or when they die—it is their discarded shells that sink to form the ocean floor. In parts of the ocean exceeding four thousand metres in depth, the skeletons disintegrate under the sheer pressure as they rain down through the depths. However, vast areas of the sea floor, especially in temperate and tropical seas, do carry a fossil record of their remains that goes back hundreds of thousands and even millions of years.

  Croll correctly intuited that some planktonic organisms live only in warm water while others survive in cold. As the global climate swings between one extreme and the other, the geographic range of these temperature-sensitive species shifts. But this evidence remained hidden for decades. A way had to be found to extract a core of the sludgy ocean sediments without interrupting the sequencing of the layers.

  The first attempts to get a sea core at the beginning of the twentieth century were based on the rudimentary principle of ramming a hollow pipe into the seabed. The longest cores that could be achieved with this type of technique reached three metres. But these cores showed three distinct bands of ooze, and the layers did not all bear the remains of the same plankton. One species of foraminifera played a key role at lower latitudes: Globorotalia menardii, which is abundant in warm water today. This was present in the top and bottom layer but not in the middle layer. The presence of G. menardii was a clear indication that the sediments were deposited during a warm interglacial period. And the change between the upper layer and the second layer without any G. menardii happened very abruptly an estimated 11,000 years ago.