by Meave Leakey
These exciting initial results soon prompted a more determined effort to develop technology that could retrieve deeper cores. In the late 1940s, a Swedish expedition sailing around the world on the Albatross was able to retrieve cores between ten and fifteen metres long that represented a time interval of up to one million years. Their results recorded at least nine glacial periods during the Pleistocene. The cores also showed that layers of high concentrations of fossils alternated with layers of low concentrations, which reflected the glacial-interglacial conditions.
A second way that ooze can tell us about global climate was developed in the 1950s. Oxygen atoms in water, like the carbon atoms in grasses, are not all the same. In its stable form, oxygen can have either sixteen or eighteen neutrons. The much rarer 18O isotope is heavier than 16O. Because 16O evaporates more easily than 18O, ice sheets are almost entirely composed of this lighter isotope (ice sheets build up when snow precipitation does not melt from one season to the next). This means that during periods of glaciation, when much of the evaporated water is locked away in miles-thick ice sheets, there will be a higher ratio of 18O to 16O isotopes in the remaining seawater than during hotter, wetter intervals. This isotope ratio is captured by the foraminifera, which use carbon dioxide dissolved in the water to build their calcium carbonate shells. In the same way that fossil tooth enamel preserved the ratio between C3 and C4 plants in the diet and documented the climate change we recorded at Lothagam, foraminifera shells preserved in the ocean sediments bear a signature of the mixture of oxygen atoms in the seawater in which they lived. As foraminifera skeletons settle on the sea bottom, changes in the ratio of 18O to 16O are recorded layer by layer in the ocean sediments and bear witness to glacial and interglacial epochs through the ages.
Engineers have now perfected sea-core drilling techniques that enable them to penetrate deep into the vertical miles of fossil-bearing sediments on the ocean floor to extract cores several kilometres long that represent millions and millions of years. But in the meantime, a second obstacle had presented itself: how could the various sea cores taken from distant points around the vast oceans be correlated with one another to build a global, composite climate picture?
This is where Giuseppe Folgheraiter enters the story. He was studying ancient pottery and noticed that the clay somehow took up the magnetic signal from the earth. In 1906, Bernard Brunhes expanded on Folgheraiter’s 1894 treatise on the magnetic alignment in baked pottery and bricks when he deduced that ancient lavas were likely to behave the same way. When Brunhes set out to measure the magnetism of ancient lavas, the result was surprising: some of the lava flows had inverted magnetic fields. His radical conclusion was that at some points in time the earth’s magnetic field had reversed (so a compass would point south, not north). But this novel and unprecedented idea went down among the scientific community like a lead balloon.
Some twenty years later, the Japanese scientist Motonori Matuyama found that ancient lava flows in Japan and Korea also showed at least one magnetic reversal in the Pleistocene. Nobody believed him either. Then, in the 1950s and 1960s, scientists found that ancient lava flows in Russia, Iceland, and Hawaii also exhibited the same phenomenon, and a critical mass of evidence seemed to persuade the sceptics. By this time, new potassium-argon dating techniques had been invented using the rate of isotopic decay of an unstable potassium isotope into an unstable isotope of argon—the dating method we used to date ash layers at Turkana. This potassium isotope has a much longer half-life than carbon, and this allowed us to obtain much older dates than using the radiocarbon dating technique (which can be used accurately only on the first 50,000 years). Since volcanoes are chockablock with easily datable materials, the timing of the magnetic reversals could now be ascertained by using the new method to date the ancient magma flows on land.
We are now in the last epoch of “normal” polarity that started in the Pleistocene, which was named the Brunhes Normal Epoch in 1963 in honour of the French geophysicist. The last reversal took place 772,000 years ago, and the preceding period of reverse magnetism is called the Matuyama Reversed Epoch. Two short periods of “normal” magnetism were also discovered that fall within the Matuyama epoch. One of these short normals, which lasted approximately 40,000 years, is called the Jaramillo normal event after a small river in Mexico. The second and older normal was first found at Olduvai Gorge, and it is called the Olduvai event. It lasted for 30,000 years around 1.8 million years ago and is a very useful marker for palaeontologists.
The work on palaeomagnetism marked a true breakthrough. For lo and behold, the sea cores also contained evidence of the same magnetic reversals. By stacking the data from cores drilled from a variety of sites in the world’s oceans and correlating the magnetic reversals, trends in the global palaeoclimate have now been reconstructed that extend all the way back to sixty-five million years ago (although the record does become less detailed in the older sediments). Scientists are now able to read the wealth of information preserved in the cores—not just from the oceans but also from the ice of the Arctic and Antarctic—to an astonishing degree. The sea cores are startling in the extent that they support Croll and Milankovitch’s theory that changes in the earth’s orbit and tilt are primary drivers of climate change.
* * *
THE PRESENT-DAY (Late Cenozoic) glacial age has borne witness to the whole history of human evolution and long predates our initial split from the ape lineage. As the polar ice has accumulated, the tropics have seen a trend in increasing aridity marked by ever more violent fluctuations in climate. The shift towards a drier climate and the consequent opening of forests and woodlands that we documented at Lothagam between seven and five million years ago—and was already evident at Kanapoi between four and three million years ago—would continue in response to the accumulation of ice in Antarctica propelled by different Milankovitch cycles. The current glacial epoch got going in earnest 2.6 million years ago and marks the beginning of the Pleistocene. At first, the ice sheets were relatively small and changed little. Between 2.58 and 0.78 million years ago, the world’s climate got cooler, ruled largely by cycles of obliquity over 41,000 years that at particular points in time generated continental ice. However, glacials and interglacials only became clear at the onset of the Middle Pleistocene when eccentricity forces shift the earth’s climate to an approximate 100,000-year cycle of much greater temperature amplitude.
These warm intermissions between mini ice ages can make the terminology horribly confusing: when people refer to the last ice age, they often do not mean the last fifty-five million years, which encompasses the present glacial age. They are usually referring to only the last glacial epoch, which ended some 10,000 years ago before the current warm interglacial period that we are enjoying today began. From 2.5 million years onwards, once both poles had iced over, a marked global cooling trend sets in, leading to increased general aridity in the tropics punctuated by ever more violent swings and upheavals in the global climate. In the last 800,000 years, there have been at least eighteen separate advances and retreats in the ice that fall in approximately 100,000-year intervals under the telling influence of the eccentricity cycle.
On the grand scale, this is the icehouse world that Homo erectus evolved in, and it must surely have played a huge role in shaping our ancestor’s evolution. Many different strands of evidence in the burgeoning field of climate science all point to this time interval as being particularly variable. Dramatic climate change also led to a corresponding sweeping turnover of flora and fauna, opening new dietary niches and closing old ones. For her PhD, which she completed in 2001, Louise looked at the fossil record of bovids in the Turkana Basin to see if she could detect any significant changes in fauna in this very time period. The idea that changes in fossil biodiversity could be correlated with climate swings was first mooted by Elisabeth Vrba in the 1980s. Vrba studied African bovids from the Miocene to the end of the Pleistocene, and she brought together palaeontologists, climatologists, oceanographers,
and geologists to look at the evidence from many different angles. It was Vrba’s hypothesis that Louise tested for her PhD thesis.
But Louise had found that it was very hard to correlate changes in fauna with specific climatic events. For starters, there are large gaps in the sedimentary record. Added to these challenges, our dating instruments are still quite crude even with all the advances that have occurred since we started working at Turkana. Unless we find a way to date the fossil-bearing sediments themselves, our techniques depend on finding a datable volcanic layer above or below the sediments in which we find the fossils. All told, there is still a significant margin of error to our calculations of any particular geological slice of time that makes it impossible to precisely correlate local habitats to specific climate cycles—for there are simply too many climate shifts. Frank Brown, who knew the geology of the Turkana Basin better than anyone, pointed out that in addition to annual seasonal cycles and Milankovitch’s three megacycles spanning thousands of years, there are also decadal (ten year) and multidecadal cycles. So changes by a mere five hundred years could mean the difference between a maximum and minimum in a millennium (thousand-year) cycle. With our blunt dating instruments, all these climatic cycles on multiple levels mean that there is always a degree of shooting in the dark involved in our reconstructions.
Moreover, whenever we try to reconstruct ancient environments, we can’t be sure if the specific area being described is representative of a larger locale or if it is highly individual. This is clearly seen around the modern Kerio River in Turkana: the vast expanse of arid, barren landscape is abruptly replaced with a lush and dense riverine forest flanking the river. This forms a wildlife corridor for creatures large and small, including migrating elephants. Since deposition only occurs where there is water, a future palaeontologist sampling the fossil record along the modern Kerio could mistakenly arrive at the conclusion that in our time the entire area was covered in enough vegetation to support these large pachyderms and a host of other wildlife when in reality the narrow green belt was flanked by barren desert.
But while Louise could detect no clear evidence that confirmed Vrba’s 2.5-million-year fauna turnover event, there seemed to be more evolutionary action going on 1.8 million years ago. Her thesis results are corroborated by subsequent studies of the fossil fauna from the Omo Valley and East and West Turkana by scientists René Bobe, Gerald Eck, and Kay Behrensmeyer (the very same Kay who found those artefacts at Koobi Fora in 1969 and after whom the famous KBS Tuff is named). They found a definite trend over time towards increasingly arid-adapted bovids that favour open habitat. Furthermore, they found three peaks in species richness—3.8 to 3.3 million years ago, 2.8 to 2.4 million years ago, and 2 to 1.4 million years ago. And they saw four episodes of relatively high faunal turnover—the biggest and most significant of which took place between 2 to 1.8 million years ago, which also coincides with the time H. erectus evolved.
This evidence of increased faunal diversity associated with shifts in the global climate corresponds with the remains that the hominins left behind at sites known as living floors. These are particular stratigraphic layers where the bones, stone tools, and other remains that indicate hominin occupation and activities have been preserved, such as the large living floor Mary excavated at the Dear Boy site at Olduvai. Although the living floors do not reveal whether the butchered remains of animals were scavenged from other predators or obtained by hunting, they do give us rare insights into the hominins’ varied diet. At Turkana, archaeological sites dated between 1.8 to 1.4 million years show that the hominins—that we assume to be H. erectus—ate a huge variety of different species of different shapes and sizes. They include one type of elephant, one species of giraffe, and several species each of rodents, monkeys, small carnivores, zebras, rhinos, hippos, pigs, and antelopes. The hominins also ate turtles, crocodiles, fish, snakes, and birds. The living floors found at Olduvai Gorge show a similar diversity. In addition to these animals, whose fossils have been preserved, the hominins would almost certainly have also been eating birds’ eggs, grubs, grasshoppers, and crustaceans. All told, as a result of the changing habitat, there were lots of delectable choices on the H. erectus menu!
Yet, in spite of the massive new feeding opportunities presented by the many flourishing species of herbivores, previously successful specialist predators—sabre-toothed cats, false sabre-tooths, and hyaenas—were pushed to extinction by a competitor also at the 1.9-million-year mark. There is interesting research into the evolution of tapeworms that suggests these hugely successful parasites decided to jump ship—or at least broaden their options—as early Homo evolved and shifted from hunted to hunter.
Tapeworms have two different hosts during their life cycle—a “definitive” host that bears the tapeworm during its reproductive phase and an “intermediate” host that acts as a vector for the parasite. I, along with others far more knowledgeable about parasitology, had always assumed that people picked up tapeworms when they began to domesticate livestock. But molecular biologist Eric Hoberg and his colleagues have used divergence data analyses to identify two separate occasions when tapeworms switched definitive hosts and evolved into new species. Both times predate animal domestication, both occurred in sub-Saharan Africa, and both times the original host was from the carnivore guild (hyaenas, jackals, wild dogs, and big cats) and the new host was a hominin. Most intriguing of all, they estimate that one of these host switches occurred between 1.71 and 0.78 million years ago. This means that H. erectus must have taken the tapeworms with them when they migrated to Asia.
Like the specialist carnivores, many of the monkeys also largely disappear, particularly the bigger, more specialised ground-dwelling colobines. H. erectus was probably eating them too. There is a fossil site called Olorgesailie, not far from Nairobi, where I sometimes took the children for weekends when we didn’t have time to go to Koobi Fora. The site is relatively young, less than one million years old, and is famous for its impressive accumulation of Acheulian hand axes, which were made by H. erectus. Not far from the vast jumble of stone tools, there is a veritable graveyard of broken bones of a species of monkey called Theropithecus, and stone tools are scattered among the broken bones. When we compared the dietary adaptations of Theropithecus and Paranthropus in chapter 10, we found that some of these monkeys became really large. Those found at Olorgesailie show that at this point in time the largest Theropithecus were the size of a female gorilla and must have been truly impressive animals. I had long ago noticed that the monkey bones all shared extremely unusual and repeated breakage patterns. These breaks are not at all typical of carnivores or natural processes. For example, one of the arm bones, the ulna, was consistently split lengthwise rather than snapped into pieces by the teeth of a carnivore, and one of the foot bones called the calcaneum was always split longitudinally. The evidence strongly suggested that hominins frequently made meals of Theropithecus, and Alan Walker’s wife, Pat Shipman, subsequently studied the break patterns and found this to be the case.
Climbing up the food chain from a predominantly herbivorous animal to a meat-favouring omnivore would have had profound implications for the home-range requirements of H. erectus. Like all predators, individual groups of hominins would have required bigger hunting ranges in their search for meat. Combined with their newfound success as hunters, the increased aridity during glacial periods would have further increased the size of territory that individual bands of hominins needed. Their ability to move into these grasslands would have been uninhibited initially. But as the Homo erectus population grew with its successful new dietary strategy, this expansion across Africa could not continue ad infinitum.
Could erratic climate and a corresponding massive faunal turnover have served as a trigger for H. erectus to move out of Africa? During the more “normal” periods of glaciations that have characterized the climate with few intermissions for some three million years, the great impenetrable dryness of the Sahara rendered much of Africa inhospit
able. But the Sahara also prevented easy access to the land bridges out of Africa that would have been exposed when the great ice sheets lowered sea levels. But during the few windows of wetter, warmer, and more hospitable interglacial periods, a corridor through the Sahara would have opened—but rising sea levels would have cut off the land bridges out of Africa. Either way, H. erectus had to find its way through the formidable physical barriers that the weather extremes presented to dispersal before it could flourish across the globe.
16
The First Explorers
I sometimes idly wonder where I would go if I could conjure a time machine for just one trip. Backwards, almost certainly, after a lifetime of trying to fathom our past—but how far back? The world of Homo erectus is one I would be sorely tempted to pick above all the others. I would want to see for myself how this ancestor actually dealt with the ferocious carnivores, how they really hunted and scavenged with their rudimentary stone tools, and how far ahead they thought to store food for lean times. I’d like to know how far the bonds of social cooperation had grown and whether grandmothers enjoyed a brief period after menopause when they taught the youngsters how to survive the hardships of life in the Icehouse.
From the cozy warmth of my time bubble, I’d love to see the frozen spectacle of the earth gripped in the vise of vast sheets of ice during glacial periods. With the greatly lowered sea levels during these times, I would want to chart the altered shapes of continents and see land bridges where today there is endless deep blue ocean—for the intrepid hominins would not have travelled very far without these periodic land bridges that opened up migration routes. My time travel would also offer me a glimpse of the other animals inhabiting the world at that time; just as we have posited that the evolution of our own species was driven by the massive upheaval and alteration of the global climate, the same forces opened up new dietary opportunities for a plethora of new species across the animal kingdom. I would want to plug all the gaps in this story and see how much of it we’ve got right. Perhaps I could even sneak a peek at the other mysterious hominins, such as Homo habilis, who for a brief time were sharing the world with Homo erectus.