Jungle

by Jungle (retail) (epub)

  As we have already seen, however, modern environments are not always a good example of what an area looked like in the past. Fossil animals found in the same geological layers as hominin fossils provided some more direct clues based on feeding preferences, and generally supported a shift to more open habitats towards the Pleistocene, though it remained hard to work out how much of a type of food a particular hominin species ate. Up stepped my own methodological field, stable isotope analysis, which relies on the fact that the abundant elements like carbon can have different atomic forms. These “isotopes” (in this case 13C and 12C) have different masses, causing them to react differently during important biological processes such as photosynthesis. Crucially for us, when carbon dioxide is processed by the different types of photosynthesis practiced by the C4 grasses that dominate subtropical and tropical savannahs and the C3 herbs, shrubs, and trees that dominate subtropical and tropical forests, the heavier isotope, 13C, is more weakly or strongly discriminated against, respectively. This leads to different isotopic ratios in the plant tissues formed that are so clear as to be measurable, not just in the plants themselves but also in the well-preserved teeth of the animals that eat them, including hominins. First pioneered using hominin fossils in South Africa in the 1980s and 1990s by my former supervisor, Professor Julia Lee-Thorp, the now significant dataset resulting from this method, covering Ardi through to Homo, shows a clear shift in reliance from forest or wooded habitats to savannahs through time. While this revolutionary work could not tell whether hominins ate animals feeding on C4 plants or the C4 plants themselves, alongside the shapes of hominin bones, fossil animals, and stone tool evidence, it apparently directly confirmed an increasing role for savannahs in our evolution between 4 million and 2 million years ago.10

  But before we dismiss tropical forests from this part of the human journey entirely, there are more than a few hints that hominins kept connections to their ancestral homes. Where limb bones are preserved, the arms and hands of a number of hominins, including Lucy, suggest that they performed a substantial amount of climbing. Even the first well-studied member of our own genus, Homo, Homo habilis, had notably strong wrist and ankle bones that indicate some lasting connection to trees. Moreover, within a gradual trend toward more C4 foods, the hominin stable isotope data shows significant variation. Data from Australopithecus anamensis indicates that it used mixed forest-woodland-grassland environments, while study of wear on its teeth reveals similarities to modern gorillas. Isotopic data from individuals of Lucy’s species show that although some individuals consumed mainly savannah foods, others lived almost entirely in woodland or forest habitats. Some hominins, such as Australopithecus africanus and Paranthropus boisei, even show massive variation in dietary isotopes associated with woodland/shrubland versus grassland habitats within a given year! In fact, Homo also breaks the linear trend toward savannah, showing a much more mixed diet than its contemporary savannah specialist Paranthropus. Clearly, therefore, even though the world around them was broadly changing from one dominated by forest to one dominated by more open grassland, many hominin species and individuals were more than capable of selectively using very different environments when they were available. As we will see in Chapter 6, the hominin clade may never actually have truly completely left these habitats, right up until the origins of our species.11

  Paleoclimatologists have also revealed complexity in the environmental context of hominin evolution when looking at broader-scale changes in the presence of grassland on the African continent from 12 million years ago. Professor Sarah Feakins at the University of Southern California has developed an innovative approach to the study of highly resistant biomolecules of plant “waxes.” Studying the carbon isotope ratios of these waxes, alongside fossil pollen, blown from across eastern Africa into the ocean and preserved in well-stratified offshore marine cores from the Gulf of Aden, Sarah and her colleagues have looked at the trajectory of vegetation from across this portion of the continent, one unaffected by biases in discovered fossil location and damaging processes of land erosion. Their findings have dealt a further blow to the “savannah hypothesis.” First, by combining pollen and plant wax isotopes, they found that 12 million years ago, C3 grasses were already extensive across the region. As a result, the later expansion of C4 grassland, from around 10 million years ago, actually replaced preexisting grassland ecosystems as much as forest habitats. Furthermore, “both of these processes took place before we see the emergence of bipedalism in the hominin fossil record,” says Sarah. Second, the expansion of C4 grasslands, when they arrived, was not a unilinear, directional one. Ocean records make clear that there were wetter and drier periods over the last 5 million years, with varying proportions of grassland ecosystems. “Ultimately, even when we ‘zoom out’ to this continental picture provided by the marine cores, it is clear that hominin evolution in Africa was accompanied by a complex environmental patchwork, from the first hominins ~7 Million Years Ago, through to the emergence of our genus ~3 Million Years Ago,” concludes Sarah.12

  Returning to land, using methods such as Sarah’s, as well as stable isotope analysis of the fossil teeth of herbivores and carbonate nodules preserved in soils, scientists have revealed similar patchworks on diverse regional and local scales. African geography and tectonics, such as steep rift valleys, resulted in wetter conditions and the formation of lakes in certain valleys and basins, even as the eastern portion of the continent was, overall, becoming more arid. There would also have always been tree corridors along rivers and lakes among the grassland or dryland ecosystems, as well as forested mountains. This regional climatic variability also meant that C4 ecosystems would have appeared and dominated at different points in time depending on where you were in eastern Africa. On a more site-specific basis, a reconstruction of past environments using animal fossils and isotopic analysis of ancient soils associated with Australopithecus afarensis fossils at the Asa Issie locality in Ethiopia suggest it was, at least in some locales, still living and dying in the same kinds of forested habitats that Ardi preferred. Mammalian fossils and ancient remains of plants also show a mixture of forest, wetlands, grasslands, and dry shrubland in association with a Pliocene hominin skull at Woranso-Mille in Ethiopia dating to approximately 3.8 million years ago. Similarly, scientists have found that tropical woodland, rather than grassland, actually surrounded some of the most important Pliocene and Pleistocene fossil and stone tool sites at the time of hominin occupation. Finally, as Sarah reminds us, “it is important to bear in mind that so-called ‘savannahs’ refer to a range of plant types.” Savannahs today have trees and even forest patches, depending on local rainfall and geology, and this would certainly have been the case in the past. Although more open, drier environments perhaps drove the evolution of our genus, variability and a persistent use of tropical forest and woodland settings also influenced it. Adaptations to this variability definitely stood early members of Homo in good stead as they went on to seek new Pleistocene horizons.13

  FROM 1.8 MILLION years ago, discoveries of fossils in the deep, dark Atapuerca Caves of Spain and the cool temperate climates of Dmanisi, Georgia, as well as stone tools in rainy Norfolk in the United Kingdom, show that some members of the striding, toolmaking, large-brained genus Homo had moved “out of Africa” to seek pastures new in northern and northwestern Eurasia. Beautiful pear-shaped “Acheulean” hand-axes, named after the site where they were first found in France, were in use across Africa by 1.7 million years ago, South Asia and the Levant by 1.5 million years ago, and Europe just after 1 million years ago. These remarkable pieces of stone are the breadcrumbs left behind by one of the most prolific of these early hominin adventurers, Homo erectus. These early hominin expansions beyond the African heartland have often been linked to grassland expansions across the Middle East, Europe, and South Asia, providing new, widespread environments for medium- and large-sized mammals that had become so important to hominin diets. Nevertheless, one major outlier in this argument has alw
ays been the remarkable fossil finds from Southeast Asia showing that Homo erectus had made it to the Asian tropics by 1.5 million years ago in Java, Indonesia, implying that perhaps these new pioneers also had more forested tastes than commonly assumed.14

  Dr. Kira Westaway of Australia’s Macquarie University is one of the leading authorities on hominin migrations into the Southeast Asian tropics. A former diving instructor, she is no stranger to working in suffocating humidity, bat-filled crevices, and long, dark, seemingly endless caves in her time investigating the environments and chronologies of the remarkable variety of different hominoids now known to have roamed this part of the world during the Pleistocene. While Africa has rightly received much attention as the center of hominin evolution, after two decades of remarkable discoveries, Southeast Asia is now not far behind in terms of yielding frequent, dramatic advances in our field. Crucially, Southeast Asia provides a test case for us to determine how early members of Homo might have adapted to or avoided tropical forest settings. Understandable caution when applying stable isotope methodologies to rare, precious hominin fossils, an absence of relevant long-term land-based climate records when compared to Africa, and a limited understanding of how preserved animal fossils relate to often isolated hominin finds have all made this problem especially difficult to explore in the region, however. One approach has simply been to look at how hominins fit into wider patterns of changing plants and animals in Southeast Asia over the course of the Pleistocene. In particular, Kira and her team have focused their attentions on the fortunes of one remarkable Pleistocene giant, Gigantopithecus, the largest ape ever to have lived.

  “Giganto,” as Kira calls it, was found across China and perhaps also parts of Southeast Asia starting 2 million years ago. Eating tropical forest seeds, fruits, and bamboo, it could weigh a maximum of nearly three hundred kilograms (around twice the size of a modern gorilla) and proved remarkably resilient, with its last appearance in eastern Asia occurring 400,000 to 300,000 years ago. “However, something happened to this gentle colossus from around 700,000 years ago, as they were pushed back into a small part of southern China,” says Kira. The range, and presumably also population size, of Gigantopithecus began to dwindle, alongside those of other early Pleistocene animals that favored dense tropical forest habitats, including an early relative of the panda (Ailuropoda wulingshanensis). This seems to have been caused by changes in sea level and atmospheric carbon dioxide that resulted in the expansion of drier grasslands and woodlands across mainland and island Southeast Asia around 1 million to 0.7 million years ago in the late-early to middle Pleistocene. Assisted by land bridges across the now exposed Sunda Shelf, which connected modern islands like Borneo to continental Asia, a whole new variety of mammals, including the ancient elephant-like Stegodon, spread across the region. These changes eventually took their toll on “Giganto.” But one ape’s misfortune was another’s opportunity. Scientists believe that these new grassland conveyor belts and their meaty herbivorous commuters may have taken Homo right into the heart of the Asian tropics. Such a scenario would fit neatly within perceptions of an increasing hominin preference for the resources of grassland habitats.15

  Figure 5.3. Map of known Gigantopithecus fossils dated to between approximately two million and three hundred thousand years ago in southern China with the reconstructed extent of tropical forest in gray. Kira Westaway

  Yet, as we have already seen, Pliocene-Pleistocene hominin habitat preferences are a little more complex than such models allow. Where actual fossil plants and animals are found alongside Homo erectus in Southeast Asia, including pygmy hippos, turtles, deer, wild pigs, and different types of Stegodon, they show that it was likely living within more mixed lake-edge marsh, grassland, and woodland environments, with some tropical forest. Complex environmental mosaics hosted another star of Pleistocene Southeast Asia, the “Hobbit.” Following discovery at Liang Bua Cave in 2003 by a team Kira was also part of, these diminutive hominins from the island of Flores, Indonesia, quickly became even more famous than their fictional namesakes from the Shire. Dating to 190,000 to 60,000 years old, these tiny hominins, growing to a maximum of about one meter in height, seem to descend from a small-bodied group of hominins that made it to the island 700,000 years ago and may, in turn, have been related to Homo erectus. The survival of this unique hominin, right up to the arrival of our own species in the region, is remarkable not only because of the potential for interaction but also because it might tell us something about their island environments. Small stature is often linked to habitation of dense tropical rainforests, and Kira and her team showed that the Hobbits’ occupation of Liang Bua was most intensive during wet, more forested periods. However, this intensification likely represented these hominins’ search for respite from rain. In fact, once again, fossil plants and animals left behind by the foraging and hunting of our diminutive cousins, including miniature Stegodon, Komodo dragons, and giant rats, indicate a preference for foraging in drier, mixed tropical woodland and grassland habitats. The change in size therefore seems to come down to the more general, well-documented process of “island dwarfism” linked to the strictly limited range of the population in an isolated location rather than to a more specific dense rainforest specialization.16

  Although the ecological situation was certainly more complex than sweeping “savannahs,” an opening of environments and an expansion of drier woodland and grassland habitats at the expense of denser rainforest clearly aided some of the earliest hominin arrivals into mainland and island Southeast Asia. This enabled them to expand and diversify, reaching beyond the Wallace Line into island settings that remained unconnected to the mainland even during periods of low sea level, like the Philippines, Flores, and Sulawesi. We can see the true importance of these more mixed environments when we consider the eventual fates of these early hominins. Kira and colleagues have now shown that Homo erectus survived in the region until approximately 100,000 years ago, while the Hobbit persisted on Flores until approximately 60,000 years ago. At around 100,000 to 80,000 years ago, communities of tropical rainforest plants and animals sprang up across Southeast Asia during a period of wetter conditions. Home to tapirs, orangutans, gibbons, and sun bears, this forest is effectively what exists in humid, undisturbed portions of the region today. In 2020, my colleague Julien Louys and I compiled all of the available stable isotope data that exists for Pleistocene fossil mammals in Southeast Asia in the journal Nature. Studying the trends in the data, we argued that the aggressive expansion of these tropical forest ecosystems across mainland and island Southeast Asia during the late Pleistocene apparently overwhelmed our more “generalist” hominin relatives, as well as the woodland and grassland animals that had walked alongside them. Indeed, while the list of our Pleistocene hominin relatives in Southeast Asia keeps growing, with remarkable finds on a near yearly basis, only one hominin actually survived this major climatic and environmental swing back to extensive tropical canopies.17

  Figure 5.4. Map of major Pleistocene hominin sites in Island Southeast Asia discussed in the text plotted against tropical forest distributions estimated on the basis of the MODIS (Moderate Resolution Imaging Spectroradiometer) Land Cover MCD12Q1 majority land cover type 1, class 2 for 2012 (spatial resolution of 500 m). Downloaded from the US Geological Survey Earth Resources Observation System (EROS) Data Center (EDC). 1. Sangiran, 2. Ngandong, 3. Bapang and Trinil, 4. Mojokerto, 5. Kedung Brubus, 6. Mata Menge, 7. Liang Bua.

  SINCE THE VICTORIAN era, savannah landscapes have been the crucial backdrop for the grand story of human evolution. All the while, tropical forests remained as outdated backwaters, unattractive for increasingly upright, innovative, hunting hominins and better associated with nonhuman great apes. New research, however, has shown that tropical forests provided the vibrant cradle for the earliest great apes and, later, the earliest hominins, experimenting at a time of increasing environmental change in the late Miocene through to the Pleistocene. Hominin bipedalism itself may have evolve
d within and below the trees rather than out in exposed, sweltering grasslands. Certainly, drying and opening landscapes stimulated hominin diversity, specialization in bipedalism, toolmaking, and, eventually, the expansion of our earliest ancestors beyond Africa and across much of Europe, the Middle East, and Asia by the middle Pleistocene. But this was not a one-way street. Many hominins emerging in Africa kept mementos of their forested beginnings, in both their skeletons and their chemical composition, often using nutritious forest or woodland patches within complex savannahs—not the uniform, sweeping plains we are used to seeing on TV today. Similarly, when some of the earlier members of our own genus ventured into the Asian tropics, mixtures of forest, dry woodland, and moist lakeside grasslands supported their populations, large and small. These populations likely avoided monotonous grasslands just as much as they did dense, stifling tropical lowland rainforests, with trees continuing to provide a source of food and refuge for our ancestors.