Jungle

by Jungle (retail) (epub)

  In fact, climatic and environmental variation framed the evolution and differentiation of our hominin ancestors. Efficient bipedalism, increased brain size and social groups, and stone tool development were all key adaptations to a rapidly changing world rather than the product of monotonous existence. The Miocene-Pleistocene saw not only grassland expansion but also increasingly dramatic swings in global climate states, driven by the powerful Milankovitch cycles discussed in Chapter 2. Our ancestors adjusted to this unpredictability by adapting to the plentiful meat opportunities drying savannas had to offer. But all the time, they kept their eyes on profitable forest habitats and woodland edges—hedging their bets. This flexibility, however, had its limits. The late middle and late Pleistocene brought more “extremes” in both climates and rapidly diversifying environments. In tropical Southeast Asia, as the late Pleistocene brought with it an expansion of dense tropical rainforests, its resident hominin generalists confronted a thick, expanding green wall—one they could not ultimately overcome. Just prior to this, a new hominin emerged into this increasingly shifting “brave new world” in Africa. It went on to march out across almost all of the planet’s continents, which by now were fully formed. It was, of course, our own species, Homo sapiens.18

  Chapter 6

  ON THE TROPICAL ORIGINS OF OUR SPECIES

  The tendrils of social media, the ubiquity of mobile phones, the cult of “celebrity” and reality TV, and our busy work and social lives all mean that we grow up in a very human-focused world. Our widespread geographical range, from the ice of Greenland to the mountains of the Himalaya, from the bustling metropolis of Beijing to the desert-fringed Dubai, leads us to simply assume that Homo sapiens is one of the most successful species to ever walk the planet. Not only that, but our huge global population, extensive cities, and dominating impacts on the natural world seem to give us a certain planetary “weight.” Yet we are relative newcomers to life on Earth, especially when compared to the greater than 300-million-year journey of tropical forests that we have been following. The first fossil skeletons with distinctive “human” (in this book I reserve the word “human” only for our own species, Homo sapiens) features, including a shorter, flatter face, round skull, and large brain (over three times bigger than Lucy’s), appear in Africa between 300,000 and 200,000 years ago. This fits with estimates based on “molecular clocks,” akin to those used for plants and mammals in Chapters 1 and 4, respectively, calculated using modern and ancient human DNA. Whichever way you look at it, this makes us practical newborns in an evolutionary context. Indeed, two extinct hominins we commonly think of as having eventually “failed,” Homo erectus and Homo neanderthalensis (or the Neanderthals), both actually lasted longer on the Earth’s surface before their demise than we have to date.1

  It may not be too surprising, then, that paleoanthropologists and archeologists have seen Homo sapiens as simply the most recent point in a longer-term evolutionary trajectory of hominins and the genus Homo toward open, savannah contexts and away from supposedly protein-poor tropical forests. In fact, anthropologists and archeologists alike have often thought of tropical forests as “barriers” to Pleistocene human presence. Small protein-poor and difficult-to-catch prey, poisonous plants, a lack of dense carbohydrate resources, tropical diseases, and rampant humidity, not to mention poor preservation of archeological sites thanks to high water flow and acidic soils, have meant that these “green hells” have often been neglected in the story of human dispersals. Between 300,000 and 60,000 years ago, the archeological record begins to show evidence for new technologies (the bow and arrow), items made for exchange to maintain social networks, and personal ornaments and artwork, including engravings and shell-bead necklaces—all of which have been considered uniquely human capacities. Yet these innovations have been linked to the need for more efficient projectile hunting and the maintenance of useful relationships between groups living in open grassland settings prone to drying in the face of ever-harsher swings between glacial and interglacial conditions, as well as shorter-term swings in climate systems, in the late Pleistocene (Chapter 2). Alternatively, they are seen as the product of growing human populations thriving on reliable marine resources in coastal settings. As with Homo erectus, the eventual dispersal of Homo sapiens into the Middle East and Eurasia and throughout Southeast Asia has also been associated with climatic periods when more challenging environments like deserts or tropical forests retreated and rich “savannah-like” hunting grounds expanded. Either that, or it has been suggested that humans tracked rich, high-protein resources from the oceans all the way from eastern Africa, around the Indian Ocean rim, to Australia by 65,000 to 45,000 years ago and to the Americas by 30,000 to 25,000 years ago, along a series of “coastal highways.” While undoubtedly new, in many ways, Homo sapiens has thus traditionally been considered as equally dependent on particular nonforested environments as its hominin forbears and contemporaries, which we met in Chapter 5.2

  These narratives rely on a single, homogeneous origin and single set of either grassland or oceanic desires for humans. But one thing that has always drawn me to studying the origins of our species is that, while we are a relatively recent evolutionary arrival, we are also the only hominin that went on to occupy all of the planet’s continents and to use all of the world’s land-based environments. Much of this seems to have happened very quickly, during the late Pleistocene (125,000 to 11,700 years ago) and appears to call into question assumptions that humans stuck to a single, easy or familiar “route.” In this chapter we will see how the latest research in archeology, paleoanthropology, genetics, and environmental science is building a more complex picture of Homo sapiens’s entrance onto the world stage, one that covers a number of different parts of Africa, including the tropical forests of West, Central, and eastern Africa. Not only that, but we will meet daring archeologists and paleoanthropologists who are beginning to explore tropical parts of the world that have often been ignored as unprofitable “blanks on the map.” Their work is demonstrating that, as we moved beyond Africa, Homo sapiens repeatedly and persistently occupied some of the most extreme sets of environments the planet had to offer. Certainly our species was a continuation of a long period of hominin adaptation to variable Pleistocene environments that we met in Chapter 5. However, it is in settings such as tropical forests, rather than sweeping savannahs or uniform coastlines, that we can see how Homo sapiens used its new cultural, technological, and social repertoire to become the most flexible hominin yet—one that could deal with all of the climatic fluctuations, between cooling and warming and drying and wetting, that the late Pleistocene world could throw at it, and one that grew into the relentless, global force we remain today.

  THE TRADITIONAL VIEW of human origins is that we evolved at one time, in one place, and in one population in Africa. In the 1980s, genetic variation in modern human mitochondria, little organelles within our cells that can only be passed down through the maternal line, was used to develop a “family tree” of human populations, linking all living people back to a single population or even just one female ancestor—“Mitochondrial Eve”—who lived in Africa between 200,000 and 140,000 years ago. Although more recent studies, using powerful modern statistics and genetic sequencers, now analyze genetic variation across the entire human genome to produce more sophisticated models, they retain the same basic principles—namely, that the variability in the DNA of modern human populations can be inserted into a simple “treelike” scheme that points back toward a single point in time and space when our species emerged and began to diversify as it dispersed across Africa and around the world. In a number of recent cases, these studies tend to zero in on KhoeSan populations, which have the most diverse human genomes seen anywhere on the planet today, suggesting that populations of Homo sapiens first emerged and started to diversify in the southern portion of the continent 300,000 to 150,000 years ago. In recent years, however, it has become clear that genetic models of our species’ origins need to become
exponentially more complex. We now know that Homo sapiens interbred with Neanderthals, with a second hominin group called Denisovans (named for the cave where they were found in Siberia, which itself was named after a hermit called “Denis” who once lived there), and perhaps even with a third hominin group. Not only that but they produced fertile offspring, complicating traditional determinations of “species” and leaving their mark on the genetic makeup of surviving members of our own species. Furthermore, only one Pleistocene fossil individual in Africa has produced ancient DNA, since generally hot conditions tend to break down ancient organic molecules. Justifiably, museum curators, archeologists, and paleoanthropologists are also inclined to protect limited fossils from destructive analyses, so they have approached this application cautiously. As modern DNA samples cannot reliably inform us about the geography of ancient populations, given long histories of human migrations between regions and interactions between different human populations, without ancient DNA, it is almost impossible to use modern DNA to “read back” exact past locations or timings of human origins with any level of accuracy.3

  Paleoanthropologists and archeologists have therefore tried to get their hands on something a little more tangible than DNA to mark the date and place of human emergence: fossils. As with DNA analyses, there has been an obsession with finding the “oldest” example of something in order to crown a new region as ruler in the story of human origins. The oldest accepted fossils with distinctly “human” features have for a long time been those found, like Ardi and Lucy, in the drylands of Ethiopia, this time at the sites of Omo Kibish and Herto. Dating to 195,000 and 160,000 years old, respectively, these skeletons look morphologically “human.” Animal bones found in the same place, with known ecological habits, climate records, and geological studies, suggest that these early humans were living in dry, grassland environments, apparently confirming our origins in the eastern African savannah. However, relatively complete hominin fossils, including those of our species, are always rare, scattered across the continent in different localities where chance (e.g., transport of remains into a sheltered cave system) or specific conditions (e.g., layers produced by volcanic eruptions or flooding) have shielded them from the ravages of time. As a result, some archeologists have suggested it may instead be better to try and find more commonly preserved material features, like art, symbols, and complex stone tools, to determine the “behavioral” appearance of humans. As mentioned earlier in the chapter, such evidence has tended to come from coastal settings in northern and southern Africa dating to around 100,000 to 70,000 years ago, suggesting a marine focus, although more recent work in Kenya hints at the appearance of sophisticated human exchange patterns and technology 300,000 years ago in a dry grassland setting. In both cases, research has focused on trying to find the first skeletons or novel material remains (e.g., specific stone tool technologies, projectile technologies, symbolic beads, and pigment use) left behind by our ancestors. In doing so, most attention has thus centered on the long hominin-associated savannahs or on coastlines where caves that have eroded into cliff faces provide their own perfect microclimates, sheltered from the elements, preserving traces of the past.4

  That is, until Dr. Eleanor Scerri and her colleagues wrote a revolutionary paper in a leading scientific journal, Trends in Ecology and Evolution, in 2018. Although other scientists had already begun to dissent from the mainstream, this paper challenged the simple traditional view that Homo sapiens appeared in one place at one time. Eleanor heads the Pan-African Evolution Research Group at my own institute in Jena, and it does exactly what it says on the tin. “We believe that between 500,000 and 200,000 years ago, the key time frame for the emergence of our species, our ancestors lived in distinct, but interconnected, populations across Africa,” says Eleanor. In other words, we did not appear at a single point in time or in a single group. Instead, millennia of partial contact between populations led to a huge raft of human diversity. The dynamic mixing of these groups was to eventually lead to our species as we know it. “To study this properly, we therefore need to look at all of Africa, not just a single mythical point of beginning,” continues Eleanor. In fact, casting the net a little wider we can start to see a number of hominin fossils that, like those from Omo Kibish and Herto in Ethiopia, have a diverse mixture of both clear Homo sapiens (e.g., flat face and rounded skull) and more “archaic” (e.g., thicker brow ridge) traits, including those from Jebel Irhoud in Morocco (c. 300,000 years ago) and from Florisbad in South Africa (c. 260,000 years ago). The “full suite” of what we often consider to be distinctively Homo sapiens features actually only really appears to take shape between 100,000 and 40,000 years ago, though this may be a product of a lack of preserved fossils from this and the preceding time interval. Significantly, in tropical West Africa, the earliest known human fossil dates to just 16,000 to 12,000 years ago at Iwo Eleru, Nigeria, and shows just how long considerable morphological diversity was maintained between different groups of Homo sapiens in different parts of the continent. A reexamination of the existing genetic evidence, in light of the fact that genetic exchange probably occurred between different hominin lineages and that modern DNA gives very little insight into geographic patterning in the past, seems to support this “African multiregionalism” proposed for the evolution of Homo sapiens by Eleanor and her colleagues.5

  Rather than a branching linear tree with a simple, single trunk, we therefore have fractured African roots. The same picture emerged when Eleanor and her colleagues looked at what they call a “patchwork” of cultural materials left behind by Homo sapiens populations across Africa in the middle and late Pleistocene. Stone tools, thanks to the materials from which they are made, are reliably preserved over deep timescales. Not only that, but archeologists can look at their shape and how they were made to infer things about behavior and technology. Archeologists have suggested that the appearance of our own species heralded a proliferation of new forms of stone tools that could be inserted into complex projectile setups and that required more sophisticated, thought-out manufacture. These so-called Middle Stone Age toolkits have been found alongside humanlike fossils at Jebel Irhoud and Florisbad, as well as at Olorgesailie in Kenya. Importantly, the Middle Stone Age toolkit varied by region. In North Africa, the rapid expansion of grasslands under wetter climates led to growing populations of Homo sapiens and the widespread appearance of unique, complex, arrow-shaped (or “tanged”) stone tools 120,000 to 80,000 years ago, alongside bone tools and shell beads suggestive of complex cultural behaviors. In eastern Africa, by contrast, there is huge variation and continuity in Middle Stone Age stone tools from 300,000 to 60,000 years ago. The eventual transitions from these Middle Stone Age forms to diminutive “microlith” stone tools, often linked to well-developed bow and arrow technology and hunting efficiency, between 80,000 and 40,000 years ago also occurs in highly diverse ways in different parts of Africa. Sometimes there is complete, rapid replacement; other times there is a gradual change. The appearance and disappearance of “human” productions like art and social display are equally varied regionally and often come and go in the Pleistocene archeological record right up until the Holocene. It appears, then, in contrast to the traditional story of our arrival on Earth, that there was no dramatic, consistent fossil or “material” advent of Homo sapiens at a particular place or time; nor was there a uniform, linear progression toward “behavioral complexity.”6

  Things get even more interesting when we look at what Eleanor and her team think drove the dynamic separation and integration of these different, evolving human populations that gave rise to all of us, wherever we live around the world. The character and chronology of the fossils, stone tools, and different material artifacts left behind by early Homo sapiens suggest that our species eventually appeared thanks to interaction between these diverse groups living in different parts of Africa. Eleanor and colleagues believe that the varied environmental settings of Pleistocene Africa, not just savannahs or coasts, either kept th
ese populations separate or brought them together at different points in time. In fact, once we move away from a presumption of a uniform, single origin and environmental association, tropical forests begin to take a much more prominent role in this early, more complex evolutionary model. Indeed, as well as the fascinating Iwo Eleru fossil evidence for morphological diversity mentioned above, some of the most unique of the Middle Stone Age regional diversity comes from the modern-day tropical forests of West and Central Africa: large, heavy-duty pick-like forms, huge scrapers, and “lance-like” points have been variously associated with digging for carbohydrate-rich tropical tuber plants, like yams, as well as hunting tropical forest animals. Many of the sites where these tools, dating to between 300,000 and 40,000 years ago, have been found are today surrounded by dense tropical evergreen rainforest or drier tropical forests and woodlands. Records of ancient plants and sediments in marine cores off the coast of West Africa contain hints that tropical forests also covered these same regions in the past. More work is, however, necessary to confirm the ecologies inhabited by the populations that made these tools and the plants and animals on which they relied for survival.7

  But while the role of dense tropical rainforests in early Homo sapiens’s evolution in Africa awaits definitive proof, there is growing evidence that expanding and contracting tropical forests may have shaped past human genetic and cultural diversity. Comparison of the DNA of different modern tropical forest hunter-gatherers in Central Africa with that of neighboring populations suggests that they diverged from each other at least 70,000 years ago. Not only that, but genetic differences found between different forest hunter-gatherer groups implies that they, too, diversified from each other as a result of the waxing and waning of forest habitats. At the cave site of Panga ya Saidi on the tropical coast of Kenya, where I was lucky enough to work on recovered archeological animal materials, we found evidence that a resistant mixture of moist tropical forests, woodlands, and grasslands buffered human populations against major climate change seen in the dry, grassland interior, allowing them to gradually experiment with their stone tool technology and symbolic material culture. Indeed, despite being just fifteen kilometers away from the present coastline, the site yielded no prominent evidence of the use of coastal resources for subsistence during the Pleistocene. Elsewhere, similar mixed environments promoted the movement of growing human populations in and around the African subtropics. Tropical forests were certainly not the only environment at play. Nonetheless, they were undoubtedly a key part of the complex “pan-African” skeletal, genetic, and behavioral appearance of our species.8