WATCHING COWS GRAZE on pastures, wildebeest flow across savannahs, and whales dive among the deep blue, it is hard for us to imagine that our earliest mammalian forebears primarily lived and flourished in dense, warm, wet forest environments. From the first experiments among the dinosaurs to their rise and domination following the K-Pg global disaster, mammals were tropical creatures, living in trees, gliding between branches, and eating insects or increasingly lush vegetation. The close relationship between mammals and the expanding angiosperms seems to have been a key part of their survival as well as their spread and diversification around the planet. It may even be that the mammals, in turn, helped these new, innovative, nutritious plants to succeed. From the peak warm period in the middle Eocene, however, and despite some brief outward flourishes, megathermal forests were on the back foot, retreating in stages, in the face of dry, seasonally adapted grasslands and woodlands, toward the equator. Horses, giraffes, and bats are but three of many more groups of mammals affected by these global environmental changes between the Eocene and Miocene, showing how the dynamism of warm, wet forests and their retreat toward their modern tropical distribution influenced some of the more characteristic mammalian lineages we know and love today—including our own lineage, the primates.
The earliest true primates to appear in the fossil record were those of the Strepsirrhini “wet-nosed” group, which includes modern lemurs and lorises. These small, arboreal mammals made the expanding lush, tropical angiosperm vegetation their own in the Paleocene and early Eocene. Diversification continued with the tarsiers (c. 58 million to 55 million years ago) and then the simians (c. 40 million years ago), a group that includes all monkeys and apes. As we have seen in the case of other mammals, primates found protection and opportunity in the flourishing tropical angiosperm vegetation of the Paleocene and Eocene. As with other mammals, the course of early primate evolution was seemingly characterized by teeth and body shapes that could better exploit the nutrient-rich fruits and seeds of these novel plants. And like other mammals, primates were forced to contend with shrinking megathermal forests from the late Eocene onward. The simians emerged in Asia before moving to Africa by 35 million years ago, where they became smarter, bigger, and more aggressive as they dealt with variable resources and threats in the fluctuating forest habitats of the Miocene. The primates are another clear example of how early mammalian evolution was tied up with the fates of tropical forests and the interlinking of land bridges between separating continents. However, the Miocene was only the beginning. Ongoing but shifting interactions with the forests and woodlands in and around the equator would set this group on an entirely new evolutionary course, one that would see them become arguably the most successful mammal that has ever existed. Us.19
Chapter 5
THE LEAFY CRADLES OF OUR ANCESTORS
When highlighting the perilous state of tropical forests today, conservation agencies will often emphasize the dangers of deforestation for chimpanzees, bonobos, gorillas, and orangutans in the hope that people will sit up and take notice of the plight of our closest living relatives and their lush surroundings. Their complex social lives and ability to use tools, to show emotion, to care for their dead, and even to perform sign language make these nonhuman great apes so easy to sympathize with. From watching them play in captivity to immersing ourselves in documentaries that follow their feuding “dynasties,” we can rapidly relate to these animals with whom, in the case of chimpanzees, we share 99 percent of our DNA. Through various iterations of the film Planet of the Apes, for example, we have grown used to seeing them as our probable planetary inheritors should we ever slip up. Somehow, however, there’s a widespread feeling that these apes ultimately represent what we were rather than what we are, that they look and behave how we used to, and that they are somehow stuck in time, living examples of an age gone by. The same is most certainly true of their tropical forest homes, which we think of as so foreign to ourselves, environments that we abandoned as soon as we were able, early on in our evolutionary journey.1
It is generally accepted that sometime between 13 million and 7 million years ago, a new type of great ape began to appear in Africa: the hominins. These novel “humanlike” apes were our great-great-great-
ancestors and, ever since Charles Darwin wrote about our “descent,” we have associated their evolution with the leaving behind of tropical forests and the other great ape species. According to this narrative, striding out across grasslands, our predecessors started to specialize in upright walking. Moving more efficiently over long distances, they could pursue and hunt increasingly large animals out on the African savannah. With their hands now free, they could make tools and use fire to cook meat and fuel their growing brains as their canine teeth became smaller and started to resemble our own. Measurements of hominin fossil limbs, the geochemistry of hominin teeth, and discoveries of plants and animals found alongside hominins have all been used to support this so-called savannah hypothesis. From approximately 7 million years ago until the origins of our own genus, Homo, within the hominin line, around 3 million to 2 million years ago, the narrative has commonly been one of decreasing climbing and use of forest plants and increasing bipedal walking and hunting of grazing animals. This idea is so deeply embedded in academic thought that even the first hominins to move “out of Africa” approximately 2 million years ago are thought to have done so only when shifts in the Earth’s climate encouraged the expansion of a grassland highway all the way from Africa to eastern Eurasia.2
Against the broad backdrop of Miocene–Pleistocene environmental change—namely, the decline of tropical forests at the expense of dramatic C4 grassland expansion—on the face of it, this all seems to make perfect sense. Yet the latest work in paleoanthropology, archeology, and environmental science shows that tropical forests may have played a far more active and persistent role in the emergence and evolution of our early ancestors than often appreciated. The hominins certainly seem to have originated within tropical forests, albeit ones that were changing quickly. Our star-studded cast of ancestors were born into a world of climatic and environmental diversity, from the earliest hominins to the specialized walker “Lucy” and, later, Homo erectus—the hominin with the greatest geographical range prior to our own species, reaching from the United Kingdom in the west to Indonesia in the east. Although the different members of our family tree undoubtedly became increasingly at home in savannahs, continued attachment to their earlier forest origins left a trace in their hands, feet, limbs, and even the chemical makeup of their bodies. In their negotiation of the diverse and fluctuating environments around them, we can see the first traces of a behavioral flexibility that was soon to reach whole new levels as we evolved and expanded across Africa and the entire world.
Figure 5.1. Map of major Miocene, Pliocene, and Pleistocene hominin sites in Africa 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).
DURING THE EARLY Miocene, between 23.04 million and c. 15.99 million years ago, the Earth truly was the “planet of the apes.” Warm global temperatures encouraged a relatively “brief” period of forest expansion. Temperate forests inhabited Greenland, while warm, wet forest environments spread across much of Africa and Eurasia. Unsurprisingly, the primates took their chance, including the relatively new ape, or “hominoid,” branch of the Oligocene simians. These hominoids, including all of the great apes as well as the “lesser apes,” or gibbons, went from strength to strength during the early Miocene. From a likely first appearance with the genus Proconsul in Kenya 31 million to 23 million years ago, by 14 million years ago there may have been over one hundred species of ape living across Africa and Eurasia, making the most of a warm planet and flourishing angiosperm fruits, leaves, and t
ubers. By 10 million years ago, however, cooler middle-late Miocene temperatures and drier conditions had forced megathermal forests and their ape inhabitants into retreat. The great apes, or “hominids,” suffered the most. The majority had disappeared or started to disappear outside Africa by just 2 million years later. Iconic species, such as Sivapithecus from Pakistan, became extinct. Orangutans were left as one of just two remaining representatives of the great apes in Asia, where they have persisted to the present day, while the ancestors of gorillas and the common ancestor of chimpanzees and bonobos began to emerge and evolve in the increasingly restricted subtropical and tropical forest belt of Africa. It was within this world of increasingly complex “mosaic” patterns of tropical forests, drier woodlands, and grasslands that the first hominins, our ancestral lineage, split off from the rest of the great apes and began to take on new, pioneering forms.3
Paleoanthropologists often identify fossil hominins on the basis of the similarity of their teeth and skull shape to our own, and these skeletal elements are often preserved best through the ravages of time. Menacing canines used by other great apes in threat, bluff, and social display reduced in size among hominins. Adaptations to “bipedalism,” or upright, two-footed walking, are also a specific feature that might define the beginnings of our family tree and reveal something about its changing environmental context. However, poor preservation means that scientists must often make do with studying a mishmash of these features when examining fossils contesting for the title of the “earliest hominin,” and where and how different hominin species moved around is frequently up for intense debate. In 1994, a team of researchers trudging through the desolate, arid environment of the Awash River basin in Ethiopia came across one of the most significant nominees for this award. Then-graduate student Dr. Yohannes Haile-Selassie of the Cleveland Museum of Natural History saw a partial hand bone poking out of the ancient silt. Subsequently, he and the rest of the team, including a seasoned veteran of hominin fossil discoveries in Africa, Professor Tim White of the University of California, Berkeley, went on to excavate the skull, teeth, pelvis, hands, and feet of a female “humanlike” skeleton dated to 4.4 million years ago. Named Ardipithecus ramidus, or “Ardi” for short, this is one of the most complete prehuman hominin skeletons ever found. In local Afar language, Ardi’s name literally means “ground floor” and “root,” and this is potentially exactly what she is. In 2001, Yohannes also went on to discover and name an even earlier ancestor of Ardi, Ardipithecus kadabba, in the same region, dating to 5.8 million to 5.5 million years ago. This means that her lineage reaches back further, approaching most genetic estimates of the last division of our line and that of chimpanzees approximately 7 million years ago, placing her right at the very beginning of hominin experimentation in Africa. However, as Yohannes tells me, “Just because ‘Ardi’ belongs to the hominin group from which our own species eventually emerged, this did not mean that she had fully committed herself to the ground. She probably spent as much time in the trees as on the ground.”4
Ardi had a small brain, one-fifth of the size of our own and slightly smaller than that of a chimpanzee. However, the shape of her pelvis and feet means that she was definitely much better suited for bipedalism than chimpanzees. Meanwhile, Ardi’s teeth and skull shape show the faintest beginnings of a trend toward those of later hominins and our own species. Yet, as Tim White argues, these features are not necessarily the most significant parts of Ardi’s discovery. “It is the environment that surrounded her that truly turns existing theories of hominin evolution on their head,” he says. The skeletons of animals found near Ardi’s fossils show that when she was alive, tropical forest, and maybe even closed spring-fed forest, covered the now desertified region. Thanks to the rare completeness of her skeleton, we can also see that Ardi had divergent big toes and long fingers well adapted for slow climbing among the trees. In other words, and contrary to beliefs widely held by both the public and academics, “bipedalism” may have originated in tropical forests, not savannahs. More detailed analysis of the animals found around her and other fossil members of Ardipithecus, as well as geochemical analysis of Ardi’s teeth, have shown that they may have inhabited forest patches within mixed forest-woodland-grassland settings. This would still be a far cry from popular assumptions of hominin origins out in the open.5
Figure 5.2. Artistic reconstruction of “Ardi” Ardipithecus ramidus. Jay Matternes / Smithsonian
Ardi is certainly not the only contender for the title of earliest hominin, and, as is usual in paleoanthropology, some have even questioned whether she is directly related to us at all. Another nomination comes in the form of Sahelanthropus tchadensis, a species largely represented by a series of jaw bones discovered in northern Chad by a French team led by Michel Brunet. Dating to approximately 7 million years ago, like Ardi’s, its teeth and skull shape show a mixture of “hominin-like” and other “ape-like” characteristics, with some tentative suggestions of bipedal movement, in a mixed forest, woodland, and grassy lake-edge environment. A further competitor, Orrorin tugenensis, found by the Franco-Anglo partnership of Brigitte Senut and Martin Pickford in the Tugen Hills of Kenya, dates to 6.1 million to 5.7 million years ago. The femur, or upper leg, of Orrorin has a shape typical of later hominins, including our own genus, Homo, suggesting that it was a frequent bipedal walker. Nonetheless, once again, it retains arm bones and fingers with shapes best associated with climbing among trees, and evidence from the shape of its teeth, as well as reconstructions of the ecological habitats of other animals found nearby, suggest that it ate leaves, fruit, seeds, roots, nuts, and insects within an area dominated by drying tropical forest. Amazingly, then, although all three challengers for the crown of first hominin were found in modern deserts or arid grasslands, they confirm the ongoing importance of tropical forests during the earliest stages of our ancestors’ evolution as both a safe haven from predators and a likely source of food.6
The fossil fragments left behind by some of the earliest proposed hominins in just a handful of locations 7 million to 5 million years ago mean that properly reconstructing our earliest evolution in Africa will always be difficult. Clearly, however, the early hominins of the late Miocene developed bipedalism at the same time as they were still climbing trees and using forest or woodland patches in the tropics. A forested origin for bipedalism is perhaps not so surprising when we look at long-term studies of modern nonhuman great ape movement. Astoundingly, the most bipedal of our living relatives are not the ground-dwelling, hunting chimpanzees but rather the orangutans, which most comfortably and frequently use bipedalism to navigate delicate branches and retrieve fruit in forest contexts. Not only that, but the fact that African nonhuman great apes only split from our own lineage around 7 million years ago, combined with the fossil record approaching this split, allows us to see that instead of being “left behind,” chimpanzees have themselves evolved, changed, and survived, ending up with a preference for living in tropical forests, a specialization that has only brought them to the brink of extinction because their close relative Homo sapiens is destroying this forest habitat. This is a very different perspective indeed on the notions that chimpanzees are good proxies for models of early hominins and that we diverged from chimpanzees simply by walking through grasslands.7
ARDI WAS NOT the only, or even the first, female fossil star to rise out of Ethiopia or the Awash River basin. In 1974, another team of paleoanthropologists, directed by Maurice Taieb, was exploring a drier part of the same valley. Two of the expedition’s members, Donald Johanson and his student Tom Gray, had spent two hours scrabbling around in the increasingly dangerous heat. Just as they were about to leave and seek shelter, Johanson noticed an arm bone poking out the side of a dried-up river gully. As more bones emerged, nearly half a complete skeleton, the two men began to realize the significance of their find. In the subsequent celebrations that evening back at an exhausted but rejoicing camp, the Beatles’ “Lucy in the Sky with Diamonds” inspired a
more personal name for the find that, until then, was known as AL 288-1. The rest, as they say, is history. This skeleton, or “Lucy,” now known as Australopithecus afarensis, showed that by 3.2 million years ago our ancestors were quite literally strutting around eastern Africa. Not only was Lucy remarkably complete, but careful study of the shape and dimensions of her hip, knee, and thigh bones show that, by this point in human history, hominins were now built to walk almost entirely upright along the ground of an increasingly open tropical landscape.8
In fact, Lucy is part of a clear, classic sequence of diverse, upright hominin bipeds from the Pliocene (5.33 million to 2.58 million years ago) to the early Pleistocene (2.58 million to 0.78 million years ago). She was preceded by Australopithecus anamensis in Ethiopia and also Kenya, an ancestor whose limited preserved limb bones tentatively hint at frequent bipedalism. She was succeeded by the delicate-looking Australopithecus bahrelghazali in Chad, Australopithecus africanus and Australopithecus robustus in South Africa, and the thick-jawed Paranthropus boisei, or “Nutcracker Man,” in eastern Africa, all of which document the spread of specialized upright hominins across the African continent. At the end of this sequence came our own genus, Homo, perhaps as early as 2.8 million years ago, represented later by the energy-efficient walking of first Homo habilis and then Homo erectus—whose name directly hints at its upstanding nature. This trajectory also entailed an increasing brain size, from around 300 to 500 cubic centimeters in Lucy to 1,000 cubic centimeters in Homo erectus. Now comfortably walking and needing to feed larger bodies and brains, hominins also began to fashion stones into tools to procure meat and marrow around 3.3 million years ago. Traditionally, this gradual but relentless movement toward upright bipedalism, bigger brains, the need for technology, and even larger social groups has been postulated as a response to the heat, openness, and larger animals, including prey but also predators, today found in the open, arid savannah-like settings where almost all of these fossils had been discovered.9
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