Chapter 14

Paleo-faunas: Rise and Fall of the Biggest Grazers

The large mammals that existed during the course of human evolution were different from those around today – not only in species, but also in genera and in size. What we know of them is rather fragmentary. The erosion that levelled Africa’s surface after its separation from Gondwana also took away not only the bones of the dinosaurs of the late Jurassic and Cretaceous periods but also those of the early mammals that replaced them during the Palaeocene and Eocene epochs. Most of the fossil record from this dawn age of mammals comes from the Fayum Province of Egypt. This northern region of Africa lay beneath a shallow sea then. Its fossil deposits include early predecessors of whales, affiliated with hippos. Also present are the fossilised bones of a short-legged, rather pig-like swamp-dweller called Moeritherium, believed to be the precursor of the elephants.

While Africa was joined with Arabia during this dawn period, it was isolated from the rest of Eurasia by the Tethys Sea. The mammal orders that evolved there in isolation loosely constitute the ‘Afrotheria’. Besides the Proboscidea (elephants and relatives), they include the Hyracoidea (hyraxes), Tubulidentata (aardvark), Sirenia (dugongs and manatees), Macroscelidea (elephant shrews) and Chrysochloridea (golden moles). Also entering Africa somehow during this early time were giant rhino-like animals called embrithopods and hippo-like anthracotheres.1 Although primates originated in Asia, they were represented in Africa by ~30 Ma, as were rodents.

Africa’s isolation from other continents ended towards the end of the Oligocene 23 Ma when a land bridge connected Arabia with Eurasia. Elephant-like proboscideans moved out and early ungulates crossed in, along with much else. Most of what we know about the evolution of Africa’s large mammals comes from sites located in rift valley troughs, beginning in Ethiopia and extending through eastern Africa (Figure 14.1AD; see also Chapter 1). Timelines for the fossil sequences are provided by succeeding layers of volcanic ash or lava, which can be dated quite precisely from the radioactive decay of certain isotopes. Most deposition followed the second, Pliocene phase of tectonic uplift, starting around 5 Ma.

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Figure 14.1

Fossil sites. (A) Olduvai Gorge in Tanzania, where deposition layers have yielded numerous fossils dated from the early Pleistocene onwards; (B) Lake Turkana region in northern Kenya, which includes several fossil-yielding locations spanning the Pliocene and Pleistocene (photo: Peter Howard); (C) Ledi Guraru region of Ethiopia, one of the sites where sedimentary deposits have yielded crucial fossils of early humans (photo: Erin DiMaggio); (D) fossil bones accumulated in sediments at Olorgesaile, Kenya; (E) Swartkrans limestone cavity entrance situated close to Sterkfontein in the Cradle of Humankind, South Africa; (F) Makapansgat limeworks located in the northern bushveld region of South Africa.

The Highveld region of South Africa provided a rather different context for fossil preservation west of Johannesburg, in the region labelled the ‘Cradle of Humankind’ (Figure 14.1E,F). Surface erosion following Miocene uplift exposed dolomitic limestone that had been formed over 2000 million years ago when this part of Africa had been under seawater, well before terrestrial life evolved. Subsequent water erosion opened caves and sinkholes, which eventually became exposed to the land surface after the early Pliocene. Animals either fell into the cavities, or were carried in by carnivores, and became encased within the mineral build-up of breccia. Other limestone caves are located at Makapansgat 200 km further north, in a region that is now savanna bushveld. They retain fossils from similarly early times from ~3.5 Ma in the mid-Pliocene.

Elsewhere in southern Africa, coastal deposits have yielded fossils at Langebaanweg in the south-western Cape, dated ~5 Ma, and at sites further north in Namibia. However, a vast region of Africa lacks fossil deposits. The resultant fragmentation of the fossil record in Africa must be borne in mind when interpreting the fossil record. The species and genera represented need to be placed in the context of the major transitions in climate (Chapter 2) and vegetation (Chapter 9) that occurred. They took place (1) between 10 and 6 Ma during the late Miocene when C4 grasslands spread, (2) after 2.7 Ma when the Pleistocene glaciations developed and grasslands became more extensive, (3) after 1.8 Ma when further drying expanded grasslands, and (4) after 0.9 Ma when glacial cycles lengthened to 100 kyr and cold extremes produced even greater aridity.

Oligocene–Miocene Giants

Following the global extinction of dinosaurs at the end of the Cretaceous period 66 Ma, the small furry mammals that had scurried from the toes of these huge reptiles began to get much larger. Most of the modern orders of mammals originated during the wet and warm environments of the Palaeocene and Eocene epochs, spanning the period from 66 to 34 Ma.2 Atmospheric levels of carbon dioxide exceeded 2000 ppm, five times current concentrations, promoting lush vegetation growth across the continent. Proboscideans and hyraxes became the predominant herbivores in Africa, with some of the latter matching the largest antelope in size. Proboscideans were especially diverse, including deinotheres with lower incisors curving downward and backward like giant hooks, shovel-tusked gomphotheres and early mastodons.3

During the Oligocene epoch (34–23 Ma), a cooling and drying trend caused forests to give way to broad-leaved deciduous woodlands. Following the start of the Miocene 23 Ma, conditions in eastern Africa became cooler and drier. Along Africa’s west coast, the Benguela current formed, conveying cold water northward and accentuating dry conditions into the Congo basin. From the Oligocene into the middle Miocene 12 Ma, Africa’s fauna was characterised by a predominance of very large animals, many in the megaherbivore size range.1,4 Enormous claw-bearing chalicotheres, related to tapirs, apparently foraged by pulling down tree branches, like giant ground sloths once did in South America. Short-necked giraffes with bony antlers, called sivatheres, entered Africa from Asia and became widespread, joined by hippopotamuses and diverse rhinoceroses.2 None of the large herbivores from the early portion of the Miocene showed dental adaptations for a grass diet, although grasses had appeared by then. The stem ruminants were apparently mainly fruit-eaters.

Ruminant Radiations

The earliest ruminant, assigned to the genus Eotragus, dispersed from Asia into Africa around 18 Ma early in the Miocene.5,6 Two distinct subfamilies within the Bovidae (horned ruminants) diverged from this shared ancestor.7 The Bovinae, encompassing wild cattle (Bovini) and spiral-horned antelope (Tragelaphini), were derived from boselaphine ancestors in Asia, represented today by the Indian nilgai (Boselaphus tragocamelus). African bovines split from Asian representatives around 7 Ma, while the tragelaphines diverged slightly later from a boselaphine stem. The lesser kudu (Tragelaphus imberbis) is closest genetically to the stem species.8 The second subfamily, the Antilopinae, encompasses the remaining antelope tribes, plus goats and sheep in the tribe Caprini. The impala, alone in the tribe Aepycerotini, had an origin close to the stem of the Antilopinae. Its closest genetic link is with the dwarf antelope called suni (Nesotragus moschatus), similarly ancient in its origin.9 The gazelle tribe (Antilopini) also had an early origin and is widely represented in Asia as well as Africa. The reduncines (waterbuck, kob, reedbuck, and allied wetland grazers) originated in Asia with early forms represented in fossil deposits in Pakistan dated to the late Miocene. The alcelaphines (wildebeest affiliates) evidently originated in Africa and had radiated into several genera by the late Miocene, all of them now either extinct or replaced by descendants.10,11 The genus Beatragus, represented by the rare hirola (or Hunter’s hartebeest, Beatragus hunteri), is closest genetically to the alcelaphine stem. Caprines were represented in South Africa by a buffalo-sized form called Makapania, now extinct. The hippotragines seemingly had an African origin.12 By the late Miocene, all of the modern antelope tribes had become distinct.

Giraffes had produced both long-necked and short-necked forms by the late Miocene. Suids (pigs) diversified into various forms, some quite huge.13 Three-toed hipparion horses (Eurygnathus sp.), with an origin in North America, appeared in Africa shortly after 10 Ma and were represented by several species.14 True elephants, derived from gomphothere ancestors, made their appearance late in the Miocene.15 White rhinos (Ceratotherium simum) diverged from a black rhino (Diceros bicornis) ancestor during the later Miocene and were distributed throughout Africa from the Cape to the Mediterranean Sea margin. Two forms of hippo coexisted, including Hippopotamus gorgops, with protruding eyes, along with the ancestral form of the modern hippo.

Emergence of Grazers

The tectonic uplift that took place in north-east Africa by 10 Ma deflected rain-bearing winds, accentuating aridity in eastern Africa. Carbon dioxide levels in the atmosphere became reduced towards 400 ppm, further restricting plant growth. Grasslands spread (see Chapter 9), enhanced nutritionally by volcanically derived soils. Shifts towards grass-based diets by large herbivores are revealed by the form of their teeth (see Chapter 10) and from the ratio of stable carbon isotopes in the collagen content of dental enamel or other body parts (see Chapter 7). The transitional period between 10 and 6 Ma is best represented in fossil deposits in the Turkana region of northern Kenya.4,16,17,18,19 Hipparion horses showed earliest indications of a C4 grass component in their diet by 9.9 Ma, although their tooth structure remained indistinct from that of browsers until 7.4 Ma. Rhinos showed an increasing proportion of C4 grasses in their diet after 9.6 Ma, and the grazing white rhino had diverged dentally from the browsing black rhino by 6 Ma. By 7.4 Ma, some bovids had diets consisting mostly of C4 grasses. Impala remained a browser in dentition and diet.20 Hippos showed a dietary trend towards an increased proportion of C4 grasses. By 6.5 Ma, some gomphotheres as well as early elephants had become mainly grazers. Even large suids had adopted C4-dominated diets by 4.2 Ma, along with short-necked giraffes.

By 4 Ma, in the early Pliocene, grazers dependent on tropical grasses for the bulk of their food had become a major component of the eastern African fauna not only among ruminants, but effectively among all large herbivore families. Tall-necked giraffe and black rhino remained mostly browsers. The tragelaphine antelope, along with the gazelles and other small antelope, consumed a variable mix of grass and browse. Hidden in the forests where few fossils formed, the duikers (Cephalophini) remained browsers on foliage and fruits. In Chad in north-central Africa, white rhinos were still consuming a mix of C3 and C4 plants around 6 Ma, but had become purely C4 grazers by 3.5 Ma after C4 grasses became dominant.21 Hipparion horses there were also exclusive grazers by that time, while both the ancestral African elephant and the gomphothere Anancus had become mainly grazers. In South Africa at Langebaanweg, dated to 5 Ma, dental adaptations for grazing were evident among early white rhino, hipparion horses and primitive buffalo (Simatherium), but stable isotopes indicated that C4 grasses had yet to make a contribution there.22,23 The early alcelaphine antelope represented at Langebaanweg (Damalacra and Parmularius spp.) were browsers or mixed feeders, based on their pointed molar cusps plus pits rather than scratches on their teeth. By 3.3 Ma, stable carbon isotopes indicated that C4 grass-dominated diets had become prevalent among all grazers inhabiting interior South Africa.24 Thus, grazers remained predominant throughout savanna Africa through the mid-Pliocene 3.5 Ma.

Plio–Pleistocene Turnover

By the commencement of the Pliocene 5 Ma, Africa’s large herbivore fauna was especially diverse, because many of the Miocene browsers still persisted alongside the more recently evolved grazers (Figure 14.2).25 Browsing ruminants remained prominent in rift valley deposits, especially in the Omo valley in southern Ethiopia, indicating the persistence of woodland mosaics near water (Figure 14.2).26 Among proboscideans, deinotheres retained a purely C3 browse diet, while the gomphothere Anancus showed a progressive shift towards consuming more C4 grasses, despite having low-crowned molars, until it vanished from the fossil record in the mid-Pliocene.3,27 The upland region around Laetoli in Tanzania supported a mix of grazing alcelaphines, hippotragines and gazelles plus hipparion horses between 3.8 and 3.6 Ma, with browsers somewhat less common.28 Also present then were several species of proboscidean as well as both species of African rhino. In the Omo Valley of Ethiopia, ruminants expanded at the expense of various monkeys and wild pigs between 4 and 2.8 Ma.25,29 The most abundant browsers or mixed feeders there were impala plus two species of tragelaphine, resembling bongo and bushbuck, respectively, while the grazers included early waterbuck (Kobus) and long-horned buffalo (Syncerus (or Pelerovis) antiquus).29,30 Early alcelaphine antelope showed greater prominence to the north-west at Hadar in Ethiopia and also near Lake Turkana in Kenya.29

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Figure 14.2

Scene representing the African Pliocene in the Lake Turkana basin. Note the presence of grazing Reck’s elephants and early equids along with sabretooth cats, as well as the wooded aspect Artwork: Mauricio Anton.

Following the climatic cooling that took place between 2.7 and 2.3 Ma during the inauguration of the Pleistocene, several essentially modern ruminant species appeared in the fossil record. They included various forms of kob, along with a large roan antelope.7 Wildebeest and greater kudu were recorded earliest in the south and exhibited dry savanna adaptations honed in arid Namibia,31,32 while sable antelope had an exclusively southern African distribution. Grazing zebra entered Africa from Eurasia around 2.6 Ma but remained less common than the coexisting hipparion horses until later. Following the transition into the Pleistocene 2.6 Ma, the species composition of large herbivore assemblages closely resembled that of modern communities. The area around the lake that formed at Olduvai remained quite densely wooded and showed a predominance of reduncine grazers plus browsing tragelaphines in its lowest beds dated to 2 Ma.

After 1.9 Ma, conditions became still cooler and more widely variable in aridity. Ruminant diversity reached its peak, augmented by further grazer radiations.25 Alcelaphine antelope became prominent everywhere, including giant wildebeest (Megalotragus) as well as common wildebeest, a large blesbok or topi (Damaliscus niro), and an extinct species in the genus Parmularius. The latter became the most common grazer both at Olduvai Gorge in eastern Africa and in cave sites in South Africa.7,33 Hippotragine grazers were rather uncommon throughout eastern Africa, except at Laetoli. At Olduvai Gorge, grazing equids increased in abundance after 1.7 Ma.33 Other grazers present then at Olduvai included giant buffalo, white rhino, a springbok (Antidorcas recki), short-necked giraffe, giant warthog along with other large suids, hipparion horse, and the gorgops hippo. Reck’s elephant (Elephas recki) was present, but scarce. Modern wildebeest and impala were rare. Alcelaphine antelope increased in representation in the lake margin habitats of east Turkana until 1.45 Ma, while reduncine grazers along with impala and gazelles became less common, indicating a continuing trend towards dry grassland.34,35 Certain large pigs became extinct locally but giant warthogs continued to flourish. The browsing deinothere was last recorded in the Omo–Turkana basin around 1.6 Ma.25 Its disappearance finally ended the tenure of genera representing the Miocene giants.

Fossil assemblages from Sterkfontein and Swartkrans in South Africa’s Cradle of Humankind dated after 1.8 Ma contain numerous large grazers, but lack the abundance of reduncine antelope and giant buffalo associated with rift valley sites.36,37 White rhino and impala are missing from cave deposits there, although they are represented further north at Makapansgat where conditions were more densely wooded.38 Elephant fossils are scarce at both sites, although frequently recorded elsewhere in South Africa around that time.

After 0.9 Ma, a further climatic shift occurred with glacial cycles lengthening to 100 kyr, making times of glacial advances more extremely cold, dry and prolonged. Around this time, dry country grazers or mixed feeders, represented by the alcelaphine antelope plus gazelles, achieved their greatest dominance in eastern Africa. The representation of mixed-feeding impala declined.39 Black wildebeest (Connochaetes gnou), endemic to treeless Highveld grasslands, originated in South Africa around 1.05 Ma,40 along with blesbok. During this time, other essentially modern species of antelope made their appearance, including hartebeest and topi in eastern Africa around 0.6 Ma.41 Numerous grazers predominated in the upland grasslands that developed on the shores of a greatly reduced Lake Victoria up until 36 ka, including forms of wildebeest, blesbok and impala along with long-horned buffalo.42 The modern African buffalo attained prominence in savanna faunas quite late in the Pleistocene, suggesting that it inhabited mainly forests until the long-horned buffalo faded out. Blesbok and springbok, typical of dry grassland or shrubland, were present further north in Zimbabwe. Lechwe occurred in parts of the Free State and even in the southern Kalahari until near the end of the Pleistocene, indicating the local presence of wetlands.43 Several grazers typical of savannas further north occurred as far south as Elandsfontein in the south-western Cape, consuming mostly C3 grasses growing among the fynbos shrubs predominant in this region.44,45

The ancestral form of the modern African elephant was the most common proboscidean in fossil deposits in northern Kenya early in the Pliocene, shifting toward a mainly grass diet through time.46 Later during the Pliocene it was replaced by Reck’s elephant, with more extreme dental adaptations for grazing, found throughout Africa until late in the Pleistocene.27,47 It seems that the modern African elephant became restricted to shrinking forests, occupied by present-day forest elephants (L. a. cyclotis), until after the disappearance of Reck’s elephant from savanna regions.

Late Pleistocene Extinctions

Starting during the sparsely recorded period between 1 and 0.5 Ma, several of the large grazers that had previously been abundant disappeared from both eastern and southern Africa.40,48,49,50,51 Among them was Reck’s elephant, last recorded in the Omo–Turkana basin at the end of the last-but-one glacial maximum 130 ka.47 Also last recorded around this time were the hipparion horse, short-necked giraffe and large gorgops hippo.52 The giant grazing gelada (Theropithecus oswaldi), previously abundant in north-eastern Africa, had disappeared from the fossil record a little earlier, after 350 ka. Notably, these were all Pliocene relicts and also all grazers.

A further wave of species extinctions took place in eastern Africa during or shortly after the LGM 20 ka, once again involving solely grazers.50 This included two forms of topi with exceptionally high-crowned molars (Damaliscus niro and D. hypsodon), which had both been locally abundant earlier, giant wildebeest and the long-horned buffalo (Figure 14.3). In southern Africa, long-horned buffalo and giant wildebeest became extinct around the same time, along with a big zebra (Equus capensis), a small springbok (Antidorcas bondi) and two suids: the giant warthog (Metridiochoerus) and grazing bushpig (Kolpochoerus).40,43,48 Several of them happened to be the largest species in their respective genera. These all represent genuine extinctions, leaving no descendants.

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Figure 14.3

Some of the large grazers that were common through the Pleistocene before becoming extinct towards the end of this epoch. (A) Grazing elephant (Elephas recki/iolensis); (B) long-horned buffalo (Syncerus antiquus); (C) giant wildebeest or hartebeest (Megalotragus priscus); (D) giant gelada (Theropithecus oswaldi).

Artwork: Roman Uchytel.

In interior South Africa, grazers that exhibited a substantial dietary contribution from C3 plants prior to 500 ka had shifted to almost purely C4 diets by the Holocene.53 Whether the C3 component represented a greater presence of C3 grasses during the cold conditions of the LGM, or low browse in the form of dwarf shrubs spreading during these conditions, remains undetermined.

White rhinos vanished from the swathe of Africa between the Zambezi and Nile rivers, separating its two subspecies, quite late in the Holocene. Cave paintings in central Tanzania,54 plus the odd tooth,55 show that white rhinos remained present in eastern Africa merely a few thousand years ago. White rhinos were abundant throughout drier savanna regions of southern Africa into historic times.56 Cave paintings show that they had been present as far north as Algeria until quite recently. The regional extirpation of this mega-grazer throughout eastern and south-central Africa cannot be ascribed to any apparent unsuitability of habitats, because other dry-country grazers that were associated with white rhinos continued to thrive.

Roan antelope provide another example of a recent local extirpation. Rock art depicts this species in the Drakensberg foothills, several hundred kilometres south of their historic distribution limit, which lay in southern Kruger NP. Roan antelope were present in the southern Cape late in the Pleistocene.48 The bluebuck (Hippotragus leucophaeus), a small version of roan antelope with a narrow distribution in the south-western Cape, persisted until around 1800 CE, when its demise was brought about by hunting plus habitat transformation following European settlement. Its favoured habitat was probably the grassy Agulhas plain before this became inundated by sea-level rise, restricting the species to grassy patches amid fynbos shrubland.

Large Carnivores

The predominant carnivores in Africa through the Oligocene into the early Miocene belonged to an extinct group called the creodonts. The first representatives of the order Carnivora made their appearance during the Miocene in the form of hyenas.57 During the late Miocene and early Pliocene, five species of hyena coexisted locally at some African sites, including a giant species weighing around 100 kg and a hunting hyena with long limbs for cursorial hunting resembling the modern brown hyena in size. Jaw adaptations for bone-cracking first appeared during the Pliocene and the modern spotted hyena had emerged by 3.5 Ma. Bears were represented by two species during the Pliocene, but they were nowhere common and soon became extinct.58 The giant hyena went extinct around 1.5 Ma and the hunting hyena around 1 Ma.

The earliest large felids were sabretooth cats (subfamily Machairodontinae), which appeared in Africa during the late Miocene around 7.5 Ma at the time of the bovid radiations. They remained the most abundant carnivores through the Pliocene, represented by three genera.57,59 Meganterion resembled the northern hemisphere Smilodon with its robust forelimbs and dagger-like canines, and presumably also specialised in killing young ‘pachyderms’ like elephants and rhinos. It was never very common in Africa and disappeared there after 1.4 Ma. Homotherium slightly exceeded a modern male lion in size, with long legs and extended serrated canines. It may have hunted by ambush like the modern tiger, targeting large ungulates that were not able to run fast in wooded savanna environments.60 However, its laterally compressed canines with serrated edges suggests that these were adapted to cutting open the carcasses of thick-skinned herbivores rather than applying lethal bites to struggling animals. It persisted until 0.7 Ma. The false sabretooth Dinofelis was a little larger than a leopard and had only slightly elongated canines. Its short but robust forelimbs seem adapted for grappling, suggesting it was an ambush predator reliant on vegetation cover and perhaps targeting young animals of the larger herbivores.61 Dinofelis remained widespread until 0.9 Ma. Big cats in the genus Panthera and ancestral cheetahs first appeared during the mid-Pliocene around 3.5 Ma, but remained rare until after the demise of the sabretooths. Canids entered Africa after 3.5 Ma in the form of jackals and foxes, but the African wild dog appeared only after 2 Ma.

Thus, during the late Pliocene and early Pleistocene, Africa supported an exceptionally rich assemblage of large carnivores, comprising up to five felids plus five hyenas, double the total that exists today.58 How did all of these carnivores coexist, and why did half of them become extinct? How did the human ancestors that had adopted a meat-augmented diet manage to persist by hunting when large carnivores that had evolved as hunters disappeared from the African fauna?

Species and Subspecies

Much of the account of faunal changes through time is based on species names. Biologically, species represent morphologically distinct segments of independently evolving lineages, isolated from genetic exchanges by mate recognition cues or by the infertility of hybrids. Recent genetic research reveals that genetic exchanges occur between morphologically distinct species placed in the same genus more frequently than had been imagined, most memorably between modern humans entering Eurasia and the Neanderthal people already present there.62 However, the potential to interbreed cannot readily be established between populations that are isolated geographically. There is a recent tendency among taxonomists to elevate populations previously regarded as subspecies to full species status, based on divergence in mostly neutral gene mutations.63 A contentious example concerns the elevation of the forest elephant (L. a. cyclotis) to full species status.64 However, all of the evidence suggests that the two elephant populations have not diverged sufficiently ecologically to coexist without genetic merging. Although the forest buffalo (Syncerus caffer nana) is more distinct physically from the savanna buffalo (S. c. caffer) than the differences between the forest and savanna elephants, they remain distinguished only at subspecies level.

This taxonomic dilemma is more acute for forms represented only by fossilised remains, too ancient to preserve their DNA. Morphological changes through time (‘anagenesis’) can justify the assignment of new species names; the former species name is extinguished, while the lineage continues unbroken. All species go extinct, but not necessarily the lineages they represent. The ancestral species can coexist sympatrically with its descendant only if sufficiently distinct ecologically in diet, habitat, or other components of ecological niches. Nevertheless, geographic isolation can form the foundation for ecological divergence.

Among Africa’s ungulates, the alcelaphine antelope exhibit numerous subspecies reflecting their patchy distribution across Africa (Table 14.1). The reduncine grazers show a different pattern, with distinct species replacing one another geographically in isolated wetlands. The gazelles are assigned to numerous species, geographically localised through parts of Asia as well as Africa. In contrast, only two impala subspecies are distinguished, from a very minor distinction in face colour in Namibia, despite the long evolutionary history of their genus. Greater kudu from the southern and north-eastern ends of Africa are distinguished only subspecifically, despite their distant geographic separation.

Table 14.1Bovid subspecies recognised (from species accounts in Kingdon & Hoffmann (2013) The Mammals of Africa)

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Do the alcelaphine subspecies indicate incipient splitting into species? Or are the criteria used to differentiate them – horn shape and coat colour – too trivial to be of any weight ecologically? I raise these issues here because they will come to the fore later when we confront the taxonomic proliferation among early hominin specimens.

Overview

Africa’s diverse assemblage of grazing ruminants originated during the late Miocene when savanna vegetation formations dominated by C4 grasses spread. Other large herbivores also shifted their diets towards C4 graminoids during this period, even some of the Miocene giants. Further diversification among the grazers, especially the alcelaphines, occurred when open savanna conditions expanded further, around 2.7 Ma and, less markedly, around 1.8 Ma. Browsing ruminants remained more conservative in species than grazers, while mixed-feeding impala retained a single lineage from the late Miocene through to the present. The greatest diversity of large herbivores was manifested during the early Pleistocene, before the last of the Miocene relicts faded out.

Comparable radiations of grazing ungulates did not occur on other continents, despite the global expansion in C4 grasslands. North America developed a diversity of grazing and browsing equids during the Miocene, later spreading into South America, along with various gomphotheres.65,66 All of America’s very large herbivores disappeared during the wave of extinctions that followed the arrival of modern human hunters in both continents during the late Pleistocene (Figure 14.4).67 The only specialist grazer that survived in South America was the vicuna, a smallish camelid restricted to grassy meadows high in the Andes ranges. No deer is an obligate grazer in its diet or anatomy, although several species include grasses in a mixed diet. Bovines including gaur (Bos gaurus), bisons (Bison bonasus and B. bison), yak (Bos grunniens), and auroch (Bos primigenius, the ancestor of domestic cattle) are the only ruminants phenotypically and dietary adapted as grazers present outside of Africa today. All of the gazelles found in Asia are mixed feeders. Australia houses a diverse assemblage of medium–small marsupials on the kangaroo theme, but with no obligate grazers among them, even before the arrival of humans. The predominance of very large herbivores in the faunal assemblages found in the Americas and northern Eurasia prior to the late Pleistocene extinctions resembles the faunal features shown in Africa during the early Miocene.67

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Figure 14.4

Browser–grazer distributions of large mammalian herbivores extant on different continents during the late Pleistocene within different body size ranges (excluding African forest duikers). The two-way division is between species that fed solely or mainly on woody plants and herbs and those that solely or mainly grazed on grasses and sedges.

(adapted from Owen-Smith (2013) Journal of Biogeography 40:1215–1224)

Africa did not completely escape the extinctions that decimated large mammal diversity on other continents around the end of the Pleistocene. Reck’s elephant, dentally specialised for grazing, disappeared, while the modern African elephant with a more diverse diet expanded in its place. Both of the mega-grazers surviving into modern times – white rhino and the hippo – have wide mouths facilitating cropping short grass. Extinctions involved several species that were the largest of their kinds: long-horned buffalo, giant wildebeest, big zebra, gorgops hippo and giant warthogs. Last appearance dates in southern Africa cluster shortly after the last glacial maximum ~20 ka,48,50 although some forms faded from the fossil record around the end of the preceding glacial maximum. Several medium-sized grazers with exceptionally high-crowned teeth that had previously been abundant in dry grasslands also went extinct. Large carnivore extinctions, involving various sabretooths and several hyenas, took place earlier, around the time when most of the relicts of the Miocene giants faded out. The grazers that did survive into modern times, like wildebeest and white rhino, have contracted distribution ranges compared with those they manifested historically.68 Why were the environmental conditions that prevailed during the last glacial extreme so inimical for large grazers that had previously thrived?

The last glacial maximum was just one in a series of glacial oscillations with 100-kyr periods between peaks that were established after 0.8 Ma. The progressive downward trend in global temperatures continued, with each glacial maximum attaining unprecedented extremes of cold, most especially the last one ~20 Ma and that which preceded it ~140 ka (Figure 19.1). Global cold means less moisture evaporated from oceans and hence less falling as rain in tropical and subtropical savannas. Lower rainfall means less grass produced and shorter grass remaining through the dry season. Abundant medium–large grazers further reduced the height of the grass remaining while human hunters contributed by setting fire to ungrazed grass to improve visibility and passage.

Reductions in amount and height of grass remaining through the critical dry season months would be especially inimical for the largest grazers.69 This seems a tenable explanation for the apparent size bias of the extinctions. Lacking resource buffers of sufficiently tall grass, the larger grazers ran out of food soonest. Their populations became reduced to zero throughout Africa, largely synchronous in time. Reck’s elephants, using their trunks to yank out tall grass tufts, would have been especially compromised. Presciently, modern African elephants turned to bark and roots to tide them through dry seasons when they extended their range from forests into savannas.

However, some contribution from early humans to these extinctions cannot be ruled out, through hunting in addition to the deployment of fire. This contentious issue will be addressed in the final set of chapters, forming Part IV. Somehow, early humans surmounted these tough times, while several larger grazers that had survived through multiple climatic extremes became extinct. What were the crucial adaptations that enabled these comparatively puny omnivores to make it through into the benign Holocene while numerous large herbivores fell by the wayside?

SUGGESTED FURTHER READING

Bobe, R. (2011) Fossil mammals and paleoenvironments in the Omo–Turkana Basin. Evolutionary Anthropology 20:254–263.

Elliot, MC; Berger, LR. (2018) A Handbook to the Cradle of Humankind. Reach Publishers, Wandsbeck.

Werdelin, L; Sanders, WJ (eds) (2010) The Cenozoic Mammals of Africa. University of California Press, Berkeley.

REFERENCES

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