Llama, Palaeolama mirifica; tapir, Tapius veroensis; and equus, Equus scotti.
By 4 million years ago, two strains of elephantine creatures (a.k.a. proboscideans)—the mammoths and the mastodons—had long since roamedthe North American continent, both deriving originally from Africa and having made their way from there into Europe and Asia. Mastodons were originally four-tusked, with both upper and lower tusks, but they lost the bottom ones and the uppers became long and curved. Otherwise, being what biologists call conservative, they changed little over time. Unlike the sociable elephants we know today, these slope-backed animals appear to have been loners for the most part. Ten to twelve feet at the shoulder, they fed chiefly on branches and other brushy material, perhaps with a preference for evergreens. Indeed, in both their habitat and their tendency to be loners, they were not unlike the moose with whom they would later share forested areas and swamps for a time. In late Pleistocene times, they were accordingly more common in the eastern portion of North America than in the more open lands of the West. In addition to Ice Age moose (and stag-moose, a moose-elk combo that was among the world's largest natural hat-racks ever) were elk, caribou, and deer. It would, in fact, take a highly expert student of deer to tell the difference between today's white-tailed deer and its direct ancestors 3 million years ago, so conservative were they. Other beasts still familiar today were numerous small mammals: badgers, shrews, moles, weasels, and both spotted and striped skunks.
Mastodon, Mammut americanum.
The other elephantine creatures here were mammoths, monsters with extravagantly long curved tusks spiraling to the point where, especially among males, the tips often crossed each other, making them no longer useful for spearing enemies, though they were most likely devastating as clubs. The first mammoths in the New World were a breed of southern mammoths that had slowly adapted to the cold of northeastern Asia but then readapted to slightly warmer climes in the southwestern parts of the United States and continued on southward, eventually populating South America as well. Wandering about in small groups not unlike today's elephant herds, they were more social than the mastodons and, some like to speculate, perhaps matriarchal, as are today's elephants (which are in fact a different strain of proboscidian altogether). Mostly creatures of open areas such as plains, they were rarely in competition with the forest-dwelling mastodons.
Columbian mammoth, Mammuthus columbi.
In addition, there were the woolly mammoths—beasts of Siberia, the frozen north, and the edge of the glaciers in North America. The woolly mammoth typically was about fourteen feet tall, more than a foot tallerthan the largest elephant ever measured, the huge African one that has long stood guard in the rotunda of the Smithsonian's National Museum of Natural History. The woolly mammoth had a shorter trunk than the southern mammoth as well as smaller ears (the better to conserve heat), a subcutaneous fat layer, and a woolly undercoat covered by a coat of long black hair. Many woolly mammoth tusks have turned up with the bottom surfaces heavily worn, evidently a result of using them to shove snow and ice aside to find morsels of pasturage underneath. As the late paleontologist Björn Kurtén wrote, the woolly mammoth is “the embodiment of the ice age. Long may it live in our imagination, a black, top-heavy shape looming up in the swirling snow, great tusks gleaming: to our forefathers, perhaps, a demigod.”
Bison, Bison antiquus.
Less demigodlike, surely, were the burro-sized mammoths that evolved on Santa Rosa Island off California, an example of the dwarfism often found in island populations where predators are few if not altogether absent. When all the mammoths (and the shorter, stockier mastodons) finally ran up against the forces unleashed at the end of the Pleistocene, they vanished—except for a population of woolly mammoths that survived in the frigid steppes of Wrangel Island off Siberia until Roman times.
Other notable Eurasian migrants to North America were gigantic bison—much larger, longer-horned, and probably less fleet than today's, but capable of hanging out in the frozen lands of Alaska until an ice-freecorridor allowed them to move south, where they found something akin to bison nirvana in the open lands of the West, spreading eastward as well as southward from there. Present in North America as early as 30,000 years ago, they were replaced by slightly smaller, slightly shorter-horned bison called Bison antiquus, which survived well after the end of the Ice Age and upon which early Americans preyed with considerable skill and effectiveness. These imperceptibly graded into the still smaller bison of today. Meanwhile musk oxen (but not, strangely enough, the equally well adapted yak) came in from Siberia across Beringia, which itself may have served as something of a hotbed of evolution when it appeared above the waves for lengthy periods. The land bridge was a swinging door that swung both ways.
One other notable no-show, at least south of the ice, was the saiga antelope, which did make it into North America but never penetrated into the interior of the continent. Today's American “antelope,” otherwise and more correctly called the pronghorn, is in fact not an antelope but a goat relative, itself the descendant of a far larger Ice Age version.
Sometime between 3 million and 4 million years ago, members of a wholly different fauna were added to what had begun in North America or found its way here from Asia. A land bridge (the current isthmus of Panama) appeared between South and North America, the result of islands being shoved upward on a rising piece of land as one gigantic tectonic plate rose up above another. This in turn gave rise to what has been called the Great Faunal Interchange—though it was but one of several intercontinental interchanges, as we have seen. For millions of years the two New World continents had been separated, not by much in the way of water but enough to prevent most mammals from crossing. Armadillos came about in South America and made it north, reaching seven feet in length. Armadillos and sloths are both of the mammalian group called edentates, meaning “toothless,” and another edentate was the huge, armored creature called the glyptodont. This oddball mammal of riverbanks looked more like an enormous turtle with its vaulted carapace, but it had a long tail, it measured ten feet in length and five feet in height, and it moved around on short, pillarlike legs. Even its tail was armored with a tubelike arrangement of rings of armor.
South America had been isolated for millions and millions of yearsafter it separated from Africa and moved west. It had its own special fauna, much of which consisted of marsupials. Among the other South American oddities was a variety of large flightless birds, at least one of which made its way to Florida once the land bridge came about. Known as Titanis, it was an immense, ostrich-sized predatory bird with thick legs and a curved beak like that of an eagle. Flightlessness often occurs among birds, but usually in places where there is little or nothing to worry about in terms of predators, in particular, on islands. Titanis and the other large flightless birds of South America flourished because the mammals (including the marsupials) remained relatively small. Once it reached North America, Titanis's career was doomed, for while it might have fended off those early hyenas, it would have been no match for the lions and other carnivores that were present.
Giant armadillo, Dasypus bellus, and giant sloth, Eremotherium rusconii.
Sloths evidently made their way north across the new land bridge, though some suggest that giant sloths, good swimmers, could have made it over before a dryland passage existed by island-hopping. In the other direction, small mammals and large invaded the south, perhaps in part to escape the deteriorating climate of the north. Mastodons, horses, camelids,bears, and deer were followed by big predators such as lions, and this new fauna virtually overwhelmed the endemic one.
In the north, in addition to the short-faced bear, lions reigned supreme. These were not the small lionlike cats of today—pumas, jaguars, and so forth—but lions that reached as much as eight feet in length, huge animals with brains that were evidently larger than those of any other lions then or now, suggesting a high degree of sociality and effectiveness in the hunt. They may have been a bit slower than t
oday's smaller, sleeker lions found in Africa (where lions originated some 2 million years ago), but they were also stronger. The brainy, brawny American strain made it south as far as Peru, making the lion at the end of the Pleistocene the most widely distributed wild animal in the world, present on five continents and missing from only Australia and Antarctica.
In North America, on the open lands south of the glacier, the lion was joined in predatory swagger by the American version of the cheetah, the fleetest land animal known today and doubtless just as fast in Pleistocene times, along with at least two forms of saber-toothed cats. One of these was the scimitar cat, a big animal with long fangs (but nothing like those of the saber-toothed “tiger”) that were sharp-edged and serrated, the perfect tool for slicing meat. These cats, called Homotherium, had long front legs and short back legs and apparently preferred to hunt baby mammoths, perhaps attacking with a slicing bite to the neck and then retreating until the dangerous adult mammoths went away. But the signature Pleistocene predator was the misnamed, fantastic saber-toothed tiger, Smilodon fatalis, a Latinate name that even without translation sounds fit for a serial killer in a thriller novel. It was the size of today's African lion but heavier and shorter of leg, its upper canine teeth thrusting down below its lower jaw by several inches. It may have used these protruding sabers with its mouth closed to slice up its prey, which it would have overcome with sheer strength, not necessarily speed. It was a loner like a tiger and less brainy than the lions, with which it must have competed to some extent. It was a heavily armed supercarnivore, and its entire jaw (which opened to 95 degrees), skull, and neck were devoted to the moment when the huge, glistening, serrated saber teeth would stab into hot flesh. At the same time, it may not have been especially bright—it was the second most frequently trapped carnivore in the La Brea Tar Pits (more than 1,000 have beenfound). The most frequent was the dire wolf (about 1,600), larger and heavier but shorter of leg than the timber wolf, with a big head and hugely strong jaws. Dire wolves, which ranged throughout unglaciated North America and as far south as Peru, probably hunted in packs and most likely hunted the large bison, Bison antiquus. One can imagine a pack of them in a snarling frenzy over some prey, driving the slow-moving Smilodon away, but one can equally imagine them slinking away upon the approach of the big-toothed monster.
By the late Pleistocene, then, North America's population of large mammals included homegrown fauna, some of which were more than a million years old, joined by Eurasian and South American migrants, some of such long standing as to be practically natives—in all, an astonishing array of prey and predator unparalleled in diversity anywhere before or since. And it was into this unbelievably rich, partly familiar but mostly exotic, promising, terrifying menagerie that the first Americans must have come. These new, bipedal arrivals, the most charismatic megafauna ever to hit the hemisphere—whenever exactly they arrived—were just as much children of the ice as were the Pleistocene giants they encountered in their migrations.
CHILDREN OF THE COLD
The trail leading to the New World ultimately began thousands of miles from Beringia in the savannas of east Africa. There, sometime around 2.6 or 2.5 million years ago, in the late Pliocene era, a diminutive hominid began to make stone tools sufficiently standardized to be recognizable as the products of human agency rather than objects shaped by natural processes. The hominid in question was either a member of the genus Australopithecus or, more likely, the earliest representative of our own genus, Homo. These first recognizable stone tools are assigned to the Oldowan tradition (after Olduvai Gorge, the place where they were first found, identified, and described, which is called their “type locality” in archaeology-speak). They constitute the earliest bona fide evidence of cultural behavior in the long history of life on the planet. These creatures doubtless made other tools of other materials—biodegradable ones such as wood, bone,horn, antler, shell, and even plant fiber—but virtually all that remains are the stone artifacts, and this is a preservational bias that has shaped our thinking ever since.
The earliest tool users whose physical remains we have found left their lithic handiwork at Kada Gona, Ethiopia, about 2.6 million years ago and slightly later at other sites in the Hadar country and around Lake Turkana in northern Kenya. They were smallish creatures, more ape than human, at least to the casual eye, and they had brains about a third the size of ours. However, in the ratio of brain volume to body mass, they were the smartest land mammals of their time. But while we tend to think of these creatures in terms of their brains, it is much more appropriate to characterize them in terms of the opposite end of their anatomy, their feet.
By the time the more gracile of the several kinds of east African australopithecines or the earliest representative of the genus Homo began manufacturing stone tools, they had already been walking on two legs for several million years. Indeed, habitual upright posture and bipedal locomotion are probably the most profound single adaptations that our remote ancestors ever made. We may well never know just what feature of the environment or other factor triggered this fundamental and revolutionary adaptation, but a mid-Pliocene cold snap between about 6.5 million and 5 million years ago probably played a role. According to this scenario, expansion of the Antarctic ice cap dropped sea levels worldwide, including that of the Mediterranean. This in turn changed the precipitation pattern of African rain forests and ultimately caused their contraction. This reduction in forest cover caused many arboreal species of primates to pass from the scene, while others became, at least in part, terrestrial.
For at least some of these new ground dwellers, bipedalism offered several advantages. Now they could cover relatively great distances with a low expenditure of energy. Life in the trees can be fairly simple, if today's monkeys are any indication. You need mostly to move around in a fairly leisurely pursuit of the regular and cyclical fruiting of the variety of forest trees. Once on the ground, however, you need to forage much more widely for adequate food. In the same vein, bipedalism frees your forelimbs to carry foods you've collected from one place to another and also to manipulate objects (sticks, for example) that could, in turn, make food getting easier. With freed forelimbs—now arms with distinct hands, infact—this bipedal creature could also carry its infant young from place to place.
There are yet other advantages of walking on two feet. Standing or walking erect puts your field of vision above the height of tall savanna grass, giving a primate that is largish a bit of an edge, to be sure, but he was no match for some of the savanna's predators. Also, it has been suggested that upright posture helps in using the body, especially the upper body and arms, in threat displays—pounding the chest or other aggressive gestures.
Whatever the primary advantage of bipedal locomotion—and I suspect it relates directly to enhanced mobility with reduced effort—it is virtually certain, as noted by Richard G. Klein, a paleoanthropologist at the University of Chicago, that all of the “benefits” of bipedalism operated in tandem. With all of bipedalism's presumed advantages, however, it did not happen overnight.
Indeed, in many ways, the earliest bipeds were far more apelike in certain respects than is usually realized. They continued for millennia to use trees as sleeping areas, foraging localities, and refuges. But also, and more basically, their bodies retained such “primitive” characteristics as a pronounced difference in size between males and females (called sexual di-morphism). Even so, as noted, by 2.5 million years ago or thereabouts these bipeds were making tools. They had crossed, at least in my view, the threshold of humanity.
At about this time, another series of dramatic environmental events again transformed the stage upon which these earliest tool-using actors and actresses performed. Between about 3 million and 2 million years ago, it appears, a series of cooling and drying episodes began in the middle latitudes of the planet. Ultimately they were linked to, if not directly caused by, the onset of glacial cycling in the Northern Hemisphere that began around this time. In Africa, as one r
esult, the forest cover diminished again, the open savanna grew, and the turnover of forest-adapted species to open-ground species continued.
While these changes are ultimately a reflection of cold glacial and warmer interglacial interludes in the north, in Africa and other subtropical settings the most important transformations were a matter not so much of increasing temperature as of a change in the patterns of precipitation. Ultimately, shifts in both the volume and the seasonality of rainfall reduced theextent of the African forests and extended the grasslands. However far removed they were, these events still mirrored—albeit with some time lag— the vicissitudes of the northern ice.
Within this changing world, largish, predominantly ground-dwelling, bipedal primates had several adaptive options. One group headed off anatomically and behaviorally in the direction of chewing tough vegetable foods. These were our Pliocene cousins, the so-called robust australopithecines (Australopithecus aethiopicus, Paranthropus boisei, andParanthropus robustus). They remained relatively small-brained, apparently made no tools, and ultimately became extinct. The second group remained more omnivorous with, if anything, a preference for the meat end of the dietary spectrum. They developed progressively larger brain cases, as well as the systematic use and manufacture of tools, and later on became you and me.
Once under way in the body of Homo habilis (“handy person”), the synergy between increased cranial capacity and enhanced production and use of tools continued and escalated, impelled in no small part by the constantly changing stimuli of a highly plastic environment. By about 1.8 million to 1.7 million years ago, at the last glimmer of the Pliocene and the dawn of the Pleistocene, a new version of us appeared in east Africa—not coincidentally, with a better tool kit.
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