A New History of Life (51 page)

As humanity crossed Asia and settled in Beringia, in a succession of small waves between perhaps thirty and twelve thousand years ago, the continental ice sheets covering large portions of North America expanded to maximum size in a long series of cooling episodes, followed by rapid warming. As the ice increased in volume, the level of the sea began to fall, causing huge land areas long underwater to become dry land—land that would in some areas serve as migratory paths between formerly isolated islands and large landmasses. But when the ice finally began to melt, the level of the sea began to rise as well. As late as fourteen thousand years ago, the continental glaciers covering most of Canada and large portions of what is now the United States were still slowly melting under gradually rising temperatures.

Soon thereafter, however, a new event accelerated the melting process. When enough ice had melted so that the glaciers no longer extended out to sea from the coast, the calving of icebergs from the eastern and western coastlines of what are now Canada and the northern portions of the United States could no longer occur. Each spring during the period of glacial maxima (about eighteen thousand to fourteen thousand years ago) great fleets of icebergs were launched into the coastal oceans, and this in turn kept the waters cool and created very cold winds that cooled the lands as well. Yet with the cessation of iceberg formation, warmer onshore winds arose, and the ice began to melt everywhere on the continents in earnest.

The melting fronts of the glaciers must have been extraordinary and extraordinarily harsh places—incessant strong winds characterized the retreating glacial walls. So strong was the wind that it created great piles of sand and silt carried by the wind, sediment called loess. The winds also carried in seeds, so that the drifting soils in front the glaciers were soon colonized by pioneering plants. First came the
ferns and then more complex plants. Willow, juniper, poplar, and a variety of shrubs were the first stable communities to transform this ancient glacial regime; and soon thereafter successive communities of plants arrived, depending on location. In the more temperate west, low forests dominated by spruce were the norm; in the middle colder parts of the continent permafrost and tundra were the norm. Yet everywhere the glaciers were in retreat, and as they migrated, or more correctly, melted north, they were followed by a front of advancing tundra, which soon was followed by vast spruce forests.

Spruce communities of large areas in North America were as much open woodland as dense forest, with copses of trees interspersed with grass and shrubs. By no means was it similar to the great, thick Douglas fir communities found in the few remaining old-growth forests of the Northwest, places where dense underbrush and fallen rotting logs make passage by large game, or humans, exceedingly difficult.

South of the ice in North America, throughout the ice age, a variety of habitats existed. There was forest tundra, grassland, and deserts, and plants sufficient to sustain enormous herds of giant mammals. With the end of ice and cold over so much of the world, human populations began to increase markedly.

By ten thousand years ago humans had successfully colonized each of the continents except Antarctica, and adaptations to the many locales led to what we now call the various human races. While it was long thought that such obvious features as skin color were purely adaptations to varying amounts of sun, more recent work suggests that much of what we call “racial” characteristics might simply be adaptations brought about by sexual selection, rather than increases to fitness in various environments. But many other adaptations, most invisible to morphologists, were happening as well.

Africa is revered for its abundance of large mammals. Nowhere else on Earth can such diversity of larger herbivores and carnivores be found. Yet this animal paradise, instead of being the exception, was the rule: all of the world’s temperate and tropical grazing regions were quite recently of African flavor. But like the forces that wiped out
the elephants in the Karoo, an extraordinary event has depleted the Earth’s biodiversity of large mammals over the past fifty thousand years.

Although the disappearance of larger animals poses a tremendous challenge to those studying extinction, a significant lesson from the past is that the extinction of larger animals has a far more important effect on the structure of ecosystems than does the extinction of smaller animals. The extinction at the end of the Cretaceous was significant not because so many small mammals died out, but because the very large dinosaurs did.

It was the removal of the very large land-dwelling dinosaurs that reconfigured terrestrial environments. In similar fashion, the removal of the majority of larger mammals species across most of the world over the last fifty thousand years is an event whose significance is only now becoming apparent, and one that should have lasting effects for additional millions of years into the future.

One time period in particular was the Late Pleistocene epoch of about fifteen thousand to twelve thousand years ago, when a significant proportion of large mammals in North America went extinct. At least thirty-five genera (and thus at least this number of species) became extinct. Six of these lived on elsewhere (such as the horse, which died out in North and South America but lived on in the Old World). The vast majority, however, died out. In fact, most belonged to a wide spectrum of taxonomic groups, being distributed across twenty-one families and seven orders. The only unifying characteristic of this rather diverse lot is that most (but certainly not all) were large animals.

The most well-known and iconic were the elephant-like animals, the Probiscideans, including mastodons and gomphotheres as well as mammoths, which were closely related to the two types of still-living Old World elephants. Of these, the most widely distributed in North America was the American mastodon, which was widespread from coast to coast across the unglaciated parts of the continent. It was most abundant in the forests and woodlands of the eastern part of the continent, where they browsed off trees and shrubs, especially spruce trees. The gomphotheres, a bizarre group quite unlike anything now alive,
are questionably recorded from deposits in Florida, but otherwise were widely distributed in South rather than North America. The last group, the elephants, was represented in North America by the mammoths, comprised of two species, the Columbian mammoth and woolly mammoth.

The other group of large herbivores iconic of the ice age in North America was the giant ground sloths and their close relatives the armadillos. Seven genera comprising this group went extinct in North America, leaving behind only the common armadillo of the American Southwest. The largest of this group were the ground-living sloths, ranging in size from the size of a black bear to the size of a mammoth. An intermediate-sized form is commonly found in the tar pits of present-day Los Angeles, while the last, the Shasta ground sloth, also the most well known, was the size of a large bear or a small elephant. Also spectacular was the North American glyptodont, a heavily armored creature ten feet in length with a heavily armored, turtle-like shell. Also going extinct was an armadillo, a genus survived by the common nine-banded armadillo.

Both even- and odd-toed ungulate animals died out as well. Among the odd-toed forms, the horse, comprising as many as ten separate species, went extinct, as did two species of tapir. Losses were greater among the even-toed ungulates. Thirteen genera belonging to five families went extinct in North America alone in the Pleistocene extinction, including two genera of peccaries (wild pigs), a camel, and two llamas, the mountain deer, the elk-moose, three types of pronghorns, the Saiga, the shrub-ox, and Harlan’s muskox.

With so many herbivores going extinct, it is no surprise that many carnivores also underwent extinction. These included the American cheetah, a large cat known as the scimitar cat, the saber-toothed tiger, the giant short-faced bear, the Florida cave bear, two types of skunks, and a dog. Some smaller animals round out the list, including three genera of rodents and the giant beaver. But these were exceptions. Most animals dying out were large in size.

The extinction in North America coincided with a drastic change in plant community makeup. Vast regions of the northern hemisphere changed from plant assemblages dominated by highly nutritional
willow, aspen, and birch trees, to far less nutritious spruce and alder groves. Even in those areas dominated by spruce (itself a relatively poorly nutritious tree), a diverse assemblage of more nutritious plants were still available. However, as the number of nutritious plants began to decrease due to the climate changes, the herbivorous mammals would have increasingly foraged on the still-remaining, more nutritional plant types, thus exacerbating their demise, and perhaps thus leading to the reduction in size of many species of mammals depending on vegetation for food. As the Pleistocene ended, the more open, high-diversity spruce forests and nutritional grass assemblages were rapidly replaced by denser forests of lower diversity and lower nutritional value. In the eastern parts of North America the spruce stands changed to large, slow-growing hardwoods such as oak, hickory, and southern pine, while in the Pacific Northwest great forests of Douglas fir began to cover the landscape. These types of forests have a far lower carrying capacity for larger mammals than the Pleistocene vegetation that preceded them.

It was not just North America that suffered such severe losses.
¹⁰
North and South America had been isolated from one another, and hence their faunas underwent quite separate evolutionary histories, until the Isthmus of Panama formed, some 2.5 million years ago. Many large and peculiar mammals evolved in South America, including the enormous, armadillo-like glyptodons, as well as the giant sloths (both of which later migrated northward and become common in North America), giant pigs, llamas, huge rodents, and some strange marsupials. When the land connection formed, free interchange between the two continents began.

As in North America, large-animal extinction occurred among South American mammals soon after the end of the ice age. Forty-six genera went extinct between fifteen thousand and ten thousand years ago. In terms of the percentage of fauna affected, the mass extinction of large animals occurring in South America was even more devastating than those in North America.

Australia suffered even greater losses, but at an earlier time than either North or South America. Since the age of dinosaurs the
Australian continent had been an isolated landmass, surrounded by ocean. It was thus cut off from the mainstream of Cenozoic-era mammals. The Australian mammals followed their own evolutionary path, resulting in a great variety of marsupials, many of large size.

The mass extinction striking the Australian fauna during the last fifty thousand years killed off forty-five species of marsupials belonging to thirteen genera. Only four of the original forty-nine large (greater than twenty pounds in weight) marsupial species present on the continent a hundred thousand years ago survived. No new arrivals from other continents bolstered the disappearing Australian fauna. The victims included large koala bears, several species of hippo-sized herbivores called
Diprotodon
, several giant kangaroos, several giant wombats, and a group of deer-like marsupials. Carnivores (also all marsupials) were lost as well, including a large lionlike creature and a doglike carnivore. In more recent times, a third predator, a catlike creature found on offshore islands, has also disappeared. Large reptiles also disappeared, including a giant monitor lizard, a giant land tortoise, a giant snake, as well as several species of large flightless birds, among others. The larger creatures that did survive were those capable of speed or those that had nocturnal habits, as noted by our great friend Tim Flannery of Australia,

The wave of extinctions affecting the faunas of Australia, North America, and South America coincides both with the first appearance of humanity in all three regions and with substantial climate change. Reliable evidence now shows that humans reached Australia between thirty-five and fifty thousand years ago. Most of the larger Australian mammals were extinct by about thirty to twenty thousand years ago.

A different pattern emerges in the areas where humankind has had a long history, such as Africa, Asia, and Europe. In Africa, modest mammalian extinctions occurred 2.5 million years ago, but later losses, compared to other regions, were far less severe. The mammals of Northern Africa, in particular, were devastated by the climatic changes that gave rise to the Sahara desert. In eastern Africa, little extinction occurred, but in southern Africa, significant climate changes occurring about twelve thousand to nine thousand years ago were coincident
with the extinction of six species of large mammals. In Europe and Asia there were also fewer extinctions than in the Americas or Australia; the major victims were the giant mammoths, mastodons, and woolly rhinos.

The extinction can thus be summarized as follows:

Large terrestrial animals were the primary creatures going extinct: smaller animals and virtually all sea animals were spared.

Large mammals survived best in Africa. The loss of large mammalian genera in North America was 73 percent; in South America, 79 percent; in Australia, 86 percent; but in Africa, only 14 percent died out during the last hundred thousand years.

The extinctions were sudden in each major land group, but occurred at different times on different continents. Powerful carbon dating techniques allow very high time resolution. These types of techniques have shown that some species of large mammals may have gone completely extinct in periods of three hundred years or less.

The extinctions were not the results of invasions by new groups of animals (other than mankind). It has long been thought that many extinctions take place when new, more highly evolved or adapted creatures suddenly arrive in new environments. Such was not the case in the ice age extinctions, for in no case can the arrival of some new fauna be linked to extinctions among the forms already living in the given region.