by Peter Ward
GETTING TO THE TENTH EXTINCTION
In 2010, a traveling exhibit from Ethiopia3 brought one of the most famous of all fossils to the United States: Lucy, the early hominid.4 At about three and a half feet tall, with remains that total only 40 percent of her original skeleton, in fact there is not a lot to Lucy. But she has told us a great deal.
Sexual dimorphism is the term used to describe the two different morphologies of males and females of a species. It is certainly not limited to hominids, and it is not the case that the larger of the dimorphs is always the male. In many animals, for instance, including a variety of cephalopods (excepting Nautilus, interestingly enough), the female morph is the larger. Apparently it takes more organ mass to produce eggs than sperm. In hominids, however, from chimps to us humans, the male is the larger. The dimorphism in humans is statistically significant, and appears to range from females being about 90 to 92 percent the height of males, depending on race. In Lucy’s kind, however, it was quite a different story.
Lucy is far from the only fossil skeleton of her kind. Her species, Australopithecus afarensis, is now far better known compared to our understanding (or lack thereof) when a team led by Don Johanson found her in 1974. One of the more recent finds is of a male skeleton of her kind that is complete enough to allow a good estimate of his height in life. He is called Big Man. He was five feet tall to Lucy’s three and a half. Her chin would have come just above his navel if standing face-to-face—except it had been face to lower chest.
If Lucy and Big Boy are representative of their genders in A. afarensis, it means that females were only 70 percent as large as their men. There had to be consequences to this—behavioral as well as cultural. In 2012, when anthropologist Patricia Kramer of the University of Washington did a detailed study5 on the relative walking speeds of males and females, based on their leg lengths, she discovered that Big Man’s optimal walking speed would have been 2.9 mph, but Lucy’s would have been a rather slower 2.3 mph. Keeping up with males would have been taxing for females—and living in a world filled with predators, being constantly in a state of anaerobic respiration would not be a very good survival tactic. Kramer thus suggested that like chimpanzees, male and female hominids spent much of the day apart, ranging separately as they foraged and hunted for food.
Other new fossil finds from Africa are also turning over some long-held views. Lucy and her kind are invariably reconstructed in dioramas or illustrations as walking upright through the Late Pliocene world of north and eastern Africa—a place with a mosaic of grassland and small patches of open forest. But for the first time ever the shoulder blades of a female of Lucy’s species—but coming from a time interval about a hundred thousand years before Lucy—show features that suggest she and her kind were tree climbers as well as adapted for walking on the ground. The question of whether these distant ancestors of ours also spent significant time in trees has been hotly debated,6 largely because until this new find, there was no way to see the morphological adaptations necessary for a tree climber. The new view seems to be that australopithecines may not have come down from the trees as early as currently believed.
While hominids are new arrivals on Earth, our group, the primates, dates well back into the Cretaceous, and we have an ancestor, Purgatorious, that survived the K-T mass extinction itself—which is lucky for us. Some of the earliest primates belonged to the lemur branch. By 45 million years ago, more advanced primates—the first true anthropoids, which today include monkeys, apes, and humans—appear in the fossil record of Asia. The oldest of these was found in China and is now named Eosimias.
About 34 million years ago, surely smarter, definitely bigger, and perhaps more aggressive monkeys evolved. One of these, named Catopithecus, has a skull the size of a small monkey’s, a relatively flat face, and is the first primate to sport the same arrangement of teeth humans have—two incisors, one canine, two premolars, and three molars. We now have a good idea of our own evolutionary tree, right up to where and when “humans” can be said to first appear—the African genesis of the australopithecines.
Paleoanthropologists have done a remarkable job of deciphering the where and when of the speciation event that produced our species. The human family, called the Hominidae, seems to begin as much as 5–6 million years ago with the appearance of Lucy and her kind, the Australopithecus afarensis described above. Since then, our family has had as many as nine species, although there is ongoing debate about this number, which seems to change years as both new discoveries and new interpretations of past bones make their way into print. But the most important descendant of the early pre-Pleistocene hominids is the first member of our genus, Homo, a species named Homo habilis (handyman) for its ability to use tools, which is about 2.5 million years old. This creature gave rise to Homo erectus about 1.5 million years ago, and H. erectus either gave rise to our species, Homo sapiens, directly about 200,000 years ago, or through an evolutionary intermediate known as Homo heidelbergensis. Our species has been further subdivided into a number of separate varieties. Some workers consider the Neanderthals to be a variety, while others interpret it as separate species, Homo neanderthalensis. A great deal of new work on recovered and decoded Neanderthal DNA7 is one of the most intriguing aspects of human paleobiology, with the latest evidence suggesting that the human and Neanderthal lineages diverged before the emergence of contemporary humans and our current DNA. They did not come from us, nor did we come from them. We both evolved from a common extinct ancestor different from both species.8
Each formation of new human species occurred when a small group of hominids somehow became separated from a larger population for many generations. In the 1960s and 1970s there was a view that modern humans came about from what has been called a candelabra pattern of evolution—that all over the planet separate stocks of archaic hominids such as Homo erectus all evolved into Homo sapiens at different times and places. This notion now seems laughable.
The fossil record tells us that the so far oldest member of our species—variably called a modern to distinguish it from more archaic forms of Homo sapiens—lived 195,000 years ago in what is now Ethiopia. It is unknown and not terribly important whether this fossil represents the oldest tribe of us or was from a group that wandered in from the true origin place and was fortuitously fossilized in Ethiopia. But very soon after, this band set out walking to the farthest southern regions of the African continent, and then to the north as well, finding a way out of Africa through Eurasia—and in so doing they spread out across the globe,9 effectively isolating themselves from others of our species, and thus adapting to the very different environmental concision in which these wanderers found themselves. Quite different adaptations, morphological to physiological, were necessary for survival in the sun-starved, ice-covered north than on the plains of Africa, as well as all areas in between. As our numbers grew, so too did our variation—and our various evolutionary changes. But all of this was within the same species.
THE LAST ICE AGE AND LIFE
Climatologists have long theorized that climate change observed over the past two and a half million years—the alteration between long periods of very cold climate with growing ice sheets and dropping sea level alternating with shorter times of warmth—were the result of the orbital changes described above as having been first articulated by Milutin Milankovic′. Until the ice cores became available, with their unprecedented resolution in discerning climate through recent time, the changes were thought to have been slow. But with that resolution a newer view became apparent.
The ice core records and other sources of climate information such as deep-sea paleontological and isotopic records indicate that over the past eight hundred thousand years the interglacial periods—the warmer times between the much cooler glacial intervals—have lasted on average about eleven thousand years. That’s almost half the Earth’s precessional cycle, orbital changes occurring every twenty-two thousand years. The current interglacial has already lasted more than eleven
thousand years and some records suggest that we have been in the warm period for as long as fourteen thousand years. Does this mean that the glaciers are advancing at this moment? The answer to that question is a decided no, for several reasons. First of all, precession is not the only orbital aspect that affects climate. Records show that between 450,000 to 350,000 years ago there was an interglacial stage that lasted much longer than eleven thousand years. This interglacial was coincident with a time when orbital eccentricity was at a minimum. Just such a pattern of minimal orbital eccentricity is under way at this time, suggesting that the present interglacial could continue for thousands or perhaps a few tens of thousands of years into the future—or it could end at any time.
The Pleistocene epoch signaled a significant kind of climate change beginning about 2.5 million years ago. The large cool grasslands and tundra of the high latitudes during the last pre-ice-age epoch of the Cenozoic era gave way to a new kind of land cover—ice. Year by year a slow excess of snow and ice caused the formations of glaciers, which slowly crawled southward. Eventually continental glaciers began to coalesce and merge with mountain glaciers, uniting in unholy matrimony to grip the land in glacial ice and glacial winter.
By no means was the entire planet gripped in ice, as seems to be popularly imagined. There were still tropics and coral reefs and warm sunny climes pleasant the year around. But probably no place on Earth was unaffected in at least some minor way; the global climate changed, causing shifts in wind and rain patterns. Even those places far from the ice were climatically changed, perhaps colder or even warmer, often quite dryer. Gigantic cold deserts and semideserts expanded in front of the advancing ice sheets, while regions normally dry, such as the Sahara desert of northern Africa, experienced increased rainfall. Conversely, the great rainforests covering the Amazon basin and equatorial Africa, regions of relative climatic stability for tens of millions of years prior to the onset of the ice age, experienced a pronounced cooling and drying such that large tracts of jungle retreated into pockets of forest surrounded by wider regions of dryer savannas.
THE SPREAD OF HUMANITY
Many of these rapid climate changes occurred while humanity was colonizing the globe. By about thirty-five thousand years ago, it appears that the final evolutionary tweaks had occurred, making us as we are now. We can call these new humans the moderns, and they conquered the world bit by bit. They arrived in each new region slowly yet inexorably. It didn’t happen in a century. It didn’t parallel the taming of North America by Europeans, when several centuries saw the transformation of a giant native-vegetation-covered continent to a giant agriculture- and concrete-covered continent. It was instead a slow conquest, with millennia falling away like leaves as the moderns slowly spread over the globe. Even the island continent of Australia had become the habitat of Homo sapiens thirty-five thousand years ago. Northern Asia, however, was still undiscovered. And beyond Asia, an even bigger territory, North and South America, had still not experienced the first human footfall.
The first people to arrive in the vast tract of what is now Siberia were Paleolithic big game hunters. They arrived as long as thirty thousand years ago, with a tradition already in place for existing in this harsh climate. Eastern Siberian stone tools show some differences from the European traditions of the time, and were clearly influenced by the flake cultures of Southeast Asia. Yet the major technology, the construction of large spearpoints, was formulated for killing large animals.
The arrival of the first humans in Siberia was set against a time of slight warming, and this warmer period, following a cooler time, may have encouraged the spread of humans into an otherwise hostile region. Yet soon after their arrival in Siberia the Earth began to cool again, and by twenty-five thousand years ago a major glacial event was well under way.
In western Europe and North America the great continental ice sheets were inexorably spreading downward to cover vast regions with ice a mile thick. In Siberia, however, there was so little moisture that the ice was unable to form. Into this vast treeless frozen territory, humans expanded ever eastward. Because there was so little wood, the hides and antlers of their prey became important resources, and the very bones of the largest quarry—the mastodons and mammoths—were used for housing. These people became—by necessity—big game hunters, and their principal prey may have been the mammoth and mastodon.
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 d
iversity 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.