The Next Species: The Future of Evolution in the Aftermath of Man

by Michael Tennesen

  Man entered Australia about forty thousand years ago, and the continent lost over 85 percent of its large mammals within a period of only about five thousand years. According to UC Berkeley’s Charles Marshall, “That’s about a one percent change every fifty years. So if you are twenty and you look at the world around you and then you are seventy and you look again, do you notice the one percent difference? Probably not. Even the extinction of large animals in Australia took five thousand years, which, though geologically fast, was still creeping on human time scales. But we’re not creeping anymore. People who do work in tropical rain forests, they go back five years later, and some of these areas are totally gone.”

  New Zealand had an assortment of very different creatures before man. Writes Jared Diamond in The Third Chimpanzee, “The scene was as close as we shall ever get to what we might see if we could reach another fertile planet on which life had evolved.” The most successful of New Zealand’s animals was the giant moa, an ostrich-like bird that stood over ten feet tall and weighed over five hundred pounds. New Zealand had different species of moa, the giant being the largest, instead of bison. It had songbirds and bats instead of mice, and huge eagles instead of leopards.

  Polynesian settlers first landed in New Zealand only about a thousand years ago, and within a few centuries managed to annihilate the local fauna. Approximately 50 percent of New Zealand species disappeared, including all the large birds and most of the flightless ones. Moas were exploited for food and their skins and bones. Their eggshells served as water containers. The remains of approximately a half million giant moas ended up in archaeological sites, many times more than would have been alive at any one time. The settlers apparently hunted giant moas for generations before they went extinct, as man almost did in North America with the buffalo.

  The extermination of many of the large animals in North America occurred from thirteen thousand to ten thousand years ago. Some scientists speculate that a sudden cold reversal of temperature that hit earth from eleven thousand to ten thousand years ago, known as the Younger Dryas event, caught millions of species unprepared. Others think it was the quickly rising temperature at the end of this event that killed off the furry creatures.

  Still others believe it was the Clovis people whose fossilized bones and other relics became dominant in archaeological excavations dated about thirteen thousand years ago. Of course, there are earlier sites with evidence of human habitation in both North America and South America before this time, but this period is notable because it is when Clovis populations gathered en masse and their fluted stone arrow points, a characteristic tool, started showing up frequently.

  Some believe that the demise of megafauna was itself the cause of sudden cold reversal. It may have cut off a large, important source of the strong greenhouse gas methane: the four-chambered stomachs of the animals themselves, which expelled the gas by burping (not passing gas). In other words, the early hunters shut off all the methane that the large animals were releasing to the environment, and this itself caused the Younger Dryas onset of cold by temporarily shutting off the greenhouse gas that was making things warmer.

  Blaire Van Valkenburgh, a vertebrate paleontologist at the University of California, Los Angeles, agrees that humans weren’t the sole cause of all the extinctions, “but they were an additional force that acted upon an ecosystem that had been in balance, but with the arrival of the Clovis people quickly became out of balance,” she told me when I visited her office.

  When Clovis man appeared, he was a new carnivore that was competing with the saber-and scimitar-toothed cats, which now had to share their meals with a skillful new predator. Groups of large cats preyed on bison and horses, though there is also evidence of them following mammoth herds and attacking young mammoths. Man was essentially an uninvited guest that the cats had to deal with, and there wasn’t enough food to go around. Humans were the tipping point. After his arrival, North America’s large animal populations started to disappear.

  Van Valkenburgh believes that further proof of the competition between man and megafauna is found in the condition of the teeth of saber-and scimitar-toothed cats and the large dire wolves. These predators show heavier tooth wear and more broken teeth than modern-day beasts like cougars and gray wolves, though some of that was present even before humans arrived. Increased competition with Homo sapiens may have aggravated an already competitive situation.

  “Large predators tend to exhibit heavier tooth wear and greater numbers of broken teeth when they consume carcasses more completely, actively feeding on bones,” Van Valkenburgh and coauthor William J. Ripple wrote in a paper in BioScience. “The predators, which were much more abundant than the humans, most likely killed the vast majority of the megafauna.” The addition of the new human predator was more the straw that broke the camel’s back rather than the lead cause of the extinction.

  How did these mammals get so big in the first place? While the dinosaurs were still alive, mammals scurried around under the feet of their much larger neighbors, occasionally taking refuge under bushes, in tree hollows, or in underground burrows. But once the asteroid fell from the sky and the dinosaurs disappeared, mammals started growing, and growing, and growing.

  It began about 65 million years ago and peaked about 30 million years later. Animals grew about eight orders of magnitude (× 10, × 10, × 10 . . . eight times) rather quickly, but it took them about 30 million years to max out. The maximum of seventeen to eighteen tons for a land mammal has remained constant over time, in different places, and with different species, says Felisa A. Smith, a professor of biology at the University of New Mexico. Indricotherium, a hornless rhino-like herbivore that weighed approximately seventeen tons and stood eighteen feet at the shoulders, lived in Eurasia about 34 million years ago. It was the largest land mammal that ever existed. Indricotherium would have towered over a modern African elephant.

  The colder the climate gets, the larger the animals, since they conserve heat better. Xiaoming Wang from the Natural History Museum of Los Angeles County and Qiang Li of the Chinese Academy of Sciences recently uncovered a large woolly rhino in the foothills of the Himalayas in the southwestern Tibetan Plateau. The animal stood perhaps six feet tall and was twelve to fourteen feet long. It had two great horns, one sprouting about three feet long from the tip of its nose, while the other arose from between its eyes. The Tibetan woolly rhino was stocky like today’s rhino but had long, thick hair. It is one of the giant mammals like woolly mammoths, giant sloths, and saber-toothed cats that became extinct. It is thought to be about 3.7 million years old. Over a million years older than the previous oldest woolly rhino fossil ever found.

  The Tibetan woolly rhino lived at a much warmer time when northern continents were free of the massive fields of ice that came later with the Ice Age. But, residing in the Tibetan Plateau, this animal grew accustomed to the cold at high elevations and was “pre-adapted” to it. Thus, when the Ice Age arrived, these cold-tolerant rhinos simply descended from the highlands and began to spread throughout Eurasia. The Tibetan Plateau might have served as breeding grounds for these and other giant Ice Age mammals.

  FAIR TO MIDDENS

  Professor Smith, whom I meet in her lab at the University of New Mexico and who towers over me in her cowboy boots, has been researching size in animals for many years and believes size is one of nature’s most important adaptations to climate change.

  Smith currently studies the size of present and ancient large pack rats (also known as wood rats) at Death Valley National Park in California, details of which she discovers by studying their middens, the refuse tossed out of a pack rat nest. She uses this evidence as clues to their environment and ecology. She can tell the size of the animal and indirect information about the climate by examining the size of the animal pellets or feces in a pack rat’s midden. She also gets teeth and bones from these middens, which are used to confirm the identity of the species constructing the nest. She showed me some samples of these midde
ns in her lab and even invited me to pick one up, which I did, cautiously. She asked what I thought of the smell, and I told her it smelled sweet, to which she responded, “You’re a born pack rat midden researcher!” since that smell was actually pack rat urine, which the animal uses to hold the pieces together.

  According to Smith, the relationship of body mass and temperature has proven so predictable it’s known as Bergmann’s rule (named for Carl Bergmann, a German biologist): For broadly distributed mammal groups, the larger-size species are found in colder environments and the smaller sizes are found in warmer places.

  Smith studies ancient pack rat middens because they provide detailed fossil evidence of the times when they were created. Pack rats are collectors of twigs, leaves, small rocks, fecal pellets, and anything they find and deposit in the large piles of debris in front of their nests. The middens provide protection from predators and insulation from climate swings. When they are constructed on rocky outcrops, they can last for thousands of years and can be carbon-dated. A single mountain may contain dozens of middens spanning thirty thousand years or more.

  The size of the fecal pellets is an indication of pack rat size and diet. Researchers are able to characterize body and genetic responses to climate in populations of pack rats spread out over thousands of years.

  Today, Death Valley holds the record for the hottest and driest place on earth, but during the last ice age, Death Valley was covered by Lake Manly, a hundred miles long and six hundred feet deep. The climate was 11 to 18 degrees Fahrenheit (6 to 10 degrees Celsius) cooler. But as the valley began to warm, pack rats adapted and slowly moved upslope. They got as high as 5,900 feet (1,800 meters) but it wasn’t high enough. By about six thousand years ago there were no more large pack rats present on the east side of Death Valley.

  THE MEAT TRAP

  Not all mammal extinctions were due to warming or man. UCLA’s Van Valkenburgh argues that over the last 50 million years, successive groups of large cat-like, wolf-like, and hyena-like mammalian carnivores diversified but then declined and went extinct. At one time the Canidae family (wolf-like carnivores) had three subfamilies. Two of those went extinct. Van Valkenburgh believes that some of this was caused by what she calls “the meat trap.” In situations where carnivores need more energy, they may switch not only to a pure carnivore diet but also to a diet in which their prey are bigger than they are. Once there, however, they have trouble going back to smaller portions.

  All three subfamilies of Canidae reached a peak about 30 million years ago, but only one subfamily survived. That included domestic dogs, wolves, foxes, and coyotes. The other two subfamilies increased in body size by 400 to 600 percent, but when their prey got sparse, they couldn’t switch back to smaller prey. They had adapted to eating all meat, all of the time, and only from animals larger than themselves. Their diets were simply unsustainable. Still, there were once twenty-five contemporaneous species of canids native to North America as opposed to seven today. Nature has indeed been much richer before. But can it ever return to the diversity it once had? Can we turn back the clock?

  RETURN OF THE CALL OF THE WILD

  For years, the golden era to which environmentalists in North America have often spoken of turning back the clock was before 1492, when the Europeans arrived. However, in the journal Nature in 2005, Josh Dolan, a biologist at Cornell University, and a group of prominent scientists expressed their desire to go back even further. They wanted to go back to a time before the Clovis people, the real starting point for human change in the Americas.

  Dolan’s group declared that Western scientists had gone into full retreat from the battle to stop biodiversity loss and were now simply struggling to diminish the rate. The team wanted to change the game from just “managing the extinction” to actively “restoring ecological and evolutionary processes.” Their idea was to restore all the big animals that once stocked North America with surrogates from other continents that could push back the time line to when there were horses, camels, elephants, and even lions stalking the land.

  One of the first things the group proposed was to restore the largest tortoise in America, the Bolson tortoise, to areas of the Southwestern United States. I accompanied the late David Morafka, a herpetologist at California State University, Dominguez Hills, to visit the Mapimí Biosphere Reserve, dedicated to the protection of the Bolson tortoise and other unique flora and fauna of the Bolsón de Mapimí, a large inland basin in the Chihuahuan Desert north of Mexico City. The Bolson tortoise, whose range had once extended across the Chihuahuan Desert in northern Mexico and the southern extremes of the US, was making a last stand here. Repatriating the Bolson tortoise to the broader expanse of the Chihuahuan desert tortoise could bring the largest of the continent’s tortoise species back to the US.

  Wild horses were another possibility for rewilding. They were introduced by Europeans to North America about five hundred years ago and have since taken up ecological niches that were held more in balance thirteen thousand years ago. Many ranchers look at wild horses and burros as large pests that foul watering holes and compete with cattle, native pronghorn antelope, and native bighorn sheep. To Dolan, the horse is just as native to this land as any other species.

  The problem is you can’t just reintroduce a large herbivore like a horse and not reintroduce predators to keep the animals under control. In 1971, the Wild Free-Roaming Horses and Burros Act made it illegal for anyone to harass, capture, or kill wild horses. Since then, populations of the animals have soared throughout the Great Basin Desert, which lies mostly in Nevada and extends to the fringes of surrounding states. But this is not the case in Montgomery Pass Wild Horse Territory, which straddles the California-Nevada border. There scientists study mountain lions, which are an effective predator control for these animals.

  Such a balanced approach could provide a lifeline for Przewalski’s horse, smaller than most domesticated horses and native to the steppes, the vast semiarid grass-covered plains of Central Asia, as well as the Asiatic wild ass, both free-roaming and critically endangered equids. Translocation to the US might save them from extinction and repatriate horses to their evolutionary home ground. Many scientists say that the trick is to introduce predators to these ranges as well, in order to keep large horse populations in check.

  Believe it or not, camels originated in North America. They migrated north from the Arizona desert and crossed the Bering Strait land bridge three to four million years ago. The IUCN currently lists the Bactrian (two-humped) camel as critically endangered. There are about 600 Bactrian camels surviving in the wild in China and 450 in Mongolia. It is the only truly wild camel. Dromedary (one-humped) camels, but for some feral animals in Australia, exist only as domestic animals. There were four species of camels and llamas in North America at the end of the last ice age. Today wild Bactrian camels are restricted to the Gobi Desert and their cousins, the llamas, to South America.

  In the 1850s, Lt. Edward Beale led the US Camel Corps, mostly dromedaries, from Texas to California, and he was amazed at how camels grazed on creosote and other brush species that now form dense monocultures across much of the Southwest desert. If we brought Bactrian camels back again or released domestic camels into the wild, the landscape of plants could be more diverse. Camels were once a vital part of the ecological community. Australia has well-managed co-grazing programs of cattle and dromedary camels, which could provide meat and milk, and an increase in the mosaic of plant species.

  Elephants would be another winner in the west. At Olduvai Gorge, I saw elephants feeding on shrub forests. Despite a plethora of sharp spines and thorns, the elephants cleared away the brush as efficiently as tractors but left the refuse to regenerate the soil. Much of the open grasslands of East African plains owe their existence to the assistance of elephants. Introducing elephants to the juniper forests on the Edwards Plateau in Texas might alleviate the juniper problem.

  Another animal that the “rewilders” would like to reintroduce to the Americ
an West is the cheetah. Cheetahs were once here. The American cheetah first appeared perhaps 2.5 million years ago but went extinct about two thousand years ago with the rest of the megafauna. They are the reason that the American pronghorn antelope is so fast. Pronghorn can travel at a top speed of sixty miles an hour, second only to the cheetah. But whereas the cheetah is a mere sprinter, the pronghorn is an endurance runner. Pronghorn can average forty miles an hour for half an hour or more, galloping across the high prairies of Wyoming. There is no living reason for pronghorn to move that fast. Biologists believe that they evolved to outrun the American cheetah, which was around when pronghorn, mammoths, and giant sloths roamed the North American plains. Returning the cheetah to America could give the pronghorn an impetus to stay fit. Also, the African cheetah, once found throughout Africa and southwestern Asia, has been greatly reduced and is not likely to survive into the next century. Moving them to the North American plains might increase their chances. This wouldn’t be another case of introducing an invasive to North America, as all these species were present long before man showed up. In many ways, we’re the worst of the invasives.

  There are currently about a thousand African cheetahs in zoo populations across the world that could act as surrogates for the American cheetah, which is closely related. I watched a cheetah from a safari car in Ngorongoro Crater in Tanzania: the animal was grooming itself in the sun. Behind the cheetah were several antelope, something that attracted the cheetah’s attention as well. The cheetah slowly got up and started to stalk an antelope, when a pair of hyenas jumped up, drawn by the cheetah’s hunting pose. They bared their enormous teeth like laughing clowns, waiting for the action to begin.

  The cheetah took a runner’s pose and bolted, leaving a huge cloud of dust behind it. The galloping cheetah quickly caught up with the antelope, knocking it over, while the hyenas danced about, wildly excited over the promise of shared meat.