A New History of Life

by Peter Ward

  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 sudde
nly 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.

  These various lines of evidence suggested to many scientists that humanity provoked this mass extinction. Others argue just as vigorously that the cause of the megamammal extinction was change in resource patterns in vegetation that occurred during the intense climate changes accompanying the end of the Pleistocene glaciation. Most discussion about this extinction deals exclusively with this argument over cause, with the two major camps being called overkill (human hunting) and climate change.

  Whatever its cause, a major reorganization of terrestrial ecosystems occurred on every continent save Africa. Today, Africa is losing its megamammals as the large herds of game become restricted to game parks and reserves, where they become easy prey to poaching within their newly restricted habitats.

  The end of the megafauna is not a clearly defined line. But then we are looking at it from the present, and it is just a moment away. Intervals of time lasting ten thousand years are insignificant and probably beyond the resolution of our technology, when viewed from times tens to hundreds of millions of years away. The end of the age of megamammals looks protracted from our current vantage point, but will look increasingly sudden as it disappears into the past, one of the odd aspects of time.

  The megamammals still left on Earth now make up the bulk of endangered species, and many more large mammalian species are now at risk. If the first phase of the modern mass extinction was the loss of megamammals, its current phase seems concentrated on plants, birds, and insects, as the planet’s ancient forests are turned into fields and cities.

  CHAPTER XX

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  The Knowable Futures of Earth Life

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  The future is a never-reachable time, the fast-moving bait to racetrack greyhounds. If there is any lesson from life’s history, it is that chance has been one of the two major players at the game of life, with evolution the other, and chance makes any attempt at prognosticating events and trends in the future history of life a very chancy proposition. But the planetary scientist and brilliant writer Don Brownlee of the University of Washington has responded to this seemingly impenetrable obfuscation of the future. Brownlee claims that there is a “knowable” future, and that seemingly paradoxically events become more knowable the further into the future they are. On this topic, Brownlee was talking about physical and predictable changes in the properties of our planet and our sun. One example of a knowable future that can be quite accurately predicted is the future history of our sun, which we know will become a red giant star with a diameter larger than the orbit of Earth and probably Mars (and thus certainly consuming the Earth and probably Mars as well) in 7.5 billion years, give or take a quarter billion.

  The study of biological evolution on Earth has increased scientists’ understanding of the distant past, and this too offers clues to the future. One characteristic is that evolutionary history has been importantly affected not only by the interplay of life (competition and predation) but also by the course of the physical evolution of Earth, its atmosphere, and its oceans. While many events will remain dictated by chance, such as the rate and future history of asteroid impact with the Earth, we can make highly refined estimates about predictable changes in global temperatures, atmospheric and oceanic chemistries, and large-scale geophysical events that will necessarily take place over Earth’s remaining lifetime.

  The concept of a habitable planet is based on planetary nurture, with life being the ultimate result of planetary formation and change. We have already looked at the most important elemental renewal systems that recycle important nutrients and maintain near-constant global temperatures, and changes in (or total cessation of) these, like the rate at which the sun expands, are knowable. For life, the most important of these fluxes are the movement and transformation of the elements carbon, nitrogen, sulfur, phosphorus, and various trace elements. The energetic underpinnings of the various systems largely come from two sources: the sun and heat generated from the breakdown of radioactive material beneath Earth’s surface. Of these, and because of its importance to life as the source of energy through photosynthesis, the sun is the more important of the two.

  The sun is a powerful nuclear reactor, but its stability is a matter of debate. As the sun evolves, the number of particles in its core decrease as hydrogen atoms are fused into helium atoms, but seemingly paradoxically, as the number of atoms in the core of the sun decrease, its energy output (as light and heat) slowly but inexorably increases.

  All stars like the sun share this same characteristic. The sun has increased in brightness by about 30 percent in the last 4.5 billion years of its life. The rise in brightness increases the intensity of the sunlight that illuminates its planets. A continuation of this change will cause the loss of oceans and create hellish conditions, similar to those that exist on Venus. (The oceans do not “boil” away, as seen in some garish depictions of the Earth’s future, but one by one, oceans’ water molecules are stripped of their hydrogen, which ascends high into the atmosphere. The oxygen stays behind.)

  For all its history, Earth has been within the “temperate zone” of the solar system. That is, Earth has been in the “right” range of distance from the sun to have surface temperatures that allow oceans and animals to exist without freezing or frying. This habitable zone (actual geography in space) extends from a well-known limit just inside Earth’s orbit to a less understood outer limit near Mars or possibly beyond. The habitable zone moves outward as the sun becomes brighter, and in the future the zone will pass Earth and leave it behind. Earth will in essence become the Venus of today. The inner edge of the habitable zone is only about 9.3 million miles (15 million km) away, and it will effectively reach Earth in half a billion or a billion years from now (or possibly less). After this time, the sun will be too bright for organisms to survive on Earth.

  The steadily rising amount of energy hitting Earth from the sun over the past 4.567 billion years should have ended life on Earth long ago, as it did on Venus (assuming that Venus ever had life), except for one of the most important of all of the planetary life support systems, the planetary thermostat described in the first chapter. For more than 3 billion years (and perhaps 4 billion years) this system has kept the global average temperature of Earth between the freezing and boiling points of water (except for the occasional snowball Earth event), thus allowing the most important requirement for life—liquid water—to continually exist on the surface of the planet for that immense amount of time. Just as important, life, which evolved within tight temperature limits, has been able to maintain essentially similar physiologies and internal chemical reactions that are temperature dependent. Rising temperature because of the sun and an increasing reduction in atmospheric carbon dioxide are the two processes that in combination will have the greatest effect on future biotic evolution.

  The rises and falls of CO2 are now fairly well documented for the last 500 million years—the time of animals. Oxygen, a requirement of all animals, is obviously important too. We have already highlighted levels of these two gases from past to present. But like the knowledge about the rate of the enlarging and ever more energetic sun, the future trajectory of both carbon dioxide and oxygen are also knowable and thus predictable.

  The long-term prediction for carbon dioxide is that it will continue in the same trend it has shown over at least the last billion years—a slow but inexorable decrease. The lowering levels are because of both life and plate tectonics: as more and more CO2 is used to make the skeletons of organisms, especially in the oceans, CO2 is consumed. If these skeletons stay in the oceans, the skeletally confined CO2 (now in calcium carbonate) will recycle. But plate tectonics makes the continents ever larger, and an increasing amount of limestone, which is the grave of atmospheric CO2, becomes locked to the continents as sedimentary deposits.

  On
e would think that the long-term trend of lowering CO2 would be a plunge into inescapable snowball Earth conditions. But it is not cooling from a lowering of the concentration of CO2 in the atmosphere that will be a hallmark of the aging Earth. It will be heating. The increasing heat from the sun will utterly dwarf the cooling effects of diminishing carbon dioxide and its greenhouse gas effects. When the average global temperature rises to perhaps 120 to 140°F (50 to 60°C), Earth will begin to lose its oceans to space.

  But long before the oceans are lost in 2 to 3 billion years, life will have died out on Earth’s surface because photosynthetic organisms, from microbes to higher plants, will no longer be able to survive in the low-CO2 atmosphere. This dwindling carbon resource will then cause a further reduction of planetary habitability, because the CO2 drop will trigger a drop in atmospheric oxygen to a level too low to support animal life.

  This process is already observable. When vascular plants first colonized Earth’s surface some 475 million years ago, they did so in an atmosphere rich in carbon dioxide. There was no need for conserving carbon in physiological processes. Even today, many plant species require a minimum of 150 ppm of CO2, and James F. Kasting pointed out in a 1997 article that there is a second large group of plants, including many of the grassy species so common in the mid-latitudes of the planet, that use a quite different form of photosynthesis and can exist at lower CO2 concentrations, sometimes as low as 10 ppm—the C4 plants described in an earlier chapter. These plants will last far longer than their more CO2-addicted cousins and will considerably extend the life of the biosphere even in a world in which CO2 levels have fallen far, far below present-day values.

  We can safely predict that the future evolution of plant life will be toward plants that can live at lower CO2 levels than that of their stock ancestral C3 plants. Also, because global temperatures will be rising, keeping water within a plant will be an increasing problem. Plants will have two conflicting needs—ever larger holes in their exterior to let the small amount of carbon dioxide in the atmosphere get into the interior, where photosynthesis can take place, at the same time trying to reduce the loss of water molecules through these same pores. At a minimum, one can expect a future flora of tough, waxy plants that would completely close down all portals to the outside world when there is no sunlight for photosynthesis.