A New History of Life (52 page)

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.

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 CO
₂
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 CO
₂
is used to make the skeletons of organisms, especially in the oceans, CO
₂
is consumed. If these skeletons stay in the oceans, the skeletally confined CO
₂
(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 CO
₂
, becomes locked to the continents as sedimentary deposits.

One would think that the long-term trend of lowering CO
₂
would be a plunge into inescapable snowball Earth conditions. But it is not cooling from a lowering of the concentration of CO
₂
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-CO
₂
atmosphere. This dwindling carbon resource will then cause a further reduction of planetary habitability, because the CO
₂
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 CO
₂
, 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 CO
₂
concentrations, sometimes as low as 10 ppm—the C4 plants described in an earlier chapter. These plants will last far longer than their more CO
₂
-addicted cousins and will considerably extend the life of the biosphere even in a world in which CO
₂
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 CO
₂
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.

With new plants with tougher exteriors, leaves—at least in their present form—might be expected to disappear. The same will happen to grass; the loss of water from plants with relatively high surface-area-to-volume ratios will doom grass blades and thin leaves alike. All of this, of course, will require a marked change in animal life.

As early as 500 million years from now, or perhaps as much as 1 billion years or so into the future, the level of CO
₂
in the atmosphere will reach a point at which familiar plant life will no longer be able to exist. The changeover at first will be in no way dramatic. All over the world, the plants will slowly die. But the planet will not immediately become brown. For as one suite of plants dies, their places will be taken immediately by another cohort of plant life that may look nearly identical to those dying. Deep inside the tissues of these two groups of plants, however, fundamental processes of photosynthesis will be radically different. After this changeover, life on Earth will continue in ways probably not too dissimilar from that which came before—at least for a time.

There is also the possibility that plants will continue to evolve other photosynthetic pathways to compensate for lower CO
₂
levels. In this case, some sort of plant life may survive at minimal CO
₂
levels. Eventually, however, even these last holdouts will die out. All models suggest that CO
₂
will continue to drop in volume, ultimately arriving at the critical level of 10 ppm.

The most important questions about any future evolution concern the future of biodiversity—the number of species on Earth. Two questions that arise are: Will there be more species than now? And if so, for how long? But as is so often the case, to begin to answer these questions one needs to look into the past.

More than just the flora on land will be traumatized by the lower CO
₂
levels. Larger marine plants and perhaps plankton as well will be
affected similarly. Marine communities thus will be strongly affected, because the base of most marine communities is phytoplankton, a single-celled plant that floats in the seas. A reduction in CO
₂
will directly affect these as well as the land plants. Yet the disappearance of land plants will also cause a drastic reduction in the biomass of marine plankton, even without accounting for CO
₂
effects on plant volumes in the seas.

Marine phytoplankton is severely nutrient limited in most ocean settings. The influx of nitrates, iron, and phosphates into the oceans each season causes phytoplankton to bloom. But the source of this phosphate and nitrate is rotting terrestrial vegetation, brought into the oceans through river runoff from the land. As land plants diminish in volume, so too will the volume of nutrients be diminished. The seas will be starved for nutrients, and the volume of plankton will decline catastrophically. This decline will never be reversed, for even if land plants rebound at low levels, as outlined above, they will never again reach the enormous mass of material that is present in a world (such as that of today) where CO
₂
starvation does not exist.

On land and sea the base of the food chains as they are constructed today will disappear. The loss of plants will suddenly cause global productivity—a measure of the amount of life on the planet—to plummet. But there will still be life: great masses of bacteria, such as cyanobacteria will continue to live, because these hardy single-celled organisms can live at lower CO
₂
levels that are below those necessary to keep multicellular plants alive, and they also do not require oxygen, something multicellular plants do.