A New History of Life (53 page)

The disappearance of plants will drastically affect landforms and the nature of the planet’s surface. As roots disappear and surface layers become less stable, the very nature of rivers will change. The large, meandering rivers of the modern era date back, at most, to the Silurian period of some 400 million years ago, when land plants first colonized the surface of the planet, for it takes root stability to maintain the banks of meandering rivers. When plants die out or are not present because of slope, soil, or other inopportune environmental conditions, a different kind of river exists—braided rivers or streams,
the kinds of flows found on desert alluvial fans or in front of glaciers, two types of environments not conducive to rooted plant life. This was the nature of rivers before the advent of land plants, and it will again be the way that rivers flow when CO
₂
drops to the plant die-off threshold.

The loss of soils will be no less dramatic. As soils are blown away, they leave behind bare rock surfaces. As this condition begins to occur over the surface of the planet, it will change the albedo—the reflectivity of Earth. Far more light will reflect back into space, thereby affecting Earth’s temperature balance. The atmosphere and its heat transfer and precipitation patterns will be radically changed. Blowing wind will begin to carry the grains of sand created by the action of heat, cold, and running water on the bare rock surfaces. While chemical weathering will lessen as a result of the loss of soil, this mechanical weathering will build up an enormous volume of blowing sand. The surface of the planet will become a giant series of dune fields.

Although this event could signal the final extinction of all plant life on land (and perhaps in the sea as well), it is more likely that a long period of time (perhaps in the hundreds of millions of years) will ensue in which CO
₂
levels hover at the level causing plant death. As the levels drop to lethal limits, plants will die off, reducing weathering and allowing CO
₂
to again accumulate in the atmosphere, once again allowing any small surviving seeds or rootstocks to germinate and, at least for some millennia, to flourish at least at low population numbers. As plant life again spreads across land surfaces, weathering rates will again increase and thus increase the rate of carbon dioxide uptake out of the atmosphere.

Animal life is dependent on an oxygen atmosphere. There are almost no animals capable of living in zero- or even low-oxygen conditions (although in 2010 a tiny invertebrate able to live with anoxia was discovered deep in the Mediterranean Sea). David C. Catling of the University of Washington has suggested that by about 15 million years after the death of plants, less than 1 percent of the atmosphere will be oxygen in contrast to the 21 percent volume that Earth’s atmosphere contains today.

Life is one of the main agents of both its evolution and its extinction. Coauthor Ward’s Medea hypothesis was based on the conclusion that life has been more enemy to itself than friend; that the various ecosystems and their species do not become ever better adapted and successful the longer they last. As we have seen, in fact, the actual killing done by the major mass extinctions was caused by various toxins produced by microbial life. Thus it seems appropriate to us to end this book with some comments about one of the most Medean of all species ever evolved: our own. What will the future of evolution be for our own species?

The science fiction trope concerning our own future is one of an even larger head, containing a
much
larger brain, high foreheads, and higher intellect. But bigger brains are probably not in humanity’s future. The fossil record shows that the days of rapid brain increase, at least based on skull sizes over the past several thousand generations, seem to be over, and those conditions causing the rise in brain size (theorized to have been largely climatic in origin) are not likely to be repeated. But if not giant brains, what
might
evolution hold for the human species? Another intriguing question is whether the human species has undergone
any
significant evolution since its formation some two hundred thousand years ago.

The surprising revelation based on genetic study is that not only has the human genome undergone some major reshuffling since the species’ formation, some two hundred thousand years ago, but it appears that the rates of human evolution, if anything, have been increasing over the past thirty millennia. A study by Henry C. Harpending and John Hawks suggests that over the past five thousand years alone, humans have evolved as much as a hundred times more quickly than at any time since the split of the earliest hominid from the ancestors of modern chimpanzees some 6 million years ago. Moreover, rather than seeing a reduction of evolution of those characteristics that in combination are used to distinguish human races, until very recently the human races in various parts of the world have become
more
, not less, distinct. Only in the past century, through the revolution in human
travel and the more open behavioral attitudes of most humans to those of other races, has this pattern slowed. There are two main reasons: agriculture and cities. Food and crowding.

Humans thus seem to be first-class evolvers, or at least they were until very recently. With that known, it is possible to speculate about what the future might hold for the human species in terms of further evolutionary change—assuming that the species gets its few million years that appears to be the average longevity of any mammal species. Because much of the observed evolutionary changes of the past five thousand years involved adaptation to particular environments, it is fair to ask how the future world, with the expectation of larger populations than now, and with larger cities and agricultural fields among the other offshoots of technology, might affect the species’ evolutionary outcome—or will it be affected at all? There are many questions: Will humans become larger or smaller, gain or lose intelligence, be it intellectual or emotional? Will humans become more or less tolerant of oncoming environmental problems, such as a dearth of freshwater, an abundance of ultraviolet radiation, and an increase in global temperatures? Will humans produce a new species, or is the species now evolutionarily sterile? Might the future evolution of humanity be not within the species’ genes, but through the addition of silicon expression and memory augmentation to human brains through neural connections with inorganic machines? Is humanity but the builder of the next dominant intelligence on Earth—the machines?

For those concerned that the “End is nigh!”—and even those who worry that life on this planet is at least under the shadow of a new mass extinction or is already in one—there should be solace. We seem to be at a high point of species numbers in all of the (at least) 3.4 billion-year-old history of life. Our view is that it is impossible to prove what percentage of life is now going extinct—the metric of deciding if a mass extinction is major (greater than 50 percent), minor (between 10 to 50 percent), or not an extinction at all—if the denominator is not
known. Clearly there are more than 1.6 million species on Earth. If it is determined that a new mass extinction is taking place, there is some slight solace in the fact that after every past mass extinction, biodiversity bounced back to even greater levels.

The latter was the argument of the great Frank Drake, in a debate with one of us years ago about whether or not Earthlike planets are rare. Author of the eponymous Drake equation, a way to try to estimate the number of other intelligent species in the galaxy, he took the view that a giant mass extinction, such as the Permian extinction, was actually a good thing for any planet. But the price to pay … It took 5–10 million years following the Permian extinction for biodiversity to finally arrive at even the pre-extinction levels. The world went back to the Proterozoic in terms of biodiversity and even the kinds of life—a situation we have elsewhere only somewhat humorously described as the empire strikes back—in this case, the Precambrian empire of anoxic and toxic microbes.

A final prediction of Ward’s Medea hypothesis is that it should pertain to every planet with life, and that there is only one way out of this suicidal box that life creates simply through existing: intelligence. The intelligence to see the future. One such future is that our species expands its habitat first to Mars, then the asteroid belts, and finally to other stars. Another future is that the carbon dioxide we are pumping into the atmosphere causes all the ice on Earth to melt, raising sea levels, slowing the thermohaline circulation patterns, bringing stagnation followed by anoxia to the ocean bottoms, and then into ever-shallower waters, at the same time liberating toxic levels of hydrogen sulfide to percolate out of every single ocean. In that future, only animals with very good gas masks will survive.

History is an early warning system.

Nothing lasts. That goes for planets to organisms to scientific careers. While funerals are among the saddest events that we humans can participate in, at least they are definitive moments marking change:
from living to dead. But perhaps even sadder is the life near its end, such as a human with a fatal malady given a highly definitive death sentence. Such is the case for the chambered nautilus, an animal featured in this book as the best model for the extinct ammonites, and an animal that dodged the bullet of the major mass extinctions, if not in its present guise, at least as a major taxonomic order. Nautiloids first appeared in the Cambrian explosion of 500 million years ago. They are still with us but in dwindling numbers, and are now on the brink of extinction in various Pacific countries because of the demand for their shell: the past mass extinctions did not kill off organisms because of their perceived beauty. The human-induced mass extinction acts otherwise.

But even before the nautiluses became the objects of commerce, such that a half million of their shells were shipped to the United States alone in the five years between 2005 and 2010, they had received their own death sentence. The nautilus body plan evolved to work in warm, shallow water. It uses an osmotic pump that empties its chambers of the liquid each is completely filled with at its formation; they evolved to grow their shells in shallow, calcium-rich seawater. But along came the Mesozoic marine revolution that we profiled earlier in this book. Nautiluses were previously impregnable in their hard outer shells until new kinds of fish evolved during the Cretaceous and later that could easily break the durable outer shell of the nautilus. Life became impossible in the shallow waters. The shallows became a death sentence.

Life is about change. The nautiluses dealt with these new evolutioary and ecological stresses over the millions of years by slowly and steadily living in ever deeper water. Our new results show that in the last five million years, they were living at average depths of 200 to 300 meters, but their design is ill-equipped for these depths. They grow slower: it used to take a year to reach full size, while now it takes ten to fifteen. Now they live as a deep sea animal, few in number, in a dark, low-resource environment that is difficult at best. And the predators are following them down. They can go no deeper, as their shells have a depth limit below which they implode, causing instant death. There is no further place to hide.

The fate of the nautilus is a metaphor for all animal life. Sooner or later evolution, competition, and the natural changing of our Earth and sun as they age will make any body plan obsolete. For us land animals, it is not the predators that will do us in, but an enlarging sun and too little carbon dioxide. There will be no place on Earth to survive. The only hope for our species, if we wish to do what the nautilus has done—or better yet, do what the cyanobacteria have done and last two to three billion years—is to leave. The last chapter here has been about the history of life—
on Earth
. But there can be a whole new book. A whole library of new books, in fact.

Perhaps life did start on Mars, our kind of life. The choice was to leave Mars or die. Survival is literally in our genes.

  
1.
   J. Loewen,
Lies My Teacher Told Me: Everything Your American History Textbook Got Wrong
(New York: Touchstone Press, 2008).

  
2.
   J. Baldwin,
Notes of a Native Son
(Boston: Beacon Press, 1955).

  
3.
   N. Cousins,
Saturday Review
, April 15, 1978.

  
4.
   P. Ward, “Impact from the Deep.”
Scientific American
(October 2006). The actual first use of the term “greenhouse mass extinction” is difficult to ferret out, although I used it overtly in a
Discover
magazine article in the 1990s.

  
5.
   G. Santayana,
The Life of Reason, Five Volumes in One
(1905).

  
6.
   Fortey’s book was (and remains) a masterpiece, not just for the “facts” but also for the stories of science in the presentation of what in lesser hands is often delivered as more dry history. And yet it is now dated (how could it not be, including much new work by Richard himself). We have used his book as a bit of a stalking horse and straw man, and hope we are forgiven. Just for starters, we take some exception to the title: assuming life is already 4 billion years old on Earth might have seemed like a good bet in the mid-1990s when the book was written, but that might not be the case now. Perhaps he is still right on this, but our own arguments are to come. The reference of the book is: R. Fortey,
Life: A Natural History of the First Four Billion Years of Life on Earth
(New York: Random House, 1997).

  
7.
   A great read on this entire aspect of the philosophy of those building the then-nascent field of geology (and its subfield of paleontology) is in M. J. Rudwick,
The Meaning of Fossils: Episodes in the History of Palaeontology
(London: Science History Publications, 1972). This book, early on hard to find, was later republished in more accessible presses. Rudwick’s take on the late 1700s into the early 1800s, when the foment over geological time and processes was intersecting with early ideas about the stratigraphic ranges of fossils as well as early ideas about evolution, is seminal and remains a must-read for anyone interested in time and natural history.