A New History of Life

by Peter Ward

  Cold water holds more oxygen than warm. In a cold world, with already high oxygen levels, life in the sea would rarely be compromised by too little oxygen. On the other hand, in a warm world where there was already relatively little oxygen, most bodies of nonflowing water would quickly go stagnant. And not just ponds to lakes—large oceans would succumb to this as well in a warm world, which is what a high CO2 world is.

  The data to date suggests that (at least for marine animals) overall, global taxonomic diversity is related to oxygen content, which might be expected, since all animals have a poor tolerance for anoxic conditions. What was unexpected was the finding that origination rates (measured either in species or genera, which are groups of related species all with the same common ancestor) appear to be inversely correlated to oxygen levels. The high levels of origination characterizing the period from 545 to roughly 500 MA (the Cambrian explosion) occurred during a protracted period of oxygen levels of between 14 and 16 percent, compared to 21 percent today. The dramatic rise in oxygen in the Silurian and again in the Carboniferous corresponds with the lowest levels of generic origination. The drop of oxygen in the Permian is correlated to a rise in origination rates but a drop in the total number of species. There seems to be a clear signal.

  Times of high oxygen are like the boom times in a country’s economy. There is little unemployment and businesses succeed and stay open—but not many new ones start up. Start-ups, it seems, are related to bad times. New ideas play out and new risks are taken in times of desperation. Yet while there are many new start-ups, few of them succeed, and at the same time many of the businesses that were succeeding during good times begin to fail at an ever-higher rate in the bad times.

  Thus we see a dichotomy: more new businesses appear, but most of them quickly go bankrupt and disappear along with many of the previously successful. There is also less money circulating. The total number of businesses plummets. The same seems to be true of species. High oxygen means good times: large numbers of species, and nothing much new comes along. But when oxygen is low, species die out at a faster rate than they are replaced, even though the actual number of emerging species is higher than in the high-oxygen times.

  There are many examples. One of the best: the long-term rise in oxygen beginning in the Jurassic and continuing to the modern day is accompanied by both a long-term drop in origination rates and a huge run-up in diversity. But what radical new designs appear? Birds, mammals, reptiles, amphibians—all of the Cenozoic forms are slight changes of body plans that originated in either Paleozoic or Mesozoic bad times, the low-oxygen times. There are no dinosaurs (the best examples of a radical innovation spawned directly from low oxygen) appearing in the Cenozoic.

  The recognition that a combination of low oxygen and elevated carbon dioxide has stimulated species formation in the past by the formation of evolutionary novelty, while at the same time vastly increasing extinction rates, has a firm biological basis. The net effect is the reduction of species during the low-oxygen periods. Dropping oxygen levels with concomitant rising temperatures is the worst kind of one-two punch, as adapting to hotter, lower-oxygen environments is never a quick fix. More hair, more feathers, and more body fat can fix increasing cold. But staying cool is far more difficult and involves much more profound evolutionary change. This is even more true in animals trying to deal with staying alive in ever-lowering oxygen, since adaptation to lower oxygen levels is necessarily profound and must occur in multiple organ systems, ranging from blood pigments to more efficient circulatory systems, to better lungs or gills.

  The most striking aspect of oxygen and its relationship to diversity was pointed out to us in 2009 by Bob Berner (of Yale), who alerted us to what he saw as a profound similarity between his latest oxygen-through-Phanerozoic time curve with the then-latest diversity curve of the John Alroy group. We show those two curves in the figure on page 158. While there is a slight direct correlation between oxygen levels and diversity when both are broken down into 10-million-year time bins, an absolutely amazing correlation is present between the change in atmospheric oxygen when plotted against the change in diversity for these same 10-million-year bins. For example, the correlation between the change of atmospheric oxygen as a percentage of total atmospheric gas content from 230 to 220 million years ago, plotted against the change in generic diversity during that same time interval, is highly significant—in other words, not by chance. The results are very strong indeed, from a statistical point of view.

  A most interesting aspect of this is that since both were introduced, the model results from the Berner group (and others as well) estimating past oxygen and carbon dioxide levels have been controversial. Equally controversial are the various Alroy curves. Each set of results (one yielding oxygen and CO2 values, the other the estimated number of animal genera through time) comes from models that have completely different inputs. None of the many values inputted into the GEOCARB and GEOCARBSULF models have anything to do with how many species there were at any given time. In similar fashion, the Alroy model is completely independent of the values used to model carbon dioxide and oxygen. Yet the near unbelievable correlation, while theoretically possible from chance, is hard to explain that way. There is no chance at work. It appears that oxygen and carbon dioxide levels (particularly oxygen) are the most important of all factors dictating animal diversity. The two independent curves in this fashion support each other in terms of that most important of scientific values: credibility.

  INSECTS AND PLANT GROUPS

  It is clear that the invasion of land unleashed the floodgates of both diversity and disparity. Our understanding of the overall diversification of life through time is that there are more kinds of life on Earth now—more kinds of species or any other way of tabulating diversity—than at any time in the past. But is this really true? What might the biases be?

  All good science has a null hypothesis, and in this, it is that marine animal life on Earth reached present-day levels at the end of the Cambrian. This was the view of Stephen Jay Gould in the 1970s, and whether he really believed this is irrelevant: his take on this issue resulted in much stronger science.

  The answer to this question, whether diversity rose rapidly or only slowly rose to present levels, relates to the relative preservation potential of modern-day organisms compared to those of the Cambrian. Today, about one out of three marine animals has hard parts that produce ready fossilization—anatomy such as shells, bones, and hard carapaces. But what if that number were one out of ten during the Cambrian? In such a case, there might be approximately equal numbers of animals during the Cambrian, compared to modern oceans. Support for this idea also came from the post-Sepkoski work of Marshall and Alroy, as their model results, while showing post-Cambrian increases in diversity, did not see the explosive runaway of post-Permian animal taxa found by Sepkoski.10 The Alroy work has since gone through several iterations with new data.11

  There are other sources of bias as well as perhaps dubious assumptions that have been made to arrive at models of diversity through time. For instance, what about the unequal sample sizes being studied? Critics of the entire diversity-through-time enterprises have noted that there is far more rock to sample of late Cenozoic or Pleistocene age than there is of Cambrian age. Furthermore, there are many more paleontologists studying late Cenozoic and Pleistocene fossils than there are professionals studying Cambrian-aged rocks and fossils. Andrew Smith of the British Museum12 and independently Mike Benton13 of the University of Bristol and Shanan Peters14 of Wisconsin have all done remarkable work on this aspect.

  It turned out that a quite simple test demonstrated that there has been an increase in marine animal taxa (be they species, genera, or families) since the Cambrian explosion. The test came from studying the number of trace fossils through time. Trace fossils are the results of animal activity, as we saw in the chapter on the Cambrian explosion, and each different trace found in strata had to come from a slightly different body plan. Their pattern of
diversity mirrors the record from body fossils. It is now agreed that the overall pattern of diversity long recognized by invertebrate paleontologists has indeed given us a fairly accurate view of how life diversified on Earth.

  By the end of the Devonian period, the major marine environments from the shallows to the deepest oceans were colonized. But this marine diversification was about to be overshadowed by a diversification that would ultimately prove far greater, creating the greatest pool of animal and plant species on Earth: the diversity of life on land.

  THE ORDOVICIAN MASS EXTINCTION

  The Ordovician period was also the time of the first of the so-called big five mass extinctions. All five involved animals and plants. There surely were mass extinctions before the Ordovician event, such as during the great oxygenation event and the various snowball Earth episodes. But animals were in the midst of differentiating at a rapid rate when something brought this increase in diversity to a halt. The best bet is that it happened when the Earth underwent a “little ice age” that turned the early coral reefs to dead piles of rubble because of sudden temperature drop. However, this is still a puzzle, as the extinction has two discrete steps, at either end of the last stage of Ordovician time, called the Hirnantian glaciation.

  There are other more fanciful suggestions about the cause of the Ordovician mass extinction. The most interesting is that the Earth was hit by a giant blast of hard radiation coming from interstellar space, called a gamma-ray burst, during the Ordovician.15 This is a most dramatic potential cause, but there is also not a single shred of evidence to support it, much as journalists have publicized it. Prior to 2011, the accepted cause for this mass extinction was that there was no accepted cause.16 Most explanations opted for some kind of rapid cooling event. One prevalent idea is that perhaps volcanic outpourings caused the atmosphere to become obscured by sulfur aerosols17 in a manner similar to that following the Krakatoa volcanic explosion of the 1800s, when Europe went through a “year without summer.” Recently, however, geologists and geochemists at Caltech18 attacked this late Ordovician glaciation problem from a superbly preserved sequence of rock on Anticosti Island, a remote Canadian island in the Gulf of St. Lawrence that was once located in the tropics. Using a new type of geochemical thermometer, they were able to measure both the relative ice volume and temperature, with unprecedented resolution. Lo and behold, they found that while ice volume changed only slowly before and after Hirnantian time, and the tropical temperature remained at a very hot but possible 32° to 37°C, there was a sharp shift at either end of it that was associated with the two steps of mass extinction. Tropical temperatures fell by ~5° to 10°C, the global ice volume peaked up to levels that equaled or topped those of the last (Pleistocene) glacial maximum, and carbon isotopes had a positive spike, suggesting a large perturbation of the global carbon cycle—in this case, presumably more organic carbon burial.

  These new data narrow the actual kill mechanisms of these two extinction pulses to two possibilities, either a fast change in climate or a fast change in the level of the oceans all over the globe. In a follow-up paper, members of the same team19 mined two enormous digital databases for North America, one showing the fossil distributions and another the volume of rock available for fossils to be found in (a necessary correction for the fossil discoveries!). Both processes were found to account for the extinctions—habitat loss from sea level drop and a sudden drop in temperature were both flagged as major elements of the die-offs. However, it is not clear if this is the entire story; the timing of the climatic perturbations, including the positive spike in carbon isotopes, is surprisingly similar to some of the events induced by the true polar wander mentioned in previous chapters. A short, sharp TPW (True Polar Wander) excursion could have triggered a short period of global cooling, producing, perhaps, a short-lived period of glaciation. This remains an enigmatic and still-to-be-researched topic. It is certainly not the traditional explanation. It is, in fact, new—the promise of our title.

  CHAPTER X

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  Tiktaalik and the Invasion of the Land: 475–300 MA

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  A long point of contention between “evolutionists” and those opposed (creationists) to the knowledge that species evolve one from another has been the supposed dissimilarity between the first amphibian and its last-known fish ancestor: the fish fossils seemed too “fishy,” and the first amphibians too “un-fish-like” to appease doubters. There indeed was merit to one aspect of this dispute: until recently, the oldest agreed-upon amphibian fossil, a Devonian-aged creature1 named Ichthyostega (which means fish-amphibian), had a fish-like body (including a quite normal fish tail) and four legs. Its immediate ancestor appeared to be a creature with a similar-looking body—but without legs. This fish, which paleontologists have deemed as the true ancestor to Ichthyostega and the other early land vertebrates (or at least living some of their life on land), belongs to a group known as the sarcopterygians, which had fins with fleshy lobes around them.2 These were the predecessors of limbs. The living fossil Latimeria (a coelacanth fish) is thought to be at least somewhat similar to the immediate ancestor of the eventual first amphibians including Ichthyostega. The critics asked: “Where are the missing links?” But a twenty-first-century fossil discovery changed all that—a fossil found in the frigid Devonian-aged strata of the high Arctic. It was named Tiktaalik, and is so transitional that its discoverers3 dubbed it a “fishopod.” This discovery is one of the most consequential of all revisions to what we call the history of life not only for filling in a large hole in our understanding (of the fossil record from water- to land-living vertebrates) but in helping solidify the entire theory of evolution.

  This large fossil proved to be the perfect antidote to creationist doubters. It was unearthed in Arctic Canada by a team of international researchers led by Neil Shubin of the University of Chicago, and when finally (and painstakingly) removed from the sarcophagus-like coating of sedimentary rock holding its bones, the first Tiktaalik fossil was deemed to be a fish, complete with scales and gills. It also showed a flattened head and fins that had thin ray bones, the most familiar kind of fish fin. However, in this new fish’s case, there were also the kind of sturdy interior bones necessary for an animal as large as this specimen (which would have been near three feet in length) to prop itself up in shallow water using its limb-like fins for support, just as four-legged animals do. With these strange fins and an amphibian (even crocodile-like) head, Tiktaalik has the combination of features that shows a perfect, step-by-step evolutionary transition between the fish and tetrapod body plans.4

  The first appearance of vertebrates on land is the most dramatic event of what was nothing less than a succession of invasions of land by aquatic animals—and plants. Yet while most relevant to us, in fact we vertebrates were among the very last to climb out of the pool and join the roster of animals making the water to land transition. To tell the story in order, we begin with the first—plants.

  THE INVASION OF LAND BY PLANTS

  It can be argued that the greatest single event in all of life’s history, save for the first formation of life itself, was life’s invention of oxygen-releasing photosynthesis. It was this that allowed life to move from its dark and dank habitats as low standing biomass and fill the shallower waters of seas and freshwater bodies alike with the living by tapping the greatest energy source that our solar system has to offer, the sun. And in so doing, as an unintended by-product, our planet radically changed its atmosphere to one with such a high concentration of oxygen that a second unintended consequence became the greatest of all dangers to living plants—grazing animals. Yet as consequential as these changes were to life on Earth by aquatic plants, even more radical changes transformed the planet when plants evolved the means to break free of their watery shackles and colonize dry land. In a relative blink of an eye in terms of Earth history to date, in a period of less than 1 percent of the total age of life itself, this great invasion of land by plants changed all
the rules—as well as the history of life on our planet.

  As we saw in an earlier chapter, there is now abundant evidence that some kind of primitive photosynthesizing organisms found a way to grow on land surfaces hundreds of millions of years before the first animal, and in fact may have been a major cause of the last of the snowball Earth episodes between 700 and 600 million years ago. We have no idea what they were. Perhaps they were simply cyanobacteria, or perhaps they had real adaptions to land life, such as the ability to stay in place, obtain nutrients, reproduce, and get and then keep water. Candidates for this seem to be the still-extant single-celled green algae.

  But even these plants from 700 million years ago may not have been first to get out of the sea, because an increasing number of geobiologists are concluding that there was land life far earlier: single-celled photosynthetic bacteria, making the water-to-land transition as much as 2.6 billion years ago. If so, these early colonists would have been long, long established when “higher” plants and animals finally climbed onto land as well.

  What is known is that in less than 100 million years after the appearance of animals in the sea, some species of green algae, probably still living in freshwater, shed the shackles of a wholly aquatic lifestyle and migrated to the land, rapidly evolving from simple leafless twiglike plants not dissimilar to many moss species of today to true giants, thanks to one of evolution’s great innovations: the leaf.