A New History of Life (25 page)

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 CO
₂
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.

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 Museum
¹²
and independently Mike Benton
¹³
of the University of Bristol and Shanan Peters
¹⁴
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 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 aerosols
¹⁷
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 Caltech
¹⁸
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 team
¹⁹
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.

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 creature
¹
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 discoverers
³
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.

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.