A New History of Life (2 page)

Second, while we may be carbon-based life, composed of “long-chain” carbon molecules (carbon atoms strung together to form proteins), it is the influence of three different kinds of molecules, simple molecules that exist as simple gases, that have had the greatest influence on the history of life: oxygen, carbon dioxide, and hydrogen sulfide. Sulfur, in fact, may have been the single most important of all elements in dictating the nature and history of life on this planet.

Finally, while the history of life may be populated by species, it has been the evolution of ecosystems that has been the most influential factor in arriving at the modern-day assemblage of life. Coral reefs, tropical forests, deep-sea “vent” faunas, and many more—each can be viewed as a play with differing actors but the same script over eons of time. Yet we know that on occasion in the deep past entirely new ecosystems appear, populated by new kinds of life. The appearance of life that can fly, for instance, or life that can swim or walk—each was a
major shift in evolutionary innovation that changed the world, and in each case helped create a new kind of ecosystem.

An author’s background affects the biases inherent in any written history. Peter Ward has been a paleobiologist since 1973, and has published extensively on modern and ancient cephalopods as well as on mass extinctions of vertebrates and invertebrates. Joseph Kirschvink is a geophysical biologist who began his work on the Precambrian-Cambrian transition, but then expanded to looking at older times (the great oxidation event) as well as being the discoverer of the snowball Earths—major parts of life’s history. Together we have subsequently worked on the Devonian, Permian, Triassic-Jurassic, and Cretaceous-Tertiary (this time interval recently renamed the Paleogene Period) mass extinctions.

We have worked together in the field since the mid-1990s. These trips included study of the Permian mass extinction in South Africa, from 1997 to 2001; the study of Upper Cretaceous ammonites in Baja California, California, and the Vancouver Island region; the study of the Triassic-Jurassic mass extinction in the Queen Charlotte Islands; the study of the K-T mass extinction in Tunisia, Vancouver Island, California, Mexico, and Antarctica; and the study of the Devonian mass extinction in Western Australia.

The voice we bring to this book is meant to be a seamless duet, but there are passages where one or the other of us self-identifies because of the nearness of the topics to some particular interest we have, or because we were integral in the history of some aspect of the science being reported.

Earlier we noted that the number of species on Earth is in the millions. Most who study life will acknowledge that the current number of formally defined species (which requires a name for both genus and
species) is probably less than 10 percent of the actual number of currently living species.
¹⁰
But how many have there been in the past? Billions, certainly. That makes the writing of a history of them a daunting process. Paleontology, biology, and geology all have entire vocabularies of highly specific jargon, and it is our job to use the English language in an understandable way to make sense of so much of the multisyllabic jargon—or, in the case of NASA, decipher their endless acronyms. Perhaps even more daunting, by necessity we will have to introduce many of the Latin names for the many creatures great and small that produced and daily continue life’s history on Earth.

Finally, a full acknowledgment of the large number of people helping us in our journey to write this book will come at the end of the text. But Ward would like to specifically shout out to two scientist-writers who have profoundly influenced him: Robert Berner, whose work on oxygen and carbon dioxide is absolutely integral to the work written here, and Nick Lane, a prolific scientist and writer whose books are pinnacles of clarity and insight, whose work profoundly influenced at least one of the coauthors, and whose books remain groundbreaking and current.
¹¹

Until recently, the history of life had an arcane time scale, measured not in years, but in the relative positions of rocks scattered about the Earth’s crust. In this chapter we will look at the geological time scale, the tool used in discovering the relative sequence of life’s history on Earth.

The geological time scale is a rickety old contraption, held together by nineteenth-century rules and current European formality. The newer generations of geologists do not love the hoary and very stuffy series of conventions involved with the time scale, and still required by an increasingly aged set of geologists who were trained in the old tradition. To this day, any change has to be approved by committees;
¹
all time units have to be associated with a “type section”—a real stack of sedimentary rocks chosen to best represent a given time interval. The type section is supposed to be readily accessible and must be undisturbed by tectonism, heating, and “structure” complexity (such as faults, folds, and other tricky mashing of the originally horizontal sedimentary beds). The section should not be upside down (which happened more often than one would think), should have lots of fossils (both macro and micro), and should also have beds, fossils, or minerals that can be dated with “absolute” ages (a date in actual years) through some combination of radiometric age dating, magnetostratigraphy, or some form of isotope age dating (such as carbon or strontium isotope stratigraphy).

The time scale is complicated and often useless in the sense that when someone says a rock is Jurassic in age, they are in reality saying the rock in question is of the same age as the designated type section for the Jurassic, which was in the Jura Mountains in Europe. But it is what we Earth and life historians have to work with to discover the age of rocks by their fossils, as well as to communicate their actual age to others. Although more modern tools than dating events and species based on their relative position in piles of sedimentary rock are
sometimes available
²
—including the determination of a fossil’s actual age through the use of isotopic dating, such as the well-known use of carbon 14 or other kinds of “radiometric” dating using the known rates of decay of various elements contained in the rock—in fact very few fossils are found in beds or are made up of materials allowing this kind of absolute age dating. Usually it is fossil content only that is available, yet from this the rock must be dated.

The geological time scale remains not only the major tool in dating all
rocks
on Earth (categorized by their age, rather than on their lithological characteristics), but also the means by which
events
in the history of life are dated. Using intricate names and seemingly random and dissimilar intervals of time, the time scale remains a thoroughly nineteenth-century tool, and more often than not is an impediment not so much because of the manner in which it was developed, but by the rigid and bureaucratic fashion in which it was formalized and codified into what we have today. Only in the last decade have new geological “periods” been put in place. The formation and common usage of these two new periods are central to our new understanding of the history of life: the Cryogenian period, from 850 to 635 million years ago, followed immediately by the Ediacaran period, from 635 to 542 million years ago.

The first half of the eighteenth century was both the time when the field of geology was born and the time when the geological time scale as we know it now was put in place. During this time, the various eras, epochs, and periods were defined and in so doing replaced a more ancient system.
³
Prior to 1800, each kind of rock observed on the Earth was thought to be of one specific age. The hard igneous and metamorphic rocks, the core of all mountains and volcanoes, were presumed to be the oldest rocks on Earth. The sedimentary rocks were younger, the result of a series of world-covering floods. This principle—called neptunism—held sway, and even developed to the point that specific kinds of sedimentary rocks themselves were thought
to have specific ages. The omnipresent white chalks that stake out the northern limits of the European subcontinent and then continue into Asia were considered of a single age, different from the sandstones, and different again from finer mudstones and shale. But in 1805 a discovery was made that changed everything. William “Strata” Smith
⁴
was the first to recognize that it was not the order of lithological types that determined their age, but the order of fossils within the rocks themselves that could be used to date and then correlate strata to distant locales. He showed that various rock types could have many different ages—and that the same succession of fossil types could be found in far separated regions.

The principle of faunal succession opened the door to the formation of the time scale in its modern sense.
⁵
Life was the key, life preserved in fossils, and the relative difference of fossil content could be used to distinguish a succession of rocks on the surface of the Earth. The largest division was of older rocks without fossils, beneath rocks where fossils were commonly present. The oldest
fossil
-bearing unit of time was named the Cambrian, after a tribe from Wales, and thus all the rocks older than this came to be known as the Precambrian. From the Cambrian onward, the fossil-bearing rocks came to be known as the Phanerozoic or “time of invisible life.” The Proterozoic era, the last before animals evolved, succeeds the older Archean and Hadean eras.

Very quickly the periods of the Phanerozoic were defined, all based on fossil content. Within decades of true scientific collection, curation, and “bookkeeping” of fossils (a compilation of the first and last occurrences of particular fossil groups in the record), it was seen that the Phanerozoic was divisible into three major intervals of time and accumulations of rock. The oldest was named Paleozoic (or old life) era, the middle the Mesozoic era, and the most recent the Cenozoic era.

Even before these eras were put in place, most of the period names still used today were in place. In successive order, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous (this is the European usage; the Carboniferous is subdivided into the Mississippian and Pennsylvanian periods in North America), and Permian comprised the
Paleozoic era; the Triassic, Jurassic, and Cretaceous comprised the Mesozoic; and the Paleogene and Neogene (formerly Tertiary), and Quaternary periods comprised the Cenozoic.

The current version of the Geological Time Scale. (Updated from Felix M. Gradstein et al., “A New Geologic Time Scale, with Special Reference to Precambrian and Neogene,”
Episodes
27, no. 2 (2004): 83–100)

By 1850 the periods were in place and new ones were rarely accepted (although many late nineteenth-century geologists tried to get the glory of defining a whole new period, which by then could only take place by cannibalizing already existing units). Only one such attempt actually succeeded, and this was by an Englishman named Charles Lapworth,
⁶
who carved out an Ordovician period by successfully claiming that some underlying Cambrian and overlying Silurian rocks deserved to be their own geological period, and he managed to persuade enough of the rest of geology to make it so in 1879. By that time the two English bulldogs who had pioneered the naming of periods—Adam Sedgwick for the Cambrian and Roderick Murchison
for the Silurian and Permian periods—had died, leaving an ownership vacuum that Lapworth exploited. All of these men had gigantic egos, and fought ferociously for “their” time periods.

The most important real change to the geological time scale in terms of the history of life came with the addition of the Cryogenian and Ediacaran periods, during the Proterozoic era, and the time when life was readying the advent of animals. But long before the evolution not only of animals, but life itself, the Earth had to undergo significant changes to support life. The Cryogenian period (from Greek “cold” and “birth”) lasted from 850 to 635 million years ago, and it was ratified by the ruling body on geological names, the International Commission on Stratigraphy and the International Union of Geological Sciences (IUGS), in 1990.
⁷
It forms the second geologic period of the Neoproterozoic era, and is followed by the Ediacaran period, also new compared to the other periods. Both of these time intervals are seminal times in the history of life, as we will see in greater detail in chapters to come. The Ediacaran period was named after the Ediacara Hills of South Australia—the last geological period of the Neoproterozoic era and of the Proterozoic eon, immediately preceding the Cambrian period, the first period of the Paleozoic era and of the Phanerozoic eon. The Ediacaran period’s status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS).
⁸

The geological time scale as constructed is a mishmash of nineteenth- through twenty-first-century science. It is analogous in this to the biological sciences dealing with the classification of organisms, as both are based on historical claims, observations, and precedence of terms and definitions, which often collide with new means of definitions—of both time and species, in the latter’s case. Just as DNA analyses have radically changed our view of evolution, so have new methods of dating rocks collided with the old “relative” time scale based on the superpositional relationships of rocks and their fossils. Quite often the collisions are monumental. We wonder what the geological time scale will look like a century from now, especially since modern universities no longer train and produce specialists capable of the high standard of fossil identification necessary to really define geological
time. This would not matter if some new
Star Trek
kind of tool allowed all rocks to be dated with the flip of a switch or scan. Sadly, that will probably never be the case. We are encased in history in both the rocks and their historical dating methods and definitions. This geological time scale has even been extended to other planets and moons, based on the number of impact craters per unit area, and each body has its unique set of geological terms that we must also learn.