Near the end of the last chapter, we pondered whether the initial diversification of the animal phyla was stimulated by evolutionary or environmental causes—specifically, the Snowball Earth events of between 800 and 600 million years ago. This same question can be posed about the subsequent Cambrian Explosion: Did it occur as late as it did in Earth history because it took that long for the establishment of an environment conducive to animals of large size, most with skeletons, or because it took that long for the necessary genes to evolve—genes that allowed the diversification of these metazoans? Genes or environment? Much new research into environmental conditions before and during the Cambrian Explosion, and into the physiological, anatomical, and genetic innovations leading to larger multicellular animals, has given us new views of this crucial moment in Earth history.
Many hypotheses have been proposed to account for the Cambrian Explosion. These can be categorized as attributing it to environmental causes or to biological causes.
• Oxygen reached some critical threshold value.
This is perhaps the most often discussed and widely favored of all environmental hypotheses. According to this hypothesis, the amount of available oxygen reached some critical level, or threshold, that made possible the great diversification of new organisms. Presumably, this level was higher than that during the first animal diversification event of 700 million years ago. Many scientists suggest that the biological response to a new, higher oxygen level was a biochemical breakthrough allowing animal life to construct hard skeletons for the first time. In the absence of abundant oxygen, organisms have a great deal of difficulty precipitating minerals as skeletal structures. As early as 1980, Heinz Lowenstam and Lynn Margulis postulated that skeletons in the form of collagen—an elastic, proteinaceous material similar to human fingernails—could have appeared as early as 2 billion years ago, for collagen formation does not require as much oxygen. Calcareous and siliceous skeletons and shells, however, were not possible at that early date.
• Nutrients became available in large amounts.
Just as a lawn needs fertilizer, ecosystems—and especially marine ecosystems—need a supply of organic and inorganic nutrients to remain at high levels of productivity and diversity. Abundant evidence suggests that the late Precambrian interval witnessed a relatively sudden and dramatic increase in nutrients, which may have had a significant effect on the evolution of organisms.
One of the mineral types most commonly found in rocks of this age is phosphorite, a mineral rich in phosphorus, one of the most important of the inorganic nutrients necessary for life. (The others are nitrate and iron.) There appears to have been a long interval during the late Precambrian when phosphates and nitrates were unavailable to organisms of the time, because they were buried in deep-water bottom sediments. However, the latest Precambrian was a time of changing oceanographic conditions,- episodes of up-welling became common, in which deep waters were brought up to the sea surface, in the process liberating nutrients formerly locked in bottom sediment. This upwelling appears to be related to changing continental configurations.
The latest Precambrian was a time of intense plate tectonic activity. In particular, a giant "supercontinent" named Rodinia began to tear apart, and as this occurred, it changed the global patterns of ocean current circulation, thereby triggering the upwelling. According to this hypothesis, the release of the phosphate nutrients accompanying that new tectonic activity sparked the Cambrian Explosion.
• Temperatures moderated following the late Precambrian "Snowball Earth" events.
Just as the evolution of humanity is set against the backdrop of a global ice age, so too is the Cambrian Explosion associated with glaciation, for it occurred soon after the cessation of the "Snowball Earth" events profiled in the last chapter. As the last of these glaciation events finally ended, it signaled a protracted warming of the planet that followed 200 million years of glacial advances and retreats. Was this the trigger that unleashed the Cambrian Explosion, as suggested in the last chapter?
• The Inertial Interchange Event
There is a final environmental possibility that borders on the fantastic— as well as the believable. For many years, paleomagnetists have known that most, if not all, of the continents underwent large amounts of continental drift during Cambrian time. As we will see in more detail in Chapter 9, continental positions have an extraordinary effect on global climate, often controlling where warm and cold currents flow, the formation of ice caps, and even the abundance of greenhouse gases in the atmosphere. During the past few years, advances in the numerical calibration of the Cambrian time scale and improvements in the paleomagnetic database have revealed something astounding: Much of this continental drift happened during the Cambrian evolutionary explosion, and the entire episode lasted no more than '0 to '5 million years. The continental shifts were quite dramatic. North America moved from a position near the south pole to the equator, and at the same time, the entire supercontinent of Gondwanaland spun around a point in Antarctica, sending North Africa from the pole to the equator as well. It is as though the continents suddenly became ice skaters, gliding about Earth's surface with unprecedented ease for a short period of time, before turning to stone once more.
In 1997 the ubiquitous Joseph Kirschvink and two colleagues published, in the prestigious journal Science, a controversial interpretation of the cause of this tectonic movement—an explanation that is either the harbinger of a revolution in our understanding of planets and their histories, or unmitigated balderdash. As of this writing, the scientific community is about evenly divided and is awaiting further developments with keen anticipation. Kirschvink, David Evans, and Robert Ripperdan proposed that the Cambrian Explosion might have been triggered by another unique event in Earth history: a 90-degree change in the direction of Earth's spin axis relative to the continents. Regions that were previously at the north and south poles were relocated to the equator, and two formerly equatorial positions on opposite sides of the globe became the new north and south poles. This interesting hypothesis will be confirmed or discredited only through the acquisition of much new paleomagnetic data.
Kirschvink and his colleagues noted that all of the world's continents experienced a major increase in continental plate motions (the "drift" movement of continental drift) during the same short interval of time when the great evolutionary diversification took place—between 600 and 500 million years ago. This rapid movement of Earth's upper surface relative to its interior is thought to have been brought about by an imbalance in the mass distribution of the planet itself. During this redistribution, the theory goes, all the solid portions of Earth move together. But because Earth also has liquid portions (its inner core, for instance), the outer layer essentially flips over relative to the spin of the planet. This phenomenon would not be limited to Earth; it may also have occurred on Mars. Kirschvink and his colleagues point to Tharsis, a large plateau located on the Martian equator. Tharsis sits atop the largest gravity anomaly (a center of high mass that creates forces of gravity greater than those in the surrounding rock) known for any planet in the solar system—a place of such high density that it creates a measurable perturbation in the planet's gravitational field. It is unlikely that Tharsis formed on the equator, according to Kirschvink and his colleagues. They believe the law of conservation of mass caused it to migrate later to its current equatorial position. Its movement would have been caused by an "inertial interchange event" similar to that posited for the Cambrian Earth. Once the volcano was at the equator, Mars would rotate so that its maximum moment of inertia was aligned with the spin axis.
The Earth's own inertial interchange event (IIE) would have taken only about 15 million years, and it is the very rapidity of this movement that causes Kirschvink and his colleagues to speculate that the IIE might have been associated with the Cambrian Explosion of life. During this period, existing life forms would have had to cope with rapidly changing climatic conditions, such as polar regions sliding to the hotter equatorial zones and warmer, low-latitude sites moving upward into the high-latitude, cold regions of the planet. These motions would have disrupted oceanic circulation patterns and would have perturbed most ecosystems on Earth. It might be only by chance that a unique tectonic event during Earth's 4.5-billion-year history coincided with a unique biological event. But how often does someone win million-dollar lotteries on two successive days?
The inertial interchange event can also explain one of the most curious aspects of Earth at this time. It is well known among geologists that the late
Precambrian and earliest Cambrian Earth underwent some sort of event that is preserved as large swings in carbon isotopes. (These are chemical signals found in the oceans in response to varying amounts of life on the planet; such signals were used to detect the first life on Earth in the Isua strata of Greenland, as described in Chapter 3.) About a dozen of these swings occurred near the end of the Precambrian time interval, and they have long puzzled geologists. The swings in these isotopes suggest that large amounts of organic carbon, long buried in ocean sediment, were suddenly exhumed and reintroduced into Earth's carbon budget. Repeated, major changes in the oceanic circulation patterns could produce these effects, yet such global changes would require massive tectonic changes in short periods of time. These changes would have fragmented ecosystems and could have prompted evolutionary diversification. The inertial interchange event would accomplish this.
If the Cambrian Explosion was necessary for animals to become so diverse on this planet, and if the inertial interchange event occurred as postulated, and if the Cambrian IIE event contributed to the Cambrian Explosion or even somehow was required for the Cambrian Explosion to take place, then Earth as a habitat for diverse animal life is rare indeed.
In his pivotal book Oases in Space, paleontologist Preston Cloud suggested that there were four biological prerequisites for the Cambrian diversification event to take place: the prior presence of life itself, the attainment of oxidative metabolism (the ability to live and grow in the presence of oxygen), the evolution of sex in the domain Eucarya, and the presence of an appropriate protozoan ancestor to give rise to more complex animals. In Cloud's view, attaining all of these milestones took nearly 4 billion years—85% of Earth's history. He thus seems to believe that biological actors were more important in creating the Cambrian Explosion event than were the environmental aspects we considered in the previous section. But other biological factors must have played a pivotal role as well.
• The advent of precipitated skeletons
Skeletons are critical to large body size for many animals. Skeletons usually perform several functions, such as protection (from predation, desiccation, and ultraviolet rays), muscle attachment (thus allowing locomotion), and maintenance of body form. Yet building such structures required many evolutionary breakthroughs. Oxygen levels would have been critical for two reasons. First, large skeletons such as a shell covering (found in the earliest trilobites and mollusks) restrict the access of seawater to the soft body parts. In most early animals, respiration took place by direct adsorption of oxygen from seawater, across the body wall. Second, the presence of a shell means that a larger area of the body is no longer available for this type of respiration. In low-oxygen conditions, animals have a difficult time getting enough oxygen as it is, and adding a body cover only makes this problem worse. Thus, skeletons such as shells would not have evolved until relatively high oxygen levels were available in seawater.
Professor Adolf Seilacher (whom we met in our discussion of the Edi-acaran fauna) is convinced that acquisition of skeletons played the dominant role in causing the sudden appearance of multicellular animal phyla. He notes that hard skeletons are not simply additions to preexisting body plans. Their very evolution modifies body plans. Seilacher argues that the Cambrian Explosion was triggered not by environmental conditions that allowed larger animals to develop but by those factors that allowed skeletons to appear. This is a subtle but important distinction. With the ability to produce hard parts, new animal groups could use these hard parts for jaws, legs, or body support, and this enabled them to exploit entirely new ways of life and new environments.
• Attainment of evolutionary thresholds made large size possible.
A second possibility is that evolutionary breakthroughs allowed, for the first time, the advent of large body size. We know that the majority of living organisms up until this time were less than a millimeter long; most were far smaller. Did genetic innovations allow larger body sizes and thus trigger the Cambrian event? Examples of such innovations include more efficient organ systems, such as improved circulatory, respiratory, and excretory systems. Each had to evolve before larger body size was attainable.
• The predation hypothesis
In 1972 paleontologist Steven Stanley (and later Mark McMenamin) proposed that the evolution of predators played a part in stimulating the Cambrian Explosion. Survival was enhanced in those animals that evolved the ability to defend themselves against predators by producing shells, burrowing deeply, or swimming or otherwise rapidly moving away from danger. And these creatures incidentally found themselves in a position to exploit food resources that had been underutilized or had not been utilized at all during the Precambrian. The evolution of shells made possible new forms of filterfeeding, and deep burrowing gave these animals access to new food resources. Cambrian predators thus forced animals to undertake new lifestyles, which turned out to be successful.
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