A New History of Life (22 page)

At this point, the god of simplicity appeared; perhaps it was not a bunch of little tectonic plates moving around, but everything on the sphere moving together, relative to the spin axis. However, this would work only if Laurentia and Australia were roughly ninety degrees away from each other at the time (which—duh—had to be true if Australia was on the equator and Laurentia on the pole!). In fact, this single-rotation hypothesis makes very specific predictions about the relative orientation and configuration of all continental landmasses, an “absolute paleogeography.” With apologies to Tolkien, “One motion to move them all, one rotation to spin them, one translation from the pole, and on the globe we’ll find them!” One simple rotation of the entire solid Earth around the spin axis brought ~90 percent of the previously scattered paleomagnetic results into a clear focus.

Everything was happening at once. A big pulse of evolution, both in the number of species and in the body plans, a huge increase in biomineralization (the number and different kinds of outer skeletons evolved by many different phyla), the first predator-prey interactions among animals, huge swings in the organic carbon budgets, and wild oscillations in the positions of the continents, leaving scientists including Kirschvink and his students to ponder whether this was coincidence or cause and effect.

As more and more paleomagnetic evidence began to accrue, not only surprising but also downright impossible motions of the ancient plates (with their entombed continents fixed in oceanic crust) were detected. Uniformitarianism tells us to use the modern to understand the past, and today we can readily measure how fast plates are currently moving. In the Atlantic, where new oceanic crust is being created along the Mid-Atlantic Ridge, the rate at which the two plates bisecting the North and South Atlantic Oceans are slowly moving away from the axial origin line is only about one inch per year. These enormous plates, while created at the oceanic spreading centers, hold the continents in their stony embrace—so as (and where) the plates go, the continents go as well. The rates are varied. For instance, today there are much faster rates seen in the plates being created in the Pacific Ocean area, with speeds of three to five inches per year. The fastest possible rate is close to ten inches per year, but even this is theoretical and controversial. Yet paleomagnetic data were yielding speeds that measured multiple
feet
per year: This is impossible if only plate tectonics was involved. Yet the data are repeatable and stark. Something revolutionary took place, or at least something so different from modern processes as to cause enormous surprise in science. So much for the Principle of Uniformitarianism!

The first reaction to encountering this data suggesting such fast motion of the surface of the Earth was to doubt the reality of the data. Fair enough. As Carl Sagan once said, extraordinary (scientific) claims required extraordinary proof. Continental motions were so fast that normal plate tectonic motions noted above, typically at most a few inches per year, could not explain them. The new data, slowly but inexorably being produced by Kirschvink and a few others, were showing that the plates were moving too fast for the conventional theory of plate tectonics. To top it all: most of this motion was happening precisely coincident with the explosive increase in diversity from the animal phyla. If it was not plate tectonics, what could it have been? And how could this affect the evolution of animals?

The answer was a surprise—but should not have been, because a similar process is known to have occurred on Mars, the moon, and
many satellites and minor planets for billions of years. Such bodies are capable of astounding changes in orientation. On Earth, the consequences for life may have been inestimable, and yet our dawning comprehension of this possibility is one of the great new revolutions brewing in our understanding of the history of life.

For over a century, geophysicists have known that the solid parts of a planet can move rather rapidly relative to the spin axis. The fundamental principle is that a spinning object would like to rotate around something called its maximum moment of inertia. A Frisbee is a good example: When thrown properly, it spins about the center point, and most of the mass at the edge of the disc keeps it rotating stably. But now put a small hunk of lead somewhere on the disk—but not at its center. The spin of the Frisbee will change as it tries to reorient itself to take this new mass situation into account, and the Frisbee will try to spin with the new heavy mass as far away from the rotational axis as possible: it wants to go to the equator. On a spinning planet, centrifugal and gravitational forces similarly tug on any anomalous mass. But on a spinning sphere, a much more orderly change takes place—the position of the spin axis will reorient so that the “weight,” which may have been located, perhaps, two thirds of the way from the equator to the pole, will not be at the equator. The spinning ball has had its axis of rotation change position—because of the strange new weight that was added.

It is very well known that the moon and Mars have both realigned themselves in this way during their geological history. Both had new masses added to their surface that originally were
not
on their equators, but then somehow ended up on the equator. For example, the gigantic Tharsis province (a geological region on the Martian surface) is composed of an enormous quantity of heavy lava. In terms of geological time it was just like the weight we added to a Frisbee or a spinning ball; it was added after the formation of the planet. In fact, it is the largest positive gravity anomaly in the solar system and lies precisely on the Martian equator—now, that is. On the moon, the pre-
Apollo
surveys detected mass concentrations associated with the lunar mare basalts, also on the equator. These processes are fairly simple to understand on the moon and on Mars, because neither of these objects have
plate tectonics. This realignment process is been termed true polar wander, or TPW. Prior to the discovery of plate tectonics back in 1966, all evidence for the poles being at different positions at earlier geological time was thought to have been a result of TPW.

A geologically rapid change of mass on a planet can happen in a number of ways, including the impact of a big asteroid or comet, or even internally through the eruption of magma from the deep interior of Earth to its surface. Similarly, big mass shifts can happen when one of the parts of the plate tectonic features, which are composed of spreading centers and, separately, subduction sites (where a plate dives down back into the deep Earth), either appear or disappear. Both of these are large enough to excite TPW as far as the Earth is concerned, as long as the masses involved are being maintained actively, not just floating buoyantly. But if they disappear, it will affect the orientation of the planet. Both subduction zones and spreading centers can disappear when one continent undergoing continental drift runs into another. Any offshore spreading centers or subduction zones between the converging continents are destroyed in the collision; only in this case it is the disappearance of a surface mass that causes reorientation, not the addition of a mass.

As it is rather unlikely that the observed biological changes associated with the Cambrian explosion were forcing the continents to move, a more plausible explanation was that the unusual burst of motion was somehow accelerating the pace of biological evolution. Several mechanisms have been discovered that might explain and connect some of these observations. First, when continents are at high latitudes they tend to build up large reservoirs of frozen methane known as clathrates, or gas hydrates, on the seafloor and in permafrost. As these areas move toward the equator, they will warm gradually and can episodically cause pulses of greenhouse gas emissions into the atmosphere, periodically warming the environment. Evolution and species diversification in particular tend to proceed faster in warmer environments, through a mechanism of accelerated metabolism.

At the time this was proposed in the literature, it was nicknamed the “methane fuse for the Cambrian explosion,” and argued that thermal
cycling of biological diversity may have been one of the major factors in promoting the proliferation of species. It is also a possible cause for the crazy oscillation in the carbon isotopes. It also turns out that geographically higher diversities exist naturally in the equatorial zone. When Ross Mitchell, a colleague of ours at Yale University, looked at the paleogeographic motions during this true polar wander event, he noticed that almost all of the newly evolving animal groups seemed to originate on the leading edges of the continents as they moved into the equatorial zone, with few to none originating as other areas moved in the high latitudes. This increase of diversity with latitude provides a stunningly simple explanation for the diversity increase, particularly if this happened when nature was experimenting with body plans via the Hox genes. It also implies that the paleontological record of this Cambrian explosion might be partially an artifact—because a side effect of true polar wander is to produce relative marine transgressions in areas moving onto the equator, and sea-level regressions in areas moving off. Sediments are preserved best during transgressions and removed during regressions. Hence, the rock record is biased during TPW events to preserve rocks that are recording a diversity increase.

Invoking true polar wander as a cause of events in the history of life is definitely a new field of research, unheard of in the twentieth century. Just as this mechanism is being used here as a new hypothesis for the Cambrian explosion, so too can TPW be used to try to explain the killing mechanisms in mass extinctions, one of which ended the Cambrian period and Cambrian explosion, killing off almost all the weird wonders described by Stephen Gould and Simon Conway Morris from the Burgess Shale. This mass extinction was given the unlikely name of SPICE.

Any history recounting the Cambrian explosion can be overwhelmed by the sheer power and importance of animal body plan
evolution—the radical change of the world’s biota from the immobile, floating, and simple larger animals of the latest Precambrian world to the diverse exuberance of the animal cargo inhabiting the world’s oceans at the end of the Cambrian. But why is there an “end of the Cambrian” at all? Here is a topic where a long-held understanding has been toppled.

Mass extinctions, the short-term periods of great mortality among both individuals and species, were variable in their severity. While the largest are included in the “big five” mass extinctions, when at least 50 percent of species died out, there were many more extinction events not as catastrophic (unless one was one of the victims, that is). One of the most celebrated of these occurred at the end of the Cambrian period.

The Late Cambrian mass extinction was actually three or four separate, smaller extinction events, mainly affecting trilobites and other marine invertebrates, especially brachiopods. It has long been accepted that these were caused by increases in warm, low-oxygen water masses affecting marine communities. Some of the earliest occurring of all trilobites, the olenellids, underwent total extinction, and in fact the entire nature of the trilobite faunas changed: trilobites of the Cambrian had many segments, primitive eyes, and no obvious defensive adaptations on the body (such as antipredatory spines), and were unable to do what modern-day pill bugs do when threatened—roll up into a tight ball. After the extinctions, and thus in the earliest times of the Ordovician period, newly evolving waves of trilobites had changed their entire body plan: now virtually all reduced the number of segments (many segments are easier for predators to break through during a predatory attack than fewer, thicker body segments) and had better eyes, defensive armor, and especially the ability to roll up into pill bug–like balls.

Warmth, low oxygen, and faunal change: that was the view of these late Cambrian extinctions. But then an entirely new series of data were recovered suggesting quite the opposite: evidence for cold water, not warmer; and evidence as well of a major burial of organic matter into the oceans—a process that caused oxygen levels to skyrocket. These changes are now named the SPICE event (after the
Steptoean positive carbon isotope excursion). But there is a huge contradiction with this new finding. It was first identified in the rock record not only because of a sudden extinction of species, but also as a major perturbation of the carbon isotope record, and hence carbon nutrient cycling. There is quite good evidence that a large percentage of trilobites died out in a succession of short-lived extinctions near the end of the Cambrian period.

One of the most interesting aspects of the SPICE event is that unlike most other mass extinctions, this one might have been accompanied not by a drop but by a short-term rise in oxygen. It is intriguing to speculate that a known volcanic eruption at about this time might have caused one of the short-term rapid continental movements mentioned above—a TPW, or True Polar Wander event. In this case more land area was moved
into
the tropics for a few million years, increasing carbon burial and spiking the atmospheric oxygen up to previously unheard-of levels. Something like
that
might have paved the way for the next major radiation of life following the Cambrian Explosion. One kind of ecosystem needs a great deal of oxygen. The coral reefs appeared soon after the SPICE event, starting in the next geological period, the Ordovician.