Scientists discover that "evolutionary big bang" may have been caused by Earth losing its balance half a billion years ago

Dr. Joseph L. Kirschvink Professor of Geobiology BS, MS, Caltech 1975; MA, Princeton University, 1978; PhD, 1979. Assistant Professor, Caltech, 1981-87; Associate Professor, 1987-92; Professor, 1992- Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander by Joseph L. Kirschvink, Robert L. Ripperdan, and David A. Evans Related Links: Figures, graphs, and quicktime movies Dr. Joseph Kirschvink David Evans Caltech Paleomagnetics Laboratory. Division of Geological and Planetary Sciences. California Institute of Technology. Caltech Office of Media Relations Science Magazine		PASADENA--Researchers at the California Institute of Technology think they have solved part of the mystery of the "evolutionary big bang" that occurred half a billion years ago. At that time, life on Earth underwent a profound diversification that saw the first appearance in the fossil record of virtually all animal phyla living today. With relative evolutionary rates of more than 20 times normal, nothing like it has occurred since. In a paper published in the July 25th issue of Science, the Caltech group reports that this evolutionary burst coincides with another apparently unique event in earth history--a 90-degree change in the direction of Earth's spin axis relative to the continents. Dr. Joseph Kirschvink, a geologist at Caltech and lead author of the study, speculates that a major reorganization of tectonic plates during latest Precambrian time changed the balance of mass within the Earth, triggering the reorientation. Thus, the regions that were previously at the north and south poles were relocated to the equator, and two antipodal points near the equator became the new poles. "Life diversified like crazy about half a billion years ago," says Kirschvink, "and nobody really knows why. It began about 530 million years ago, and was over about 15 million years later. It is one of the outstanding mysteries of the biosphere. "The geophysical evidence that we've collected from rocks deposited before, during, and after this event demonstrate that all of the major continents experienced a burst of motion during the same interval of time." David Evans, a co-author on the paper and graduate student at Caltech, notes that it is very difficult to make large continents travel at speeds exceeding several feet per year; typical rates today are only a few inches per year. "Earth has followed a 'plate-tectonic speed limit' for the past 200 million years or so, with nothing approaching the rates needed for this early Cambrian reorganization." Evans said. "Some other tectonic process must have been operating that would not require the continents to slide so rapidly over the upper part of Earth's mantle." In fact, geophysicists have known for over half a century that the solid, elastic part of a planet can move rapidly with respect to its spin axis through a process known as "true polar wander." True polar wander, Kirschvink explains, is not the same as the more familiar plate motion that is responsible for earthquakes and volcanism. While the latter is driven by heat convection in Earth's mantle, true polar wander is caused by an imbalance in the mass distribution of the planet itself, which the laws of physics force to equalize in comparatively rapid time scales. During this redistribution, the entire solid part of the planet moves together, avoiding the internal shearing effects which impose the speed limit on conventional plate motions. (While his happens, of course, the entire Earth maintains the original spin axis in relation to plane of the solar system.) Thus, true polar wander can result in land masses moving at rates hundreds of times faster than tectonic motion caused by convection. An analogy of the effect can be seen by cementing lead weights at the antipodal (or opposite) ends of a basketball. If the ball is then set on a slick floor and spun with the weights along the equator, the ball will spin in a manner as one would normally expect, with the weights remaining on the equator. If the ball is spun on one of the lead weights, however, the axis of rotation will tend to migrate until the weights are again on the ball's equator. In this configuration, the spinning ball has aligned its maximum moment of inertia with the spin axis, as required by the laws of physics. As for astronomical evidence that such a phenomenon can occur, the authors point to Mars. Along the equator of the Red Planet is a gigantic volcano known as Tharsis, which is known to be the largest gravity anomaly in the solar system. Tharsis could have formed on the equator, but more likely formed elsewhere on the planet and then migrated to the equator via true polar wander because of rotational torques on its excess mass. Something similar must have happened to Earth, says Kirschvink. At about 550 million years ago, 20 million years before the evolutionary burst, one or more major subduction zones in the ancient oceans closed down during the final stages of assembly of the supercontinent of Gondwanaland, leading to a major reorganization of plate tectonic boundaries. Geophysicists have known for many years that this type of reorganization could, in theory, yield a sharp burst of true polar wander. In particular, if Earth were slightly "football shaped," with a major and stable mass anomaly on the equator and a more equal distribution of mass elsewhere, only slight changes of the smaller masses would be needed to produce large motions. A burst of motion up to 90 degrees in magnitude could even be generated if the maximum moment of inertia (about which the planet spins) became less than the intermediate moment (which is always on the equator). The massive plate motions observed by the Kirschvink group fit the predictions of this "inertial interchange" event rather closely. Over the 15 million year duration of this true polar wander event, the existing life forms would be forced to cope with rapidly changing climatic conditions as tropical lands slid up to the cold polar regions, and cold lands became warm. "Ocean circulation patterns are sensitive to even slight changes in the location of the continents," says co-author Robert Ripperdan, a geochemist at the University of Puerto Rico and a Caltech alumnus. "A progressive shift of this magnitude could cause oceanic circulation patterns to become rather unpredictable, jumping from one semi-stable configuration to another on a million-year time scale. "Imagine the havoc which would result in Europe if the Gulf Stream were to disappear suddenly. These jumps offer an explanation for yet another unique mystery of the Cambrian explosion, which is a series of nearly a dozen large swings in the marine record of carbon isotopes. "Repeated changes in global oceanic circulation patterns should ventilate organic carbon buried in the deep oceans, producing these carbon wiggles," Ripperdan says. "We used to think that they were somehow due to repeated expansion and contraction of the entire biosphere, but no one could think of a mechanism to do that. All of the evidence suddenly makes sense with this true polar wander model." But what caused the evolutionary burst? Kirschvink notes that these global shifts in oceanic circulation will also act to disrupt regional ecosystems, breaking them down into smaller, more isolated communities. "Evolutionary innovations are much more likely to survive in a small, inbreeding population, rather than in large, freely interbreeding groups," he notes. "And the carbon cycles are telling us that major changes in ocean circulation happened about every million years or so. That is certainly enough time for natural selection to weed through the fragments left by the last disruption, and to form new, regional-scale ecosystems. "Then, Wham! They're hit again and the process repeats itself. That is a great script for increasing diversity, particularly as it seems to have happened shortly after the evolution of major gene systems which regulate animal development." The end result was that evolution proceeded nearly 20 times faster than its normal rate, and the life of the planet diversified into many groups still living today. Kirschvink and his collaborators base their conclusions on data collected from 20 years of work on numerous well-exposed sections of the Precambrian-Cambrian and Cambrian-Ordovician eras. By studying the weak fossil magnetism (paleomagnetism) left in many rocks as they form using ultrasensitive superconducting magnetometers, they can recover the direction of the ancient geomagnetic field. This provides information concerning the direction of ancient north, for the same reason that a small hand-held magnetic compass can be used to find the approximate north direction today. This remanent magnetism can also provide an estimate of the ancient latitude in which the sediments were deposited, as the inclination or dip of the magnetic field changes smoothly with latitude--it points vertically at the poles and is horizontal (tangent to the earth's surface) on the equator. Therefore, the fact that magnetic materials are found pointing in other directions is evidence that the ground itself has moved in relation to Earth's magnetic North, which is locked over time to the spin axis. Geological samples collected by the Caltech group in Australia (which has some of the best-preserved sediments of this age from all of Gondwanaland) demonstrate that this entire continent rotated counterclockwise by nearly 90 degrees, starting at about 534 million years ago (coincident with onset of the major radiation event in the Early Cambrian), and was finished sometime during Middle Cambrian time. North America, on the other hand, moved rapidly from a latest Precambrian position deep in the southern hemisphere, and achieved a position straddling the equator before the beginning of the Middle Cambrian, about 518 million years ago. Even the type of marine rocks deposited on the various continents--carbonates in the tropics, and clays and clastics in high latitudes--agree with these paleomagnetically-determined motions. The paleomagnetic directions are accurate within about 5 degrees, the authors write. Latitudes are quite reliable, but because the poles moved so rapidly, even the relative longitude between blocks can be determined. This true polar wander analysis predicts a unique "absolute" map of the major continental masses during this event, an animation of which can be viewed at http://www.gps.caltech.edu/~devans/iitpw/science.html "This hypothesis relating abrupt changes in polar wander to evolutionary innovations could be tested in many ways," notes Kirschvink, "as there are some interesting events in the paleontological record during the following 200 million years which might have been triggered by similar processes. "There's lots of work to do."