A New History of Life (21 page)

Ten different Hox genes were all that were necessary to utterly change and diversify the arthropods. Their secret was discovered by comparing the distribution of the product of Hox genes—proteins that are specific to a particular Hox gene—and where these proteins can be found on a developing embryo. The old idea that some gene or genes
of an arthropod coded for the construction of a leg is false. The Hox genes make proteins. These proteins then become the means of starting and stopping the growth of particular regions of a developing embryo. Some of these proteins are concerned with making specific kinds of appendages. If those Hox gene proteins are somehow moved to different geographic regions on the developing embryo, the product that is produced will move as well. In this way a leg that was formerly in one part of the body might suddenly be found in a totally new place—if, however, the Hox gene protein was somehow moved to the corresponding place on the embryo long before the leg was formed. Innovation came from shifting the geographic places or “zones” on an embryo that a specific Hox gene protein could be found in.

Shifting the Hox gene zones in arthropod embryos resulted in the many different kinds of arthropods that we see. There are thousands, perhaps millions of different kinds of arthropod morphologies—and all of this was evolved using the same tool kit of ten genes. Arthropods are nothing if not body plans with repetitive parts. The specialization of these parts requires that each falls into a separate Hox gene zone.

There has been no end of ideas about why there was a Cambrian explosion at all. Sometimes events of the past seem as if they could not have been otherwise. Yet why not a long slow formation of the many animal phyla, instead of the seemingly compressed duration that we do see? And just how diverse were the major animal players in the Cambrian explosion? All of the current animal phyla (variably listed as about thirty-two) first appeared in the Cambrian explosion. Surprisingly, there has not been a single animal phylum added to the world since, even after the devastating Permian extinction of 252 MA. But were there many
more
phyla in the Cambrian than now? Were there strange, fundamentally different kinds of animals in the Cambrian than now? That has been a very contentious issue, culminated in a late 1990s feud
¹⁶
of memorable bile between the late great evolutionist Stephen
Jay Gould and Cambridge University’s Simon Conway Morris, who remains, essentially, Britain’s paleontologist laureate.

In his
Wonderful Life
, Gould asserted that the Cambrian was full of “weird wonders,” which he defined as body plans now longer present on the Earth. His view is that the Cambrian explosion was just that—an explosion of new body types, body plans, numbers of species. But to slightly mix metaphors, most explosions are deadly. In fact many of the new kinds of body plans—in Gould’s view, new kinds of phyla—did not make it out of the Cambrian. Killed by the explosion, but not in the original sense. The effects of the vast increase in kinds of animals killed them by competition. With so many body plans, only some would stand the test of natural selection. Gould’s view is that the diversification of body plans can be modeled by a pyramidal shape; the great diversification of body plans was fast, creating a fat base of the pyramid of numbers of body plans—also known as disparity (the diversity of body plans, not species). But as the Cambrian progressed, that base diminished, until there were far fewer phyla at the end of the Cambrian than soon after its start.

Many others disagreed that disparity has, in fact, increased since the Cambrian. Simon Conway Morris is the leading proponent of this point of view, one that is in direct contradiction to that of Stephen Gould. In Morris’s view, the weird wonders were not separate phyla at all, just early and not yet recognizable members of well-known and still-living phyla. The consensus since this late-twentieth-century argument, one that was heated to unseemly levels between scientists, seems to be that Gould was wrong, and we can add little to this argument. But if this once-boiling scientific dispute has cooled to a low simmer, other aspects of the Cambrian explosion remain frontline science, the best science—controversial science.

The Cambrian explosion was obviously one of the most important and until recently least understood of major events in life’s history as well. Much of the uncertainty came from dating—or lack thereof, at least in any sort of precision—and the older the rock, the greater the
uncertainty. When he first defined the base of the Cambrian as the beds with the first trilobites within them, the early eighteenth century’s Adam Sedgwick had no idea that actual age dating in years—rather than the relative appearance of fossils—would ever become available to his brethren (but we are sure he must have dreamed of the possibility). For almost two hundred years, in fact, an accurate date for the base of the Cambrian was a case in point. A major problem was that it had never really been defined either in biological terms or with respect to the actual rock record, and numerically dated calibration points were few and far between. Unlike a mass extinction event or other biological innovations, the Cambrian radiation did not have a specific obvious well-defined starting point. The global definition of the terms was chosen instead by a special committee of international specialists organized by UNESCO, under the auspices of the International Geological Correlation Program (coauthor Kirschvink was a voting member of this committee).

At issue was the actual position of whatever boundary was to be chosen, and how to date it. By the 1960s and 1970s, age guesses (for they were nearly that) for when the Cambrian explosion happened varied from over 600 million years ago to as young as 500 million years ago. It took the development of incredibly sensitive—and precise—radiometric dating techniques before progress could be made. The problem with dating was that in order to obtain a radiometric age date, volcanic rocks had to be interbedded with the sedimentary beds as ashes, for it is only the volcanic ashes—and only some of them—that contain the mineral zircon (which locks in uranium and lead ratios to form beautiful geological clocks). And almost none of these kinds of beds within beds were known from any Cambrian-aged rocks around the globe.

In an attempt to try something else, a prominent Australian geochronologist named William Compston (at the Australian National University in Canberra) developed a technique in the mid-1900s using rubidium-strontium isotopes in shale (a sedimentary, not volcanic rock) that gave age estimates of 610 million years for the first trilobites in China. We now know that his technique was totally wrong, and that techniques based on dating the mineral zircon with the uranium-lead are the way to go. Nevertheless, until the 1980s the “official” date for
the base of the Cambrian was listed as 570 million years ago, and that date is occasionally still found in many compilations of the geological time scale online and in books.

But the second problem, not “when” so much as “what”—what first or last fossil occurrence should mark the base of the Cambrian—was more intransigent. As noted above, by the 1960s, paleontologists had improved their collecting methods and instrumentation, and it became increasingly clear that in fact a great deal of animal evolution, including animals with hard parts that could and did fossilize, predated the trilobites by great periods of time. The oldest hard-part fossils in strata beneath those with trilobites were tiny but recognizable parts of shells (the “small shelly fossils”). Some looked like tiny spines, some like small snail shells, some simply chunks of what looked like armor from some archaic mollusk or echinoderm. But at question were their actual ages of formation and existence.

International agreement was finally reached
¹⁷
in the early 1990s. Of the four-part appearance of animals known from the fossil record, the first, the Ediacarans, were kicked out of the Cambrian period altogether. Their time received its own name: the newly defined Ediacaran period of the Proterozoic era. The base of the Cambrian System was defined as strata containing the lowest, vertically burrowing trace fossils, thus predating the successive strata with small shelly fossils, which in turn underlay the strata with trilobites. The ability to burrow vertically through sediments is thought to imply the existence of a hydrostatic skeleton and the neuromuscular connections to control it, but this horizon was nearly 20 million years older than the actual Cambrian explosion (as recorded by the fossil record itself). Yet if finally sorted out, the dates when these strata were deposited was still unknown.

Without reliable radiometric dating, the extent of this interval—between the oldest recoverable animal fossils and the first appearance of trilobites—could (in some regions) be measured in tens of thousands of meters of strata between the Ediacarans and trilobites. This suggested that tens of millions of years separated them—but the 1980-era mass spectrometers (the instruments that can determine ages from rocks) needed large numbers of zircons to do the analyses properly.
However, technology advanced, and by the late 1980s, new, better instruments began to be used on the rare but crucial volcanic horizons that occasionally could be found in the sedimentary beds thought to be Cambrian in age. One such locality, discovered long after Sedgwick and all his contemporaries went to the great fossil record in the sky (or wherever paleontologists go), was located in the Anti-Atlas Mountains of Morocco. Here was the potential Rosetta stone for determining the age of the four acts of the Cambrian explosion.

It was in the late 1980s that coauthor Kirschvink collected samples of a volcanic ash from the Anti-Atlas Mountains of Morocco. This ash layer was stratigraphically located about fifty meters below the first occurrence of Cambrian trilobites in this great pile of sedimentary strata. But how long did it take for those critical fifty meters of underwater-derived strata to form? Unfortunately this volcanic ash produced only a tiny number of zircon grains, far too little to be dated using techniques that were conventional at that time. However, by that time Compston had developed an incredible instrument known as the super-high-resolution ion microprobe (SHRIMP), which was able to focus a collimated beam of cesium ions onto a small spot on a mineral grain. The plasma generated by this process was fed into a mass spectrometer, and with a few subtle manipulations they were able to produce an extremely high resolution uranium-lead date.

The result was stunning. The dates emerging for these Morocco samples were about 520 million years, rather than being older than 600 million years in age!
¹⁸
Compston did everything he could to try and make the age older, but it did not work. There was at least an 80-million-year error in the age of the base of the Cambrian. This meant that the Cambrian explosion—at least the massive diversification of the animal phyla that is seen when the first shelly fossils appear—was more like a nuclear explosion, at least twenty-five times faster than supposed. Other groups at MIT (Sam Bowring) and elsewhere have since replicated these findings with additional volcanic ashes from Morocco, as well as others
from exotic places like Namibia and the northern part of the Anabar Uplift in Siberia.
¹⁹
There was now a date for the appearance of the trilobites, and it was far younger than previously supposed. The paleontologists charged with selecting the formal base panicked when they thought the entire Cambrian would be only 10 million years long, so they abandoned the first trilobites as their guide and chose an older event—the first burrowing trace fossil—that ultimately was calibrated at about 542 MA.

It turns out that this unusual interval of the evolutionary activity and innovation has some other rather unusual features as well. Studies of the carbon isotopes across the Proterozoic-Cambrian boundary show that something rather strange was happening, with huge oscillations that lasted for hundreds of thousands to millions of years (these are now known as the Cambrian carbon cycles).
²⁰
The magnitude of these is wild—the equivalent of grinding up and burning all of the existing biomass on Earth every few million years. Either that or something was causing extremely light carbon (which occurs in methane) to erupt into the atmosphere on a massive scale, with all of the associated greenhouse effects. Did the Earth go through a succession of short-term heating events? Mild heating can actually increase biological diversity by shortening generation times—an effect observed in the modern biota. Too much, of course, can be lethal!

Another oddity is that the Cambrian has long been known as having some extremely large apparent plate motion (plates are the enormous sheets of crust that compose the Earth’s surface, and that move, diverge, or collide with other of these Earth tectonic plates). These motions can be tracked using the technique known as paleomagnetism, which can determine ancient latitudes of rocks as well as the directions of plate motions. It was using this tool that coauthor Kirschvink first proved the snowball Earth episodes of previous chapters. New paleomagnetic analyses coming out of multiple paleomagnetism laboratories were showing something seemingly impossible: that the continents were scooting across the surface of the global at great speed—or that the entire globe was rapidly moving under its poles of rotation. The north and south poles were staying where they always were: it was the globe beneath them that was moving.

This information came from samples taken from Australia, for example, indicating that while it straddled the equator, it underwent a nearly seventy-degree clockwise rotation between Early Cambrian and Late Cambrian time—in less than 10 million years, and perhaps much less time than this. However, because Australia was a part of the supercontinent of Gondwana, which included Antarctica, Greater India, Madagascar, Africa, and South America, this rotation must have involved well over half of the continental landmass at the time. Now data from virtually all over Gondwana tell a similar story—it was spinning counterclockwise precisely during the Cambrian explosion interval, 530 to 520 million years ago. Similar results from the large North American continent called Laurentia indicated that it moved from the chilly South Pole all the way up to the equator at essentially the same time.