by Peter Ward
Switches are the key here; they tell various parts of the body when and where to grow. One of the great discoveries is that the exact sequence of different body regions on an arthropod from its head to midregion to abdomen are lined up first on chromosomes in the same geographic pattern, and then on the developing embryo itself. Much of this is done by the crown jewels of the evo-devo kingdom: the Hox genes, and their differently named but equivalents in other taxonomic groups.
The many new discoveries of evo-devo have certainly been brought to bear on the many questions to be solved about that central mystery in the history of life, the Cambrian explosion, and the most important understandings of all: the timing of when and how the various animal phyla and thus separate body plans that we see today originated.
There have long been two schools of thought. The first is that the fossil record gives us a true picture of when the great differentiation of animals actually took place—phyletic divergence somewhere about 550 to perhaps 600 million years ago. But the second line of evidence comes from comparing genes of extant members of the ancient phyla, and using the concept of the “molecular clock,” mentioned earlier. At issue is when the most fundamental divisions in the animal kingdom take place—the split between an aggregate of phyla called protostomes and those called deuterostomes. These two groups are separated by fundamental anatomical and developmental differences in embryos.
The protostomes are composed of the arthropods, mollusks, and annelids among others, and they are characterized by embryos that as they develop and grow following fertilization form a mouth out of a central opening in the growing larva called the blastopore. In deuterostomes (echinoderms, us vertebrates, and a number of minor phyla), the mouth and the blastopore remain separate. There is a third group, the very primitive phyla that split off from the main stem of animal evolution prior to the great protostome-deuterostomes split: these include the Cnidaria, sponges, and other jellyfish-like minor phyla.
The first to appear were the simplest forms, the cnidarians and sponges, which appear to be represented, as we have seen, in the Ediacaran assemblages of as much as 570 million years ago, the time interval before the Cambrian period (which began at 542 million years ago). But recognizable protostomes and deuterostomes are not seen until a short interval into the Cambrian period itself.
If the protostomes and deuterostomes split, what was the last animal before that split like? Many lines of evidence indicate that this creature was bilaterally symmetrical and was capable of locomotion. Many who ponder this time and its animals imagine this last common ancestor of both the protostomes and deuterostomes as a small featureless worm, perhaps like the modern-day Planaria, or the tiny and extant nematodes. But one of the great new discoveries is that this last member of the as yet undivided stock already had a genetic tool kit allowing it to begin some radical new engineering—and had such a tool kit for at least 50 million years before it was put into use! This worm would have had a mouth at front, anus at the rear, and a long tubelike digestive system in between. It may have had stubby projections sticking out of its side, perhaps for sensory information (touch and chemical sensing?). But the point is that all of this was set up in such a way that rapid transformation could—and did—take place. This is new. All the tools and features necessary for the Cambrian explosion sat around for 50 million years.
As noted above, the base of the Cambrian is dated now at 542 million years ago. The base of the period has been defined as the place in rock where the first identifiable locomotion marks are found in strata—a certain kind of trace fossil showing that animals, moving animals, were present and could make vertical burrows in the mud. Yet for the next 15 million years, there seems to have been little formation of new body plans at all—or at least that we can find evidence of in the fossil record. The first real indication that a great diversification was taking place comes from the spectacular fossil beds only recently discovered in Chengjiang, China,15 dated as 520 to 525 million years in age and mentioned above. It is an older version of the Burgess Shale in having common preservation of soft parts.
Both the Chengjiang and Burgess Shale faunas are dominated by arthropods—lots and lots of different kinds of arthropods. They soon became the most diverse animals on Earth—and have stayed that way ever since. There are some estimates that in our modern day, there may be as many as 30 million separate species of beetles alone!
Evo-devo tells us why. Of all the body plans, none can be so easily, quickly, and radically changed as arthropods. The reasons are just those listed above by Carroll: arthropods have modular parts, they have redundant morphologies that can be co-opted for new functions, and they have a series of Hox genes that allow ready transformation of specific regions in the overall body plan of segments throughout.
The old view has been that new animals mean that there must have been new genes coming into existence. There is sound logic in this. Surely a primitive sponge or jellyfish would have fewer genes than the more complex arthropods: it was argued that the common ancestor of all arthropod groups somehow added new genes—new Hox genes, as these are those that are the “switches” that tell the various parts of a body how to form and when. But such is not the case. Carroll and others showed that the last common ancestor of the arthropods did not evolve new genes; it already had them, and that the subsequent and amazing diversification of so many kinds of arthropods was done with existing genes. As Carroll put it: “The evolution of forms is not so much about what genes you have, but about how you used them.”
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.
STEPHEN GOULD VS. SIMON CONWAY MORRIS: THE SHAPE OF DISPARITY
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 feud16 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.
NEW DATING OF THE CAMBRIAN EXPLOSION
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 reached17 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.
AGE BREAKTHROUGH—AND AGE SURPRISE
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 explosi
on—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.