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Appearance of multicellular organisms

^ Appearance of oxygen in atmosphere ■4 Origin of photosynthesis

< Anaerobic metabolism

First living things

Chemical evolution

Consolidation of Earth

Figure 6.1 Time line of major evolutionary events in the history of the Earth. Width of figures representing the respective kingdoms indicates the relative amount of diversity. The density of shading in the "Oxygen" column indicates the concentration of oxygen in the atmosphere. [After Hickman, Roberts, and Larson (1993)]

Appearance of multicellular organisms

^ Appearance of oxygen in atmosphere ■4 Origin of photosynthesis

< Anaerobic metabolism

First living things

Chemical evolution

Consolidation of Earth

Figure 6.1 Time line of major evolutionary events in the history of the Earth. Width of figures representing the respective kingdoms indicates the relative amount of diversity. The density of shading in the "Oxygen" column indicates the concentration of oxygen in the atmosphere. [After Hickman, Roberts, and Larson (1993)]

1. Trace fossils that look like burrows and tracks have been found in rocks as old as 1.5 billion years, but the animal origins of these are controversial. For example, any trace fossil in rocks older than about 1.2 billion years cannot have arisen from an animal, since the first Metazoa did not appear until then. Other record of this time indicates that burrows, and the burrowing animals that made them, were rapidly becoming more complex. For example, simple impressions were among the earliest trace fossils, left by animals resting on a soft mud. Very early in the Precambrian, these simple fossils were supplanted by tracks made by ambulatory animals slithering across soft bottom muds. After this, horizontal tunnels and short vertical tunnels began to appear. Finally, these were supplanted by vertical tunnels that penetrate deeper into muds. Some were simple vertical shafts, but complex networks of branching tunnels, chambers, and conduits also began to appear.

The Burrowing "Arms Race"

What drove this diversification and elaboration of Precambrian burrows? There are (no surprise, this) lots of possible explanations. One favored by many paleontologists suggests it was driven by the emergence of entirely new body plans, which gave organisms entirely new ways to dig in mud (Box 6A). This set in motion a self-perpetuating dynamic relationship between the burrowing animals and the new macropredators trying to eat them, a relationship that has been called an evolutionary arms race. The concept of an arms race is unsettlingly familiar to us. Having seen at least one, the nuclear arms race, play itself out, we can identify some common features of such contests. One is the evolution of increasingly complex methods of attack and defense, whereby an aggressive measure promotes a defensive counter-measure, which in turn promotes a new aggressive measure—a counter-counter-measure, if you will. The nuclear arms race, for example, began with the simple goal of supposed trace fossils have nonbiological explanations: they might be desiccation cracks or crystallization structures arising in drying sediments. The oldest trace fossils that nearly all agree were left by animals are circular impressions left in muds, designated by paleontologists as Bergaueria, which probably are impressions of sea anemone-like creatures resting on soft sediments. Bergaueria have not been found in sediments older than about 700 million years.

dropping really powerful bombs on enemies. From there, it evolved into a complex, almost baroque dance of intelligence and counter-intelligence, better warheads and guidance systems, more precisely targeted projectiles with explosive yields more precisely calibrated to specific military objectives, culminating in the 1980s in the remarkable Strategic Defense Initiative by the United States. Nuclear war is now so complex that we have had to take the job of killing large numbers of people and destroying their cities and crops away from human beings and turn it over to computers (although it sadly must be said that the more personalized forms of barbarism are alive and flourishing).

If the proverbial archaeologist from Mars came down in several thousand years and tried to piece together the history of the twentieth century from the artifacts we left behind, I suspect he would have no trouble doing it: the nuclear arms race has left its fingerprints all over our society, our culture, our thinking. Similarly, the paleontologist sometimes sees fossil artifacts of an ever-escalating cycle of measure and counter-measure among animals. This seems to be the case for the Precambrian macropredators and their prey. One of the first defensive measures was an increased body size—it is harder for a predator to choke down a body bigger than its own, so getting big is an obvious defense. Getting bigger still if you are a predator is an equally obvious counter-measure. Another defensive measure is evident in the appearance of the "shelly fauna," animals that secreted external armor in the form of calcite or silica shells. These were met by counter-measures from the predators, like the development of hard mouth parts that could crush or drill through their prey's defenses. These, in turn, were met by counter-counter-measures from the prey, like spines or other devices, and so on, and so on, culminating in the origin of the major animal phyla at the beginning of the Cambrian.

Increasingly diverse types of burrows probably were part of this Precambrian arms race. The best evidence for this proposition comes from a count of new types

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