The Evolution of Animals Biological Breakthrough or Environmental Stimulus

Complex animals surely cannot appear on any planet without following some evolutionary pathway from simpler, single-celled organisms. The change from single-celled microbes to multicellular creatures must be the common route on any planet, and even if the molecules of life are different from world to world, the pathway from simple to complex may be universal. Because of this, the example of how animals evolved on our Earth may be of the utmost importance in understanding the frequency with which animals occur on other planets.

If we are to understand how animals evolved from single-celled ancestors, we must first understand the environments where these monumental evolutionary advances were made. We know well the "when" of this change— it took place during a 500-million-year interval from 1 billion to 550 million years ago. The second event, the Cambrian Explosion of between 550 and 500 million years ago, included the morphological diversification of the phyla into subdivisions based on body plans, as well as the appearance, within the various phyla, of species with skeletons and large size (see Figure 5.3).

During this interval of time, Earth went though major environmental changes, among them ice ages of unprecedented severity, rapid continental movements, and drastic changes in ocean chemistry. We are thus left with perplexing questions: Did the environmental changes of this interval (which are described in more detail below) somehow trigger the diversification of an-

Million Millior

Million Millior

Figure 5.3 Differing views of metazoan phylogeny. Most paleontologists follow the "traditional view" (left), accepting the fossil record as a fairly reliable indicator of original events. Molecular clocks are interpreted by Wray et al. (center) as indicating very deep origins for the principal metazoan phyla. The recognition that some molecular clocks run much faster than others suggests a "compromise view" (right), which implies that our search strategy for the first metazoans should be concentrated in the interval from about 750 million years onward. Perm, Permian, Carb, Carboniferous, Dev, Devonian, Sil, Silurian, Ord, Ordovician, Cam, Cambrian.

Figure 5.3 Differing views of metazoan phylogeny. Most paleontologists follow the "traditional view" (left), accepting the fossil record as a fairly reliable indicator of original events. Molecular clocks are interpreted by Wray et al. (center) as indicating very deep origins for the principal metazoan phyla. The recognition that some molecular clocks run much faster than others suggests a "compromise view" (right), which implies that our search strategy for the first metazoans should be concentrated in the interval from about 750 million years onward. Perm, Permian, Carb, Carboniferous, Dev, Devonian, Sil, Silurian, Ord, Ordovician, Cam, Cambrian.

imals? or would the rise of animals have occurred even in the absence of these profound environmental changes? These questions, which of course are central to understanding the evolution of life on our planet, have great relevance to understanding the frequency of animal life on other planets as well. Does animal life always (or even commonly) evolve, once a suitable ancestor appears? Or does there need to be an additional trigger of some sort, a sequence of environmental steps? We might compare this whole process to baking a cake. Say the ingredients for the batter were all assembled and mixed by 1 billion years ago. Does the cake need to be cooked for a given time at a highly restricted temperature in order to rise? or will any amount of cooking at any temperature accomplish the task just as well? Or will our cake be completed without any cooking at all? (That is, does simply assembling the ingredients into a batter ensure success?)

The beginning of this fecund period in Earth history is marked by the appearance not of new types of animals, but of plants. Around 1 billion years ago, many types of algae begin to appear in the fossil record, including the green and red algae still so prominent on Earth today. These were not the ancestors of animals, of course, but their appearance was the opening salvo of an evolutionary assault that was the most significant up to that time. It was followed, hundreds of millions of years later, first by the initial diversification of animal phyla and then (after more hundreds of millions of years) by the Cambrian Explosion of animal life.

What were the environmental events of this interval of time 1 billion to 600 million years ago? By this period, land masses approaching the size of today's continents had formed, and the total area of land on the planet may not have been significantly different from what we see in the present day. The land, however, was not a tranquil place. The period was one of significant mountain building and continental drift. It was also marked by episodes of continental glaciation unmatched in severity since that time. Did these events have anything to do with the diversification of animals? One school of thought says yes. Work by Martin Brasier and others suggests that rapid changes in sea level, and especially the formation of broad, shallow seas within the new continents, would have opened up many new habitats very hospitable in terms of temperature and nutrients. This, in turn, may have stimulated the diversification of animals and plants. There are dissenters, notably James Valentine, who cautions, "the link between plate tectonics . . . and the origins and radiation of animals remains to be demonstrated." But, as Harvard paleobiologist Andy Knoll points out, there is another way in which the new and active tectonic events could have influenced the initial radiation of animals that occurred during this time. In 1995 Knoll noted that "tectonic processes could have influenced one or more of the great radiations (of animals) . . . through their participation in the biogeochemical cycles that regulate Earth's surface environments."

Examples of such effects include the role of hydrothermal influences on ocean chemistry. The hydrothermal vents, as we saw in Chapter 1, are submarine regions where great volumes of hot and chemically distinctive water are mixed with seawater. The amount of this volcanically derived water entering the oceans fluctuated during the interval of 1 billion to 550 million years ago, and these fluctuations had marked effects on the chemistry of the seawater, on the composition of the atmosphere, and on climate. The tectonics events also affected the rate of burial and exhumation of organic carbon in sediments. Oxygen and carbon dioxide values shifted, and as they did so, major changes in the temperature and oxygenation of the planet ensued.

Yet another environmental stimulus may also have contributed to the initial animal diversification. Changes in ocean chemistry caused by increased tectonic activity beginning a billion years ago facilitated the evolution of skeletons. This period is marked by the appearance of rocks called phosphorites. Some authors credit these rocks with bringing about an increase in the fertility of the oceans at this time, which may in turn have helped trigger the sudden appearance of many diverse animals beginning about 600 million years ago. Phosphorus is much more concentrated in living things than in the environment, so it is a limiting nutrient. The sudden presence of abundant sources of this element could have acted as a veritable fertilizer for growth.

Knoll has discussed all of these disparate factors and has proposed three alternatives. First, it may be that the complex physical events and the equally complex series of biological events that occurred from 1 billion to 550 mil lion years ago are simply coincidental—they had nothing to do with one another. If this is true, the great biological diversification must be attributed solely to biological innovations (such as the ability of cells to bind together, build an outer cell wall, and evolve internal cooperation between contained cells) that were unrelated to concurrent environment changes.

The second alternative is that evolution was indeed facilitated by changes in the physical environment. The most important of these changes may have been in levels of oxygen. The first appearance of larger metazoans, the ediacarans, about 600 million years ago occurred immediately after a sudden increase in atmospheric oxygen (evidence for this comes from stable isotopes). Thus it may be that the initial animal diversification of around 700 million years ago was itself a response to the oxygen level reaching some critical threshold.

The third alternative is that the biological revolutions themselves somehow triggered some of the physical events—just the opposite of alternative two! In this scenario, the common use of calcium carbonate shells by newly evolved animals changed the way calcium was distributed in the oceans. Similarly, organisms may have favored the formation of phosphorus, not the other way around: The presence of many organisms may have changed the physical chemistry of the ocean environment, boosting the formation of this mineral type.

Knoll leans toward the last alternative. He stresses that the first major evolutionary radiation among protists and algae (about 1 billion years ago) may have occurred because of the first evolution of sexual reproduction. The invention of sex, rather than an environmental trigger, stoked the fires of diversification. But Knoll also acknowledges the central role of oxygenation in the evolution of larger animals. Without oxygen, larger animals could never have evolved, and oxygenation during this interval was facilitated by tectonic processes—specifically, the role of changes in sea level and erosion of continents in complex geochemical cycles. For a variety of physiological reasons, oxygen is a key to the appearance of larger animals; the metabolism of animals requires oxygen.

Indeed, we may well ask whether oxygenation, and hence the rise of animals, would ever have occurred on a world where there were no continents to erode. Perhaps "water worlds" are ultimately inimical to animal life. But there may have been even more sudden and catastrophic changes than those listed by Knoll—most important, dramatic changes in planetary temperature. Evidence uncovered in the late 1990s has led to a radical new concept: that Earth almost completely froze over at least twice in its history—once 2.5 billion years ago and a second time (perhaps repeatedly) during the interval from about 800 to some 600 million years ago. These times of intense global cold, when even the oceans were covered with ice, are known as Snowball Earth. Their biological significance is explored in the next chapter.

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