Origin and Early Evolution of Animals

by

Kevin J. Peterson

• LAST REVIEWED: 13 January 2014
• LAST MODIFIED: 13 January 2014
• DOI: 10.1093/obo/9780199941728-0043

Introduction

Coming at the end of one of the most intensive glaciation periods in Earth history (glaciers at sea level on the equator!), the explosive rise of animals 530 million years ago is one of the few major events in the history of life that combines the evolution of novel developmental regulatory circuitry with the emergence of unique environmental circumstances. This “Cambrian explosion” is truly a remarkable event in the history of life, as the Earth’s biota went from an essentially static system billions of years in existence to the one we enjoy today, a dynamic and awesomely complex system that forever changed the Earth’s biota, and ultimately the Earth itself. Part of the intrigue with the Cambrian explosion is that numerous phyla with very distinct body plans (whether arthropod, annelid, mollusk, echinoderm, or chordate) arrive on the scene in a geological blink of the eye, with little or no warning of what is to come in rocks that predate this interval of time. The abruptness of the transition between the “Precambrian” to the Cambrian was apparent early on with the publication of Murchison’s The Silurian System (1839), and is still apparent today. Indeed, the abruptness of the explosion has only gotten more pronounced since Murchison’s time as more and more of the Earth’s geological record has been explored, and accurate and precise dates have been placed on many of these outcrops. The books and papers in this article highlight the multifarious nature of the origin and early evolution of animals, ranging from morphology, to phylogeny, to paleontology, to developmental biology, and even to geochemistry and molecular biology.

The Cambrian Explosion

As with most things in ecology and evolution, the first statement of the problem that the origin of animals had for modern evolutionary theory can be found in Darwin 1859. Since that time, though numerous “precambrian” forms were described, they were virtually all debunked in Cloud 1968, making the starkness of the Cambrian explosion very real. Runnegar 1982 in many ways defined the essence of the problem that the Cambrian explosion represents, and concluded, based on early molecular clock dates, that the Cambrian explosion likely represents the first appearance of fossils. In contrast, Budd and Jensen 2000 argued that bilaterians were not likely older than about 555 Ma, and thus the Cambrian explosion records the evolution of animals themselves and not just the first appearance of animal fossils. Gould 1989 also saw the Cambrian explosion as an evolutionary event. Erwin, et al. 2011, using a molecular clock, substantiated Runnegar’s general thesis by showing that the origin of bilaterian animals was nearly 200 million years before the Cambrian explosion itself. Nonetheless the explosion is still very real in terms of the ecological revolution that takes place (Erwin and Valentine 2013), a revolution that some, for example Knoll 2003, see as driven by the advent of relatively higher oxygen levels.

• Budd, G. E., and S. Jensen. 2000. A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews of the Cambridge Philosophical Society 75:253–295.

  DOI: 10.1017/S000632310000548X

  Here, Budd and Jensen argue that in essence the Cambrian explosion simply reflects the origin of bilaterian animals themselves, but that the origin of the “body plans” of most phyla evolved later in geological time. Further, they argue that because the last common ancestor of bilaterians possessed coelom(s), the trace fossil record then constrains the age of this clade to younger than 555 Ma.

• Cloud, P. E. J. 1968. Pre-metazoan evolution and the origins of the Metazoa. In Evolution and environment. Edited by E. T. Drake, 1–72. New Haven, CT: Yale Univ. Press.

  Preston Cloud, by rejecting numerous instances of purported Precambrian metazoan fossils, defined the problem of the Cambrian explosion by showing that authentic metazoan remains seemed restricted to Phanerozoic rocks.

• Darwin, C. 1859. On the origin of species. London: John Murray.

  In chapter 9 (pp. 279–311) Darwin considers the known fossil record. He goes on to detail why the fossil record is likely incomplete, not preserving the ancestors of these animals. Then, using this insight, he addresses the problem of the Cambrian explosion (p. 306) and suggests that the ancestors of Cambrian animals swarmed the Precambrian seas but simply had not (yet) been found. Republished by Harvard Univ. Press, Cambridge, MA, 1964.

• Erwin, D. H., M. LaFlamme, S. M. Tweedt, E. A. Sperling, D. Pisani, and K. J. Peterson. 2011. The Cambrian conundrum: Early divergence and later ecological success in the early history of animals. Science 334:1091–1097.

  DOI: 10.1126/science.1206375

  Erwin, et al. present the latest molecular clock estimates for animal diversification and the newest compendium of the known fossil record calibrated to the current geochronological record. These authors suggest that because the origin of bilaterian metazoans predates the Cambrian explosion by tens of millions of years, the explosion must largely be the result of novel ecological interactions within permissive environmental circumstances.

• Erwin, D. H., and J. W. Valentine. 2013. The Cambrian explosion: The construction of animal biodiversity. Greenwood Village, CO: Roberts.

  An outstanding treatment of the multifarious suite of problems and questions that constitute the research avenue that is the Cambrian explosion. For each of the sections in this article, the interested reader should also consult the specific chapter for a synthetic, up-to-date, and highly readable treatment.

• Gould, S. J. 1989. Wonderful life. New York and London: W. W. Norton.

  Gould argues that the Cambrian was a special time in animal evolutionary history in that body plans were highly plastic, but quickly settled into relatively few major design themes, what we today call “phyla.” Further, Gould tackles the theme of “contingency,” such that if the tape of Earth’s history were rewound (the source of the title of the book), a different set of winners might populate the world’s oceans.

• Knoll, A. H. 2003. Life on a young planet. Princeton, NJ, and Oxford: Princeton Univ. Press.

  Andy Knoll, one of the leading researchers on the Cambrian explosion and the evolution of life, places the Cambrian explosion in the context of Earth history and highlights many of the critical features about the explosion. A highly entertaining and informative read.

• Runnegar, B. 1982. The Cambrian explosion: Animals or fossils? Journal of the Geological Society of Australia 29:395–411.

  DOI: 10.1080/00167618208729222

  A masterly treatment of the metazoan fossil record: Runnegar defined the problem of the Cambrian explosion for the modern era and suggested that a sharp rise in the level of atmospheric oxygen played a crucial role in the appearance of animals.