Causes of the Cambrian Explosion M. Paul Smith1, David A. T. Harper2 Science Podcast: 20 September Show Science 20 September 2013: 1409. Many hypotheses have been invoked to explain the rapid diversification of animal species in the early Cambrian (541 million to 515 million years ago), ranging from starbursts in the Milky Way to intrinsic genomic reorganization and developmental patterning. Recent hypotheses for the Cambrian explosion fall into three main categories: developmental/genetic, ecologic, and abiotic/environmental, with geochemical hypotheses forming an abundant and distinctive subset of the last (1). Most of these hypotheses have been posited as stand-alone processes that were the main cause of the explosion, yet many of them are tightly interlinked and codependent. The rapid diversification of animals in the early Cambrian is likely to have been the result of a complex interplay of biotic and abiotic processes (see the first figure). One challenge relates to the precise definition of the explosion. Is it the first appearance of animal groups, their diversification, the emergence of marine ecosystems with “modern” trophic structures, or all of these? The timing of the diversification of animal groups is now fairly well known, allowing a clear distinction to be made between the first appearances of high-level animal crown groups in the Neoproterozoic (1000 million to 541 million years ago), followed by the main diversification of animal groups, a substantial increase in morphological disparity, and the emergence of complex food webs in the early Cambrian (2–4). Molecular clock estimates predict that the earliest members of many animal groups, including sponges, cnidarians, and bilaterians, lived 850 million to 635 million years ago. Yet molecular clocks and the fossil record together indicate that more than 100 extant animal phyla and classes first appeared in the Cambrian; only a handful predate the start of the Cambrian. Two events are thus distinguishable, with the origin of high-level animal groups temporally distant to the abrupt increase in diversity and disparity within the Cambrian—the Cambrian explosion in the strict sense (see the second figure). One essential component of the Cambrian explosion is the advent of bilaterian developmental systems. Bilaterians are animals with a longitudinal plane of symmetry and specialized internal organ systems, and include most living animals with the notable exceptions of sponges, cnidarians, and some minor groups. It has been argued that the origin of the bilaterian gut and the ability to feed on large prey items (macrophagy) around 650 million years ago in turn enabled the evolution of large body sizes and skeletons in response to seabed predation pressures (5). This ignores, however, an apparent >100-million-year gap between the evolutionary innovation and its consequences. Developmental systems must have been in place to enable the macroevolutionary cascade, but the clues for the causes of the Cambrian diversification must lie closer to 540 million years. By then, stem bilaterians had already evolved the developmental tool kit to exploit the complex mosaic of opportunities that arose (6). With macrophagy in place, the emergence of complex food webs was a crucial driver for diversity increase in the Cambrian explosion (1, 7), partly because of the tendency of burrowing organisms to modify the physicochemical properties of their substrates (ecosystem engineering) (8). In addition, it has been argued that the inherent “evolvability” of bilaterian animals and their tendency to induce escalatory arms races may account for much extant diversity (7). These types of feedback are manifest in the origins of planktic and free-swimming animals, burrowing, and biomineralization, which initially were new evolutionary products but rapidly became entrained in the diversification. Earth system, developmental, and ecological processes have been hypothesized as isolated, singular causes of the major diversification of marine taxa early in the Cambrian. Instead, many of these processes sit within a series of cascading and nested feedback loops that together generated the Cambrian explosion. Each box corresponds broadly to a stand-alone hypothesis or suite of related hypotheses (red, geological; blue, geochemical; green, biological). The figure represents a narrow interval of time at the beginning of the Cambrian (541 million to 521 million years ago). In the case of biomineralization, the presence of feedback loops is evidenced by the near-simultaneous appearance of both predatory and defensive hard tissues across a wide range of animal groups (9, 10). These hard tissues mainly consist of two types of calcium biomineral, suggesting that the availability of calcium is an important aspect of the event. It has been argued that the emergence of complex food webs is the result of crossing a threshold or tipping point (7), but it may have been an end product of complex feedback loops (see the first figure). Recently, attention has returned to abiotic processes as a possible cause of the Cambrian explosion. A long period of Neoproterozoic erosion had resulted in very low-relief continental interiors with highly weathered crystalline basement rock at the surface, together with associated soils (regolith). Major sea-level rise in the early, but not earliest, Cambrian led to the flooding of these interiors and triggered a range of Earth system responses (11), including the extensive erosion and mobilization of weathered rock and regolith and the rapid input of calcium (11), phosphate (12, 13), and other ions into the oceans. Calcium concentrations in seawater increased almost threefold in the early Cambrian, and this input may have directly facilitated the origin of biomineralization (14). The input of phosphate provided simultaneous nutrient flux to shallow-water areas (12, 13). Each hypothesis outlined here is a viable mechanism for increasing mean species diversity within habitat, differentiation between habitats, and/or total regional biodiversity. However, it is unlikely that any single casual mechanism can explain the Cambrian explosion, with many of the individual hypotheses instead acting as components of interacting feedback loops between Earth systems and biological processes. Together, these interacting processes generated an evolutionary cascade that led to the rapid rise in diversity. The initiating event is likely to have been the early Cambrian sea-level rise that led to inundation of continental margins and interiors and the rapid input of erosional by-products (11). This sea-level rise would also have generated a very large increase in habitable area lying between the base of wave turbulence and the depth to which light penetrates, providing a further driver for large increases in diversity. These early events then segue into the complex interaction of abiotic and biotic processes shown in the first figure. It would be valuable, therefore, to model the diversification in a holistic and interdisciplinary way, rather than focusing on individual causal factors. A substantial challenge then lies in determining the relative position of each component process upstream or downstream in the cascade of events that produced the Cambrian explosion. Figure View larger version: In this page In a new window Download PowerPoint Slide for Teaching Times of change. The major diversification of marine taxa at high taxonomic levels between 635 and 443 million years ago [after (2)]. The red box indicates the time interval discussed in the text. EB, Ediacaran biota. References ↵ C. R. Marshall, Annu. Rev. Earth Planet. Sci. 34, 355 (2006). CrossRefWeb of Science ↵ D. H. Erwinet al., Science 334, 1091 (2011). Abstract/FREE Full Text D. H. Erwin, J. W. Valentine, The Cambrian Explosion: The Construction of Animal Diversity (Roberts, Greenwood Village, CO, 2013). Search Google Scholar ↵ C. J. Lowe, Science 340, 1170 (2013). Abstract/FREE Full Text ↵ K. J. Petersonet al., Paleobiology 31 (suppl.), 36 (2005). Abstract/FREE Full Text ↵ D. H. Erwin, E. H. Davidson, Development 129, 3021 (2002). Abstract/FREE Full Text ↵ N. J. Butterfield, Trends Ecol. Evol. 26, 81 (2011). CrossRefMedlineWeb of Science ↵ R. H. T. Callow, M. D. Brasier, Earth Sci. Rev. 96, 207 (2009). CrossRefWeb of Science ↵ D. J. E. Murdock, P. C. J. Donoghue, Cells Tiss. Organs 194, 98 (2011). CrossRefMedlineWeb of Science ↵ R. Wood, A. Y. Zhuravlev, Earth Sci. Rev. 115, 249 (2012). CrossRefWeb of Science ↵ S. E. Peters, R. R. Gaines, Nature 484, 363 (2012). CrossRefMedlineWeb of Science ↵ P. J. Cook, J. H. Shergold, Nature 308, 231 (1984). CrossRef ↵ M. D. Brasier, R. H. T. Callow, Mem. Ass. Austral. Palaeontol. 34, 377 (2007). Search Google Scholar ↵ S. T. Brennan, T. K. Lowenstein, J. Horita, Geology 32, 473 (2004). Abstract/FREE Full Text The editors suggest the following Related Resources on Science sites sciencemag.org/content/341/6152/1355.full
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