New perspectives on Cambrian Explosion: construction of the first animal consumer-driven marine ecosystem on Earth
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Abstract:
The Cambrian Explosion represents one of the most profound phases in the history of life, with nearsimultaneous emergence of most modern animal phyla, including all Bilateria. Since this time, the proliferation of animals across all ecospaces, including terrestrial environments, suggests that the Earth has become increasingly habitable for animal life. Along with the sudden appearance of essentially all the readily fossilized major animal groups, the Cambrian Explosion (540–510 Ma) also resulted in the first appearance of macro-consumers and complex trophic interactions, and established patterns of emergence and diversification that define the modern animal tree of life. Following the Cambrian, despite numerous major geological events and the migration of both plants and animals into the terrestrial realm, there is no apperance of new animal phyla. The Cambrian Explosion can therefore be considered a unique event in the history of life, representing both a key transition in global habitability and the starting point for animal consumer-driven marine ecosystems. Previous work by multiple research teams, from both China and abroad, has explored the Cambrian Explosion from many different perspectives, and has markedly increased our understanding of this major evolutionary event. Based on this work, combined with research progress on Precambrian biotas, this paper proposes three macroevolutionary phases in the evolution of early life. Each phase is defined by both organismal “hierarchical level” and geological settings, with the Cambrian Explosion representing the culmination of the three phases. These three phases are: molecular-level evolution during the origin of early continents, cellular-level evolution along with the supercontinent evolution of Nuna (Columbia) and Rodinia, and tissue-level evolution coupled with the early assembly and breakup of Pangea. At the earliest stages of Earth’s history, macro-level prokaryotes, derived from protocell-forming molecules, first formed (Molecular Phase). The first appearance of eukaryotic cells, derived from prokaryotes, represents the next milestone in biological evolution (Cellular Phase). The subsequent emergence of multicellular eukaryotes (metazoans) represents the third and final phase (Tissues Phase). Combined with the advent of multicellularity comes a revolution in body-plan organization, facilitated by the innovative capacity to develop specialized tissues. This includes the spontaneous development of mineralized epithelial exoskeletons and connective tissue endoskeletons. Based on these developments, a ‘Lego Blocks’ hypothesis for the Cambrian Explosion is proposed, driven by the initiation of the modern plate tectonic regime characterized by full-plate deep subductions and the associated changes in Earth’s multi-sphere interactions, and bio-litho-atmospheric cycles during Rodinia breakup and Gondwana assembly. During this period of geological disruptions, the diversity of marine niches expanded, resulting in both accelerated phylogenetic diversity and ecological expansion, as organisms developed both a wide range of specialized biological tissues and tissue arrangements to exploit the newly available vacant niche space. The failure of new animal phyla to emerge following the Cambrian Explosion can thus possibly be explained by both a maximization of potential organismal complexity and the relative stabilization of ecospaces. Following the breakup of the Neoproterozoic supercontinent Rodinia, major palaeogeographic changes during the Phanerozoic are limited to the opening and closing of the NeoPaleo-Tethys Ocean, neither of which created novel ecological niches not previously in existence.
