VI. Earth Life Emergence: Development of Body, Brain, Selves and Societies
5. Multicellular Fauna and Flora Organisms
Ravasz, E., et al. Hierarchical Organization of Modularity in Metabolic Networks. Science. 297/1551, 2002. The universal properties of scale-free systems apply in developing cellular organization which forms many functional modules arranged in interconnected webs with a nested hierarchical iteration.
Here, we show that the metabolic networks of 43 distinct organism are organized into many small, highly connected topologic modules that combine in a hierarchical manner into larger, less cohesive units, with their number and degree of clustering following a power law. (1551)
Reinhard, Christopher, et al. Earth’s Oxygen Cycle and the Evolution of Animal Life. Proceedings of the National Academy of Sciences. 113/8933, 2016. A senior team of geoscientists including Douglas Erwin, and edited by Andrew Knoll, quantify life’s billion year span before the Cambrian profusion some 600 million years ago. Our rare planetary surface balance of both sea and land is seen as a crucial reason that Earth life has developed beyond an anoxic marine phase which is not conducive.
Earth is currently the only planet known to harbor complex life. Understanding whether terrestrial biotic complexity is a unique phenomenon or can be expected to be widespread in the universe depends on a mechanistic understanding of the factors that led to the emergence of complex life on Earth. Here, we use geochemical constraints and quantitative models to suggest that marine environments may have been unfavorable for the emergence and large-scale proliferation of motile multicellular life for most of Earth’s history. Further, we argue that a holistic evaluation of environmental variability, organismal life history, and spatial ecological dynamics is essential for a full accounting of the factors that have allowed for the emergence of biological complexity on Earth. (Significance)
Rokas, Antonis. The Origins of Multicellularity and the Early History of the Genetic Toolkit for Animal Development. Annual Review of Genetics. 42/235, 2008. A Vanderbilt University biologist reports the latest findings for the persistent, multiple appearance of cellular organisms from biomolecular substrates, which is then seen to convey group survival advantages. But as usual mechanical terms prevail, along with no inkling of a deeper source which could drive such increasing complexity. A icrobiology blog on the article takes Rokas to task for using the phrase “reasons” for multicellularity – to wit, don’t you know there are no reasons for anything. Not to be on his case, since this is the prevailing paradigm, but a 2012 blog citation on Rokas' website is "The Global Proteome Machine."
Multicellularity appeared early and repeatedly in life's history; its instantiations presumably required the confluence of environmental, ecological, and genetic factors. Comparisons of several independently evolved pairs of multicellular and unicellular relatives indicate that transitions to multicellularity are typically associated with increases in the numbers of genes involved in cell differentiation, cell-cell communication, and adhesion. Further examination of the DNA record suggests that these increases in gene complexity are the product of evolutionary innovation, tinkering, and expansion of genetic material. (Abstract, 235)
Ruiz-Trillo, Inaki and Aurora Nedelcu, eds. Evolutionary Transitions to Multicellular Life. Berlin: Springer, 2015. As an example of how much the major transitions model is now accepted and availed to structure and explain life’s episodic ascent, the many chapters herein explore its presence and effect across all manner of microbial forms such as biofilms and algae. The main sections are Multicellularity in the Tree of Life, Model Systems, Theoretical Approaches, Genomic Insights, and Molecular Mechanisms. See also a companion Springer volume Multicellularity: Origins and Evolution edited by Karl Niklas and Stuart Newman (2016).
The book integrates our understanding of the factors and processes underlying the evolution of multicellularity by providing several complementary perspectives (both theoretical and experimental) and using examples from various lineages in which multicellularity evolved. Recent years marked an increased interest in understanding how and why these transitions occurred, and data from various fields are providing new insights into the forces driving the several independent transitions to multicellular life as well as into the genetic and molecular basis for the evolution of this phenotype. The ultimate goal of this book is to facilitate the identification of general and unifying principles and mechanisms. (Publisher)
Ruiz-Trillo, Iraki, et al. The Origins of Multicellularity: a Multi-taxon Genome Initiative. Trends in Genetics. 23/3, 2007. A multi-author paper that lays out the project introduced in the abstract below.
The emergence of multicellular organisms from single-celled ancestors – which occurred several times, independently in different branches of the eukaryotic tree – is one of the most profound evolutionary transitions in the history of life. These events not only radically changed the course of life on Earth but also created new challenges, including the need for cooperation and communication between cells, and the division of labor among different cell types. However, the genetic changes that accompanied the several origins of multicellularity remain elusive. Recently, the National Human Genome Research Institute endorsed a multi-taxon genome-sequencing initiative that aims to gain insights into how multicellularity first evolved. This initiative (which we have termed UNICORN) will generate extensive genomic data from some of the closest extant unicellular relatives of both animals and fungi. (113)
Sachs, Joel and James Bull. Experimental Evolution of Conflict Mediation Between Genomes. Proceedings of the National Academy of Sciences. 102/390, 2005. Transitions to a new level of biological complexity requires cooperation among members, but a selection for individual advantage would seem to thwart that. In this study, an evolutionary life cycle for two bacteriophages inherently produced a salutary balance of interaction and independence. These findings, along with evidence from other stages, infer an innate propensity in emergent nature for cooperative behavior.
Specifically, the two phages evolved to copackage their genomes into one protein coat, ensuring cotransmission with each other and virtually eliminating conflict. (390) Our results parallel a variety of conflict mediation mechanisms existing in nature: evolution of reduced genomes in symbionts, cotransmission of partners, and obligate coexistence between cooperating species. (390)
Schopf, William. Cradle of Life. Princeton: Princeton University Press, 1999. A chatty exposition about Precambrian cellular organisms as “the discovery of earth’s earliest fossils,” with paleontologist Schopf as a major player.
Sebe-Pedros, Arnau, et al. The Origin of Metazoa: A Unicellular Perspective. Nature Reviews Genetics. 18/8, 2017. Not only has multicellularity evolved independently multiple times in eukaryotes, but each of these transitions also occurred at different time in the history of life. (499) Akin to van Gestel and Tarnita 2017, Weizmann Institute of Science, University of Queensland, and University of Barcelona researchers provide a wide and deep study of life’s persistent tendency to evolve into more complex unitary organisms. With an emphasis on a “unicellular urmetazoan genome,” a common cyclical process can be identified that occurs each many times. Figure 5 depicts “the origin of multicellularity as a transition from temporal to spatiotemporal cell differentiation.” Phylogenetic relationships of unicellular Holozoa and animals are then traced from fungi to mammals and we human beings whom collectively (another MET) in retrospect are able to learn all this. Some 150 references are cited in support.
The first animals evolved from an unknown single-celled ancestor in the Precambrian period. Recently, the identification and characterization of the genomic and cellular traits of the protists most closely related to animals have shed light on the origin of animals. Comparisons of animals with these unicellular relatives allow us to reconstruct the first evolutionary steps towards animal multicellularity. Here, we review the results of these investigations and discuss their implications for understanding the earliest stages of animal evolution, including the origin of metazoan genes and genome function. (Abstract)
Simpson, Carl. The Evolutionary History of Division of Labour. Proceedings of the Royal Society B. 279/116, 2012. A Duke University biologist contributes to the recognition that living systems at any and all scales evolve and emerge in complexity and cognizance by virtue of a reciprocal diversification of functional roles, as they proceed with a further formation of a bounded, individuated whole.
Functional specialization, or division of labour (DOL), of parts within organisms and colonies is common in most multi-cellular, colonial and social organisms, but it is far from ubiquitous. Several mechanisms have been proposed to explain the evolutionary origins of DOL; the basic feature common to all of them is that functional differences can arise easily. These mechanisms cannot explain the many groups of colonial and social animals that exhibit no DOL despite up to 500 million years of evolution. Here, I propose a new hypothesis, based on a multi-level selection theory, which predicts that a reproductive DOL is required to evolve prior to subsequent functional specialization. I test this hypothesis using a dataset consisting of the type of DOL for living and extinct colonial and social animals. The frequency distribution of DOL and the sequence of its acquisition confirm that reproductive specialization evolves prior to functional specialization. A corollary of this hypothesis is observed in colonial, social and also within multi-cellular organisms; those species without a reproductive DOL have a smaller range of internal variation, in terms of the number of polymorphs or cell types, than species with a reproductive DOL. (Abstract, 116)
Smith, Cameron, et al. Emergence of Self-Reinforcing Information Bottlenecks in Multilevel Selection. arXiv:1506.04611. As the Abstract explains Smith, Albert Einstein College of Medicine, with Matthieu Laneuville and Nicholas Guttenberg, Earth-Life Science Institute, Tokyo, provide more insights into evolution’s persistence to recurrently form into more complex, inclusive, viable organisms. See similar works by Kapheim, Rehan, Van Gestel, and others as this transitions model grows in veracity.
We explain how hierarchical organization of biological systems emerges naturally during evolution, through a transition in the units of individuality. We will show how these transitions are the result of competing selective forces operating at different levels of organization, each level having different units of individuality. Such a transition represents a singular point in the evolutionary process, which we will show corresponds to a phase transition in the way information is encoded, with the formation of self-reinforcing information bottlenecks. We present an abstract model for characterizing these transitions that is quite general, applicable to many different versions of such transitions. As a concrete example, we consider the transition to multicellularity. Specifically, we study a stochastic model where isolated communities of interacting individuals (e.g. cells) undergo a transition to higher-order individuality (e.g. multicellularity). This transition is indicated by the marked decrease in the number of cells utilized to generate new communities from pre-existing ones. In this sense, the community begins to reproduce as a whole via a decreasing number of cells. We show that the fitness barrier to this transition is strongly reduced by horizontal gene transfer. These features capture two of the most prominent aspects of the transition to multicellularity: the evolution of a developmental process and reproduction through a unicellular bottleneck. (Abstract)
Smith, Felisa, et al. Body Size Evolution Across the Geozoic. Annual Review of Earth and Planetary Sciences. 44/523, 2016. A dozen scientists from geologists to paleontologists at American universities including Daniel McShea take a billion year retrospective review of life’s complex anatomy and physiology development across the bacteria, archaea, and eukarya domains. Re the second quote, it is evident that this course indeed concludes with a global humanity whom altogether can reconstruct its ancient heritage.
Sogabe, Shunsuke, et al. Pluripotency and the Origin of Animal Multicellularity. Nature. 570/519, 2019. Nine University of Queensland biologists including Sandie and Bernard Degnan contribute to a revisionary understanding of how organisms got going on their evolutionary way. Instead of a single step via clumped unicells, ancestral ur-cells are seen to differentiate at various stages in a life cycle before actual multicellularity. This effect is conveyed by the term pluripotency for cellular material capable to developing into several forms such as stem cells. See a commentary on this work as Scientists Debate the Origin of Cell Types in the First Animals by Jordana Cepelewicz in Quanta Magazine (Online July 17, 2019).
A widely held but rarely tested hypothesis for the origin of animals is that they evolved from a unicellular ancestor that structurally resembled modern sponge choanocytes and choanoflagellates. Here we test this by comparing the transcriptomes, fates and behaviours of the three primary sponge cell types. Together, these analyses argue against homology of sponge choanocytes and choanoflagellates, and the view that the first multicellular animals were simple balls of cells with limited capacity to differentiate. Instead, our results are consistent with the first animal cell being able to transition between multiple states in a manner similar to modern transdifferentiating and stem cells. (Abstract excerpt)