VI. Earth Life Emergence: Development of Body, Brain, Selves and Societies
5. Multicellular Fauna and Flora Organisms
Parfrey, Laura Wegener and Daniel Lahr. Multicellularity Arose Several Times in the Evolution of Eukaryotes. BioEssays. 35/4, 2013. University of Colorado and University of Sao Paulo system zoologists contribute to findings of a “strikingly similar” impetus and tendency across flora and fauna to evolve and join into increasing complex organismic assemblies. This propensity then converges in a way that repeats in kind the biomolecular mechanisms of its unicellular ancestors.
The cellular slime mold Dictyostelium has cell-cell connections similar in structure, function, and underlying molecular mechanisms to animal epithelial cells. These similarities form the basis for the proposal that multicellularity is ancestral to the clade containing animals, fungi, and Amoebozoa (including Dictyostelium): Amorphea (formerly “unikonts”). This hypothesis is intriguing and if true could precipitate a paradigm shift. However, phylogenetic analyses of two key genes reveal patterns inconsistent with a single origin of multicellularity. A single origin in Amorphea would also require loss of multicellularity in each of the many unicellular lineages within this clade. Further, there are numerous other origins of multicellularity within eukaryotes, including three within Amorphea, that are not characterized by these structural and mechanistic similarities. Instead, convergent evolution resulting from similar selective pressures for forming multicellular structures with motile and differentiated cells is the most likely explanation for the observed similarities between animal and dictyostelid cell-cell connections. (Abstract)
Pennisi, Elizabeth. The Power of Many. Science. 360/1388, 2018. A series of simple steps can explain the momentous transition from single cells to multicellular life. A science journalist gathers the work of Laszlo Nagy, Ben Kerr, Nicole King, Nicholas Butterfield, William Ratcliff and others to report new findings about how this evolutionary ascent unto complex organisms seems meant to readily proceed. An innate natural affinity for rudimentary cells to band together, repurpose, diversify, divide labor, and more so to gain group level benefits appears to be written in.
Pfeiffer, Thomas and Sebastian Bonhoeffer. An Evolutionary Scenario for the Transition to Undifferentiated Multicellularity. Proceedings of the National Academy of Sciences. 100/1095, 2003. The initial phase in this emergence is seen as the formation of simple, undifferentiated cell clusters wherein cooperative behavior became more advantageous for survival than competition.
The first step in the evolutionary transition to multicellularity likely was the evolution of simple, undifferentiated cell clusters….Here we argue that in populations of unicellular organisms with cooperative behavior, clustering may be beneficial by reducing interactions with noncooperative individuals. (1095)
Back to Our Roots.
A News Report chronicles the latest theories on an original “urmetazoan” from which all multicellular life arose. This ancestor had a “toolkit of genes” which specified four features: body-plan genes, specialized cell types, cells “glued” together, and a communication system.
Raff, Rudolf. The Shape of Life: Genes, Development and the Evolution of Animal Form. Chicago: University of Chicago Press, 1996. An often cited work on the stability and ontogenetic recurrence of bodily plans.
Rainey, Paul and Silvia De Monte. Resolving Conflicts During the Evolutionary Transition to Multicellular Life. Annual Review of Ecology, Evolution, and Systematics. 45/599, 2014. By late 2014, Massey University, Auckland, and Ecole Normale Superieure, Paris, biologists are able to well describe this distinct and persistent advance of life’s unicellular phase within the major transitions scale. Two major themes are its achievement by a mutual balance between each component and the whole organism, i.e. competition and cooperation, along with acquiring a novel degree of individuality. See also Katrin Hammerschmidt, et al (2014) for a similar contribution.
Although the evolution both of eukaryotes and of multicellularity mark major transitions between levels of organization, they are often distinguished on the basis of the nature of the alliance among lower-level entities. Evolution of eukaryotic life from two different and once free-living bacteria counts as an “egalitarian” transition characterized by fairness in reproduction and a mutual coming together of disparate entities. The origin of chromosomes from independently replicating genes marks another egalitarian transition. In contrast, the evolution of multicellular life as well as the evolution of eusocial insect societies are “fraternal” transitions, which originate with an alliance of entities that were most likely identical from the outset and in which a division of labor evolved through epigenetic means. (600).
Ratcliff, William, et al. Experimental Evolution of Multicellularity. Proceedings of the National Academy of Sciences. 109/1595, 2012. University of Minnesota, Evolution and Behavior and BioTechnology Institute, ecologists start their project by situating it within the popular “major transitions” scale and sequence from proteins to people. By this view, life’s emergent passage from eukaryotic cells to cellular assemblies was a natural next stage of “sophisticated, higher-level functionality via cooperation among component cells with complementary behaviors.” See herein concurrent work by Iaroslav Ispolatov, et al, Carl Simpson below, and Cooperative Alliances in the Emergent Individuality section.
Multicellularity was one of the most significant innovations in the history of life, but its initial evolution remains poorly understood. Using experimental evolution, we show that key steps in this transition could have occurred quickly. We subjected the unicellular yeast Saccharomyces cerevisiae to an environment in which we expected multicellularity to be adaptive. We observed the rapid evolution of clustering genotypes that display a novel multicellular life history characterized by reproduction via multicellular propagules, a juvenile phase, and determinate growth. The multicellular clusters are uniclonal, minimizing within-cluster genetic conflicts of interest. Simple among-cell division of labor rapidly evolved. Early multicellular strains were composed of physiologically similar cells, but these subsequently evolved higher rates of programmed cell death (apoptosis), an adaptation that increases propagule production. These results show that key aspects ofmulticellular complexity, a subject of central importance to biology, can readily evolve from unicellular eukaryotes. (Abstract, 1595)
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)
Retallack, Gregory. Great Moments in Plant Evolution. PNAS. 118/17, 2021. A University of Oregon paleontologist reviews notable work, as below, with regard to our whole scale reconstruction of how life proceeded through all the fauna and flora to arrive at a global speciesphere whom is able to do this. Our take, as the Abstract cites, is to muse that these past creaturely occasions appear as if they are intended, indeed expected, stages along the ascendant way.
Just as dinosaurs can be regarded as protobirds (1) and synapsids as protomammals (2), understanding the evolution of plants depends on extinct fossil groups, such as those linking spore and seed plants. A profound paleobotanical surprise in 1904 was discovery of pteridosperms, commonly called seed ferns, because of their unexpected combination of seeds and fern-like leaves. Another paleobotanical surprise in 1960 was discovery of progymnosperms, spore plants with woody anatomy comparable to modern conifer trees. Now, Wang et al. in PNAS (118/11) report a Chinese Permian fossil plant, Paratinga wuhaia. These permineralized fossils convincingly expand the progymnosperm clade to include the enigmatic Noeggerathiales, an extinct order of vascular plants.
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)