Category: Papers

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Figure 2 from Bozdag et al. 2026. a, Multicellular groups form either through daughter cells remaining attached after division (clonal multicellularity) or through aggregation of independent cells (aggregative multicellularity). b, Simple relatives of more complex multicellular organisms display diverse group formation mechanisms: S. rosetta forms chain colonies (through intercellular bridges) and rosette colonies (through bridges, ECM and filopodia); C. flexa develops sheets through collar microvilli adhesion; S. arctica develops epithelium-like layers through coenocytic growth and cellularization; T. socialis forms flattened colonies within mother cell walls; E. elegans creates spheroidal colonies through encapsulation within a colonial boundary and production of ECM; and Aspergillus fumigatus develops hyphae through connected septate divisions. c, Laboratory evolution experiments demonstrate rapid, convergent evolution of multicellular groups in yeasts, algae and holozoans under diverse selective conditions. For each experiment, the text under the cartoons shows the fraction of replicate populations that evolved multicellular groups (for example, 10/10 or 2/5), and the average time in generations it took for group formation to evolve. The blue and red circles indicate clonal and aggregative multicellularity, respectively, with some organisms using both strategies.

Abstract:

Dozens of independent origins of multicellularity have profoundly shaped Earth’s evolutionary and ecological history. These transitions have given rise to massive radiations encompassing thousands to millions of species and the evolution of increasingly complex bodies. Multicellular taxa now constitute the majority of Earth’s biomass and have fundamentally transformed global ecosystems, underpinning diverse biomes from forests and grasslands to coral reefs. While these macroevolutionary patterns have long been recognized, the microevolutionary processes driving the transition to multicellularity have remained unclear. Questions remain on how solitary cells form cohesive groups, how these groups become evolutionary units capable of adaptation at the multicellular level, and how novel cell types and developmental patterns emerge. Recent discoveries of previously unknown multicellular species and the development of experimental models have provided important insights into these questions. This work reveals that seemingly insurmountable evolutionary hurdles are often readily overcome either through the repurposing of ancestral unicellular traits or through emergent properties of multicellular collectives that arise spontaneously. Here we review key breakthroughs in our understanding of the microevolutionary processes that underpin the evolution of increasingly complex multicellularity, focusing on mechanisms through which single cells can evolve into functionally integrated multicellular organisms.

Bozdag, G. O., K. Tong, P. J. Yunker, M. D. Herron, and W. C. Ratcliff. 2026. Overcoming key challenges in the evolutionary transition to multicellularity. Nature 658:327–336, doi: 10.1038/s41586-026-11038-w, shareable link: https://rdcu.be/MTdQv9MkJq4a

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Large organism size reduces the fitness costs of somatic specialization. Contour lines show the fold reduction in per-generation exponential growth rate for different organism sizes and germ cell proportions. The black dotted line marks the theoretical minimum of one germ cell; the gray horizontal dotted line illustrates that at a fixed , larger organisms pay lower growth rate costs. Colored symbols show calculated growth-rate costs for volvocine algae species.

Abstract:

The evolution of reproductive specialization, in which somatic cells forfeit reproduction, represents a fundamental innovation in complex multicellular life. This specialization imposes a fitness cost: because somatic cells do not produce offspring, organisms that invest in soma have reduced fecundity. The magnitude of this cost might be expected to depend simply on the proportion of cells allocated to soma. Here, we show that these costs also decrease with the logarithm of organism size, because larger organisms require proportionally more cell divisions for development, diluting the rate at which reproductive costs compound across multicellular generations. We derive this result analytically and validate it with data from the volvocine green algae. When somatic cells provide a compensating survival benefit, a positive feedback emerges: larger organisms can afford greater somatic investment, which in turn favors further size increases. This size-scaling relationship helps explain the broad association between large organism size and multicellular complexity.

Zhang, C., E. Libby, A. Burnetti, M. D. Herron, and W. C. Ratcliff. 2026. The fitness costs of reproductive specialization scale inversely with organismal size. Proceedings of the National Academy of Sciences, USA 123: e2536055123, doi: 10.1073/pnas.2536055123

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The Evolution of Multicellularity, co-edited with Peter Conlin and Will Ratcliff, has been published by CRC Press. It’s available on Amazon, but cheaper to order direct, and for the time being you can save 20% with discount code FLA22 (I don’t know how long that will last).

The goal of this book is to provide an overview of the evolution of multicellularity: the types of multicellular groups that exist, their evolutionary relationships, the processes that led to their origins and subsequent evolution, and the conceptual frameworks in which their evolution is understood. In four main sections, the contributors review the philosophical issues and theoretical approaches to understanding the evolution of multicellularity, the evolution of aggregative multicellularity, the evolution of clonal multicellularity, and the evolution of multicellular life cycles and development. While the subject is too broad to cover in a truly comprehensive way, the contributors have done an outstanding job of synthesizing the critical information on their respective topics. We hope that this book will serve as a starting point for readers interested in the evolution of multicellularity, a reference for researchers on the subject, and a jumping-off point to stimulate future research.

The publisher has put pretty strict limits on what we can share (they want to sell books, after all), so I won’t be posting a downloadable version (I don’t, in fact, have one). However, the Foreword and Chapter 1 (together) can be downloaded for free, and some of the authors posted preprints of their chapters (which the publisher allowed). I have linked to these in the table of contents below. If I learn of others, I’ll update this post.

I’m biased, of course, but I really do think the authors have done an outstanding job with their respective chapters. I hope you think so, too!

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Jim Umen and I have published an article in the newest issue of Annual Review of Genetics. We review some green algae that are or have the potential to be models for the evolution of multicellularity, including Volvox, Ulva, Chara, and Caulerpa. Transitions from unicellular to multicellular (or, in the case of Caulerpa, giant, multinucleate unicellular) have been frequent and varied within the green algae, and we argue that studying diverse examples is necessary to understand how and why these transitions have taken place.

Abstract:

The repeated evolution of multicellularity across the tree of life has profoundly affected the ecology and evolution of nearly all life on Earth. Many of these origins were in different groups of photosynthetic eukaryotes, or algae. Here, we review the evolution and genetics of multicellularity in several groups of green algae, which include the closest relatives of land plants. These include millimeter-scale, motile spheroids of up to 50,000 cells in the volvocine algae; decimeter-scale seaweeds in the genus Ulva (sea lettuce); and very plantlike, meter-scale freshwater algae in the genus Chara (stoneworts). We also describe algae in the genus Caulerpa, which are giant, multinucleate, morphologically complex single cells. In each case, we review the life cycle, phylogeny, and genetics of traits relevant to the evolution of multicellularity, and genetic and genomic resources available for the group in question. Finally, we suggest routes toward developing these groups as model organisms for the evolution of multicellularity.

Umen, J. & M.D. Herron. 2021. Green algal models for multicellularity. Annual Review of Genetics  55:603-632. doi: 10.1146/annurev-genet-032321-091533 Free e-print

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Postdoc Kimberly Chen has published a lay summary of our recent Scientific Reports paper, in which we showed that predation can drive the evolution of multicellularity in the green alga Chlamydomonas:

Multicellular life is one of the most astonishing wonders on Earth, but why and how does it arise in the first place, and at what cost? To help answer these questions, we exposed single-celled algae to predators and watched them evolve into multicellular life. Within a year, they had formed groups of cells to avoid being eaten – but at a price.

Chen, I-C. K. & M. D. Herron. 2019. Predators drive the evolution of multicellularity. The Science Breaker 257. doi: 10.25250/thescbr.brk257

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A new paper describing the results of a yeast evolution experiment has been published in Evolution. Jordan Gulli exposed nascent multicellular “snowflake yeast” to an environment in which solitary multicellular clusters experienced low survival. In response, snowflake yeast evolved to form cooperative groups composed of thousands of multicellular clusters.

Gulli et al. 2019 Fig. 2
Figure 2 from Gulli et al. 2019. Evolution of proteinaceous aggregates that bind many multicellular clusters. When subjected to strong settling selection, snowflake yeast evolved to form cooperative aggregates composed of hundreds of clusters (A). A composite image (B) reveals the aggregates are composed of both protein (C, green, Qubit fluorescent protein stain) and DNA (D, red, propidium iodide). Cells embedded within the aggregate are shown in blue (E, Cell Tracker Blue). Scale bars are 500 μm.

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Our Director of Communications, Maureen Rouhi, has written a press release to accompany the new Scientific Reports paper, “De novo origins of multicellularity in response to predation.”

Coauthors currently at Georgia Tech in front of the Ramblin’ Wreck (left to right): Kimberly Chen, Will Ratcliff, Frank Rosenzweig, and me. Photo by Jennifer Pentz. Not shown: Josh Borin, Jacob Boswell, Jillian Walker, Alex Knox, and Maggie Boyd.

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A new paper describing the results of a microbial evolution experiment has been published in Scientific Reports. Predation by the filter-feeding predator Paramecium tetraurelia drove the evolution of simple multicellular structures in the green alga Chlamydomonas reinhardtii:

Herron et al. 2019 Fig. 2
Figure 2 from Herron et al. 2019. Depiction of C. reinhardtii life cycles following evolution with (B2, B5) or without (K1) predators for 50 weeks. Categories (A–D) show a variety of life cycle characteristics, from unicellular to various multicellular forms. Briefly, A shows the ancestral, wild-type life cycle; in B this is modified with cells embedded in an extracellular matrix; C is similar to B but forms much larger multicellular structures; while D shows a fully multicellular life cycle in which multicellular clusters release multicellular propagules. Representative microscopic images of each life cycle category are at the bottom (Depicted strain in boldface).

From the abstract:

Here we show that de novo origins of simple multicellularity can evolve in response to predation. We subjected outcrossed populations of the unicellular green alga Chlamydomonas reinhardtii to selection by the filter-feeding predator Paramecium tetraurelia. Two of five experimental populations evolved multicellular structures not observed in unselected control populations within ~750 asexual generations. Considerable variation exists in the evolved multicellular life cycles, with both cell number and propagule size varying among isolates. survival assays show that evolved multicellular traits provide effective protection against predation. These results support the hypothesis that selection imposed by predators may have played a role in some origins of multicellularity.

Herron MD, Borin JM, Boswell JC, Walker J, Knox CA, Boyd M, Rosenzweig F, Ratcliff WC. 2019 De novo origins of multicellularity in response to predation. Sci. Rep. 9, 2328. (doi: 10.1038/s41598-019-39558-8)