The rise of complex multicellularity: towards a comparative theory of biological complexity

Seminar organized by Susana Coelho et Dolf Weigers from 6th to 11th of April 2026.

Participants

Agathe CHAIGNE (Utrecht University – Utrecht, Pays-Bas), Gautam DEY (European Molecular Biology Laboratory – Heidelberg, Allemagne), Alba DIZ-MUÑOZ (European Molecular Biology Laboratory – Heidelberg, Allemagne), Georg HOCHBERG (Marburg University – Marburg, Allemagne), Alfonso MARTINEZ ARIAS (Institute of Evolutionary Biology – Barcelona, Espagne), Laszlo NAGY  (HUN-REN Biological Research Center – Szeged, Hongrie), Naomi NAKAYAMA (Okinawa Institute of Science and Technology – Okinawa, Japon), Inaki RUIZ-TRILLO (Institute of Evolutionary Biology – Barcelona, Espagne), Berend SNEL (Utrecht University – Utrecht, Pays-Bas), Renske VROOMANS (Sainsbury Laboratory, University of Cambridge – Cambridge, Royaume-Uni), Dolf WEIJERS (Wageningen University – Wageningen, Pays-Bas), Susana COELHO (Max Planck Institute for Biology Tuebingen – Tubingen, Allemagne), Núria ROS (Institut Pasteur – Paris, France), Máté VIRÁGH (HUN-REN, Biological Research Centre, Institute of Biochemistry – Szeged, Hongrie), Zohar MEIR (Gregor Mendel Institute for Molecular Plant Biology – Wien, Autriche).

Summary

Eukaryotic cell and diverse organisms

The seminar « The Rise of Complex Multicellularity: Uncovering Core Principles through Deep Lineage Comparisons » brought together researchers studying complex multicellularity in animals, plants, fungi and algae, together with experts in biophysics, evolutionary biology, comparative genomics and theoretical modelling. Combining scientific presentations with extensive discussion sessions, the meeting highlighted the absence of a common conceptual framework for comparing the independent origins of complex multicellularity across eukaryotes. These exchanges culminated in the development of a comparative roadmap centred on three complementary levels – ancestral states, evolutionary trajectories and regulatory lock-ins, providing a general framework for understanding how biological complexity emerges, stabilizes and evolves across deeply divergent lineages.

Résumé

Le séminaire « L’émergence de la multicellularité complexe : mise en lumière des principes fondamentaux par des comparaisons approfondies de lignées » a réuni des chercheurs étudiant la multicellularité complexe chez les animaux, les plantes, les champignons et les algues, ainsi que des spécialistes de biophysique, de biologie évolutive, de génomique comparative et de modélisation théorique. Alternant présentations scientifiques et longues discussions collectives, la rencontre a permis d’identifier l’absence d’un cadre conceptuel commun pour comparer les origines indépendantes de la multicellularité complexe. Les échanges ont conduit à l’élaboration d’une nouvelle feuille de route comparative fondée sur trois niveaux d’analyse — les états ancestraux, les trajectoires évolutives et les verrouillages régulateurs (« regulatory lock-ins ») — offrant un cadre général pour étudier l’émergence, la stabilisation et l’évolution de la complexité biologique à travers les différentes lignées eucaryotes.

Report

How did complex multicellular life arise? This deceptively simple question lies at the heart of evolutionary biology. Complex multicellularity evolved only a handful of times during the history of life, giving rise independently to animals, land plants, fungi, brown algae and red algae. Yet each of these lineages followed its own evolutionary trajectory over hundreds of millions of years, developing distinct body plans, developmental strategies and molecular toolkits. Despite decades of remarkable progress within individual fields, there is still no unified framework that allows these independent origins to be compared in a meaningful way.

This question formed the focus of the Fondation des Treilles seminar The Rise of Complex Multicellularity: Uncovering Core Principles through Deep Lineage Comparisons. Bringing together developmental biologists, evolutionary biologists, physicists, theoreticians and computational scientists, the meeting sought to move beyond traditional lineage-specific perspectives and ask whether common principles underlie the repeated emergence of biological complexity.

The seminar combined scientific presentations with extensive discussion sessions, a format designed to maximize interaction across disciplines. The first part of the meeting was organized around invited talks, during which participants presented recent unpublished advances from their laboratories, providing a unique opportunity for open scientific exchange. In parallel, a set of major conceptual questions had been identified in advance by the organizers and served as the basis for structured discussions throughout the week. The final day was devoted entirely to synthesizing these exchanges, identifying common themes, and drafting the outline of a collaborative white paper that will form the basis of a forthcoming perspective article.

A defining feature of the seminar was the diversity of biological systems represented. Discussions ranged across animals, land plants, fungi, brown algae and their unicellular relatives, with each participant bringing the conceptual tools and experimental approaches of their own field. Rather than emphasizing differences between these systems, the discussions explored how they could be compared within a common evolutionary framework. Throughout the week, it became increasingly apparent that the greatest obstacle is no longer the lack of data, but the absence of a shared conceptual language.

A recurring theme was the realization that “complex multicellularity” itself remains surprisingly difficult to define. Traditional distinctions between simple and complex multicellularity often fail to capture the continuous spectrum of multicellular organization observed across eukaryotes. Participants therefore argued that multicellularity should not be viewed as a binary trait, but rather as a multidimensional continuum encompassing different degrees of cellular specialization, developmental integration and organismal complexity.

Throughout the discussions, several broader themes repeatedly emerged. One focused on the balance between historical contingency and evolutionary convergence. To what extent do independent lineages repeatedly recruit similar molecular solutions? Are there universal developmental principles that inevitably emerge when multicellular organisms become sufficiently complex, or does each lineage remain fundamentally shaped by its unique evolutionary history? Recent discoveries, including convergent recruitment of similar transcription factors, shared developmental dynamics such as transcriptomic hourglass patterns, and common biophysical constraints governing tissue organization, suggest that both historical contingency and universal principles contribute to evolutionary innovation.

The seminar also highlighted the increasing importance of integrating disciplines that have traditionally developed independently. Developmental biology, evolutionary genomics, biophysics and mathematical modelling each provide only partial perspectives on multicellular evolution. Bringing these approaches together allows entirely new questions to be addressed. Physical constraints, tissue mechanics and information processing are increasingly recognized not simply as consequences of multicellular organization, but as active drivers of developmental evolution. Likewise, comparative evolutionary studies benefit enormously from theoretical models capable of distinguishing lineage-specific innovations from more general organizing principles.

These discussions converged on the need for a shared conceptual framework capable of comparing independently evolved multicellular lineages while preserving their distinct evolutionary histories. A major outcome of the meeting was therefore the development of a comparative roadmap intended to guide future research, stimulate genuinely comparative studies across eukaryotes, and foster new collaborations between disciplines that have traditionally evolved independently.

The intimate setting of the Fondation des Treilles played a central role in fostering these exchanges. Extended discussions continued well beyond the formal sessions, often bringing together researchers who would rarely meet within conventional disciplinary conferences. The relatively small number of participants encouraged open debate, the development of new conceptual ideas and the formation of collaborations that are expected to continue well beyond the meeting itself.

Perhaps the most important conclusion of the seminar was the recognition that the field is entering a new phase. The question is no longer simply how multicellularity evolved in animals, plants, fungi or algae individually. Instead, the challenge is to understand why fundamentally different evolutionary histories repeatedly generated organisms sharing comparable levels of developmental complexity. Addressing this question will require moving beyond lineage-centred perspectives towards genuinely comparative approaches spanning the breadth of eukaryotic diversity.

To help catalyse this transition, participants agreed to prepare a collaborative perspective article proposing a roadmap for future research. The manuscript will synthesize the conceptual advances emerging from the meeting and establish a common framework for comparative studies of independently evolved multicellular lineages. It is currently in preparation and is expected to be submitted by the end of 2026.

The meeting also laid the foundations for a long-term international network dedicated to the comparative study of complex multicellularity. Participants agreed in principle to organize a second meeting in 2027, tentatively in Barcelona, followed by a third meeting in Japan in 2029. To foster the next generation of researchers in this emerging field, each participant will be encouraged to bring one junior scientist (PhD student or postdoctoral researcher), while preserving the small, highly interactive format that proved essential for the open discussions and intellectual exchanges that characterized the Treilles meeting.

The seminar highlighted the power of comparative thinking across deeply divergent evolutionary lineages. By bringing together researchers who rarely interact within conventional disciplinary boundaries, the Fondation des Treilles provided an exceptional environment in which to challenge established ideas and develop new conceptual approaches. The collaborations and scientific agenda that emerged from this meeting are expected to shape future research on the evolution of complex multicellularity for years to come.

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