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Updated: Jul 8, 2026
12:59
Three and Four-Dimensional Visualization and Analysis Approaches to Study Vertebrate Axial Elongation and Segmentation
Published on: February 28, 2021
Back in time: a new systematic proposal for the Bilateria
Jaume Baguñà1, Pere Martinez, Jordi Paps
1Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain. jbaguna@ub.edu
Summary
Phylogenetic analyses suggest that early bilaterians were small, simple, benthic organisms, evolving from planuloid ancestors before the split with cnidarians. This challenges previous theories on the origin of bilateral symmetry.
Area of Science:
- Evolutionary biology
- Developmental biology
- Phylogenetics
Background:
- The origin of bilateral symmetry in animals remains a key question in evolutionary biology.
- Two main hypotheses exist: the planuloid-acoeloid theory and the archicoelomate theory.
- Recent data suggest cnidarians may also possess bilateral characteristics.
Purpose of the Study:
- To investigate the phylogenetic relationships of bilaterian metazoans.
- To determine the nature of the earliest bilaterian ancestors.
- To propose a revised systematic scheme for Bilateria.
Main Methods:
- Phylogenetic analyses using quantitative markers (ribosomal, nuclear, expressed sequence tag sequences).
- Phylogenetic analyses using qualitative markers (HOX cluster genes, microRNA sets).
- Integration of morphological and gene expression data.
Main Results:
- Phylogenetic trees support acoel and nemertodermatid flatworms as the earliest branching Bilateria.
- Acoelomate and pseudocoelomate clades emerge as early branching Lophotrochozoans and Deuterostomes.
- Results indicate stem bilaterians were small, acoelomate/pseudocoelomate, benthic organisms.
Conclusions:
- Stem bilaterians likely originated from planuloid-like organisms.
- The last common bilaterian ancestor predates the cnidarian-bilaterian split.
- A revised systematic scheme for Bilateria, including Cnidaria, is proposed.
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