A new map of arthropod evolution, from fossils to embryos

A new study by Prof. Ariel Chipman of The Alexander Silberman Institute of Life Science at the Hebrew University of Jerusalem provides a novel model for understanding the development and evolution of arthropod body plans—specifically the arrangement of their segmented body parts known as tagmata.

Arthropods, a group that includes insects, crustaceans, spiders, and centipedes, represent the most diverse animal phylum on Earth. Each group of arthropods is defined by a unique arrangement of tagmata: for instance, insects have a head, thorax, and abdomen, while spiders possess a cephalothorax and abdomen. Although these body plans are widely recognized, how they evolved has long remained a mystery.

Prof. Chipman's research, published in Proceedings of the Royal Society B, proposes a new developmental model that traces the evolutionary origin of these body structures back to distinct processes occurring during embryonic development.

Drawing on comparative developmental biology, classical embryological studies, and data from the arthropod fossil record, the study identifies three ancestral developmental zones from which all arthropod tagmata are derived. These include a unique anterior segment set, a mid-body region formed within a pre-existing developmental field, and a posterior region generated sequentially from a specialized growth zone.

This model not only challenges traditional distinctions between short-germ and long-germ development—terms that refer to how many segments are defined early in an embryo's life—but also reframes the role of well-known genetic regulators such as Hox genes in defining segment and tagma identity.

By linking developmental mechanisms to the fossil history of arthropods, the study offers a comprehensive explanation for the vast diversity of arthropod forms. It provides a clear, unified framework for understanding how the complex and varied body plans of arthropods arose through evolutionary time.

Prof. Chipman emphasizes that "these insights open up new questions and directions for research, especially in identifying the genetic and molecular drivers of tagma formation across species."

The study is a culmination of over a decade of work in Prof. Chipman's lab on modes of development in different arthropods. It represents a synthesis of many data points coming together to give a broad picture. The study also underscores the importance of interdisciplinary approaches—bridging developmental biology, paleontology, and evolutionary theory—to uncover the deep patterns that have shaped life's most diverse group of animals.