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Origin of the Metazoa.

¹Molecular Biology Institute, University of California, Los Angeles 90024.

Proceedings of the National Academy of Sciences of the United States of America

|January 1, 1990

Summary

Investigating the origin of multicellular animals using 18S rRNA sequences reveals Metazoa and Deuterostomia as monophyletic groups. The Annelida-Mollusca lineage likely evolved from a segmented ancestor with an open circulatory system.

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Area of Science:

Evolutionary Biology
Molecular Phylogenetics
Animal Origins

Background:

Understanding the evolutionary history of multicellular animals (Metazoa) is a fundamental question in biology.
Phylogenetic analyses using molecular data, such as ribosomal RNA (rRNA) sequences, are crucial for reconstructing evolutionary relationships.

Purpose of the Study:

To investigate the origin and early diversification of multicellular animals.
To determine the phylogenetic position of major animal lineages, including Deuterostomia and the Annelida-Mollusca group.
To infer the characteristics of the last common ancestor of the Annelida-Mollusca lineage.

Main Methods:

Rate invariant analysis of 18S ribosomal RNA (rRNA) gene sequences.
Phylogenetic inference to reconstruct evolutionary relationships among animal taxa.

Main Results:

The Metazoa (multicellular animals) were confirmed as a monophyletic taxon.
The Deuterostomia were also found to be a monophyletic group.
The Annelida-Mollusca lineage was identified as the sister group to a subgroup of arthropods.
The last common ancestor of the Annelida-Mollusca lineage is most parsimoniously inferred to be a segmented, hemocoelic ancestor with an open circulatory system.

Conclusions:

The study provides strong molecular evidence for the monophyly of Metazoa and Deuterostomia.
The findings suggest a specific sister-group relationship between Annelida-Mollusca and arthropods.
The ancestral state of the Annelida-Mollusca lineage likely involved segmentation and an open circulatory system, offering insights into early animal body plan evolution.

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