Bodily Complexity: Integrated Multicellular Organizations for Contraction-Based Motility

Area of Science:

• Evolutionary Biology
• Developmental Biology
• Cellular Biology

Background:

• Animal multicellularity is uniquely characterized by highly integrated cell collectives acting as unitary, mobile entities.
• This complex organization is often assumed to be an inevitable evolutionary outcome driven by environmental pressures favoring large, mobile organisms.

Purpose of the Study:

• To challenge the notion of evolutionary inevitability in animal origins.
• To introduce and support the "thesis of bodily complexity" regarding animal multicellularity.
• To highlight the underappreciated role of intermediate organizational steps in animal evolution.

Main Methods:

• Conceptual analysis and theoretical argumentation based on existing biological knowledge.
• Focus on the transition from cilia-based to contraction/muscle-based motility.
• Examination of basal animal phyla (Porifera, Ctenophores, Placozoans, Cnidarians) to illustrate key features.

Main Results:

• The evolution of complex animal bodies required integrating sensorimotor, physiological, and developmental aspects.
• Contraction-based tissue and cellular events, particularly muscle and myoepithelial contraction, provided a physical platform for mechanical force transmission and multicellular signaling.
• This transition from cilia-based to contraction-based motility involved crucial intermediate organizational steps.

Conclusions:

• Animal evolution is not a simple, inevitable path but involved specific "bodily complexity" requirements.
• Muscle-based contraction is a foundational element for animal motility, sensing, maintenance, and development.
• Understanding these intermediate steps is key to comprehending the evolution of basal animal forms and subsequent animal diversification.