Early transitions in the evolution of cognition.

This paper examines the early evolution of cognition in animals through the lens of the Transitions in Structural Complexity approach. By focusing on the emergence and transformation of coordination systems, the study identifies three progressive stages: collective, specialised, and integrated coordination. Each stage is characterised by distinct structural innovations-ranging from contractile epithelia and cytoskeletal coupling to the development of neurons, neurosecretory cells, and integrated nervous tissues-and is shaped by the processes of modularisation, subfunctionalisation, and integration. By conceptualising coordination as a transferable biological function, this approach offers an analysis of the evolution of this capacity without presupposing specific mechanisms or structures. Rather than defining cognition in advance, this approach tracks how functions associated with cognition become structurally embodied in multicellular systems through the progressive reorganisation of coordination across levels of biological organisation. Besides offering a general schema for analysing the early stages of cognitive evolution in metazoans, this approach has broader implications for the comparative study of cognition and the structural focus within the major evolutionary transitions framework.