Some unanswered questions about consciousness as a biological phenomenon lie in its evolutionary origin and distribution among living organisms. Which animals are conscious and when did consciousness appear in the history of life on earth? The purpose of this thesis is to shed light on these long-standing questions by reviewing the literature on the evolutionary approaches to the fundamental concept of phenomenal consciousness. In agreement with the “Cambridge Declaration on Consciousness,” this study aims to introduce three recently developed theories, with a particular emphasis on examining one model. The “Cellular Basis of Consciousness” (CBC) is a reductionist, cellular-based model that argues all organisms from unicellular to humans are sentient. Another theory based on neuroevolutionary arguments is called the “Neurobiological Naturalism.” It suggests that consciousness first appeared duringthe Cambrian period about 550 million years ago. This model considers vertebrates, arthropods and cephalopods as conscious animals. An alternative framework, which is the main focus of this thesis, is the “Unlimited Associative Learning” (UAL). The UAL model employs a novel method to formulate a transition marker as an indicator of consciousness. Although the UAL framework is a promising method for tracing the evolution of consciousness, it also has significant limitations. Nevertheless, the literature review reveals that as an innovative framework, UAL has the potential to generate fruitful research programs. Rather than providing a final answer, it can be considered a significant starting point for unraveling the origin of consciousness.

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