Most multicellular animals have a nervous system that is based on the following three components: (1) sensory cells gather information and send it to processing units; (2) the processing units use the information to decide what action to take; and (3) effector neurons activate the appropriate muscles. Due to the importance of making the right decisions, evolution made profound advances in processing units. I review present knowledge regarding the evolution of neurological tools for making decisions, here referred to as strategies or algorithms. Consciousness can be understood as a particularly sophisticated strategy. It may have evolved to allow for the use of feelings as a ‘common currency’ to evaluate behavioral options. The advanced cognitive capacity of species such as humans further improved the usefulness of consciousness, yet in biological terms, it does not seem to be an optimal, fitness-enhancing strategy. A model for the gradual evolution of consciousness is presented. There is a somewhat arbitrary cutoff as to which animals have consciousness, but based on current information, it seems reasonable to restrict the term to amniotes.
No other phenomenon in nature has captured as much interest, and from such a variety of disciplines, than the question of what consciousness is about. I believe an evolutionary perspective can help inform the debate and that this perspective is best cared for by considering the role of nervous systems (NS) in facilitating survival.
NS allow an animal to behave, which generally means a coordinated use of muscles—typically to move around. The key purpose is presumably to find food, but NS also serve other purposes, such as finding mates and controlling internal organs. Plants rely on sunshine for energy and therefore do not have the same need to move.
NS have a sensory branch and an executive branch; between them lies a processing unit (Figure 1). The processing units analyze sensory input and harbor the necessary algorithms for responding to challenges by making behavioral decisions. The arguably most difficult aspect of behavior is to decide what to do or where to go; consequently, the processing units tend to be the most complex part of NS. The units have evolved from simple nerve nets to small aggregates of neurons (ganglia) and on to advanced, centralized brains. A key question for the present review is when and why evolution incorporated consciousness into the decision-making toolkit.
Figure 1. Overview of nervous systems. The majority of NS can be divided into three components: sensory cells detect environmental or internal information; this is sent to processing units (ganglia or brains) that decide on a response and execute the decision by sending signals to muscles via effector neurons.
Vertebrates have a brain, but ganglia-like structures, such as those positioned along the spinal cord, are also involved in responses. At least two more phyla, arthropods and mollusks (annelids may also be included) evolved centralized nervous structures of sufficient complexity to warrant the term brain [1
]. The importance of having an advanced processing unit is reflected in the observation that arthropods (for example, insects), mollusks (for example, octopuses), and vertebrates are by far the most successful, in terms of biomass and species variety, of the more than forty phyla present on Earth today. Large processing units offer the opportunity to utilize more sensory input, store more information in the form of memory, and make better behavioral decisions by using more advanced algorithms.
The simplest form of behavioral response is a monosynaptic reflex, where a signal from the sensory system directly activates muscle contraction. In most situations, there tend to be many signals that are relevant, and movement depends on the coordinated contraction of several muscles; thus, evolution moved in the direction of more intricate behavioral strategies. An increase in the complexity of sensory organs and processing units evolved concomitantly with an increase in the complexity of decision-making tools. The overall purpose was survival and procreation, but this was typically cared for by equipping the organisms with more proximate targets, such as eating edible things and mating with appropriate partners [2
I argue that to understand consciousness, it is useful to consider its role in NS. The evolutionary trajectory leading to its presence in humans is of particular interest. I start with defining key terms; then, I add some cautionary remarks, followed by a brief history of NS. In the remaining sections, I consider information pertaining to the evolution of consciousness.