Early Animal Evolution: From Filter Feeders to Nervous Systems

The earliest marine creatures, akin to modern sponges, thrived by filtering water for sustenance, devoid of sensory perception or response mechanisms. Gradually, diffuse nerve nets emerged in jellyfish-like organisms, enabling basic touch and chemical detection but lacking precise spatial discrimination.

Approximately 550 million years ago, the theoretical urbilaterian emerged, exhibiting bilateral symmetry and concentrated sensory and nervous structures at its anterior end. This ancestral form is believed to have given rise to various lineages, including vertebrates, worms, and insects, despite lacking fossil evidence.

Early scholars, like the Ancient Greeks, speculated on the brain's influence on muscles, attributing nerve function to "animal spirits." Later, the discovery of animal electricity by Luigi Galvani in 1791, through studies on frogs, revealed the role of electrical currents in nerve-muscle interactions. Julius Bernstein's 1902 proposition on nerve cell currents furthered understanding, attributing them to voltage differences caused by charged particle distribution.

According to the neuron doctrine, neurons function as discrete units, separated by synapses, physical gaps between neurons and muscles. Electrical impulses facilitate long-distance communication within neurons, while neurotransmitters transmit messages across synapses. The release of neurotransmitters in response to electrical impulses facilitates communication between nerves and muscles.

The illustration portrays neurotransmitter-mediated message transmission between two neurons across a synaptic gap.