Predation and the Origin of Neurones

Abstract

The core design of spiking neurones is remarkably similar throughout the animal kingdom. Their basic function as fast-signalling thresholding cells might have been established very early in their evolutionary history. Identifying the selection pressures that drove animals to evolve spiking neurones could help us interpret their design and function today. We review fossil, ecological and molecular evidence to investigate when and why animals evolved spiking neurones. Fossils suggest that animals evolved nervous systems soon after the advent of animal-on-animal predation, 550 million years ago (MYa). Between 550 and 525 MYa, we see the first fossil appearances of many animal innovations, including eyes. Animal behavioural complexity increased during this period as well, as evidenced by their traces, suggesting that nervous systems were an innovation of that time. Fossils further suggest that, before 550 MYa, animals were either filter feeders or microbial mat grazers. Extant sponges and Trichoplax perform these tasks using energetically cheaper alternatives than spiking neurones. Genetic evidence testifies that nervous systems evolved before the protostome-deuterostome split. It is less clear whether nervous systems evolved before the cnidarian-bilaterian split, so cnidarians and bilaterians might have evolved their nervous systems independently. The fossil record indicates that the advent of predation could fit into the window of time between those two splits, though molecular clock studies dispute this claim. Collectively, these lines of evidence indicate that animals evolved spiking neurones soon after they started eating each other. The first sensory neurones could have been threshold detectors that spiked in response to other animals in their proximity, alerting them to perform precisely timed actions, such as striking or fleeing.

Liquid brains, solid brains

“…From this viewpoint, prediction is both a cause and a consequence of animal movement, and its implementation required learning networks to emerge [5]. Predation was likely the most important selective pressure to create learning networks [6]. This powerful evolutionary innovation was not limited to neural networks, and we suggest here that the many kinds of networks of interacting agents that evolved to process information have characteristic similarities and differences.…”

The brain: a concept in flux

“…Interestingly, it seems that the evolutionary origin of neurons-an event that happened approximately 500 million years ago-coincided with the initiation of hostilities between animal species for nutritional purposes. In other words, neurons appeared shortly before animals began pursuing and eating each other [51]. Incidentally, this information will likely play a role in the sponges-ctenophores controversy (see above), as ctenophores are true predators, a characterization that tends to indicate a relatively sophisticated nervous system.…”

Complex Homology and the Evolution of Nervous Systems

“…Although many of these genomic events cannot yet be dated precisely, they most likely followed the end of the worldwide glaciation events, contemporaneously with the rise of oceanic oxygen and macroscopic animal forms. The rise of inter-animal predation in the early Cambrian probably provided selective pressure to evolve complex behaviors, neural organization [67], and musculature [68]. It is perhaps not meaningful, therefore, to assign a single date to the origin of nervous systems, especially when one considers the genomic continuity between sensu stricto neurons and proto-neurons, and the large degree of homoplasy in animal nervous systems as a whole.…”