Early animal evolution and the origins of nervous systems

Abstract

Understanding the evolution of early nervous systems is hazardous because we lack good criteria for determining homology between the systems of distant taxa; the timing of the evolutionary events is contested, and thus the relevant ecological and geological settings for them are also unclear. Here I argue that no simple approach will resolve the first issue, but that it remains likely that animals evolved relatively late, and that their nervous systems thus arose during the late Ediacaran, in a context provided by the changing planktonic and benthic environments of the time. The early trace fossil provides the most concrete evidence for early behavioural diversification, but it cannot simply be translated into increasing nervous system complexity: behavioural complexity does not map on a one-to-one basis onto nervous system complexity, both because of possible limitations to behaviour caused by the environment and because we know that even organisms without nervous systems are capable of relatively complex behaviour.

Keywords: Cambrian explosion; Ediacaran; environmental change; homoplasy; nervous system evolution.

Figure 1.

The optimization problem. If a complex and very similar character such as a brain (marked by the black circle) is shared by taxa B, F and K, can it be considered to have evolved at the base of the tree and lost (in a non-parsimonious way) in the clades of A, C–D, E, G, H–I and J, or should it be considered a convergence between B and K?

Figure 2.

A large specimen (in the field) of the lower Cambrian trace fossil Plagiogmus arcuatus from the Uratana Formation near Alice Springs (photograph kindly provided by Sören Jensen). The organism making the trace made a large loop and on completing it began to burrow down into the sediment, with its siphon-like structure rocking from side to side to make the sinusoidal pattern. Such a complex trace is extremely likely to be of (unknown) bilaterian origin. See [35] for an analysis of its mode of formation. The oval structure in the centre is an eroded ripple suggesting a shallow water environment. Scale bar, 5 cm.