Abstract:One contribution of 16 to a discussion meeting issue 'Homology and convergence in nervous system evolution'. Older evolutionary scenarios for the origin of vertebrates often gave nervous systems top billing in accordance with the notion that a big-brained Homo sapiens crowned a tree of life shaped mainly by progressive evolution. Now, however, tree thinking positions all extant organisms equidistant from the tree's root, and molecular phylogenies indicate that regressive evolution is more common than previousl… Show more
“…The point of view stating that the planarian brain is a true representative of what 'the first brain' must have looked like is an interesting starting point to try to gain insights on vertebrate nervous systems. However, it must be pointed out that planarians are not the most basal animals [39,40], meaning that they cannot be the absolute first example of a brain, broadly defined. Current ideas about the phylogeny of animals are also in flux; this field is undergoing a minor controversy in specialized circles.…”
One of the most important aspects of the scientific endeavour is the definition of specific concepts as precisely as possible. However, it is also important not to lose sight of two facts: (i) we divide the study of nature into manageable parts in order to better understand it owing to our limited cognitive capacities and (ii) definitions are inherently arbitrary and heavily influenced by cultural norms, language, the current political climate, and even personal preferences, among many other factors. As a consequence of these facts, clear-cut definitions, despite their evident importance, are oftentimes quite difficult to formulate. One of the most illustrative examples about the difficulty of articulating precise scientific definitions is trying to define the concept of a brain. Even though the current thinking about the brain is beginning to take into account a variety of organisms, a vertebrocentric bias still tends to dominate the scientific discourse about this concept. Here I will briefly explore the evolution of our ‘thoughts about the brain’, highlighting the difficulty of constructing a universally (or even a generally) accepted formal definition of it and using planarians as one of the earliest examples of organisms proposed to possess a ‘traditional’, vertebrate-style brain. I also suggest that the time is right to attempt to expand our view of what a brain is, going beyond exclusively structural and taxa-specific criteria. Thus, I propose a classification that could represent a starting point in an effort to expand our current definitions of the brain, hopefully to help initiate conversations leading to changes of perspective on how we think about this concept.
This article is part of the theme issue ‘Liquid brains, solid brains: How distributed cognitive architectures process information’.
“…The point of view stating that the planarian brain is a true representative of what 'the first brain' must have looked like is an interesting starting point to try to gain insights on vertebrate nervous systems. However, it must be pointed out that planarians are not the most basal animals [39,40], meaning that they cannot be the absolute first example of a brain, broadly defined. Current ideas about the phylogeny of animals are also in flux; this field is undergoing a minor controversy in specialized circles.…”
One of the most important aspects of the scientific endeavour is the definition of specific concepts as precisely as possible. However, it is also important not to lose sight of two facts: (i) we divide the study of nature into manageable parts in order to better understand it owing to our limited cognitive capacities and (ii) definitions are inherently arbitrary and heavily influenced by cultural norms, language, the current political climate, and even personal preferences, among many other factors. As a consequence of these facts, clear-cut definitions, despite their evident importance, are oftentimes quite difficult to formulate. One of the most illustrative examples about the difficulty of articulating precise scientific definitions is trying to define the concept of a brain. Even though the current thinking about the brain is beginning to take into account a variety of organisms, a vertebrocentric bias still tends to dominate the scientific discourse about this concept. Here I will briefly explore the evolution of our ‘thoughts about the brain’, highlighting the difficulty of constructing a universally (or even a generally) accepted formal definition of it and using planarians as one of the earliest examples of organisms proposed to possess a ‘traditional’, vertebrate-style brain. I also suggest that the time is right to attempt to expand our view of what a brain is, going beyond exclusively structural and taxa-specific criteria. Thus, I propose a classification that could represent a starting point in an effort to expand our current definitions of the brain, hopefully to help initiate conversations leading to changes of perspective on how we think about this concept.
This article is part of the theme issue ‘Liquid brains, solid brains: How distributed cognitive architectures process information’.
“…Due to shared computational and functional constraints on the evolutionary development of complex neural systems, phyletically distant animals often exhibit ‘phenotypic’ similarity in their neural organization ( Farris, 2008 ; Roth, 2013 ; Wolff and Strausfeld, 2016 ; Shigeno, 2017 ). However, the origin and evolution of neural systems across animal phyla remains uncertain ( Moroz, 2009 ; Northcutt, 2012 ; Holland et al, 2013 ; Holland, 2016 ). For example, centralization of nervous systems has occurred on more than five occasions during evolution (e.g., molluscs, annelids, nematodes, arthropods and chordates; see discussion in Moroz, 2009 ), and the acquisition of behavioral ‘capabilities’ such as the need for foraging strategies, spatial-, social- and instrumental-learning are all considered major driving forces in the evolution of complex brains and “high intelligence” several times independently in the animal kingdom ( Roth, 2015 ).…”
Cephalopod and vertebrate neural-systems are often highlighted as a traditional example of convergent evolution. Their large brains, relative to body size, and complexity of sensory-motor systems and behavioral repertoires offer opportunities for comparative analysis. Despite various attempts, questions on how cephalopod ‘brains’ evolved and to what extent it is possible to identify a vertebrate-equivalence, assuming it exists, remain unanswered. Here, we summarize recent molecular, anatomical and developmental data to explore certain features in the neural organization of cephalopods and vertebrates to investigate to what extent an evolutionary convergence is likely. Furthermore, and based on whole body and brain axes as defined in early-stage embryos using the expression patterns of homeodomain-containing transcription factors and axonal tractography, we describe a critical analysis of cephalopod neural systems showing similarities to the cerebral cortex, thalamus, basal ganglia, midbrain, cerebellum, hypothalamus, brain stem, and spinal cord of vertebrates. Our overall aim is to promote and facilitate further, hypothesis-driven, studies of cephalopod neural systems evolution.
“…In particular, functional studies of neural specification and patterning in xenacoelomorphs are needed to infer how genetic pathways underlying these processes have evolved. These studies would also inform the evolution of animal nervous systems [ 28 – 30 ] in the alternative scenario that places xenacoelomorphs as sister to Ambulacraria [ 31 , 32 ]. Figure 1.…”
The origin of bilateral symmetry, a major transition in animal evolution, coincided with the evolution of organized nervous systems that show regionalization along major body axes. Studies of Xenacoelomorpha, the likely outgroup lineage to all other animals with bilateral symmetry, can inform the evolutionary history of animal nervous systems. Here, we characterized the neural anatomy of the acoel
Hofstenia miamia
. Our analysis of transcriptomic data uncovered orthologues of enzymes for all major neurotransmitter synthesis pathways. Expression patterns of these enzymes revealed the presence of a nerve net and an anterior condensation of neural cells. The anterior condensation was layered, containing several cell types with distinct molecular identities organized in spatially distinct territories. Using these anterior cell types and structures as landmarks, we obtained a detailed timeline for regeneration of the
H
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miamia
nervous system, showing that the anterior condensation is restored by eight days after amputation. Our work detailing neural anatomy in
H
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miamia
will enable mechanistic studies of neural cell type diversity and regeneration and provide insight into the evolution of these processes.
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