In the Light of Evolution Volume VI: Brain and Behavior (2013) / Chapter Skim
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3 Evolution of Centralized Nervous Systems: Two Schools of Evolutionary Thought--R. Glenn Northcutt
3 Evolution of Centralized Nervous Systems: Two Schools of Evolutionary Thought--R. Glenn Northcutt
Pages 37-56
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From page 37... ...
Analysis of brain–body complexity among extant bilateri ans indicates that diffuse nerve nets and, possibly, ganglionated cephalic neural systems existed in Ediacaran organisms. An outgroup analysis of cephalic neural characters among extant metazoans also indicates that the last common bilaterian ancestor possessed a diffuse nerve plexus and that brains evolved independently at least four times.
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From page 38... ...
The molecular phylogenetic studies of the last 20 years are particularly important in understanding metazoan interrelationships as well as the time frame in which these animals arose and radiated, and we now have increased insights into the genetics underlying the development of CNSs. First, I will review the fossil history of the earliest putative metazoans, and then, I will discuss different comparative approaches to analyzing both molecular and morphological data: the molecular clock hypothesis, which has yielded a range of possible dates for the origin and divergence of metazoans; developmental genetics and its contribution to our understanding of the patterning of metazoan bodies, particularly patterning of the CNS; and conclusions based on the first outgroup analysis of metazoan central neural characters.
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From page 39... ...
. After this burst of descriptions, Ediacaran anatomy was reevaluated; claims were made that all Ediacarans were organized on a quilt-like pattern and represented an independent experiment of nonmetazoan animals, termed the Vendobionta, that failed with the evolution of macrophagous bilaterian metazoans (Seilacher, 1989; Buss and Seilacher, 1994; McMenamin, 1998)
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From page 40... ...
biotas. The Ediacaran biota is reconstructed to convey maximal morphological complexity.
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From page 41... ...
. The White Sea assemblage represents the peak diversity of Ediacaran biota, including all of the taxa in Fig.
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From page 42... ...
One or more of the three radiations of the small vagile organisms may be close relatives, or even stem members, of three clades of extant metazoans. Cambrian Explosion The close of the Ediacaran was marked by a massive reduction in the Ediacaran biota, with only a small number of Ediacaran taxa continuing into the Early Cambrian (Conway Morris, 1993)
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From page 43... ...
. Unlike the Burgess Shale biota, the Chengjiang biota contains a rich variety of chordates.
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From page 44... ...
, divide similar characters among organisms into three categories: shared primitive characters, shared derived characters, and uniquely derived
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From page 45... ...
Many shared primitive characters are, in fact, shared derived characters at some lower level of the tree of life and thus linked as transformational homologs to shared derived characters at a higher level. In addition, analysis of homoplasious characters can reveal structural and functional constraints in phylogeny.
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From page 46... ...
The phenetic approach is still widely used, however, in developmental genetic studies, in which evolutionary statements are based on a two-taxon approach, possibly because until recently, it has been difficult to explore the genetic basis of phenotypic characters widely among different taxa. The roles of both cladistic and phenetic approaches are examined in the next three sections dealing with the molecular clock hypothesis, the genetic basis of bilaterian body plans, and an outgroup analysis of metazoan neural characters.
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From page 47... ...
, bilaterians arose and began to radiate during the Ediacaran Period. Genetic Basis of Bilaterian Body Plan In the last 20 years, developmental biologists have made spectacular strides in revealing the genetic basis of the regulatory networks that underlie anterior–posterior and dorsoventral patterning of body organization in many bilaterian metazoans (Lewis, 1978; Nüsslein-Volhard and Wieschaus, 1980; Bopp et al., 1986; Cohen and Jürgens, 1990; McGinnis and Krumlauf, 1992; Holley et al., 1995)
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From page 48... ...
Work on body patterning in an enteropneust hemichordate Saccoglossus, which has a diffuse nerve net, reveals the same expression of homeobox genes in the anterior–posterior body axis as in other bilaterians, but the antagonistic actions of sog and dpp do not restrict neural development to the dorsal body surface of this bilaterian (Lowe et al., 2003; Lowe, 2008)
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From page 49... ...
If an outgroup analysis of central neural characters reveals that a brain is a shared primitive character for bilaterians, then the tripartite brain hypothesis might be supported. This finding would be the case, however, only if a brain divided into three parts is a shared primitive character.
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From page 50... ...
The following characters -- or levels of increasing morphological and functional complexity in the cephalic CNSs of extant metazoans -- are recognized in the present outgroup analysis: (i) diffuse nerve nets or subepidermal
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From page 51... ...
. Diffuse nerve nets range from those nets of cnidarians and ctenophores to those nets in enteropneust hemichordates, in which neural cell bodies occupy a subepidermal nerve plexus with centralized bundles of fastconducting axons forming dorsal and ventral nerve cords.
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From page 52... ...
Because the third neural character, simple cerebral ganglion, is the primitive condition in protostome bilaterians, additional examination of the outgroups, xenocoelmorphs and the basal metazoan clades, would still indicate that the last common bilaterian ancestor and the last common metazoan ancestor possessed a diffuse nerve plexus. Outgroup analysis of intracladal variation in central cephalic neural characters also indicates that brains have evolved numerous times independently.
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From page 53... ...
Both molecular clock and paleontological data indicate that bilaterian metazoans arose ~600–700 Mya during the Ediacaran, and they radiated rapidly into most bilaterian crown clades by the end of the Cambrian (Erwin et al., 2011)
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From page 54... ...
3.2) supports an LCBA model with a diffuse nerve plexus, which subsequently coalesced into a number of cephalic ganglia and nerve cords or a dorsal hollow neural tube.
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From page 55... ...
Evolution of Centralized Nervous Systems / 55 the memory of Sue Commerford, a good friend and superb assistant until her death during the preparation of this manuscript. I thank Jo Griffith for assistance with the illustrations and Mary Sue Northcutt for help with many phases of the research and manuscript preparation.
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