A hypothesis for the evolution of the ..

Another interesting aspects of the hypothesis that the neocortex derived from the superposition of lateral and dorsal cortex, is that it may account for some hodological features that the UL shares with the olfactory cortex but not with the reptilian dorsal cortex. For instance, the olfactory cortex of tetrapods possesses homotopic projections to the contralateral hemisphere passing through the anterior commissure (Zeier and Karten, ; Butler, ; Hoogland and Vermeulen-Vanderzee, ; Sassoè-Pognetto et al., ; Suárez et al., ). Inter-hemispheric projections arising from UL neurons still decussate exclusively through this commissure in monotreme and marsupials, and this is generally thought to represent the basal condition in mammals (Ashwell et al., ; Suárez et al., ). Nonetheless, in sauropsids inter-hemispheric connections of DP and MP derivatives decussate at the nearby pallial/hippocampal commissure (Butler, ; Martinez-Garcia et al., ; Atoji et al., ). Thus, our hypothesis may account for the strange evolutionary history of the inter-hemispheric connections of the mammalian DP derivatives that at first flipped their direction and coursed a long lateral trip to decussate with fibers of the olfactory cortex at the anterior commissure. Only in eutherian mammals most, but not all, of the neocortical inter-hemispheric connections turned medially again decussating at the corpus callosum (Suárez et al., ). Another interesting similarity between the connections of olfactory cortex and UL is that they are both the source of feed-forward projections that flow to a series of hierarchical areas progressively defining sensory objects and ultimately converging on the LEC (Felleman and Van Essen, ; Haberly, ; Gilbert and Sigman, ; Wilson and Sullivan, ). In summary, the idea that the six layered neocortex originated from the superposition of lateral and dorsal cortex is consistent with the fossil record and may account not only for the topological position of the neocortex, but also for its basic cytoarchitecthural and hodological features. Unfortunately very little is known about the embryonic development of this putative six-layered primordium in modern reptiles. Guirado and Davila identified radial glial processes crossing both dorsal and lateral cortex in the lateral superposition of the lizard Podarcis Hispanica (Guirado and Dávila, ) and we made similar observations in Golgi stains of Lacerta Sicula (Luzzati unpublished observation). These authors raised the possibility that an independent progenitor domain giving rise to neurons of both dorsal and lateral cortex may actually exist in some living reptiles. In contrast to this interpretation however, Ulinsky reported that during development the layer II of the reptilian dorsal and lateral cortex is a continuous stratum of cells that is secondarily ruptured during differentiation (Ulinsky, ). Starting from this latter observation, a possible scenario for the evolution of the neocortex may be that in early mammaliaforms the homologs of UL and DL cells organized in a proto-neocortical column that was initially produced by spatially segregated progenitors. At some point a spatial to temporal patterning switch, together with the evolution of the inside-out neurogenic gradient, led to the generation of the proto-neocortical module from a single population of progenitors (Figure ). This crucial event enabled the tangential expansion of this module providing the basis for the establishment of the modern neocortex (Rakic, ; Lewitus et al., ; Figure ). According to the growth rings hypothesis of Sanides, during this tangential expansion the internal parts of the neocortical island progressively lost their allocortical features with the addition of stellate cells in layer IV and a reduction of cell density in layer II (Sanides, ; Sanides and Sanides, ). An intriguing aspect of this model is that it implies that the early neocortex worked as an higher order association cortex and that primary sensory areas appeared only subsequently. This latter idea has been also recently proposed based on functional models of both mammalian and reptilian allocortices (Fournier et al., ).

25/09/2012 · The Origin of Virions and Virocells: The Escape Hypothesis ..

Biologists recognize many evolutionary mechanisms, including not only natural selection and mutation, but the effects of chance fluctuations in gene frequency (genetic drift), the effects of genetic rearrangements on a chromosome (recombination), the effects of migration of genetic variants into and out of a population (gene flow) and the effects of wholesale incorporation of genetic material by one species from another species (endosymbiosis). There is an ongoing debate within evolutionary biology over whether genetic drift is more influential than natural selection on the course of evolution. Other biologists have suggested that endosymbiosis may be even more important, and continue to test that hypothesis. If really intended to present ongoing scientific controversies regarding evolution, those debates over evolutionary mechanisms would have a prominent place. And yet, in discussing "the creative power of natural selection," states:

Chapter 01 - Exploring Life | CourseNotes

The only structural approach of exploring the co-evolution of viruses and prokaryotes ..

It's important to keep in mind that changes to these hypotheses are a normal part of the process of science and that they do not represent a change in the basis of evolutionary theory.