The Evolution of Human Brain Development

The human brain is a large and complex organ, setting us apart from other primates. It allows us to exhibit highly sophisticated cognitive and behavioral abilities. Therefore, our brain??s size and morphology are defining features of our species and our fossil ancestors and relatives. Endocasts, i.e., internal casts of the bony braincase, provide evidence about brain size and morphology in fossils. Based on endocasts, we know that our ancestors?? brains increased overall in size and underwent several reorganizational changes. However, it is difficult to relate evolutionary changes of size and shape of endocasts to evolutionary changes of cognition and behavior. We argue here that an understanding of the tempo and mode of brain development can help to interpret the evolution of our brain and the associated cognitive and behavioral changes. To do so, we review structural brain development, cognitive development, and ontogenetic changes of endocranial size and shape in living individuals on the one hand, and ontogenetic patterns (size increase and shape change) in fossil hominins and their evolutionary change on the other hand. Tightly integrating our knowledge on these different levels will be the key of future work on the evolution of human brain development.     

The Development of Radial and Biradial Symmetry: The Evolution of Bilaterality

SYNOPSIS. Understanding the evolutionary origin of novel metazoanbody plans continues to be one of the most sought after answersin biology. Perhaps the most profound change that may have occurredin the Metazoa is the appearance of bilaterally symmetricalforms from a presumably radially symmetrical ancestor. The symmetryproperties of bilaterally symmetrical larval and adult metazoansare generally set up during the cleavage period while most "radially"symmetrical cnidarians do not display a stereotyped cleavageprogram. Ctenophores display biradial symmetry and may representone intermediate form in the transition to bilateral symmetry.The early development of cnidarians and ctenophores is comparedwith respect to the timing and mechanisms of axial determination.The origin of the dorsal-ventral axis, and indeed the relationshipsof the major longitudinal axes, in cnidarians, ctenophores,and bilaterian animals are far from certain. The realizationthat many of the molecular mechanisms of axial determinationare conserved throughout the Bilateria allows one to formulatea set of predictions as to their possible role in the originsof bilaterian ancestors.     

The Evolution and Development of Cephalopod Chambers and Their Shape

The Ammonoidea is a group of extinct cephalopods ideal to study evolution through deep time. The evolution of the planispiral shell and complexly folded septa in ammonoids has been thought to have increased the functional surface area of the chambers permitting enhanced metabolic functions such as: chamber emptying, rate of mineralization and increased growth rates throughout ontogeny. Using nano-computed tomography and synchrotron radiation based micro-computed tomography, we present the first study of ontogenetic changes in surface area to volume ratios in the phragmocone chambers of several phylogenetically distant ammonoids and extant cephalopods. Contrary to the initial hypothesis, ammonoids do not possess a persistently high relative chamber surface area. Instead, the functional surface area of the chambers is higher in earliest ontogeny when compared to Spirula spirula. The higher the functional surface area the quicker the potential emptying rate of the chamber; quicker chamber emptying rates would theoretically permit faster growth. This is supported by the persistently higher siphuncular surface area to chamber volume ratio we collected for the ammonite Amauroceras sp. compared to either S. spirula or nautilids. We demonstrate that the curvature of the surface of the chamber increases with greater septal complexity increasing the potential refilling rates. We further show a unique relationship between ammonoid chamber shape and size that does not exist in S. spirula or nautilids. This view of chamber function also has implications for the evolution of the internal shell of coleoids, relating this event to the decoupling of soft-body growth and shell growth.     

Heterochrony: the Evolution of Development

Heterochrony can be defined as change to the timing or rate of development relative to the ancestor. Because organisms generally change in shape as well as increase in size during their development, any variation to the duration of growth or to the rate of growth of different parts of the organism can cause morphological changes in the descendant form. Heterochrony takes the form of both increased and decreased degrees of development, known as “peramorphosis” and “paedomorphosis,” respectively. These are the morphological consequences of the operation of processes that change the duration of the period of an individual’s growth, either starting or stopping it earlier or later than in the ancestor, or by extending or contracting the period of growth. Heterochrony operates both intra- and interspecifically and is the source of much intraspecific variation. It is often also the cause of sexual dimorphism. Selection of a sequence of species with a specific heterochronic trait can produce evolutionary trends in the form of pera- or paedomorphoclines. Many different life history traits arise from the operation of heterochronic processes, and these may sometimes be the targets of selection rather than morphological features themselves. It has been suggested that some significant steps in evolution, such as the evolution of vertebrates, were engendered by heterochrony. Human evolution was fuelled by heterochrony, with some traits, such as a large brain, being peramorphic, whereas others, such as reduced jaw size, are paedomorphic.     

How Development May Direct Evolution

A framework is presented in which the role ofdevelopmental rules in phenotypic evolution canbe studied for some simple situations. Usingtwo different implicit models of development,characterized by different developmental mapsfrom genotypes to phenotypes, it is shown bysimulation that developmental rules and driftcan result in directional phenotypic evolutionwithout selection. For both models thesimulations show that the critical parameterthat drives the final phenotypic distributionis the cardinality of the set of genotypes thatmap to each phenotype. Details of thedevelopmental map do not matter. If phenotypesare randomly assigned to genotypes, the lastresult can also be proved analytically.     

Bridges between Development and Evolution

Adaptive evolution is usually assumed to be directed by selective processes, development by instructive processes; evolution involves random genetic changes, development involves induced epigenetic changes. However, these distinctions are no longer unequivocal. Selection of genetic changes is a normal part of development in some organisms, and through the epigenetic system external factors can induce selectable heritable variations. Incorporating the effects of instructive processes into evolutionary thinking alters ideas about the way environmental changes lead to evolutionary change, and about the interplay between genetic and epigenetic systems.     

非线性森林发展系统研究进展

就近年来国内外对非线性森林发展系统的研究进展做一综述，介绍了一些非线性森林发展系统模型，分析了非线性森林系统的研究结果。     

Waddington's Legacy in Development and Evolution

This paper provides an overview of the life and works of ConradHal Waddington(1905-1975). After an early life spent apart fromhis parents pursuing ammonites, naturalhistory, geology andarchaeology, Waddington took a degree in Geology at Cambridge(1926).Genetics and experimental embryology soon replaced palaeontologyas he began his experimentalstudies on the chemical nature ofthe primary organizer discovered by Spemannand Mangold in 1924.It was during this period of collaboration with Joseph and DorothyNeedhamthat Waddington developed the concepts of evocation and individuation.Theestablishment of a Unit on Animal Breeding and Genetics inEdinburgh after the secondWorld War provided Waddington withhis professional home for the rest of his career as he soughtto integrate genetics and development into an evolutionarilyrelevant discipline.Our conception of embryonic developmentas a highly integrated series of canalized pathways owes muchto Waddington's development of the concepts of canalization,chreods, epigenetics and the epigenotype. The metaphorical epigeneticlandscape became the way that most developmental biologists"saw" the organization of embryonic development. The conceptof supragenomic, integrated, heritable, epigenetic organizationof embryonic development is arguably Waddington's lasting legacyto development and evolution. The integration of his epigeneticlegacy into a quantitative developmental genetics model of thedevelopmental and evolutionary origin of phenotypes is now beingundertaken. It has still to be proven whether genetic assimilation,which Waddington demonstrated to be a real phenomenon in laboratoryexperiments, has been a force in evolutionary adaptation.     

The Geography of Evolution and the Evolution of Geography

Insights into the geography of life have played a fundamental role in motivating major developments in evolutionary biology. The focus here is on outlining some of these major developments, specifically in the context of paleontology, by emphasizing the significance of geographic isolation and allopatric speciation, punctuated equilibria, and the Turnover Pulse Hypothesis to evolutionary theory. One of the major debates in evolution concerns the relative contributions of abiotic and biotic factors to macroevolution, and each one of these developments increasingly suggested that it was climatic and geologic factors, rather than competition, that played the primary role in motivating macroevolution. New technical developments, including in the area of Geographic Information Systems, allow continued detailed testing of the relative roles that biotic as opposed to abiotic factors play in causing evolution, and some of the work in this area will also be described.     

Protein Evolution in the Context of Drosophila Development

The tempo at which a protein evolves depends not only on the rate at which mutations arise but also on the selective effects that those mutations have at the organismal level. It is intuitive that proteins functioning during different stages of development may be predisposed to having mutations of different selective effects. For example, it has been hypothesized that changes to proteins expressed during early development should have larger phenotypic consequences because later stages depend on them. Conversely, changes to proteins expressed much later in development should have smaller consequences at the organismal level. Here we assess whether proteins expressed at different times during Drosophila development vary systematically in their rates of evolution. We find that proteins expressed early in development and particularly during mid–late embryonic development evolve unusually slowly. In addition, proteins expressed in adult males show an elevated evolutionary rate. These two trends are independent of each other and cannot be explained by peculiar rates of mutation or levels of codon bias. Moreover, the observed patterns appear to hold across several functional classes of genes, although the exact developmental time of the slowest protein evolution differs among each class. We discuss our results in connection with data on the evolution of development.This revised article was published online in June 2005 with the final version of the content.Jerel C. Davis and Onn Brandman contributed equally to this work. Reviewing Editor: ManYuan Long     

植物MADS盒基因与花器官的进化发育

已在维管植物中发现百余种MADS盒基因,此种基因家族由表达模式和功能关系密切的基因构成多种特定的亚族和DEF／GLO类（B功能）,AG类（C和D功能）等,植物花器官发生和花形态多样性的遗传机理和进化规律都可能与MADS盒基因的结构,表达以及功能进行相关。     

Maize Transposable Elements in Development and Evolution

Transposable elements were first discovered in maize by BarbaraMcClintock more than 40 years ago. Today it is apparent thattransposable elements are a common component of the geneticmaterial in virtually all organisms. The best studied maizetransposable elements belong to the Activator-Dissociation andSuppressor-mutator families. They are short DNA sequences thatconsist of genes required for mobility and regulation. Boththe expression and the mobility of transposable elements areregulated in development by a mechanism that relies on the methylationof element sequences critical for expression. Elements can bestably inactivated by the same mechanism, persisting in thegenome in a cryptic form for long periods. The ability of thehost organism to regulate the highly mutagenic transposableelements may be critical to their survival, as well as theirutility as agents of genomic change.