User:Hmacintyre/sandbox

Human Evo Devo is the subset of Evo Devo dealing with human specific studies and aspects. Evo-Devo, in general, is the study of biology with the purpose of linking evolutionary and developmental processes through the study of different organisms. It is utilized within multiple disciplines, primarily evolutionary biology and anthropology. Ground work for the theory that “evolutionary modifications in primate development might have led to … modern humans” was laid by Geoffrory Saint- Hilaire, Ernst Haeckel, Louis Bolk, and Adolph Schultz (Gunz and Mitteroecker, 2009)[1]. Evo Devo is primarily concerned with the ways by which evolution affects the machinations of development (Muller, 2007)[2] and was born of evolutionary biology studies. It differs from traditional evolutionary approaches in that the focus is on unraveling the causal factors that produce evolutionary innovations (Ploeger and Galis, 2012)[3] Although Evo Devo as a discipline is relatively new, there are significant roots to the discipline. Schultz, in The physical distinctions of man, began exploring distinctualirzing factors of humans in the 1940s. Shultz argued for the need to utilize broad comparative studies as a means by which to identify uniquely human traits from those that might simply be more or less frequent in populations (Shultz,1950).

History
Brian Hall traces the roots of Evo Devo in his 2012 paper on its past present and future. He begins with the basis in Darwin's theory and the nineteenth century genetic research of Mendel and the tendency of both groups of researches to not only follow separate paths, but to also black box that which were deemed inexplicable aspects (Hall, 2012)[4] Greater understanding of genotypic and phenotypic structures began to break into these previously inaccessible aspects (Hall, 2012)[5] This has lead to a virtual explosion of research in the field with expansions and specializations in a multitude of areas including those specific to humans.

Human and Primate Development
Many of the human evo-devo studies have been modeled after primate studies and consider the two together in a comparative model. Brain ontogeny and human life history evolution were looked at by Leigh, in a 2006 paper. He compares brain growth patterns for Homo erectus and Homo sapiens to get at the evolution of brain size and weight. Leight found three different patterns, all of which pointed to the growth rate of H. erectus either matching or exceeding H. erectus (Leigh, 2006)[6] He makes the case that this finding had wide application and relevance t the overall study of human evolution. It is pertinent specifically to the connections between energy expenditure and brain development. These finding are of specific utility in studies on maternal energy expenditure. (Leigh, 2006)[7]   Comparative study of nonhuman primates, fossils and modern humans to study patterns of brain growth to correlate human life history and brain growth. (Leigh, 2006)[8]

Jeremy De Silva and Julie Lesnik examined chimpanzee neonatal brain size to identify implications for brain growth in Homo erectus. This changed the understanding of differences and similarities of post-natal brain growth in humans and chimpanzees. The study found that there was a distinction necessary between growth time and growth rate. The times of growth were strikingly similar, but the rates were not (DeSilva and Lesnik, 2005)[9] The paper further advocates the use of fossils to assess brain size in general and in relation to cranial capacity.

Utilization of endocranial volume as a measure for brain size has been a popular methodology with the fossil record since Darwin in the mid 1800s. This measure has been used to access the metabolic requirements for brain growth and the subsequent trade-offs.

Neoteny
Some of the work on human Evo Devo has centered around the neotenous features that present in humans, but are not shared across the primate spectrum. Steven J Gould in his work Ontogeny and Phylogeny studied the presentation of neoteny with "terminal additions" in humans (Gould, 1977)[10] Neoteny is defined as the delayed or slowed development in humans when compared with their non-human primate counter parts. The "terminal additions" were extensions or reductions in the rate and scope of stages of development and growth (Gould, 1977)[11] Gould hypothesized that this process and production of neoteny in humans might be the key feature that ultimately lead to the emotional and communicative nature of humans. He credits this factor as an integral facet of human evolution. However, there have also been cautions against the application of this aspect to group ranking during it inappropriate as a measure of evolutionary achievement (Pievani, 2012)[12]

Fossil record
Many of the initial forays into studies, both comparative and human, have utilized the fossil record in an attempt to infer information otherwise unavailable. A lot of this has taken the form of studies on cranial sizes and capacities to infer brain size, growth rate, total growth and potential implications for energy expenditure. Although there is much to be learned about human evolutionary development from the fossil record, the static nature of individual fossils present its own challenge. The phylogenic fossil line is in and of itself a hypothesis, therefore anything based upon it is equally hypothetical. (McNulty, 2005) This is true and yet this uncertainty does not negate the utility of the comparison or the correlations drawn from the fossil record. Nor does it negate the inferences about underlying human evolutionary development. It is, however, an essential consideration and must be included with these types of studies.

Using the fossil record of Neanderthals, modern humans, and chimpanzees, Gunz et al examined that patterns of endocranial development. (Guns,2012[13]). They found that there are common features shared between the three. However, they also found that modern humans diverge from these common patterns in the first year of life. They concluded that even though much of the developmental results are similar insofar as brain size, the trajectories by which they arrived are not shared. Most of the differences between the two arise post-natally, in the first year, with cognitive development. (Guns, 2012)[14]

There have been a number of studies that not only take incomplete fossil records into consideration, but have attempt to specifically identify the barriers presented by this condition. For example, Kieran McNulty covers the potential utilities and constraints of using incomplete fossil taxa to examine longitudinal development in Australopithecus africanis (McNulty, 2005)[15]

Many studies on development have been human-specific almost entirely in their scope. In his 2011 paper Bernard Crespi focused on what could be gained in understanding of childhood diseases. Adaptation and genomic conflict are the key features of this paper. He considers the evolution of childhood diseases and the risk levels within, along-side a larger framework of evolution, and finds that both risk and disease have evolved (Crespi, 2011) [16] Although there is much to be gained from research guided by evolutionary perspective. It is not without its own challenges. In addition to the nebulous nature of genomic behavior and phenotypic presentation, often there is a lack of data (Crespi, 2011)[17] The last of these issues at least is being addressed somewhat, or will be with the continued research and publications of studies on evolution and disease, especially when coupled with ever-improving technological measures.

Not all of the research has focused on development in utero. Indeed there have been a range of ages and conditions examined. Hotchberg and Belsky incorporate a life-history perspective and their work focuses on adolescence rather than childhood. Substantial variation in phenotypic paths and presentations suggest the potential for significant environmental influence. They focus on plasticity between stages of development and the factors that shape it. Rate of maturation, fecundity, and fertility were all impacted by environmental circumstances. They argue that early maturation can actually be a positive thing, reflecting opportunistic actions within specific conditions (Hotchberg and Belsky, 2013)[18]

Genetic and Epigenetic Basis of Developmental Changes
Technological advances that have allowed better and better access to the growth of the human form in utero have proven particularly formative in studies involving focus on genetic and epigenetic development. This ability has been, and continues to be well utilized in human eve devo studies. and Bakker et al look at the interconnected nature of developmental processes and attempt to use fetal vertebral abnormalities as an indicator for other malformations. They found that the origin of the cells was not nearly as highly correlated as the observed developmental signals (Broek and Bakker et al, 2012)[19] Not only did they find a connection between in utero development and malformations, but they were also correlated in their level of severity. The paper further covers specific connections and associations. The authors argue that their finding could be of particular use in the identification of other physical malformations through examination of vertebral development and function. These things that were not previously accessible through an ultrasound could be predicted and even mitigated with earlier detection. (Broek and Bakker et al, 2012)[20]

The connections between development of abnormalities has also been explored by Freiston and Galis. In An evolutionary and developmental perspective on congenital abnormalities, they specifically look at ribs, digits, and mammalian asymmetry. They make this argument that these connections are fruitful and important based on the intrinsically linked nature of human tissue in utero. The paper argues that this construction is relevant and useful for both the study of disease instances, the consistency in evolution of body plans, and full explorations of developmental constraints. (Friestone and Galis, 2010)[21] They not only assert the utility of this connection, but also its reliability for medical diagnosis. (Friestone and Galis, 2010)[22] This is a similar argument as made in the larger body of Evo Dego studies, but illustrates the human specific application or potential applications for findings. There is specific coverage of sexual dimorphic prenatal digit ratio. Sexual dimorphism was found as early as 14 weeks and maintained whether or not the fleshy finger part was included. (Friestone and Galis, 2010)[23

Language and Cognitive Studies
The use of evolutionary theories and eve devo in particular have not been limited to the tangible. Languages and cognitive function have also been subjects of evolutionary studies. Insofar as language and Evo Devo, there is tension from the gate. Much of this contention has centered around whether to view and study language as an adaptation in and of its self, or as a by-product of other adaptations. Jackendoff and Pinker have argued for language as an adaptation owing to the interdependent social nature of humans. To support these claims, he points to things like the bi-directionality in language usage and comprehension. (Jackendoff and Pinker, 2005)[24] This is a counter to the claims by theorists like Chomsky[25], who argued against language as a human specific adaptation.

Adaptation and adaptive theory has been argued even separate from its utility in the study of language. Gould and Lewontin directly engage with what were viewed as major flaws within adaptive theory in examination of the spandrels of San Marco. Among the issues identified is the lack of distinction between what trait developed and how it is used, and the underlying reasons or forces that created the novel trait initially (Gould and Lewontin)[26] This is particularly difficult to access in intangible language and cognition.

This debate has continued over decades and most often presents in the form of a response and published dialogue between theorists. This continued debate has prompted efforts to marry the two perspectives in a useful way. In Skyhooks and Spandrels: What can Evo-Devo tell about the evolution of syntax, 2009, Fitch argues that these two approaches can be rectified with the study of “neutral computation and mammalian brain development” (Fitch, 2009) It may be more useful to consider specific components of neural computation and development, what has been selected for, and to what end? (Fitch, 2009)[27]

Ploeger and Galis tackled issues of modular evolvability and developmental constraints within human and primate evolutionary trajectories. They argue that research in the cognitive sciences should consider these and should be approached interdisciplinary. They frame this in the focus on three things: 1. Modularity- the ability of a system to organize individuals for the benefit of the whole 2. Evolvability- ability of organism or organisms to adapt through evolution and 3. Developmental constraints- those things that act as barriers to evolutionary adaptations. (Ploeger and Galis, 2011)[28]

Also See
Evo Devo

Neoteny

Regulation of gene expression