A Century of Development

“The first step in the argument is one which will scarcely be denied but is perhaps often overlooked. The capacity to respond to an external stimulus by some developmental reaction, such as the formation of a callosity, must itself be under genetic control”

(Waddington, 1942)

This brief account of the role of developmental biology in anthropology largely reflects my personal journey as I followed my intellectual curiosity and professional interests. If you asked others who have brought developmental biology to anthropological inquiry to write such an essay, you might-in fact you would- get a very different account. Relative to anthropology, the field of developmental biology is new, but it is mature and thriving. My essay is long on impressions and does not attempt to describe or document a complete history of research or basic themes in the fields of developmental biology or evolutionary developmental biology. Rather, it is the story of the influence of developmental biology on anthropological inquiry from my personal perspective as well as an accounting of what I think anthropology offers to developmental biology. For more comprehensive reviews of developmental biology from various perspectives, see (Fraser and Harland, 2000; Galloway and Tabin, 2008; Gilbert, 1991, 2010; Hamburger, 1988; Horder et al., 1986; Lovejoy et al., 2003; Maienschein et al., 2004; Oppenheimer, 1967; Scott, 2000).

Though standardized definitions are hard to come by, it is probably fair to say that most anthropologists think of “development” in terms of early, prenatal changes in morphology that occur due to molecularly driven processes, while “growth” is thought of as changes in dimensions of form, with the most profound changes in dimensions occurring postnatally. Even before the transformation of the biological sciences triggered by the birth of molecular biology, Stanley Garn realized the close relationship between growth and development, but acknowledged that specific aspects of development can take place without changes in magnitude. Garn, a key figure in physical anthropology of the 20^th century (Smith et al., 2009), stated that “growth” refers mainly to changes in magnitude, increments in the size of organs, increases in the thickness of tissues, or changes in the size of individuals as a whole, while “development” implies differentiation, changes in proportions, and changes in complexity (Garn, 1952). Distinct from this perspective, development as contemplated by developmental biologists focuses largely on the identification of specific genetic instructions, their role in larger regulatory networks, and how those instructions are used to change the function or behavior of cells. Researchers in both disciplines share an interest in how changes in phenotypes are produced by developmental processes driven by genes thus linking development with evolution. But modern developmental biology is more focused on the molecular bases of development (identifying molecular pathways, signaling networks and their component processes) while anthropologists use the explanation of those mechanisms as an entry into understanding the formation of complex traits and their variation.

Developmental biologists define mechanism at the level of the molecule while anthropologists recognize the importance of molecular underpinnings while placing equal emphasis on the production of form. Anthropologists study variation as a source of the marked diversity of animal life on our planet. We tend, as a discipline, to describe that variation, estimate its trajectory and its boundaries and hypothesize about its potential for the evolution of forms. But to understand phenotypic variation we need to know how it is created and how it is constrained. And this brings us to the developmental basis of the production of structure: a study of the mechanisms, processes, and events that underlie the generation of morphological variation on which natural selection acts.

In this essay, I will focus on how the historic study of growth and development evolved in the field of anthropology being fully equipped to embrace and inform the field of developmental biology as it emerged. Those interested in the history of the study of growth in physical anthropology should consult the special issue of AJPA (volume 150; introduced by (Sherwood and Duren, 2013)) that includes papers from a symposium focused on growth research, as well as other essays in this volume [e.g., Bogin et al].

THE BIOGENIC LAW, ITS FALL, AND THE RELEVANCE OF DEVELOPMENT IN EARLY ANTHROPOLOGY

“The inputs to heterochronic change are simple quantitative alterations in rate and timing of underlying processes that generate altered morphologies.”

(Gould, 1982)

Scholars of the early 19^th century proposed that “advanced” species were produced by changes in development. However, their ideas about development were linear. Meckel’s recapitulation theory, later rebranded as the Meckel-Serres law, stated that embryos pass through successive stages that represent the adult forms of less complex organisms and that these developmental stages chronologically replayed the scala naturae (Barnes, 2014). Later, Ernst Haeckel proposed the term heterochrony and expanded the Meckel-Serres Law to incorporate evolutionary principles into what has become known as the recapitulation theory that produced the Biogenic law, that ontogeny recapitulates phylogeny (Gould, 1977). The Biogenic law introduced timing as a fundamental player in evolutionary process and proposed that evolution occurs primarily by changes in the timing of development; specifically by the recapitulation of phylogeny by ontogeny. In short, Haeckel’s theory rejected the rungs of the scala naturae, which was accepted at that time, and instead believed that evolutionary stages were repeated in the embryonic development of an animal. However, Haeckel’s Biogenic law was restricted in the way that it operated: it only allowed acceleration of development, and recapitulation only included the shoving of adult ancestral forms into the developmental stages of descendants by terminal addition. The assertion that embryos pass through successive stages that represent the adult forms of less complex organisms meant that adult stages were added to the developmental sequences of descendants, and that earlier stages were deleted from each of these new developmental features so that ontogeny becomes a sequence of adult forms ordered from less to more “advanced” organisms (Gould, 1977)(Figure 1). As a result, each successive stage in the development of advanced (descendant) species is characterized by the adult form of all species that appeared in its evolutionary history.

Figure 1.

Ontogeny recapitulates phylogeny or the Recapitulation Theory as espoused by Ernst Haeckel claims that the development of advanced species (species D) passes through stages represented by adult organisms of more primitive species (species A, B, C). Consequently, each successive stage in the development of an individual in an “advanced” species represents one of the adult forms that appeared in its evolutionary history. Thus, the rate of change is increased in more advanced species in which adult ancestral forms are ordered into the developmental stages of descendants by terminal addition. Change in evolving lineages occurred through change in developmental timing of appearance and increased rates of development for characters already present in an ancestor by driving the ancestral adult forms into the juvenile stages of descendants.

This “law”, of course, could not hold. A series of essays published in 1828 by Karl Ernst von Baer (founder of the Russian school of Anthropology) in reaction to the Meckel-Serres Law cast doubt on recapitulation theory (Gould, 1977). The von Baer essays were simple and based largely on a key observation that embryonic development proceeds from general to specific through differentiation of structure, and that the variation seen among adult morphologies conflicts with observations of embryos (Gould, 1977). In short, at no stage is the early embryo of an “advanced” (descendent) species like the adult of a “lower” (ancestor) species. Eventually, the idea of adult forms being pushed into embryonic stages was too difficult to maintain. The developing field of microscopy would contribute to the solution of this controversy by providing new kinds of evidence that everyone, and not just a chosen few of science, could see. Once people were able to see embryos develop, they could search for the supposed adult structures and correctly identify them as embryonic features that are common to early development of all vertebrates (i.e., pharyngeal grooves were not adult gill slits but instead structures that separate the pharyngeal arches). Changes in development were not restricted in the way they operated: novel characters could be produced at any time during development, development could be accelerated or slowed, and the developmental appearance of features in an ancestor could be shifted to an earlier or later stage of development in a descendent (displacement) (de Beer, 1940; Gould, 1977).

Persuasive as von Baer’s essays and new data sets might have been, the recapitulation theory continued to diffuse across disciplines and was still in vogue in the early part of the 20^th century (see (Gould, 1977)). William King Gregory argued astutely against the biogenetic law and its application to the shape of the forehead of primitive man in AJPA (Gregory, 1925), positing that forehead shape was likely driven by the evolution of the brain and insisting that classification of any feature as primitive requires independent evidence. Recapitulation was eventually abandoned on the basis of von Baer’s arguments, the findings of experimental embryologists, the growing acceptance of evolution, and the rise of genetics (Gould, 1977), but heterochrony –developmental change in the timing or rate of events leading to morphological change- would remain relevant.

It is well known that embryology and developmental genetics were left out of the forging of genetics, paleontology, and systematics to build the “Modern Synthesis” (Carroll, 2008; Gilbert, 2010; Gilbert et al., 1996; Gould, 1992; Hendrikse et al., 2007; Wilkins, 2002), but anthropologists actively acknowledged the value of early development in understanding the human condition in the early part of the 20^th century. In 1913 Franklin P Mall established the Department of Embryology at the Carnegie Institution of Washington (CIW) around an original series of human embryos that he collected and that would be housed at Johns Hopkins University. Mall, an anatomist, fundamentally changed the way that anatomy was taught at Johns Hopkins Medical School and pioneered the scientific study of normal and abnormal human embryonic development (Buettner, 2007). The nascent CIW quickly amassed hundreds of human embryos for study (Maienschein et al., 2004) and by the 1960s would collect and permanently store more than 10,000 embryos.

As the number of human embryonic specimens grew yielding valuable information, many were used to develop the formal classification of embryo development known as the Carnegie Stages (Hill, 2017), which remains the global standard for human embryological research. But the original collection of preserved human embryos did not include data from the earliest stages of development -from fertilization to implantation- and at that time, it was impossible to collect those data from humans (Maienschein et al., 2004). Given knowledge of comparative anatomy, it was reasoned that a complete series of non-human primate (monkey) embryos of known gestational age would complete the human series and aid in the interpretation of the human material making the CIW human embryo collection ever more valuable. In the summer of 1924 the CIW sponsored an expedition to Nicaragua that was led by Adolf Schultz where he collected 191 monkey specimens including embryos and fetuses that served as the start of the Carnegie non-human primate embryo collection. Soon after, a colony was established in order to guarantee a continual supply of more precisely aged embryos from timed matings (Hanson, 2004).

Schultz made use of the Carnegie Collection of human embryos by systematically accumulating anthropometric measures of developing fetuses (Schultz, 1923). Special notes were made on differences in prenatal human development between the races (i.e., “whites” and “negroes”) with the observed patterns thought to favor a monophyletic origin “at least for these two human races”(Schultz, 1923). This was a revolutionary conclusion published one year before the Scopes trial into a violently racist post-WWI America. These early, intensive efforts by Schultz to report the growth and development of particular populations by precise measurements gradually included interpretive summaries of the correspondence between prenatal development and phenotypic variation in human and non-human primates leaving an enduring impression on the thinking of primatologists (Stewart, 1983) and presaging the influence of developmental biology in anthropology.

ANTHROPOLOGISTS AS ANATOMISTS

Certain it is that physical anthropologists and human geneticists are - or should be - complementary members of a team focusing upon human biology in all its possible medicodental ramifications.

(Krogman, 1951)

The maturation of cell biology in the 1960s and 1970’s, and ensuing advances in genetics and molecular biology provided the backdrop for big changes that occurred in academic departments beginning in the 1970s and early 1980s. The study of human gross anatomy played a significant role in the medical school curricula of the first half of the 20^th century, with first year medical students studying anatomy for a full year using only Grey’s Anatomy as their dissector. Most medical schools had Departments of Anatomy whose faculty was charged with teaching these dissection-heavy courses. As new trends within disciplines emerged (“histochemistry” appeared in anatomy departments in the late 1940s according to Stewart (Stewart, 1983)), the names of those departments transformed to be more descriptive of the faculty housed within (e.g., Department of Cell Biology and Anatomy, Department of Pathology and Anatomical Sciences, etc). This reflected an overall trend in biological sciences away from whole organism research, towards reductionist approaches that focused on phenomena at the cellular and subcellular levels. As time went on, fewer and fewer of the new faculty whose research focused on cell biology and later, molecular mechanisms of the cell had interest in, or the required training to teach human gross anatomy. The active use of gross dissection in the field of Medicine became increasingly limited to those seeking more advanced training for surgical specialties. Biological anthropologists have and continue to actively use anatomy in research in ways that can enrich their knowledge and provide information for the research questions posed. Consequently, the search for faculty to teach human gross anatomy logically turned to experts from the fields of anthropology and human paleontology as they were trained in comparative anatomy, had a keen knowledge of human traits, and conducted research in whole organism biology. At Johns Hopkins University School of Medicine, the creation of a graduate program in Functional Anatomy and Evolution by Alan Walker, originally housed within the Department of Cell Biology and Anatomy, was a form of history repeating itself. Johns Hopkins University had partially financed one of Adolph Schultz’ trips to Nicaragua and subsequently invited him to join their faculty as associate professor of physical anthropology in the Department of Anatomy, the first such position in any American medical school (Stewart, 1983).

And so during the 1970s and 1980s anthropologists and vertebrate paleontologists were eagerly sought to join the faculty of some of the most prestigious medical schools in the USA. Some dental, physical therapy, and osteopathic programs followed the same model, resulting in even more diverse faculty placements for biological anthropologists trained in comparative and human gross anatomy. In the program in which I was trained, and in the one that I joined as a new Assistant Professor, development and embryology were deeply embedded within the presentation of the anatomical curriculum to medical students. Dissecting the adult abdomen is a nightmare of twisted bowel and peritoneal sacs. But if you understand how the gut forms as a straight tube suspended by mesenteries, that goes through an umbilical herniation turning 90 degrees counterclockwise on the axis of the superior mesenteric artery, and then returns to the confines of the abdomen while completing a further 180-degree counterclockwise rotation for a total of 270 degrees, the adult anatomy becomes fairly straight forward. Knowledge of embryological development explains many details of adult anatomy including the postnatal anatomy of the face, the limbs, the heart, and even the notoriously difficult pelvis and perineum.

This change in the institutional affiliation of some groups of physical anthropologists resulted in the initiation of new graduate programs in evolutionary-based studies of anatomy and anthropological science that produced many fine scholars, some of whom have gone on to train additional generations of anatomically-focused biological anthropologists. In many of these programs, the medical and dental students received an evolutionary- and/or functional anatomy-based program of study taught by anthropologists and paleontologists, fulfilling a prophecy outlined by Wilton Marion Krogman in his dinner address to the AAPA in 1951 (Krogman, 1951). On the flip side, PhD students in Anthropological sciences were exposed to pre-clinical training of future physicians and to graduate students in the basic sciences who were conducting research in genetics, biochemistry, molecular biology, neuroscience, and bioengineering. This movement also put anthropology faculty in physical proximity to molecular labs and within ear shot of research and seminars pertaining to new developments in genetics, molecular biology and developmental biology. It didn’t take many stirs of this pot to produce novel, cross disciplinary research alliances. Collaborations born of collegiality and curiosity about stimulating research problems relevant to evolution, genetics, and biomedicine developed organically.

ANTHROPOLOGY AND DEVELOPMENT AND THE RE-DISCOVERY OF HETEROCHRONY

…..the mystery of human origins is expanding beyond the description and history of human traits, towards the genetic mechanisms underlying their formation and evolution.

(Carroll, 2003).

Stephen Jay Gould’s publication of Ontogeny and Phylogeny (Gould, 1977) demonstrated the potential importance of changes in development as a key to evolutionary change. Gould provided historical detail to demonstrate why the biogenetic law failed, while reintroducing heterochrony as an explanation for evolutionary change. Gould’s efforts to demonstrate how changes in rate and timing of development foster small changes that can cascade into large phenotypic effects, potentially resulting in rapid, large-scale variation with the capacity for phyletic change were driven in part by his dissatisfaction with adaptation as a common explanation for evolutionary change (Gould and Lewontin, 1979). He wanted to understand the developmental basis for the appearance and evolution of adaptive features rather than prolong the “strict Darwinian idea that evolution may be traced as insensibly transitional adaptations to external environments” (Gould, 1992). But in 1977 Gould could only describe the potential significance of developmental programs to the evolutionary pathways of morphological change (Gould, 1992). Their genetic basis had yet to be discovered.

Dissatisfaction with Gould’s clock model prompted a formalism for heterochronic analysis proposed by Alberch et al. (1979), and data reveal an obvious surge in heterochronic research in the biological sciences after these publications (Hanken, 2015). Additional quantitative methods for the study of heterochrony were later introduced by anthropologists (e.g., (Godfrey and Sutherland, 1996; Richtsmeier and Lele, 1993)), but the immediate response to the re-introduction of heterochrony in anthropology was an increase in comparative analyses of postnatal growth patterns of extant species. The thinking went something like: If differences in growth patterns between sexes, species, ethnic groups, etc., could be precisely determined, the “heterochronic process” responsible for the change in growth pattern could be identified and inform us about how the change came about.

Shea (1983) integrated allometry and heterochrony to investigate differing growth patterns to explain inter-specific differences in adult morphologies of the African apes citing the importance of differentiating between changes in timing and changes in rate of growth (parameters almost impossible to estimate without precise data on chronological or developmental age). Ultimately, Shea concluded that heterochrony had played a central role in our evolutionary history, but stressed that rigorous testing would be required to determine whether neoteny was the heterochronic process central to human evolution (Shea, 1989). Godfrey and Sutherland (Godfrey and Sutherland, 1996) proposed a matrix representation of growth and shape change specifically to test hypotheses about the role of neoteny in human evolution. Leigh (1996) investigated subadult growth spurts in anthropoid primates and found that humans exhibit growth spurts that are broadly comparable to other primates, but are shifted to late absolute ages, again emphasizing the importance of distinguishing chronological and developmental age. And though it was recognized that certain primates, or specific organs in certain primates, were relatively more or less mature at birth, most datasets put forward to test heterochronic hypotheses about human evolution were postnatal. Not until later would embryonic specimens be included in studies of heterochronic processes aimed at understanding the emergence of phylogenetically derived modern human morphologies (e.g., (Jeffery, 2002; Jeffery and Spoor, 2004b, 2004a)).

Even with the valuable addition of embryonic specimens, the proposed systems of study were classificatory, able only to recognize patterns associated with morphological diversification. None of the above methods developed to formalize heterochronic analyses offered a way to get at the mechanism(s) underlying heterochronic change. Knowledge of the genetics of development was sketchy at best in the late 1970s and even the discovery of Hox genes in the early 1980s did not have a rapid, direct effect on heterochronic analyses in anthropology. Investigators were focused on questions pertaining to the relative heritability of traits (Cheverud and Buikstra, 1981, 1982; Corruccini et al., 1986; Frisancho et al., 1981; Nichol; Townsend, 1980), the influence of prenatal stressors on the development of complex traits (Harris and Nweeia, 1980; Mooney et al., 1985; Siegel et al., 1977; Siegel and Mooney, 1987), how evolutionary change in complex traits might occur (Black et al., 1980; Bondioli et al., 1986; Gilligan et al., 1985; Karafet et al., 1997), and how to infer differences in biomechanical inputs and function from morphological variation (Daegling, 1989; Daegling and Grine, 1991; Spencer and Demes, 1993; Strasser, 1992). By the end of the 20^th century anthropologists were investigating the influence of genetic variation on phenotypic variation in complex traits; e.g., morphological features, physiological characteristics, and even aspects of behavior (Rogers et al., 1999) [see Weiss, this volume], but the focus remained on the pattern of inheritance rather than the heritable developmental mechanisms that produced morphological diversity.

THE BIRTH OF DEVELOPMENTAL BIOLOGY AND EVO-DEVO

“Developmental biology is a celebration of natural beauty combined with the elegance of molecular control systems”

(Scott, 2000: 38)

Developmental biology arose from the fields of embryology and genetics, quite separately from anthropology. Developmental biology seeks to understand how a single cell, the fertilized egg, gives rise to all the cells in the body, how each cell is directed to differentiate into a specific cell type that executes a specific function, and how a specific genetic change triggers changes at the cellular level. If all cells within the body are derived from a single cell, and they all carry the same set of genes, how do cells differentially receive and respond to genetic information to function as blood, bone, or muscle cells? How and why do bone cells respond to a genetic mutation in one way and neural cells respond in another way, while other cells don’t seem to have an unusual response and function normally? Molecular genetics figures critically into experiments designed to answer these questions, as the focus in the most general sense, is on trying to understand how genes direct development, both normal and abnormal.

Evolutionary developmental biology (evo-devo) focuses on how changes in embryonic development during single generations relate to the evolutionary changes that occur between generations (Hall, 2012; Hendrikse et al., 2007). The focus is on mechanism with the goal of identifying and explaining the specific developmental events that generate morphological diversity (Wilkins, 2002). Evo-devo arose primarily from the fields of embryology, developmental biology, evolutionary biology, and genetics - fields of study that operated in parallel for decades with little interaction, despite the realization by some early scholars (e.g., J. Huxley, C.H. Waddington, G. de Beer, J.T. Bonner) that the study of inherited developmental processes was required to understand evolution. The field is based on a fairly simple idea: that mechanisms of evolutionary change that modify genetic variation (natural selection, genetic drift, mutation, migration (gene flow)) and cause changes in the frequency of particular phenotypes do so through changes in genes that provide instructions for development.

The emergence of evo-devo as a discipline is typically associated with the discovery of the homeobox, a 60-amino acid-encoding DNA sequence contained within a gene (e.g., Hox gene). The importance of embryology in evolution had been recognized by Darwin (Darwin, 1859)(chapter 13), but very little developmental research entered into mainstream evolutionary biology until the discovery of Hox genes. Hox genes were originally discovered in the genome of the fruit fly, Drosophila melanogaster (McGinnis et al., 1984; Scott and Weiner, 1984), and subsequently clusters of related Hox genes were identified throughout the three kingdoms of multicellular organisms, verifying that these codes for a protein domain were highly conserved (Mark et al., 1997) (Figure 2). Before the discovery of Hox genes, biologists thought that development of each species was unique and therefore separate investigations of development of each species would be required to understand the full complexity of development (Carroll, 2017). It was not expected that the genes providing instructions for building specific body parts and organs in the fruit fly would have equivalents in humans (Carroll, 2017). The discovery of Hox genes verified the intuition of many biologists that genes must provide explanation for both development and evolution and fostered the realization that in spite of observed morphological diversity, there is a deep similarity in the key molecular regulators of development and the way in which organisms use them (Rolian, 2014; Wilkins, 2002).

Figure 2.

Humans and fruit flies look nothing alike, but they construct their anatomies using remarkably similar genetic circuitry, most of which was inherited from a common ancestor that lived ~600 million years ago. Schematic representation of the genes (colored boxes showing the anterior-most expression domain of a given gene subfamily), genomic organization, and collinear expression patterns of Drosophila HOM genes and mammalian Hox genes schematized in a fly and a human fetus using data extrapolated from mouse. The Drosophila homeotic complex, the four human Hox complexes, and a hypothetical ancestral homeotic complex and their proposed phylogenetic relationships are shown. For details and figure as originally published see (Mark et al., 1997).

Homeobox-containing genes encode DNA-binding proteins that regulate gene expression and control various aspects of morphogenesis and cell differentiation, providing cells with regional information along the main body axis. These gene networks and the instructions that they code provide cells with positional information as they concurrently receive instructions to differentiate and contribute to the formation of a body part. Considerable information about the function of Hox genes was gained when it was shown that mutations in fruit fly Hox genes result in “homeotic transformations”, where the positioning of the morphogenesis of complete body parts is changed resulting in the transformation of one of the insect’s body segments into the likeness of another (Mark et al., 1997). The discovery of Hox genes also reinforced a principal of heterochrony, dissociation, in Gould’s model the ability for size increase and shape change to become uncoupled as the organism ages. Dissociation could only be addressed morphologically prior to the discovery of Hox genes, but once detected, Hox genes established a logic for how specific traits conserved developmental programs, revealed the mechanism responsible for the independence of development of organs, and stimulated further research into modularity by providing a potential explanation for its role.

One of the most important contributions of evo-devo is the demonstration that regulation of established genetic networks could play a central role in the production of diversity by changing their contribution to morphogenetic processes of development (Carroll, 2005). The potential importance of the regulation of genes in the evolution of phenotypes was famously suggested by King and Wilson (1975) as they presented and then proposed an explanation for the difference in estimates of chimp to human distance based on molecular and organismal evidence. They hypothesized that a relatively small number of genetic changes in systems controlling the expression of genes present in both species could account for the major organismal differences between humans and chimpanzees (King and Wilson, 1975). This hypothesis presaged investigations that would reveal the fundamentals of regulation, now comprise a field that focuses on how minor changes in the timing or spatial distribution of gene expression produce phenotypic variation, including changes that could initiate phyletic change (Carroll, 2003, 2005; Carroll et al., 2001). By 2000, the human genome project exposed the lack of correspondence between organismal complexity and gene quantity, reaffirming the proposition that the genetic basis of the origin of human traits was not going to be solved by the discovery of a great deal of novel genes.

The field of evo-devo grew out of these discoveries that encouraged interested scientists to investigate the mechanisms of inherited developmental programs through which phenotypic novelty and variation are generated (Carroll, 2008). Questions posed by researchers in this new field did not focus on what features or traits evolved to be similar or different among organisms, but rather on how developmental processes evolved to produce variation in those traits while employing much the same molecular machinery.

EVO-DEVO CONTEXT OF A NONMETRIC TRAIT: CRANIAL SUTURE CLOSURE

“One of the major problems confronting modern biology is to understand how complex morphological structures arise during development and how they are altered during evolution”

(Atchley and Hall, 1991: 151)

My entry into evolutionary developmental biology research occurred through the study of nonmetric traits. Nonmetric, discontinuous, or discrete traits are anomalies in the normal anatomy of the skeleton that are recorded as present or absent and used as descriptive features of the skeleton (Hooten, 1918), as indicators of stress experienced by the individual (Deol and Truslove, 1957; Grüneberg, 1963; Pucciarelli, 1974; Searle, 1954), and of biological relationships between populations (Brothwell, 1959; Buikstra, 1980; Ossenberg, 1976). Cheverud and Buikstra (Cheverud and Buikstra, 1981) noted a difference in heritability values for two classes of nonmetric traits (hyper/hypostotic traits and foraminal traits) and suggested that the observed pattern might be related to differences in the determinants of trait ontogeny. We set out to test a definitive developmental biology hypothesis that the relative degree of inheritance of certain traits could be traced directly to genetic variants operative during development (Richtsmeier and McGrath, 1986). Operating unaware of the new field of developmental biology, I knew only to seek the developmental basis of these traits by reviewing published embryological accounts (e.g., (Bennett, 1965; Dodo, 1986; Grüneberg, 1963; O’Rahilly and Muller, 1984; Ossenberg, 1974)) and by dissecting these traits in adult (!) laboratory mice.

As an anthropologist, I was well trained to observe and analyze the skeleton and was familiar with the relevant gross anatomy, but I did not recognize the importance of the coordinated development of soft and hard tissues. The functional matrix (FM) hypothesis (Moss, 1969, 1971; Moss M.L., 1997; Moss and Young, 1960) proposed that the morphology of bone is dependent upon the surrounding soft tissues and functional spaces, and caused me to think about bone in a different way. Neurovascular bundles didn’t burrow through bone – the skeletal system formed after the vascular and neural systems were established, mineralizing in coordination with already established soft tissues. How could this happen? How did the bone “know” to mineralize around arteries, veins and nerves with such accuracy? We now know that the genes operative during the differentiation of osteoblasts (e.g., Sox9, Runx2, Osx) are expressed in a synchronized fashion as osteoprogenitor cells proliferate, differentiate, and change function, and that cell-to-cell signaling underlies this type of developmental coordination (Long, 2011), but we have yet to determine how bone discerns precisely where and when to form and where and when not to. Genes are not alone in their supervision of bone formation: subtle changes in developmental conditions like a change in the rate of growth of a soft tissue might bring about phenotypic change through altered mechanotransduction (e.g., (Adachi et al., 2003; Bacabac et al., 2008; Carter et al., 1998; Love, 2015)).

One “etiological” category of nonmetric traits under consideration was “fusion” traits. Anthropologists had long recognized suture irregularities in human and nonhuman skeletal populations (e.g.,(Bennett, 1965; El-Najjar and Dawson, 1977; Cheverud and Buikstra, 1981; Corner and Richtsmeier, 1992; McGrath et al., 1984; Mooney and Siegel, 1991)), but little concrete evidence existed regarding their specific patterns of inheritance or why sutures close prematurely in some instances. Craniosynostosis, which always involves the premature fusion of one or more of the cranial vault sutures and can include many associated craniofacial dysmorphologies, is a relatively common congenital malformation (Cohen and MacLean, 2000; Heuzé et al., 2014). The identification of mutations in people diagnosed with the most prevalent craniosynostosis syndromes in genes encoding fibroblast growth factor receptors 1, 2 and 3 (FGFR1, FGFR2 and FGFR3) (Heuzé et al., 2014; Johnson and Wilkie, 2011; Wilkie et al., 2017) made it possible to insert these genetic variants into laboratory mice creating living models of the developmental genetic processes responsible for premature suture closure (Figure 3). The FGF/FGFR signaling system is conserved across metazoans (Bertrand et al., 2011) and is critical to normal bone growth and development (Ornitz and Marie, 2002). But evolution of these gene families has enabled functional diversification of the FGF/FGFR signaling system and modifications in regulatory sequences resulting in their almost ubiquitous involvement in developmental processes of most tissues (Itoh and Ornitz, 2004; Ornitz and Itoh, 2015). Mouse models of craniosynostosis enabled interrogation of the genetics of bone development local to the suture (Figure 4), but also allowed analysis of the effects of these mutations on non-osseous tissues, and the response of cells and tissues to changes in signals generated or communicated by neighboring tissues (e.g., meninges, brain, chondrocranium) (e.g., (Kawasaki and Richtsmeier, 2017; Lee et al., 2017; Motch Perrine et al., 2017; Opperman, 2000; Richtsmeier and Flaherty, 2013)).

Figure 3.

Lateral view of 3D reconstruction of micro computed tomography (μCT) and magnetic resonance microscopy (MRM) images of the heads of: Fgfr2c^C342Y/+ mouse model for Crouzon syndrome (upper left); Fgfr2^+/S252W mouse model for Apert syndrome (upper right); Fgfr2^+P253R mouse model for Apert syndrome (lower left); and an unaffected littermate (lower right). All mice are newborn showing soft and hard tissue segmented from μCT and MRM for each animal superimposed. Mouse nasopharynx (in pink), vitreous humor (in green), and vestibular canal and cochlea (as one unit in purple) are segmented separately. Note that the coronal suture is open in the typically developing mouse (unaffected littermate) while the mouse models for craniosynostosis syndromes show varying states of premature closure of the coronal suture. Each scale bars measures 1 mm. Figure composed by Susan Motch Perrine.

Figure 4.

Suture anatomy. A) Two-photon laser scanning microscopy (2PLSM) image of a mouse inter parietal suture with bone labeled fluorescently with calcein, two days postnatal (P2), coronal section. B) 2PLSM image of a mouse coronal suture with bone labeled fluorescently with calcein (P2), para sagittal section, frontal at left, parietal at right. Note that the interparietal (sagittal) suture is an abutting suture and the coronal is an overlapping suture. C) Cell composition of interparietal suture, with meningeal layers below (pink (dura mater), gray (arachnoid mater), and black (pia mater)). D) Cell composition of coronal suture with dura mater below (pink). Original artwork by Kevin Flaherty (Flaherty et al., 2016).

The evolution of the vertebrates is marked by the appearance of distinguishing morphological and functional features concentrated in the head (de Beer, 1937; Gregory, 1929; Moore, 1981) made possible by the evolution of the neural crest (Gans and Northcutt, 1983). With the evolution of the neural crest (Bronner and LeDouarin, 2012; Hall, 1999), bones of the dermatocranium that mineralize intramembranously (Percival and Richtsmeier, 2013) arose to protect the expanding neural mass and associated sensory organs (Brugmann et al., 2006; Cordero et al., 2011; Marcucio et al., 2005). The emergence of the vertebrates marks the rise of mobile predation, a way of life that required the advent of coordinated specializations in respiratory gas exchange and modifications in brain structure and function, special senses, neural circuitry, the skeleton, and pharyngeal anatomy (Kardong, 2012). For this trait complex to arise, developmental genetic networks needed to emerge that fostered their integrated development. Table 1 provides a list of the characteristics that together define the vertebrates (Kardong, 2012) along with examples from the literature demonstrating that FGF/FGFR signaling was part of this developmental system (Bertrand et al., 2011).

Table 1.

Vertebrate characteristics as defined by (Kardong, 2012) with evidence of the role of FGF/FGFR signaling in the formation of each characteristic as presented in the relavent literature.

Vertebrate characteristic	Reference
bilateral symmetry	(Deng et al., 1994)
two pairs of jointed locomotor appendages	(Sun et al., 2002)
outer protective covering (skin, feathers, etc)	(Wang et al., 2012)
metamerism (repeated segments)	(Aragon and Pujades, 2009; Graham et al., 2014)
well-developed coelom or body cavity	(Okazawa et al., 2015)
well-developed endoskeleton made of cartilage and bone	(Ornitz, 2002)
developed, protected nervous system	(Aldridge et al., 2010; Trokovic et al., 2003)
well-developed cephalic sense organs	(Hébert, 2011)
respiratory system associated with pharynx	(Frisdal and Trainor, 2014)
closely related genital and excretory systems	(Polanska et al., 2011)
digestive tract with two major glands (liver, pancreas)	(Danopoulos et al., 2017)

The dermatocranium of early vertebrates consisted of several skeletal series (facial, orbital, temporal, vault, palatal)(Kardong, 2012) with sutures forming boundaries between bones that functioned as joints mediating cranial mechanics (Herring, 2000). As mobile predation emerged, a decline in cranial kinesis evolved over approximately 150 million years of synapsid history through the loss of cranial bones and sutures (Sidor, 2001). This can reflect one of two processes: either an ossification center for a particular bone is lost (fails to form), or two previously independent ossification centers coalesce by mineralization of the sutural mesenchyme that previously separated them. This evolutionary trend that continued throughout the Cenozoic suggests that the genetic networks that supervise cells that function to build vertebrate heads fostered a molecular system that could direct atypical closure of sutures in typically developing individuals.

When we consider premature suture fusion associated with an FGFR mutation with the knowledge of the evolution of FGF/FGFR signaling, it seems unlikely that suture fusion is an isolated event. Though scored as dichotomous, anyone who has collected data on nonmetric traits knows that there are many grey areas between present and absent or open and closed, suggesting a degree of developmental complexity that underlies their expression. An FGFR mutation could lead to changes in a number of the downstream intracellular pathways that vary on the basis of cell type and time of development, changing the strength or timing of cell-to-cell signaling, or shifting boundaries that affect the patterning of character complexes. This would result in phenotypic variation in soft tissue morphologies and bone shapes that could be subtle or so marked as to evoke phyletic change.

Many mutations on at least ten genes (many of them transcription factors) have been identified in association with syndromic forms of craniosynostosis (Heuzé et al., 2014; Wilkie et al., 2017) confirming that premature suture closure can result from diverse molecular changes that operate in complex networks. Understanding variation in suture closure patterns requires that we appreciate a suture as a component of a larger evolutionary developmental system. Suture formation (like growth plate development, tooth germ formation, etc.) is a part of normal development and genetic networks regulating suture patency are highly complex. How genetic instructions are used to direct the integrated development of tissues, and how developing tissues physically affect the growth of other tissues are equally important aspects of the evolutionary developmental context that supervises trait expression.

ANTHROPOLOGY MEETS DEVELOPMENTAL BIOLOGY: FOCUS ON THE PRODUCTION OF PHENOTYPIC VARIATION

“If groups so disparate as arthropods and vertebrates bear substantial homology in basic morphological design, then constraints on pathways are legion and life’s commonality will be resolved more by the study of development than by adaptation.”

Gould, 1992: 279

Developmental biology grew out of a comparative, experimental embryology where embryos were perturbed and then compared to test hypotheses of underlying mechanism (Fraser and Harland, 2000). Historically, understanding changes in development was sought at the organ level, but once genes that control development were identified, the focus of embryology was molecularized (Fraser and Harland, 2000). As it became possible and then relatively straightforward to identify molecular signals that operate during development, embryology turned from its roots in organism- or organ-based inquiry to a focus on genes that control development and what was considered their explanatory power.

But detailed information about molecular pathways does not provide an explanation for the production of phenotypes in most cases (Buchanan et al., 2009). What is lacking from a molecularized developmental biology is context of the genotype to phenotype transition: an appreciation of the developmental basis of the production of phenotypic variation, or what connects biological phenotypes with their underlying genetic basis. Focus on molecules that alter timing, direction, and reaction to genetic input provides us with very specific knowledge of the function of a particular gene or gene network, but does not provide developmental context – what Weiss (Weiss, 2005; Weiss and Buchanan, 2004) calls the phenogenetic logic of life- how instructions provided by genes produce phenotypes through basic developmental processes. Context of developing structures (anatomical, biophysical, biochemical) is needed to understand these basic developmental processes that are the building blocks of phenotypes. Specific molecules are exchangeable across species, but these building blocks (e.g., regional differentiation by dynamic inductive signaling; repetitive patterning by quantitative interactions (Weiss, 2005)) are a constant, high order, “emergent” result.

Since its inception, biological anthropology has insisted on the quantitative study of phenotypic variation and excelled in the origination and improvement of appropriate statistical methods for parameter estimation and hypothesis testing. During the latter half of the twentieth century, there was a significant burst of new methods of form comparison, some of which were developed in anthropology and others developed outside of anthropology but heavily used in anthropological investigations including Procrustes superimposition methods (Rohlf and Slice, 1990; Sneath, 1967), finite element scaling analysis (Cheverud et al., 1983; Richtsmeier and Cheverud, 1986), thin-plate splines and relative warp analysis (Bookstein, 1989, 1991), eigenshape analysis (MacLeod, 1999), and Euclidean distance matrix analysis (Lele and Richtsmeier, 2001; Richtsmeier and Lele, 1993), among others (e.g., (Lestrel, 1989)), with refinements developed and reviewed thereafter (e.g., (Klingenberg, 2002, 2008, 2016; Richtsmeier et al., 2002; Rohlf and Bookstein, 1990; Rohlf and Marcus, 1993)). Many of the new techniques were identified as belonging to a specific class of methods called “geometric morphometrics” that explicitly preserve the geometric relationships among corresponding landmarks as reflected in their positions in two- or three-dimensional Cartesian coordinate space. However, as described expertly by Tim Cole (Cole, 1996), there is a long, but relatively unappreciated history of geometric morphometrics in anthropology. Franz Boas recognized the problems of superimposition (in this case, the problem of using the Frankfort horizontal plane in comparing skull shapes) in a publication in 1905 and worked through a solution of least-squares fitting (Boas, 1905). His student, Eleanor Phelps extended his method in interesting ways in a comparison of male skulls from three different populations (Phelps, 1932), anticipating many of the Procrustes-based techniques introduced in the latter half of the twentieth century. The growing number of quantitative methods introduced in the 1980s and 1990s would enrich the studies of development carried out by anthropologists, enabling precise, quantitative study of the determinants and constraints on the generation of phenotypic variation (see (Hallgrimsson and Hall, 2005) for examples).

Anthropologists interested in development used their knowledge of evolution, their curiosity about phenotypic variation, and their quantitative skills to apply morphometric and statistical approaches in ways that had not been envisioned by most developmental biologists. Data-based investigations of concepts and theories of interest to anthropology (modularity, developmental stability, segmentation, morphological integration, canalization) were executed by researchers seeking to understand the developmental rules that oversee the generation of phenotypes (e.g., (Blomquist, 2009; Cheverud, 1982; Gonzalez et al., 2014; González-José et al., 2004; Hallgrimsson et al., 2002; Kieser et al., 1986; Klingenberg, 2008; Young et al., 2015)). These analyses were grounded in the idea that an organism’s phenotype is an organized, integrated, functional whole (in part a response to the surge of cladistics in systematics) but that there were principles of development, structure, and function that could be recovered through analytical design.

Biological anthropologists were typically well trained in anatomy, embryology, evolutionary theory, and quantitative methods, and adept at 3D imaging. Consequently, they brought a toolkit for the quantitative and statistical comparison of biological shapes and their variation to developmental biology, matching the precision of phenotypic analysis with already precise molecular work. But anthropologists also brought a scientific perspective that expanded the scope and types of questions being investigated with developmental datasets. Biological anthropologists collaborated with cell, molecular, and developmental biologists to apply morphometric methods to assess the outcomes of experiments, using their insight to explore questions about how molecular mechanisms that determine cell behaviors (migration, differentiation, proliferation, apoptosis) drive phenotypic outcomes (e.g., (Baxter et al., 2000; Hallgrimsson and Lieberman, 2008; Hu et al., 2015; Olson et al., 2004; Richtsmeier et al., 2000; Singh et al., 2015; Young et al., 2010)). Currently a new generation of researchers trained in anthropology and developmental biology are designing precise experiments and using genomic tools, laboratory animals, and developmental genetics to elucidate how variation in and evolution of these traits might arise and contribute to the evolution of the human condition (e.g., (Capellini, 2006; Capellini et al., 2010, 2017; Lovejoy et al., 2003; Marcucio et al., 2015; McLean et al., 2011; Reno et al., 2007, 2016; Young et al., 2010)).

DEVELOPMENTAL BIOLOGY OFFERS OPPORTUNITIES FOR DISCOVERY

“We shall not cease from exploration and the end of all our exploring will be to arrive where we started and know the place for the first time”

T.S. Eliot’s Little Gidding

We have gained understanding of many key traits thought to be uniquely human through the study of the fossil record and comparative anatomy. Recent examination of the development of human traits (or their homologues) using model organisms reveals that their construction involves complex coordination among many genetic networks, developmental pathways, and elements of developmental mechanobiology that are common across vertebrates. The use of model organisms greatly informs our search for the mechanisms that underlie the evolution of these traits but also underscores the fact that morphological evolution in hominins, though creating a unique lineage, was the product of genetic regulators of development and developmental changes that occur in other animals (Carroll, 2003; Lovejoy et al., 2003; Wilkins, 2002). Anthropologists are now working productively with zebrafish, mice, and other model and naturally occurring animal populations to focus on the role of development in the evolution of human traits (see the work of Terrence Cappellini, Nathan Young, Phil Reno, and Jukka Jernval for current examples), using methods from developmental biology and other disciplines to understand additional aspects of the production of normal variation. At least four symposia organized for our annual meetings over the past six years have focused on developmental biology in anthropology (AAPA 2012 meetings, Session 25; AAPA 2014 meetings, Sessions 1, 12 and 26), with edited volumes resulting from two of these (Boughner and Rolian, 2016; Percival and Richtsmeier, 2017). As we move forward, evaluating the causative role of newly discovered genetic variants in the production of complex traits, it is critical to remember that genes routinely function in complex pathways that are involved in the development of many tissues at diverse times in development and that a dynamic architecture of interacting factors—genetic and nongenetic—underlies the development of, and variation in any complex trait. With our eye on human evolutionary trajectories and the realization that common developmental complexes underlie corresponding traits across the vertebrates, anthropologists are in a strong position to contribute significantly to our understanding of human and primate evolution by their work in developmental biology.