As part of the 2009 Darwin celebrations, we have seen the emergence and widespread acceptance of a standard narrative of the history of evolutionary biology that construes a more or less direct line from Darwin to present-day evolutionary developmental biology, or evo-devo (Mayr 1982; Larson 2004; Carroll 2005; Zimmer 2006, 2009; Ruse and Travis 2009). It is a story of completions and syntheses that not only celebrates Darwin’s genius but also implies an implicit progression of ideas, with inclusion of new empirical facts and methodological approaches within the general framework of Darwinism leading to an increasingly more complete understanding of the evolutionary process. This narrative involves both scientific and public discourses. It can be found in textbooks of evolutionary biology and in popular accounts of evolution; it is also the basis the many efforts to construct a more inclusive evolutionary worldview.

But the standard narrative “From Darwin to Evo Devo” is also woefully incomplete as it leaves out several important traditions within the history of evolutionary biology (Laubichler and Maienschein 2007). These neglected traditions are not fringe ideas with no relevance to current understanding of evolutionary processes that can therefore be relegated to the dustbin of history. Quite the contrary. The ideas and approaches that are part of a complementary tradition – namely, to explain the evolution of organisms in reference to the developmental mechanisms that first generate phenotypes and phenotypic variation – have informed some of the most important current evolutionary biology research programs, those in developmental evolution (devo-evo) and synthetic experimental evolution (SEE) (Wagner, Chiu, et al. 2000; Davidson 2006; Davidson and Erwin 2006; Laubichler 2007; Erwin and Davidson 2009).

The success of grand historical narratives and conceptual reflections rests, in no small part, on the selection of their central characters and features. If, as in our case here, the focus is on the development and integration of those ideas that have shaped our understanding of the living world, it is hardly possible to think of one pair of concepts more appropriate to structure a longue durée narrative than form and function. These linked concepts are uniquely suited to organizing the rich diversity and idiosyncratic developments within the history and philosophy of the life sciences in general, and within evolutionary developmental biology in particular.

The concepts of form and function, and everything connected to them, have been used as an organizing principle in several influential analyses of the development of the biological sciences. Their symbiotic and often dialectic relationship forms the backbone of such treatises as Ernst Mayr's The Growth of Biological Thought, William Coleman's Biology in the Nineteenth Century, Ernst Cassirer's Theory of Knowledge (specifically the section on the biological sciences), Edmund Beecher Wilson's The Cell in Development and Heredity, and, of course, Edward Stuart Russell's Form and Function: A Contribution to the History of Animal Morphology (Cassirer 1950; Coleman 1971; Mayr 1982; Russell 1916; Wilson 1925).

When philosopher of biology Michael Ruse moved from Guelph, Ontario, to Florida State University, a very good thing happened for our understanding of the conceptual, historical, and philosophical foundations of biology. Ruse became the William H. and Lucyle T. Werkmeister Professor, which brought with it an endowment that allowed him to organize conferences on a regular basis. As usual, Michael Ruse lost no time, and the resulting series of conferences has brought together biologists, historians, and philosophers in lively discussion of a number of important topics. A favorite image of those events is that of Ernst Mayr, sitting in the boat during a swamp tour on a drowsy Florida afternoon. The aging but ever-intense Mayr seemed to be dozing, when suddenly he pointed and declared a noteworthy bird, then another, and another. That led to a discussion of whether biodiversity is declining; then to philosophical questions about how we count diversity; and finally to ethical and policy questions of why we care. Other conferences have led to debates about science and religion. And so on.

In 2005, a group of leading biologists joined philosophers and historians for four days of thinking about form and function. For this meeting, Ruse followed his usual approach. He provided the general theme, brought together a mix of enthusiastic scholars, and waited to see what happened. In this case, it was something very interesting.

Evolutionary developmental biology is an emerging new research area that explores the links between two fundamental processes of life: development of individual organisms (ontogeny) and evolutionary transformation in the course of the history of life (phylogeny). For some of its more ardent proponents evolutionary developmental biology, or evo-devo for short, represents a new paradigm that completes the “Modern Synthesis” of the 1930s and 1940s, while others, often those with a more astute sense of the history of biology, have emphasized the long-standing connections between these two areas of study. But all agree that evo-devo offers some of the most promising theoretical perspectives in evolutionary biology at the beginning of the twenty-first century (see for example Amundson 2005, Carroll 2005, Carroll et al. 2005, Hall 1998, Kirschner and Gerhart 2005, Laubichler 2005, Müller 2005, Wagner et al. 2000). In this essay I will first sketch the emergence of present day evolutionary developmental biology during the last decades of the twentieth century followed by a brief overview of the central questions and research programs of evo-devo. I will conclude with a discussion of the one problem - the issue of how to explain evolutionary innovations and novelties - that has the most profound implications for the philosophy of biology.

Here, by Chapter 196 of a volume exploring the endothelium in medicine, it has become clear that this is an astonishingly diverse and far-reaching subject. Identified for the first time only at the end of the nineteenth century, the once seemingly simple and passive cell layer is now understood to represent a dynamic system that reflects and determines much about health and function for humans and other vertebrate species. This collection of chapters shows how far we have come in our understanding, and also how rich the opportunities are for further study and more effective translation of our understanding into clinical practice.

There is great virtue in bringing together all these topics that would normally reside in the archives of separate disciplines. We have gained many different perspectives of what the endothelium is and how endothelial cells (ECs) come together as a functioning system. By reflecting on the way the endothelium develops and by comparing phenotypes across different species, we gain insights into the alternatives available and what any particular developmental pathway means for the organism. This helps us understand the endothelium as a highly evolved, integrated system. The system is by no means perfect. Rather it is an interlocking “bundle of adaptations,” some of which were born of trade-offs that are no longer applicable. Indeed, the extreme path-dependence of evolutionary change has undoubtedly engendered design flaws that render the modern-day human endothelium highly vulnerable to disease.

The use of metaphors to conceptualize the endothelium is a powerful tool for understanding an otherwise hidden and under-appreciated entity. We hope that the reader will agree that the analogies between endothelium and weather, landscape, and urban design are at the same time provocative and illuminating.

WHY STUDY EVOLUTION?

An understanding of the evolutionary origins of the endothelium has important implications for human health and disease. As discussed by Nesse and Weder (Chapter 15), when approaching mechanisms of structure and function, evolutionary explanations (“why”) complement proximate considerations (“how”). They provide important insights into the design constraints, path dependence, trade-offs, and selective pressures that underlie endothelial function and vulnerability to disease. Another critical and often underappreciated consideration is that, in the course of human evolution, our body (including the endothelium) has evolved to maximize fitness in a far earlier era, perhaps tens of thousands of years ago, which is the time it takes for the gene pool to be filtered by natural selection. In Chapter 16, Eaton and his colleagues write about the environment and lifestyle of the hunter-gatherer, and remind us about the importance of cultural (as distinct from genetic) evolution in mediating the modern-day predisposition to atherosclerosis. As another example of how evolutionary theory may be applied to an understanding of human disease, Haig discusses how evolutionary conflicts between maternal and fetal genes may underlie the pathophysiology of preeclampsia (Chapter 17).

Piecing together the evolutionary origins of the endothelium is challenging and necessarily speculative. Because the human endothelium does not fossilize, interpretations rely on a combination of molecular phylogeny and comparative biology/physiology, with the assumption that what works for extant organisms may have worked for ancestral species. In Chapter 14, McVey demonstrates how molecular phylogenetic approaches may be used to gain insights into the evolutionary history of the endothelium. In Chapter 3, Burggren and Reiber provide a comprehensive overview of the comparative biology of cardiovascular systems.

The endothelium is only now beginning to gain acceptance as a physiologically relevant organ with potential clinical significance. Yet the cell layer called the endothelium was first identified well over a century ago. In this chapter, we explore the circumstances leading to the slow recognition of the endothelium as a system with untapped diagnostic and therapeutic potential. We trace historically important steps toward increased interest in the endothelium, beginning with ancient discussions of the heart and blood vessels, and the conviction that blood derives from nutrition and is continually used up by the body. We see that, in Western medicine, the dominant culture of the Catholic Church impeded new discovery and instead emphasized reliance on accepted ideas for nearly 1,500 years. Only in the context of the Scientific Revolution of the seventeenth century could anatomists such as William Harvey challenge prevailing dogma and reach the conclusion that blood circulates and that it does so through a system of connected vascular vessels.

In this chapter, we examine those contributions and the developments that followed, slowly and gradually, the rise of new technologies for observation and the framing of new questions. We ask what caused researchers to focus on cells and tissues, and then, during the last part of the nineteenth century, to identify endothelial cells (ECs) as a unique structure, distinguishable structurally, physiologically, and developmentally from the epithelium that researchers initially had seen as closely connected to it. Then, we explore what implications the identification and naming of this particular set of cells had for the biomedical sciences.