Cells in Evolutionary Biology

Translating Genotypes into Phenotypes - Past, Present, Future

Brian K. Hall, Sally A. Moody

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351652025
• Edition: 1
• Topic: Biowissenschaften
• Subtopic: Zellbiologie

This book is the first in a projected series on Evolutionary Cell Biology, the intent of which is to demonstrate the essential role of cellular mechanisms in transforming the genotype into the phenotype by transforming gene activity into evolutionary change in morphology. This book — Cells in Evolutionary Biology — evaluates the evolution of cells themselves and the role cells have been viewed to play as agents of change at other levels of biological organization. Chapters explore Darwin's use of cells in his theory of evolution and how Weismann's theory of the separation of germ plasm from body cells brought cells to center stage in understanding how acquired changes to cells within generations are not passed on to future generations.

Chapter 7 of this book is freely available as a downloadable Open Access PDF at http://www.taylorfrancis.com under a Creative Commons Attribution-Non Commercial-No Derivatives (CC-BY-NC-ND) 4.0 license.

1	The Role of Cells and Cell Theory in Evolutionary Thought, ca. 1840–1872
	Andrew Reynolds

CONTENTS

1.1 Introduction

1.2 Cells As Anatomical–Physiological–Developmental Units: 1838–1861

A The Cell Theory

B The Cell Concept

1.3 Cells As Evolutionary Units: 1844–1868

A The Subcellular Menagerie and Explanations of the Life of the Cell

B Spencer’s “Physiological Units” (1867)

C Darwin’s “Gemmules” and the Pangenesis Hypothesis (1868)

1.4 Cells as Units Of Ontogeny and Phylogeny: Recapitulation and Haeckel’s Gastraea Theory (ca. 1872)

1.5 Conclusion

References

1.1 INTRODUCTION

The modern student of biology is likely to take for granted that all living things are composed of cells; that cells are the fundamental units of anatomy, physiology, reproduction, and development; and that the evolutionary history of life on earth is characterized by a diversification and specialization of cell types, ranging from primitive and comparatively simple single-celled bacteria to the specialized tissue cells of the multicellular plants and animals—or to use the popular nineteenth century slogan: that evolution proceeds “from monad to man.”

Given the ever-expanding warehouse of facts the student of biology must assimilate in order to get up to speed with to the advancing edge of scientific research, this is quite understandable. One consequence, however, is that there is little time for learning about the history of biology, and a student might be forgiven for wondering what purpose there could be in learning about how scientists in the past got things wrong and failed to recognize what we think we know today. But for those actively engaged in research and hoping to move their field forward, an awareness of the history of their subject can help to identify unstated assumptions and provide a valuable range of alternative possibilities as to how to conceptualize and frame their investigations and research questions. Because nature itself does not tell us the terms with which we should describe it, nor how we should understand the relationships and connections between various areas of scientific research and because science is every bit as much a creative activity as it is an analytic exercise, anything that can assist the scientific imagination is to be encouraged. These are just some reasons why scientists might regard the history of science as a topic suitable for more than extra credit alone.¹

This chapter is concerned with the history of scientific thought about cells,—particularly changes in how the cell has been defined, what have been taken to be its essential properties, and how it stands in relation to larger organisms as a whole (i.e., considerations of anatomy, physiology, and development). In addition, this chapter considers how the cell theory merged with the theory of evolution, following the publication of Darwin’s On the Origin of Species in 1859. It then became relevant to ask how cells themselves have evolved, both as individual living units and as components of larger social aggregates which form supracellular organisms exhibiting novel features and capacities previously inaccessible to the ancestral cell lineages from which they descended. The discussion is limited roughly to the middle third of the nineteenth century (from 1838 to 1872 or so), ranging from the establishment of what is typically known as “the cell theory” of Schleiden and Schwann to Ernst Haeckel’s speculations about the relationship between ontogeny and phylogeny, including an examination of Darwin’s thoughts about the cell theory as expressed in his theory of pangenesis of the late 1860s.

While Darwin focused largely on the origin of higher-level taxa such as species, Haeckel and other embryologists paid special attention to the development of individual organisms. As envisioned by Haeckel and his colleagues, comparative embryology would help fill in the gaps in the fossil record of the evolutionary history of the higher taxa, with the premise being that the development of the individual organism from its component cells reveals a brief and shortened version of the evolution of the species to which it belonged—a thesis expressed more memorably by the slogan “ontogeny recapitulates phylogeny.” Study of development in representatives of the “lowest” stages in the evolutionary tree (colony-forming protists and sponges for instance) promised to give a glimpse into the evolution of the very first plants and animals, and perhaps explain the basic anatomy and physiology of “higher” organisms such as ourselves.

Attitudes about the nature and the significance of the cell, however, changed considerably over the nineteenth century. Even among those committed to the theory of evolution, it was a valid question to ask whether life arose coincidentally with the first cells or whether life was older than cells?

While initially conceived as a unit of anatomical, physiological, and developmental organization chiefly of significance for understanding current plant and animal life, by mid-nineteenth century the cell came to be regarded as an “elementary organism” in its own right, leading biologists and philosophers to ask whether cells themselves might not be composed of yet smaller submicroscopic units of an even more fundamental nature and ancient evolutionary history. Having reduced the bodies of plants and animals to living cells, some questioned whether the phenomena associated with life might not be the properties of some smaller unit, or perhaps was inherent in the essentially amorphous and homogeneous chemical substance known as “protoplasm.” The cell concept, therefore, faced challenges as the fundamental carrier of life both from below—by “protoplasmists” and proponents of various subcellular entities or molecular structures (e.g., Herbert Spencer’s “physiological units,” Darwin’s “gemmules,” or Haeckel’s “plastidules”)—and from above by “holists” and “organicists” who prioritized the “organism as a whole” over any of its cellular parts.

By the beginning of the twentieth century, our current view of life as an emergent systems-level property arising from the heterogeneous elements that collectively make up the cell began to crystallize. Aside from vitalists, who insisted life is the result of some special force superadded to the regular material forces of physics and chemistry, life was considered either to be immanent in material particles or to have emerged from complex interactions among a vast number of molecules under favorable conditions.

For nineteenth century biologists cells presented both an opportunity and a challenge: an opportunity to unify the diversity of living organisms under one fundamental form and a challenge to explain how these fundamental living units themselves arose and evolved in the first place. If made out to be too simple (e.g., as homogeneous and structureless clumps of protoplasm) it is difficult to explain how they manage to carry out all the various vital functions with which they are credited. Similarly, if made out to be too elemental (or “irreducibly complex”), it is difficult to explain how they managed to evolve from any simpler components. In this regard, the emergent systems conception follows a middle path—macromolecular components of the cell (e.g., enzymes) may be ascribed chemical activity while the cell as a whole system is said to be properly alive. In the words of the pioneering biochemist, Francis Gowland Hopkins (1861–1947): “we cannot, without gross misuse of terms, speak of the cell life as being associated with any one particular type of molecule…‘life,’ as we instinctively define it, is a property of the cell as a whole, because it depends on the organization of processes, upon the equilibrium displayed by the totality of the coexisting phases” (Hopkins 1913, 715).² The challenge then, as now, was to explain what that organization is and how it comes about.

1.2 CELLS AS ANATOMICAL–PHYSIOLOGICAL–DEVELOPMENTAL UNITS: 1838–1861

The development of the first microscopes in the seventeenth century made it possible to see details of living tissue previously unwitnessed by anatomists and naturalists. In 1665, Robert Hooke described seeing a great number of minute “boxes,” “pores,” “chambers,” or “cells” in sections of cork plant. Motivated as much by speculation about the ultimate composition of living bodies as by new optical technology, some natural philosophers sought to resolve organic bodies into more elementary structures such as fibers, globules, or cells. Imperfections in the design of microscope lenses that prevented seeing clear and reliable images of the fine anatomical structures were overcome by the end of the 1830s.

A THE CELL THEORY

For purposes of pedagogical convenience, the brief histories at the beginning of textbooks typically credit the German biologists Matthias Schleiden (1804–1881) and Theodor Schwann (1810–1882) alone with creating the cell theory. But they were neither the first to propose that plants and animals are composed of cells, nor were their specific ideas about the nature and genesis of cells reflective of what is today understood to be the cell theory (Sapp 2003; Dröscher 2014). What Schleiden (a botanist) and Schwann (an animal physiologist) did was to articulate and to popularize a theory that the cell is the basic unit of life. Schleiden (1838) argued that all plants are composed of—and more importantly—by cells, which are the elementary individuals through whose developmental activity the larger plant body is constructed. Schwann extended this idea to animals in his essay of 1839, illustrating by means of his own microscopic investigations of animal development how the various tissues and organs arise from the multiplication, modification, and in some instances the amalgamation of originally separate and distinct cells.

Schleiden was of the opinion that new cells arise endogenously from within existing cells, growing around a preexisting nucleus, the structure first described by Robert Brown in 1833. The nucleus itself (or cytoblast as Schleiden called it), he believed first arose from granules in a chemical “mother liquid,” in a process akin to the formation of crystals from within a supersaturated fluid medium. Schwann, on the other hand, maintained that cells grow exogenously from an extracellular liquid (cytoblastema) in the space between cells. Both men were familiar with the claims of other naturalists that new cells were created by the division of existing cells, but they remained unconvinced of these observations and retained their belief in the “free formation” of cells from a nutritive liquid. For Schwann, the attribution of cell formation to physical–chemical forces rather than to the vital action of preexisting cells satisfied a conviction that a scientific account of organismal development should be consistent with the rest of science and fueled a belief in spontaneous generation as opposed to special creation by a supernatural agent (Parnes 2000).

Robert Remak’s (1815–1865) study of development in vertebrate embryos in the 1850s helped to make popular the thesis that new cells arose by cell division. This thesis became closely associated, however, with Rudolf Virchow (1821–1902), whose famous dictum “Omnis cellula e cellula,” states that all cells come from previously existing cells (Virchow 1855). This amendment of the theory of Schleiden and Schwann established what is typically understood today to be the cell theory: that all living organisms are composed of one or more cells; that cells are the fundamental living units; and that all cells arise from preexisting cells by binary fission. Virchow championed this version of the cell theory through his influential book Cellularpathologie (1858), in which he provided a new account of health and disease firmly rooted in normal and abnormal cell activity.

In the following year, Charles Darwin (1809–1882) published his views on the common evolutionary origins of all living things (Darwin 1859). As the historian Thomas S. Hall explained, Virchow’s “Omnis cellula e cellula” statement was significant for Darwin’s theory because it “supplied the physical basis for that larger continuity of life as a whole which began, according to Darwin, when God first breathed life into an original cell or cells, which has culminated in the variety of forms that inhabit the earth today. The cell, for those who saw it as the irreducible life unit, was thus the basis of the whole history of life” (cited from Hall 1969, 206–207).³

B THE CELL CONCEPT

As the cell theory was becoming better established, researchers were reconsidering the cell concept itself.

Originally used by Hooke to emphasize an empty space or chamber characterized by a solid enveloping wall, investigators were beginning to note that many so-called “cells” lacked any discernible membrane let alone a rigid wall (e.g., the amoeboid “swarmer” cells of fungi and algae, the ova, blood, and even tissue cells of higher animals).⁴ Given the vast diversity in cell morphology throughout the organic kingdoms, rejection of an outer wall as an essential characteristic made defining the cell more difficult. Attention turned to the contents of the cell vesicle, to the sticky semifluid substance within. This was variously known as “sarcode,” with respect to the infusoria or protozoa, and as “protoplasm” in the case of plants and (rather confusingly) animal embryonal cells. Eventually, it was agreed by most that sarcode and protoplasm were one and the same substance, which provided a means for unifying all the various forms of living beings.

In 1861, Max Schultze proposed a new definition, whereby, a cell was understood to mean essentially “a naked clump of protoplasm containing a nucleus” (Schultze 1861). This protoplasmic cell concept avoided reference to any specific morphological feature aside from a nucleus. In the same year, Ernst Brücke, made popular yet another perspective on cells when he referred to them as “elementary organisms” (Brücke 1861). The suggestion that cells, including those of which human and other animal bodies are composed, are themselves organisms, had obvious resonance with the thesis that all living things share a common evolutionary origin from some more ancient and less complex form of life. Additionally Darwin’s younger colleague and disciple T. H. Huxley (1825–1895) had been quite critical of the original cell theory (cf. Huxley 1853), he would go on to champion—against vitalists and opponents of evolution—the idea that protoplasm is the “physical basis of life” (Huxley 1868).⁵

1.3 CELLS AS EVOLUTIONARY UNITS: 1844–1868

Having started as a significantly morphological concept, the cell was now increasingly thought of as an elementary organism composed of protoplasm, the fundamental physical–chemical stuff of life. Textbook illustrations of the cell concept from the mid-nineteenth century on frequently featured amoebae, the supposedly simplest and most primitive cells and organism...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Preface
• Preface
• Editors
• Contributors
• Chapter 1 The Role of Cells and Cell Theory in Evolutionary Thought, ca. 1840–1872
• Chapter 2 Germ Cells and Somatic Cells in Evolutionary Biology: August Weismann’s Germ Plasm Theory
• Chapter 3 Cell Lineages in Ontogeny and Phylogeny from 1900
• Chapter 4 Protists and Multiple Routes to the Evolution of Multicellularity
• Chapter 5 Symbiosis in Eukaryotic Cell Evolution: Genomic Consequences and Changing Classification
• Chapter 6 Cellular Signaling Centers and the Maintenance and Evolution of Morphological Patterns in Vertebrates
• Chapter 7 Cellular Control of Time, Size, and Shape in Development and Evolution
• Chapter 8 Cellular Basis of Evolution in Animals: An Evo-Devo Perspective
• Chapter 9 Dynamical Patterning Modules Link Genotypes to Morphological Phenotypes in Multicellular Evolution
• Index