Monkey See, Monkey Do? Deciphering the Structure-Function Relationship in the Fossil Record
An organism’s survival is contingent on the way it moves and interacts with the environment. We can get at the relationship between a living organism’s morphology and the way it moves through direct observation and experimentation. This relationship, however, is more clandestine in fossil organisms. Teasing out the relationship between form, function, and their evolutionary significance has been a major avenue of research for over 200 years. In our last blog post, Ben touched on the use of comparative anatomy to infer the structure-function relationship in the fossil record. In this post, I briefly explore this topic from a historical perspective and discuss its potential for evolutionary analysis.
Apples to Apples: Using Analogy to Infer Function
The use of comparative anatomy as a means of organismal classification can be traced back to Aristotle. However, it was Georges Cuvier who first applied these principals to the fossil record (Russell 1917). Cuvier’s approach to classifying organisms, whether living or extinct, relied on the functional relationship among organs. He argued that comparative anatomy could provide a framework for inferring organismal structure from the functional requirements of the animal and vice versa (Racine 2013). Since Cuvier’s method was based on the correlation among organs in a single organism, it could not explain differences in structure between species. This issue was addressed by Geoffroy’s proposal to focus on structural similarity as a means of classification. He posited that an animal’s structure limited the functions it could perform, and that similarities of anatomical features among different species were an indication of an ideal pattern for the environment in which they lived (Russell 1917; Racine 2013). The work of both Cuvier and Geoffroy established the approaches used by early naturalists and more contemporary researchers to infer locomotor behavior of fossil organisms: Cuvier provided a framework to infer function from form, while Geoffroy set forth a method to place fossil material in a historical or evolutionary context.
Figure 14 from Adolph Schultz's 1930 Human Biology paper.
It is no surprise, then, that early methods for estimating locomotion in fossil organisms approached the problem using analogy within a comparative anatomy framework. The structure of traits associated with known locomotor behaviors in living organisms were likened to similar structures in extinct organisms, with the assumption that like structures resulted in like movement (Plotnick and Baumiller 2000). As I explore in more detail below, in the absence of living correlates, structures in fossil organisms are often compared to simple machines to infer how the extinct organism moved (Rudwick 1964). Anthropologists have used comparative anatomy to better understand locomotor and behavioral differences among primates (i.e. Schultz 1930) and the effect of those differences on scapular shape (Oxnard 1967, 1968), upper limb proportions (Cartmill 1974; Ashton et al. 1976; Fleagle et al. 1981), lower limb proportions (Robinson 1972; Gebo 1996) and cranial morphology (Lieberman et al. 2000). An advantage of this approach is that it assumes similarities in form indicate the same function, but this requires detailed knowledge of the functional morphology in living creatures, and it makes a significant assumption about equivalency between extant and extinct form. While comparative anatomy has been essential towards building a foundation of how structure and function are related, it cannot account for functional differences among similar structures, nor does it provide testable hypotheses of function in fossil organisms. However, this recognition led to methods that could.
The Formation and Testing of Functional Hypotheses
An early treatise on biomechanics. Giovanni Alfonso Borelli, 1680. De Motu Animalium.
Rudwick (1964) sought to provide testable hypotheses to infer functional behaviors from the structure of living or fossil organisms without relying on existing forms as analogues. By comparing simple machines to structural features of an organism, researchers estimated a trait’s potential to perform a particular function (Rudwick 1964; Paul 1975). The degree to which a structure could perform the function of interest was used as an indication of how well adapted the organism was to perform the ascribed task (Rudwick 1964), though this approach tends to favor natural selection over other evolutionary processes as an explanation of trait variation (as argued in the infamous “Spandrels” paper by Gould and Lewontin in 1979). This method has been criticized as being too mechanistic and as ignoring the fossil as a living organism (Paul 1975). This concern, however, can be mitigated through biomechanical analyses.
Biomechanical approaches apply engineering theory to models of biological structure. Studies using this approach place the skeleton and muscles into a framework by which to understand their reaction to the stresses and strains placed upon them. Biomechanical analyses facilitate our understanding of how bone functional adaptation, muscle mechanics, and organismal body form work together to resist mechanical loading forces and avoid the possibility of mechanical failure (McGowan 1999; Carter and Beaupré 2001). A great advantage to this method is that it allows for the direct assessment of body form within a theoretical framework dictated by mechanical engineering. That is, it tests whether form is a product of function. A major advantage in the application of biomechanical analyses over pure comparative analogy is that material properties and the forces of physics work under the assumption of uniformitarianism—the reactions of bone to mechanical forces seen in present day organisms were the same that were in place millions of years ago (McGowan 1999).
But How Does This Relate to Evolution?
While most of the approaches can be applied without invoking evolutionary theory, a primary goal of the application of the form-function relationship to fossil organisms is to better understand the evolutionary relationships among these and living organisms. Placing structural or functional traits into a historical or phylogenetic perspective allows for us to create and test evolutionary hypotheses (Lauder 1990). To date, this application has been used to demonstrate patterns of the evolution of form and function, with a few exceptions appearing in the last decade that use model-bound approaches. These patterns provide a hypothesis for the evolution of these traits, but they are not, inherently, tests of these hypotheses. The next step is to use evolutionary models—such as those discussed in previous posts on this blog by Sam and myself—to assess the hypotheses derived from those patterns. Such an approach will allow us to better understand through what evolutionary processes trait complexes evolve to meet mechanical and behavioral needs. This is something I’m working towards with my dissertation, and I will be presenting on my preliminary analyses at the American Association of Physical Anthropologists Annual Meeting next week in New Orleans, and at the American Association of Anatomists Annual Meeting the week after in Chicago. Come check out my research if you’d like to know more!
References
Cartmill M. 1974. Rethinking primate origins. Science 184(4135):436-443.
McGowan C. 1999. A practical guide to vertebrate mechanics. Cambridge: Cambridge University Press.
Oxnard CE. 1968. The architecture of the shoulder in some mammals. J Morph 126:249-290.
Rudwick MJS. 1964. The inference of function from structure in fossils. Br J Phil Sci 15(57):27-40.
Schultz AH. 1930. The skeleton of the trunk and limbs of higher primates. Hum Biol 2(3):303-438.