Randall R. Skelton

Evolutionary Relationships among Early Hominids

Although cladistic analysis provides one of the most useful approaches to discovering the phyletic relatiosships among the species of Australopithecus and early Homo, methodological problems continue to beset any attempt to apply it in this context. Two of the most pressing problems are redundancy of traits due to similarity of underlying function and overrepresentation of some anatomical or functional systems in trait lists. In an attempt to mitigate these problems, we collapse our list of 77 traits into sets of traits using two methods. The first method segregates traits into seven groups by anatomical region. The second method segregates traits by function into five complexes which correspond fairly well with recognized trends in the evolution of early hominids (adaptation for heavy chewing, reduction of the anterior dentition, basicranial flexion, increased orthognathism and encephalization). The most parsimonious cladogram describing the relationships among the early hominid species obtained using the original 77 traits or the summary scores from the functional complexes, places Australopithecus afarensis as a sister group to all other hominids. Australopithecus aethiopicus occupies the next branch, leaving A. africanus, A. robustus, A. boisei and Homo as a monophyletic group. The cladogram next separates A. africanus from the remaining hominids and finally divides Homo from A. robustus and A. boisei. Summary scores for anatomical regions produced three equally parsimonious cladograms, one of which was identical to that described above. This result implies that there was a large amount of parallelism in hominid evolution, and that adaptations for heavy chewing evolved separately in the lineage leading to A. aethiopicus and in the lineage leading to A. robustus and A. boisei. Another implication is that Homo descended from an A. africanus-like ancestor and diverged from A. robustus and A. boisei relatively late in hominid evolution by reducing the extent of its adaptation to heavy chewing. Most current phylogenies are not compatible with the cladogram obtained in this study, but are instead compatible with a cladogram obtained when traits related to heavy chewing are used exclusively. The “heavy chewing” cladogram reverses the positions of A. aethiopicus and Homo. Perhaps the reason why most current cladograms resemble the “heavy chewing” cladogram is the over-representation of traits related to heavy chewing in most trait lists.

Phylogenetic Analysis of Early Hominids

The proposal of the new australopithecine species Australopithecus afarensis has led to a multiplicity of hypotheses concerning the evolutionary relationships between the known Pliocene and Pleistocene hominid species. We use phylogenetic analysis to gain a new perspective on the subject. Using 69 traits, we construct a series of 12 complexes, each with a defining polarized morphocline. Four mutually exclusive cladograms are derived from these complexes, the most parsimonious of which implies that Homo habilis and A. robustus/boisei are more closely related to each other than either is to A. africanus and that these three species form a distinct evolutionary group relative to the more primitive A. afarensis. We advocate a phylogeny wherein A. afarensis is ancestral to A. africanus, which is in turn ancestral to A. robustus/boisei and H. habilis. We believe that the evolutionary transition from Australopithecus to Homo involved reduction in the size of the chewing teeth and associated traits leading to a unique derived condition in H. habilis that superficially resembles the primitive condition of A. afarensis. Five other current phylogenies are treated as critiques of this one.

Get Rad! The Evolution of the Skateboard Deck

Today there is growing interest in material culture studies among a wide range of social and biological scientists. Researchers recognize that some concepts drawn from biology can be useful in understanding aspects of material culture evolution. Indeed, recent research has demonstrated that material culture can evolve in a branching manner (vertical transmission) similar to that of biological species. However, there are many complicating factors as well, particularly the human penchant for borrowing and resurrecting old ideas resulting in extensive blending and hybridization (lateral transmission). But blending and hybridization occurs in biology as well depending upon the nature and scale of interacting organisms. There is far more lateral information transfer between populations within species than between species (although there are always exceptions). History can also be expected to play a role in the degree to which evolution is affected by vertical versus lateral transmission processes. All things equal, we should expect branching to be most important early in the history of a cultural system since blending could not become significant without the early development of distinct lineages. This is different from most biological systems in the sense that the development of distinct lineages would significantly reduce (or prevent) opportunities for blending. We explore these ideas with an analysis of skateboard decks spanning the history of professional skateboards since 1963. We apply cladistic and networking models in order to develop an understanding of the degree by which skateboard evolution was affected by branching and blending/hybridization processes. The study is enhanced by a historical record that provides significant insight into the actual innovation and borrowing processes associated with skateboard evolution. Results confirm that both branching and blending played important roles and that branching was most critical early in professional skateboard history. The study offers the important implication that while cultural systems will typically incorporate far more horizontal transmission in the evolutionary process (particularly in later stages) than many biological systems, general principles governing early stage branching and disparity may apply to both.

Mosaic Evolution in Cultural Frameworks: Skateboard Decks and Projectile Points

There has been significant debate in paleoanthropology and more recently, archaeology, over the concept of mosaic evolution. Essentially, proponents of the concept argue that different aspects of organisms evolve separately while others argue that organisms evolve as integrated entities. Similarly, archaeologists debate the relevance of cultural evolution as a complex multi-scalar process. In this paper we conduct two cladistic analyses of cultural phenomena focusing on skateboard decks and projectile points from an archaeological site to examine variability in the evolutionary process. We find evidence for mosaic evolution in both studies and conclude that modularity likely is an important factor in cultural evolution, at least at the level of artifact design. We caution future investigators of evolution in ancient stone tools that modularity could have complicating effects on phylogenetic outcomes unless explicitly considered.

A priori assumptions about characters as a cause of incongruence between molecular and morphological hypotheses of primate interrelationships

When molecules and morphology produce incongruent hypotheses of primate interrelationships, the data are typically viewed as incompatible, and molecular hypotheses are often considered to be better indicators of phylogenetic history. However, it has been demonstrated that the choice of which taxa to include in cladistic analysis as well as assumptions about character weighting, character state transformation order, and outgroup choice all influence hypotheses of relationships and may positively influence tree topology, so that relationships between extant taxa are consistent with those found using molecular data. Thus, the source of incongruence between morphological and molecular trees may lie not in the morphological data themselves but in assumptions surrounding the ways characters evolve and their impact on cladistic analysis. In this study, we investigate the role that assumptions about character polarity and transformation order play in creating incongruence between primate phylogenies based on morphological data and those supported by multiple lines of molecular data. By releasing constraints imposed on published morphological analyses of primates from disparate clades and subjecting those data to parsimony analysis, we test the hypothesis that incongruence between morphology and molecules results from inherent flaws in morphological data. To quantify the difference between incongruent trees, we introduce a new method called branch slide distance (BSD). BSD mitigates many of the limitations attributed to other tree comparison methods, thus allowing for a more accurate measure of topological similarity. We find that releasing a priori constraints on character behavior often produces trees that are consistent with molecular trees. Case studies are presented that illustrate how congruence between molecules and unconstrained morphological data may provide insight into issues of polarity, transformation order, homology, and homoplasy.