Developmental Biology and Human Evolution

Abstract

▪ Abstract Our understanding of developmental biology burgeoned during the last decade. This review summarizes recent advances, provides definitions and explanations of some basic principles, and does so in a way that will aid anthropologists in understanding their profound implications. Crucial concepts, such as developmental fields, selector and realizator genes, cell signaling mechanisms, and gene regulatory elements are briefly described and then integrated with the emergence of skeletal morphology. For the postcranium, a summary of events from limb bud formation, the appearance of anlagen, the expression of Hox genes, and the fundamentals of growth plate dynamics are briefly summarized. Of particular importance are revelations that bony morphology is largely determined by pattern formation, that growth foci such as physes and synovial joints appear to be regulated principally by positional information, and that variation in these fields is most likely determined by cis-regulatory elements acting on restricted numbers of anabolic genes downstream of selectors (such as Hox). The implications of these discoveries for the interpretation of both contemporary and ancient human skeletal morphology are profound. One of the most salient is that strain transduction now appears to play a much reduced role in shaping the human skeleton. Indeed, the entirety of “Wolff's Law” must now be reassessed in light of new knowledge about pattern formation. The review concludes with a brief discussion of some implications of these findings, including their impact on cladistics and homology, as well as on biomechanical and morphometric analyses of both ancient and modern human skeletal material.

Understanding Entheses: Bridging the Gap Between Clinical and Anthropological Perspectives

“…In fact, Enlow (1990) noted that enthesial development is insignificant during adolescence, suggesting that mechanical strain during growth and development merely provides threshold levels for bone growth. Therefore, threshold levels set during adolescence preserve throughout adult life with mechanical strain maintaining bone strength; thus the focus is on bone retention not deposition (Lovejoy et al, 2003). Hypothetically, these observations account for deposition and resorption reduction observed in adult long bones when exposed to increased strain levels, since mechanical mechanisms are bound to predetermined threshold levels set during initial growth and development.…”

Histomorphological Variation in the Appendicular Skeleton

“…As has been shown here, there appears to be no significant effect in adults of asymmetric loading of the forelimb. Persistent scars of union of fractured bones in human adults are a further indication of the lack of response of bone tissue to mechanical loading in adults [5]. Further, it has been shown that when long bone length, age and sex are fully considered, perceived populational differences in femoral cortical thickness are nonsignificant [43].…”

“…This raises the strong possibility that osteoblast response protocols are highly conserved, and do not vary substantially from one mammal to another. Such a view has also received strong support from studies of limb bud dynamics [4,5]. These suggest that differences in the structure and form of the skeleton can be traced to early initial differences in the disposition of positional information (PI).…”

“…However, in recent years several aspects of that process have become significantly clarified. Among the most important is an accumulation of evidence suggesting that osteoblast behavior is highly conserved [3][4][5]. Despite an enormous range of body mass, actual microstrain experienced by mammalian bones has been found to fall within a very narrow range [6][7][8][9][10][11].…”

“…The standard paradigm of skeletal remodeling, often based around assumptions of the so-called "Wolff's Law," [12] portrays remodeling of skeletal elements as an adaptive response of bone tissue to the mechanical loads imposed upon it. In recent years, the validity of this assumption as the sole basis of skeletal remodeling has come into question [5,13,14]. To test some of these assumptions, we here examine variation within and among skeletal elements and across species, both at the tissue level and the whole bone level.…”