Testing hypotheses about tinkering in the fossil record: the case of the human skull

Abstract

Efforts to test hypotheses about small-scale shifts in development (tinkering) that can only be observed in the fossil record pose many challenges. Here we use the origin of modern human craniofacial form to explore a series of analytical steps with which to propose and test evolutionary developmental hypotheses about the basic modules of evolutionary change. Using factor and geometric morphometric analyses of craniofacial variation in modern humans, fossil hominids, and chimpanzee crania, we identify several key shifts in integration (defined as patterns of covariation that result from interactions between components of a system) among units of the cranium that underlie the unique shape of the modern human cranium. The results indicate that facial retraction in modern humans is largely a product of three derived changes: a relatively longer anterior cranial base, a more flexed cranial base angle, and a relatively shorter upper face. By applying the Atchley-Hall model of morphogenesis, we show that these shifts are most likely the result of changes in epigenetic interactions between the cranial base and both the brain and the face. Changes in the size of the skeletal precursors to these regions may also have played some role. This kind of phenotype-to-genotype approach is a useful and important complement to more standard genotype-to-phenotype approaches, and may help to identify candidate genes involved in the origin of modern human craniofacial form.

A geometric morphometric analysis of heterochrony in the cranium of chimpanzees and bonobos

“…Such diagnoses still need to be interpreted biologically using developmental data from other kinds of analyses. In fact, these diagnoses are primarily useful for generating hypotheses about developmental shifts (Lieberman et al, 2004).…”

Statistical and biological definitions of “anatomically modern” humans: Suggestions for a unified approach to modern morphology

“…They noted that the crania of most living humans and ''modern'' fossil individuals share a short, high vault; a long, high parietal arch that expands in length superiorly; a high, fairly vertical frontal bone; and a gently curved rather than angled occipital bone. These morphological features continue to figure prominently in more recent appraisals of cranial vault globularity, 3,4 which emphasize the developmental roots and morphological integration of these dimensions. This approach to globularity fits among the ''biological'' definitions of modernity and demonstrates that ''statistical'' and ''biological'' definitions are partially related.…”

“…Many of the proponents of these models stress that one or, at most, a few key changes in growth and development produced modern cranial morphology. Lieberman, 3,4,68 Krovitz, 69 Ackermann, 52,70,71 Williams,72 Ponce de Leó n and Zollikofer, [73][74][75][76] and Mitteroecker and colleagues 77 have been prominent recent advocates of this approach, but it is rooted in previous work. [78][79][80][81] Much of the early work on cranial ontogeny in archaic Homo focused on contrasts between anatomically modern children and Neanderthals, the best-known group of premodern Homo.…”

“…3,4,[69][70][71][72][73][74][75][76]101 These studies admit that an increase in brain size during the Middle Pleistocene may have also been an important factor that necessitated some additional alterations in cranial architecture. Lieberman and colleagues 3,4 described the primary developmental shifts that distinguish ''modern'' crania from archaic specimens: (1) a more globular cranial vault in the sagittal, coronal, and transverse planes; (2) reduced facial projection linked with a more flexed cranial base and a longer anterior cranial fossa; and (3) a more clearly developed canine fossa. Cranial globularity is hypothesized to develop from an interaction among brain size, cranial base length and width, and cranial base angle, which also correlates with facial size.…”

Ontogenetic covariation in the endocranium of Pan troglodytes

“…Nonetheless, different facial structures are also likely to promote variation and covariation in the chimpanzee's endocranium. Rev Arg Antrop Biol 16(2): 79-91, 2014. La evolución morfológica en el linaje hominino involucró numerosos cambios en el neurocráneo, tales como el aumento de la capacidad craneana (en relación con el tamaño cerebral), la forma globular de la bóveda, el ángulo sagital de la base, la posición central del foramen magnum en la base y la reducción de las superestructuras óseas (Aiello y Dean, 2002;Lieberman et al, 2002Lieberman et al, , 2004. Estas autapomorfías han sido centro del debate sobre las diferencias heterocróni-cas entre el ser humano y el chimpancé (Gould, 1977).…”