The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


Miocene-to-Recent evolution of the hominin foramen magnum

GARY D. RICHARDS1, REBECCA S. JABBOUR2, CAITLIN L. IBARRA3, CAROLINE F. HORTON4 and HOMER ASADI1.

1Department of Biomedical Sciences, A. A. Dugoni School of Dentistry, University of the Pacific, 2Department of Biology, Saint Mary’s College of California, 3Department of Anthropology, California State University East Bay, 4Department of Anthropology, University of California, San Diego, 5Department of Biomedical Sciences, A. A. Dugoni School of Dentistry, University of the Pacific

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Recent work demonstrates that the human foramen magnum (FM) comprises two functional matrices, ventral and dorsal. The ventral matrix is related to locomotion while the dorsal matrix is related to neural, hemodynamic, and hydrodynamic functions. How these matrices responded to the Mio-Pliocene transition in locomotor function and the Pleistocene transformation in cranial capacity is unknown. As an initial step in addressing these questions, we compared FM and cranial size and shape across fossil (Miocene through Pleistocene) and modern hominin species.

We measured or compiled from the literature lengths and widths of crania and FM, and calculated shape indices, in 92 fossil specimens and 471 modern humans. Standard statistics were employed to compare taxa and explore the relationship between cranial and FM size and shape.

Miocene-to-Recent trends in the cranial index are complicated by non-comparable sample sizes and geographic and temporal variation. In general, pre-Homo-erectus hominins have long and narrow crania whereas Homo erectus-Anatomically modern humans have longer but also broader crania. Although cranial capacity increases with H. habilis, it is not until H. erectus that FM size increases dramatically; it’s then relatively stable throughout Pleistocene-to-Recent times. An exception is the increased FM breadths, but not lengths, in East African robusts.

Modification of FM matrices has important selective consequences. However, in pre-Homo-erectus hominins exclusive of East African robusts, FM size sufficiently accommodated locomotor changes and brain size increases. With the evolution of H. erectus, FM size initially increases but stabilizes, even though cranial size and shape continue to undergo significant modifications.

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