Evolution of cranial blood drainage in hominids: Enlarged occipital/marginal sinuses and emissary foramina

Abstract

Physiological studies of cranial blood flow in humans in reclining vs. upright postures suggest that selection for bipedalism was correlated with the establishment of epigenetic adaptations for delivering blood preferentially to the vertebral plexus of veins, depending upon momentary respiratory and postural constraints. The frequencies of vascular/osteological channels used to deliver blood to the vertebral plexus of veins were determined for samples of African pongids, various taxa of fossil hominids, and extant Homo sapiens. These channels include an enlarged occipital/marginal (O/M) sinus system, multiple hypoglossal canals, and foramina that conduct emissary veins: posterior condyloid, mastoid, occipital, and parietal. The African pongid, and therefore presumably the ancestral prebipedal hominoid, condition is characterized by low frequencies of all of these routes except multiple hypoglossal canals. The earliest known bipeds (Australopithecus afarensis) and robust australopithecines are characterized by fixation of enlarged O/M sinus systems. Robust australopithecines are also characterized by apparently low frequencies of mastoid and parietal foramina, and high frequencies of multiple hypoglossal canals and posterior condyloid foramina. In gracile australopithecines and subsequently living hominids, trends towards increased frequencies of mastoid and (later) parietal emissary foramina coincide with a trend towards decreased frequencies of an enlarged O/M sinus system and multiple hypoglossal canals. These findings suggest that selection for bipedalism initially resulted in epigenetic adaptations for routes to deliver blood to the vertebral plexus including an enlarged O/M sinus system and hypoglossal canals, but that the pressures underlying these adaptations relaxed as bipedalism became established, and other routes for delivering blood to the vertebral plexus of veins were either directly or indirectly selected for, perhaps in conjunction with a changing architecture of the skull. A systematic implication of this study is that robust australopithecines are descendants of A. afarensis rather than of A. africanus.

Emissary Foramens of the Human Skull: Anatomical Characteristics and its Relations with Clinical Neurosurgery

“…In homo sapiens, the volume of this drainage was reduced occurring, thus, the development of new veins. Among these new veins, the emissary mastoid vein led to the emergence of the mastoid foramen in the modern human skull (Falk, 1986). …”

Missing Omo L338y‐6 occipital‐marginal sinus drainage pattern: Ground sectioning, computer tomography scanning, and the original fossil fail to show it

“…We believe that as this morphological landmark is variably present in different taxa, and even the same specimen (see below), considerable caution must be exercised in attempting to define these features. We note that Falk (1986) identified an enlarged O/M in the Swanscombe occipital but could not find it on the Tautavel composite endocast. Actually, the occipital of the Tautavel composite was the Swanscombe occipital (see Arsuaga et al, 1997, p 269 -270, for a description of this matter).…”

“…Actually, the occipital of the Tautavel composite was the Swanscombe occipital (see Arsuaga et al, 1997, p 269 -270, for a description of this matter). We note that Falk (1986) and White and Falk (1999) are adamant that "robust" australopithecines all have one or more enlarged O/Ms, including the new Konso endocast (Suwa et al, 1997), which does not show such a feature (Holloway, manuscript in preparation). We also suspect that since the Omo L338y-6 endocast was smaller than any other "robust" Australopithecus, and since Falk et al (2000) have recently claimed that all other "robust" Australopithecus endocast volumes have been inflated, the presence of an enlarged O/M on Omo L338y-6 would buttress their argument for its taxonomic assignment to Australopithecus robustus.…”

“…They claim that the enlarged left O/M is readily visible and delineate its suggested presence by arrow markers in their Figure 2 illustration (White and Falk, 1999, p 403). As the enlarged O/M has been consistently used by Falk and her colleagues (Falk and Conroy, 1983;Falk, 1986) to assign taxonomic status, and whereas it appears to be present on several different genera (e.g., Kimbel, 1984;Arsuaga et al, 1997;see also O'Loughlin, 1996), it is important that this feature be accurately identified. In line with R.L.H.…”

High occurrence of a basicranial feature in Homo erectus: Anatomical description of the preglenoid tubercle

“…Even if the problem of causation arises when differences occur between populations in the frequency of discrete traits, can the differences be attributed to biological distance, or have events played a significance? A more in-depth study is needed to understand the significance of the feature, although some works have already focused on the phylogenetic interest of discrete traits (Anthony, 1946;Falk, 1986;Barriel, 1991Barriel, , 1994Barriel and Tassy, 1991;Braga, 1995;Msuya and Harrisson, 1996). To support the use of any feature to build phylogeny, one should be reminded that problems of inheritability are relevant because discrete traits are based on phenotypic expression, and the question is to determine just how far appearances reflect their genetic base.…”