Protohominids who gave rise to the common ancestor around five million years ago were at least facultative bipeds who were adapted for bipedality by their primate ancestry. During the transition to full bipedality, vascular systems changed and evolved along with many other characters. The primary force behind the change would have been the hydrostatic pressures associated with erect posture. But another, larger change was occurring in the environment in which these animals lived.

Between five and seven million years ago the global climate gradually became warmer and drier. Forested areas began receding, making way for the grassland characters of savanna environments. Cerling et al. (1993) clearly illustrates this shift using 13C isotopes. Between 20 and ~ 7 million years ago the predominant vegetation was C4 which includes shrubs and bushes which thrive in colder, wetter climates. By five million years ago a sharp and clearly evident shift to C4 grasses adapted to warm, dry climates is seen.

Current theory suggests bipedality a in response to this new adaptive zone, but recent fossil finds and paleoenvironmental reconstructions are showing that early hominids were mostly likely already experimenting with a bipedal stance and locomotion prior to and while the climate was changing.

By three million years ago, early robust-like hominids had evolved the highly derived trait of an enlarged occipital/marginal sinus as an adaptation to the changes in hydrostatic pressures associated with a partially bipedal existence. Falk (1992) suggest that because these hominids were living at the forested margins of the savanna the threat of hyperthermia did not exist and therefore they did not possess a network of radiator veins to keep the brain cool.

While lying supine, or in a quadrupedal stance, the blood flows from the brain through the jugulars. Upon assuming an upright stance the blood shifts away from the jugulars into the vertebral plexus. The shift occurs to compensate for he change in gravity which produces added pressure to the lower portion of the venous system. The function of the O/M sinus is to deliver blood to the vertebral plexus when standing erect. It is the major channel of cerebral venous drainage in an upright position. Enlarged O/M sinuses occur in all Hadar early hominids and robust australopithecines. Outgroup comparisons show this to be a derived condition (Falk and Gage 1995)

A transverse sigmoid sinus which is unsupplemented by an enlarged O/M is a primitive condition that is retained in high frequencies in African apes, gracile australopithecines (not including the Taung specimen (it may be a juvenile robust), fossil Homo and in living people who do not manifest altered skull shape due to cultural practices (Falk and Gage 1995).

As the robust australopithecines evolved one enlarged route to shift blood to the vertebral plexus, graciles were experimenting and perfecting a more diverse and complicated network of small veins to carry out this function, and to cool their brains.

Frequencies for the O/M sinus are extremely low in gracile australopithecines with the overall trend toward reduction. But frequencies for mastoid and parietal emissary foramina increased dramatically. Data regarding these emissary veins suggests that a different cranial vascular system developed in graciles who lived in thermally stressful environments and gave rise to Homo (Falk 1992).

Why was the evolution of a prototype network of cranial veins in gracile australopithecines so important in our evolutionary history? The brain can only be as big as the venous system can cool.

Robust australopithecines exhibited cranial capacities from 450 to 750 cc's, on the average. Falk (1992) proposes that by continuing to inhabit the cooler forested areas it was unnecessary for the robust's to evolve a "radiator" network of veins to keep the brain cool and this eventually constrained their brain evolution. The O/M sinus function to reduce the hydrostatic pressure of an upright stance and that was all they required.

Gracile hominids did evolve an advanced radiator network that was modifiable and able to keep up with the increasing thermolytic needs of and enlarged brain. Michel Cabanac, a physiologist at Laval University in Quebec agrees with Falk. He also researched the emissary veins and concluded that it is highly possible that they evolved for the defense of the brain temperature.

Falk plotted hominid cranial capacities against data for emissary veins. Her results show that for the past two million years the increase in frequencies of emissary foramina kept exact pace with the sharp increase in brain size (Falk 1992)

The implications of the Radiator Theory have a profound effect on the hypothesizing of hominid phylogenies. The classic or most widely recognized tree has A. afarensis giving rise to A. africanus and H.habilis with A. robustus diverging from A. africanus and A.boisei diverging from A. afarensis. The problem arises when considering the venous systems of these hominds. A. afarensis possesses the highly derived condition of an enlarged O/M sinus and both A. africanus and H. habilis have the primitive condition of a venous network.

In conclusion, the evolution of a mechanism for human brain cooling has been debated by physiologists for some time. The research done by Falk and others provides considerable evidence that hominids were evolving selective brain cooling beginning about five million years ago in response to a change in climate and habitat. Although it has also been shown that not all early hominids required this character. Unfortunately, the lack of selective brain cooling may have inhibited the evolution of robust australopithecine brains and contributed to their extinction as a genus . Likewise, possession of selective brain cooling by our direct ancestors is likely to have been the releaser that allowed for Homo's rapid brain expansion and ultimate success.

References and further reading:

Falk, Dean 1992: Braindance. Henry Holt and Company, N.Y.

Falk and Conroy 1983: The cranial venous system in Australopithecus afarensis. Nature 306(22/29):779-781

Wheeler, Pete 1988: Stand tall and stay cool. American Scientist. May 12 pps. 62-65.