The Great Brain Drain: A Controversial Theory Takes Ancestral Brain Growth in Vein
Bower, Bruce, Science News
The Great Brain Drain
This mechanical metaphor has generated plenty of heat among anthropologists and physiologists since Dean Falk proposed her "radiator theory" of brain expansion in June. Falk insists it offers an important insight into human evolution, a contention some researchers support and others vigorously challenge.
Accumulating evidence suggests that an evolving cerebral "radiator" enabled brain size in the Homo lineage -- including modern Homo sapiens -- to double over the last 2 million years, says Falk, an anthropologist at the State University of New York at Albany. Her metaphor refers to a dense network of veins that, in her opinion, cools the heat-sensitive brain during intense exercise or heat exposure.
The burgeoning brain of successive species leading to modern humans has long intrigued anthropologists. Many have suggested that one key behavior, such as language, hunting or throwing, served as a critical influence on the evolution and growth of our ancestors' brains. However, the causes of brain growth in humans and other mammals remain unclear.
Into this murky milieu strides Falk with her radiator theory, which manages to stoke ongoing debates in both anthropology and physiology.
Much of the controversy emerges in the June Behavioral and Brain Sciences, where Falk presents the theory and 26 scientiests from around the world critique her proposal. Physiologists skirmish over the contention that a special venous thermostat authomatically cools the brain when body temperature rises. Falk's theory also fires up anthropoligical arguments over lines of descent among human ancestors and flushes out conflicting versions of how our evolutionary forerunners made a living in Africa.
Falk's scenario of human brain growth begins 4 million to 5 million years ago, when hominids -- members of the evolutionary family that includes modern humans -- abandoned knuckle-walking for an errect posture and a two-legged gait. Prior to this ambulatory innovation, in Falk's view, blood returned from the brain to the heart through the jugular vein, which today provides the main thoroughfare for cerebral blood drainage in reclining humans. But in an upright position, blood streaming straight downward in a gravity-induced rush would damage jugular walls, she maintains.
To lighten the jugular load, two new drainage systems for the brain evolved in early hominids, Falk contends. Both divert cerebral blood to the vertebral plexus, an extensive network of veins running down the torso that returns blood to the heart. Analyses of bony grooves on the inside of fossilized hominid crania, performed by Falk and several other investigators (NS: 7/2/83, p.8), reveal the two venous patterns.
Fossil evidence of one drainage system appears in the famous "Lucy" and other members of Australopithecus afarensis discovered at the Hadar site in Ethiopia, who lived about 3.2 million years ago. "Robust" australopithecines, consisting of several small-brained hominid species dating to between approximately 2.5 million and 1 million years ago, display the same drainage setup.
A pair of channels for venous blood at the back of the head -- the occipital and marginal sinuses -- show considerable enlargement in these species and feed into the vertebral plexus. They would have helped to show the outward flow of cerebral blood among the upright hominids, Falk argues. This arrangement appears in all 13 crania of Hadar A. afarensis and robust hominids with identifiable bony grooves, she says.
In contrast, large occipital and marginal sinuses rarely appear in "gracile" australopithecines and members of the Homo lineage, Falk argues. The graciles include 3.5-million-year old fossils from Laetoli, Tanzania -- placed alongside Lucy in the species A. afarensis by many other anthropoligists -- and the more than 2-million-year-old South African species A. africanus. …