Hominid skull has human-like drainage plan.

Hominid skull has human-like drainage plan

An expanding network of veins controlling the flow of blood from the brain characterized the more than 2-million-year-old hominid species Australopithecus africanus, a pivotal but poorly understood member of the evolutionary family that includes modern humans. This finding, confirming work with other A. africanus skulls, suggests that the sprouting of these veins and the reorganization of other venous channels may have set the groundwork for the dramatic increases in brain size observed in the Homo lineage that led to modern humans, glenn C. Conroy of Washington University Medical School in St. Louis and his colleagues assert in the Feb. 16 SCIENCE.

The researchers literally looked right through the rocky filling clogging an A. africanus skull by using computerized tomography (CT) scans whose special computer software produced two- and three-dimensional images (SN: 12/19& 26/87, p.408). The computer imaging technique also reconstructed a missing portion of the cranium based on a statistical analysis of the endocranium, or inner surface of the braincase.

The endocranial capacity of the specimen, which provides an estimate of brain size, is fairly small, Conroy says, and confirms a previous calculation based on indirect comparisons with another A. africanus skull.

CT analysis of the endocranium reveals a venous drainage system similar to that observed on four of the five additional A. africanus skulls that have been discovered, the researchers note. The other skulls do not have a hardened filling to obstruct endocranial measurements. The drainage pattern, which occurs in more pronounced form among modern humans, consists of enlarged sinus grooves running laterally down the temporal bone, as well as marks made by a network of veins around the foramen magnum, the large opening in the skull through which the spinal cord passes.

Prior studies of A. afarensis, the 3.5-million-year-old hominid species to which the famous Lucy belongs, and the robust australopithecines -- a hominid lineage that emerged around 2 million years ago and died out 1 million years later -- show a contrasting venous drainage pattern. Enlarged sinus grooves run down the occipital bone, from the back of the head to the foramen magnum; branching veins around the foramen magnum are not apparent.

The new CT study adds to a growing body of evidence that A. africanus shared an evolving mechanism for clearing blood from the brain with later members of the Homo lineage, Conroy maintains.

The finding also fuels a theory developed by Conroy and two others -- Phillip V. Tobias of the University of the Witwatersrand in Johannesburg, South Africa, and Dean Falk of the State University of New York in Albany. They propose that early hominids developed an altered blood blow from the brain when they began walking upright. In some species, such as A. africanus, a network of veins was established to cool the brain and allow it to increase greatly in size. Other species, as the robus australopithecines, had no extensive cooling system and thus remained small-brained.

However, the evolutionary role of A. africans -- which dates to between 3 million and nearly 2 million years ago -- remains unclear, Conroy adds. Most researchers class it as a sister species or direct ancestor of the Homo lineage.