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Paleoneurology is the study of brain evolution by analysis of endocranial traits and volumes. The cranium is unique in that it grows in response to the growth of brain tissue rather than genetic guidance, as is the case with bones that support movement. Fossil skulls can be compared with each other as well as the skulls of recently deceased individuals to make inferences about functional anatomy, physiology and phylogeny. Paleoneurology is in a large part influenced by current developments in neuroscience as a whole; without substantial knowledge about current functionality, it is impossible to make inferences about the functionality of ancient brains. Advances in technology are also necessary for proper analysis to be made.

History

Tilly Edinger

Ottilie "Tilly" Edinger was born in Frankfurt, Germany in 1897. Her father Ludwig Edinger, himself a pioneer in comparative neurology, provided Tilly with invaluable exposure to his field and the scientific community at large. Tilly had many private tutors before attending Schiller-Schule, the only secondary school for girls in Frankfurt at that time. Tilly Edinger continued her schooling with university studies in zoology, geology, and paleontology. While preparing her doctoral dissertation, Edinger encountered a natural brain endocast of Nothosaurus, a marine reptile from the Mesozoic era. Edinger's first paper, published in 1921, centered on the characteristics of the Nothosaurus specimen. Prior to the publication of her work, inferences about the evolution of the vertebrate brain were made exclusively through comparative anatomy of extant fish, amphibian, reptile, bird, and mammal brains. Tilly Edinger's background in neurology and paleontology paved the way for her to integrate comparative anatomy and stratigraphic sequence, thus introducing the concept of time to neurology and creating the field of paleoneurology. The field was formally defined with the publication of Die fossilen Gehirne (Fossil Brains) in 1929 which compiled knowledge on the subject that had previously been scattered in a wide variety of journals and treated as isolated events.

While still in Germany, Edinger began studying extant species from a paleoneurological perspective by making inferences about evolutionary brain development in seacows using stratigraphic and comparative anatomical evidence. Edinger continued her research in Nazi Germany until the night of November 9, 1938 when thousands of Jews were killed or imprisoned in what became known as Kristallnacht. Although a visa was not immediately available for immigration to the United States, with the help of friends and colleagues who valued her work, Edinger was able to immigrate to London where she translated German medical texts into English. Eventually her visa quota number was called and she was able to immigrate to the United States where she took on a position as a research fellow at Harvard's Museum of Comparative Zoology.

Her contributions to the field of paleoneurology include determining the extent to which endocasts reflect the anatomy of ancient brains, the adequacy of comparative anatomy to interpret brain evolution, the ability of brain endocasts to predict the lifestyles of extinct organisms, and if brain size has increased over geological time; topics which are still being explored today. In her later years, Edinger corresponded with the next generation of paleoneurologists, which insured that the work from her 50 year career continued into the future. The pinnacle accomplishment of her career was the compilation of an annotated bibliography of paleoneurological papers published between 1804 and 1966. The bibliography, Paleoneurology 1804-1966, was completed and published by colleagues posthumously in 1975 due to the untimely death of Edinger from injuries sustained during a traffic accident in 1967.

Holloway and Falk Conflict

Paleoneurologists Ralph L. Holloway and Dean Falk disagree about the interpretation of a depression on the Australopithecus afarensis AL 162-28 endocast. Holloway argues that the depression is a result of lipping at the lambdoid suture and that the sulcal patterns indicate cerebral organization moving toward a more human pattern while Falk insists that the depression is the lunate sulcus in a position that is indicative of an ape-like sulcal pattern. The debate between these two scientists is not hinged solely on the AL 162-28 endocast, but rather extends to all australopithecine fossils with Holloway insisting on the presence of hominid sulcal features while Falk maintains the features are pongid in nature. The debate between Holloway and Falk is so intense that between 1983 and 1985 they published four papers on the identification of the medial end of the lunate sulcus of the Taung endocast (Australopithecus africanus) that only further strengthened the division between each scientist's respective opinion. Although there have been no definitive conclusions about the fossils in question, many techniques were created or critically analyzed and refined as a result of the conflict.

Brain Endocasts

A brain endocast is the imprintation of the inner features of a cranium that captures the details created from pressure exerted on the skull by the brain itself. Endocasts can be formed naturally by sedimentation through the cranial foramina which becomes rock hard due to calcium deposition over time, or artificially by creating a mold from silicon or latex that is then filled with plaster-of-Paris while sitting in a water bath to equalize forces and retain the original shape. Natural endocasts are very rare and most of those that are studied are the result of artificial methods. Although the name implies that it is a copy of the once living brain, convolutions rarely appear on endocasts due to having been buffered by the pia mater, arachnoid mater, and dura mater that once surrounded and protected the brain tissue. Furthermore, not all endocasts are created from a complete cranial fossil and the missing parts are approximated based on similar fossils. In some cases, fragments from several fossils of the same species are used to construct a single endocast.

More recently, computed tomography has played a large role in reconstructing endocasts. The procedure is non-invasive and has the advantage of being able to analyze a fossil in record time with little risk of damaging the fossil under review. CT imaging is achieved through the application of x-rays to produce tomographs, or sectional density images, are similar to the images produced during MRI scans. CT scans use slices approximately 1 mm thick to reconstruct a virtual model of the specimen. This method is especially useful when a fossil cranium is occupied by a natural endocast that cannot be removed without destroying the skeletal portions of the fossil. Because the cranium and its contents are of different densities, the endocranial cavity and its unique traits can be reconstructed virtually.

Analysis

Historically, many primitive methods were used to assess brain endocasts. For example, cranial capacity was once gauged by filling the fossil cranium with mustard seed and measuring the volume of the mustard seed in a graduated cylinder. Many of these procedures have been replaced by more accurate techniques developed through scientific and technological advancement.

Volume

Cranial capacity or brain volume is prominent in the scientific literature for discussing taxonomic identification, behavioral complexity, intelligence, and dissimilar rates of evolution despite being ill-suited for these purposes. In modern humans, cranial capacity can vary by as much as 1000 cc without any correlation to behavior. This degree of variation is almost equivalent to the total increase in volume from australopithecine fossils to modern humans and brings into question the validity of relying on cranial capacity as a measurement of sophistication.

Many paleoneurologists measure cranial capacity via the submersion method in which displacement of water in a beaker is taken as the volume of the endocast. Scientists who believe that this method is not accurate enough will use a similar procedure in which a beaker with a spout is filled until it is full. The water displaced by the endocast is then weighed to determine the endocast volume. Although both of these techniques are significantly more precise than previous methods, scientists are optimistic that advanced techniques such as computed tomography will provide greater accuracy of volume measurements.

Encephalization Quotient

Morphometric Analysis

Morphometric Analysis relies on chord and arc measurements of the endocast surface. Length, width, bregma-basion, and height meausrements of an endocast are taken with spreading calipers. Frontal, parietal, and occipital lobe chord length (the length of the lobe at its widest point along the midsagittal plane) are measured using a dioptograph in which landmarks are projected onto a two dimensional surface. Measurements may be skewed if the orientation of the endocast has not been properly determined before the dioptograph is made. Geometric morphometrics, systems of coordinates superimposed over the measurements of the endocast, are often applied to allow comparison between specimens of varying size. Measurements may also be taken in reference to Broca's area, height of the endocast at 25% intervals of the maximum length, and the vault module (mean of maximum length, width, and middle height). Although other measurements may be taken, the choice of landmarks are not always consistent between studies.

Asymmetries

Assymmetries occur due to hemispherical specialization and are observed in both a qualitative and quantitative manner. The unevenness of the hemispheres, known as a petalia, is characterized by a lobe that is wider and/or protruding beyond the contralateral lobe. For example, a right handed person typically has larger left occipital and right frontal lobes than the contralateral lobes. Petalias also occur due to specialization in the communication centers of the brain in modern humans. Petalias in the occipital lobe are easier to detect than those in the frontal lobe. Some gorillas have shown strong petalias, but they are not found in combination with other petalias as is almost always the case in humans. Scientists use the presence of petalias to show sophistication, but they are not a definitive indicator of evolution toward a more human brain.

Convolutions

Convolutions, composed of the individual sulci and gyri that compose the folds of the brain, are the most difficult aspect of an endocast to accurately assess due to obscuration by the meninges as well as the vasculature of the brain's surface. Because some cranial fossils such as the robust australopithecine fossils show these details, convolutions are included in the study of endocasts whenever possible.

Meningeal Patterns

Although the meninges have no link to behavior, they are still studied within the realm of paleoneurology due to the high degree of conservation of meningeal patterns within a species which may serve as a way to determine taxonomy and phylogeny.

Endocranial Vasculature

Because meningeal blood vessels comprise part of the outermost layer of the brain, they often leave vascular grooves in the cranial cavity that are captured in endocasts. Endocranial vasculature originates around the foramina in the skull and in a living body would supply blood to the calvaria and dura mater. The vasculature is so well preserved in some fossils that terminal branches of the circulatory system can be observed. Analysis of cranial vasculature concentrates on the anterior meningeal system of the frontal region, the middle meningeal system of the parieto-temporal and part of the anterior occipital region, and the cerebellar fossa system of the cerebellar region. In the course of hominid evolution, the middle meningeal system has undergone the most change. Although cranial vasculature has been exhaustively studied in the last century, there has been no consensus on an identification scheme for the branches and patterns of the vascular system resulting from little overlap of results between studies. As such, endocranial vasculature is better suited for inferring the amount of blood delivered to different parts of the brain.

Limitations

The limited scale and completeness of the fossil record inhibits the ability of paleoneurology to accurately document the course of brain evolution.

See Also

Archaeology

Paleontology

Neuroscience