paleoneurology

Paleoneurobiology is the study of brain evolution by analysis of brain endocasts to determine endocranial traits and volumes. Considered a subdivision of neuroscience, paleoneurobiology combines techniques from other fields of study including paleontology and archaeology. It reveals specific insight concerning human evolution. 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 and their endocasts can be compared to each other, to the skulls and fossils of recently deceased individuals, and even compared to those of other species to make inferences about functional anatomy, physiology and phylogeny. Paleoneurobiology is in large part influenced by developments in neuroscience as a whole; without substantial knowledge about current functionality, it would be impossible to make inferences about the functionality of ancient brains.

Hominid paleoneurobiology refers specifically to the study of brain evolution by directly examining the fossil record of humans and their closest hominid relatives (defined as species more closely related to humans than chimpanzees). Paleoneurobiologists analyze endocasts that reproduce details of the external morphology of brains that have been imprinted on the internal surfaces of skulls.

History

Humans have had a long interest in the brain and its functions. The first recorded study of the brain and its functions was from a papyrus text written by the ancient Egyptians during the 17th century BCE. The document details 48 medical ailments and makes references to how to deal with head wounds. Much later in the 6th century BCE the ancient Greeks began to focus on studies of the brain and the relationship between the optic nerve and the brain. Studies of brain evolution, however, did not come about until much later in human history.

Comparative anatomy began its emergence in the latter part of the 19th century. Two main views of life sprung forth; rationalism and transcendentalism. These formed the basis for the thought of scientists in this period. Georges Cuvier and Étienne Geoffroy St. Hilaire were leaders in the new field of comparative anatomy. Cuvier believed in the ability to create a functional morphology based simply on empirical evidence. He stressed function of the organ must coincide with its form. Geoffroy, in contrast, put a heavy emphasis on intuition as a method of understanding. His thought was based on two principles: the principle of connections and the principle of unity of plan. Geoffroy was one of the first to look for homologies in organs across species, though he believed that this was evidence of a universal plan rather than descent with modification.

The late part of the 19th century in comparative anatomy was heavily influenced by the work of Charles Darwin in the ''On the Origin of Species'' in 1859. This work completely changed the views of comparative anatomists. Within 8 years of Darwin's release of the ''Origin of Species'', his views on descent from a common ancestor were widely accepted. This led to a shift in trying to understand how different parts of the brain evolved. The next major innovation that helped to bring about paleoneurobiology was the microscope. Although the microscope was invented in the 17th century, it was only used in biology in the beginning in the late 19th century. The techniques of observing brain cells under a microscope took a long time to refine. In 1873, with this tool in hand, Camillo Golgi began to cellularly detail the brain and employ techniques to perfect axonal microscoping. Ludwig Edinger took advantage of this and came up with a new branch of anatomy called comparative neuroanatomy. Edinger held that vertebrates evolved in a linear progressive series. He also thought that changes in the brain were based on a series of additions and differentiations and that the most highly, complex brains were those that were the most encephalized. The period of 1885-1935 was an explosion of ideas in comparative neuroanatomy. This era culminated in the publication of "The Comparative Anatomy of the Nervous System" by Arienns, Kappers, Huber, and Cosby. This paper influenced Tilly Edinger and she later became the founder of paleoneurobiology.

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 paleoneurobiology. 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 paleoneurobiological 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 paleoneurobiology 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 paleoneurobiologists, 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 paleoneurobiological 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.

Conflict between Holloway and Falk

Paleoneurobiologists 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, and Falk maintaining that 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''), which 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. These new techniques in endocast analysis included the use of stereoplotting to transfer sulci between differently shaped endocasts, measurement of indexes from photographs rather than directly from specimens, and confounding of measurements taken directly from specimens and those taken from photographs.

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; 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, endocasts rarely exhibit convolutions due to buffering by the pia mater

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