Subregions of the Hippocampus

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The overall structure and connectivity of the Hippocampus is similar across mammals from monotremes to humans. Though allometry studies have shown the hippocampus proportionally increases with body size across phylogeny, it does not increase at the same rate as neocortical expansion. Therefore while the adult human hippocampus is larger than other higher order primate hippocampi, it occupies a smaller overall percentage of the brain. In particular, of each of the subregions of the hippocampus (i.e. Dentate Gyrus, Hilus, CA3, CA2, and CA1), only the hilus and CA1 have been demonstrated to be of greater size when normalized to the overall volume of the hippocampus. Soft tissues like the brain are fossilized therefore little is known about further differences between humans and other hominids.

In addition to its importance in episodic memory formation, the hippocampus plays an important role in spatial navigation. Neurons in the mammalian hippocampus have been shown to have “place fields” in which they increase their activity at particular locations within an environment. Old world monkeys, and presumably humans, appear to utilize these fields differently. Rather than activating when the animal located in particular location, macaque hippocampal neurons increase activity when viewing a spatial location, as if they projecting themselves into the location. The developmental differences in these two forms of place fields is not well understood however it may be linked to intrinsic anatomical differences between higher primates and other mammals.

Differences in the size of hippocampal subregions has played an important role in the early debate of evolutionary theory and human uniqueness. In 1859, Sir Richard Owens—an accomplished comparative anatomist, member of the royal society, and superintendent of the Natural History Department of the British Museum—revised the Linnean classification system to create a new subclass—just below that of Mammalia—exclusively for humans with the hope of preempting the theory of common descent published by Darwin in On the Origin of Species later that year. Owens summarized this new system as: “Man is the sole species of his genus, the sole representative of his order and subclass. Thus I trust has been furnished the confutation of the notion of a transformation of the ape into man, which appears from a favourite old author to have been entertained by some in his day.” Key among his arguments for this distinction of uniqueness was the existence of the hippocampus minor solely in humans, a region outside the hippocampus proper, now known as the calcar avis. A debate between Owens and Sir Thomas Huxley, known as the “Great Hippocampus Question,” raged through the 1860s over the theory of common descent with the existence of the calcar avis at its center. Huxley ultimately demonstrated the existence of the calcar avis in other primates, discrediting Owens ideas and stature, and increasing acceptance for evolutionary theory in 19th century England.

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