page_title: Hippocampus
section_title: Evolution
hierarchical_section_title: Hippocampus / Evolution
context_page_description: The hippocampus is part of the mammalian brain, and belongs to the limbic system. Humans and other mammals have two, one in each side of the brain. The hippocampus is under the cerebral cortex.
  It is important in spatial memory and navigation, and helps turn short-term memory into long-term memory. The hippocampus is named after the seahorse because its shape is similar.
  In Alzheimer's disease, the hippocampus is one of the first regions of the brain to suffer damage; memory loss and disorientation are included among the early symptoms. People with extensive, bilateral hippocampal damage may experience anterograde amnesia – the inability to form or retain new memories.
  The different neuronal cell types are neatly organized into layers in the hippocampus. It has often been used as a model system for studying neurophysiology. Long-term potentiation, a neural mechanism for storing memory, was first discovered to occur in the hippocampus.
context_section_description: The hippocampus has a generally similar appearance across the mammals from monotremes such as the echidna to primates such as humans. The hippocampal size to body-size ratio increases: it is about twice as large for primates as for the echidna. It does not, however, increase at anywhere close to the rate of the neocortex to body-size ratio. Therefore, the hippocampus takes up a larger fraction of the cortex in rodents than in primates.
  Other vertebrates have areas which may be homologous to the hippocampus of mammals. Some insects, and cephalopods such as the octopus, have strong spatial learning and navigation abilities. These appear to work differently from the mammalian spatial system, and have evolved independently of the mammalian system.
caption: Drawing by Camillo Golgi of a hippocampus stained using the silver nitrate method
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