From one of Jeff’s presentations I got the impression that the hippocampus evolved first, and then the neocortex evolved later, between the sense organs and the hippocampus. I’m curious to know if anyone has any context for why that order of evolution appears to be the case. Is there a fossil record maybe? Or is it based on other animals maybe? Is it purely a logical inference? It sounds reasonable to me, I just don’t know the story of the supports for that position, if there is one.
I’m not a neuroscientist, but I think part of the support for the logical inference is that as one observes organisms going from the simple to the complex; one sees neural structures being added on top of previous structures instead of replacing entirely?
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I can’t point to any references but this is something I have read on numerous occasions. The hippocampus and the entorhinal cortex are both simpler structures (3 layers) than neocortex (6 layers). Reptiles, which don’t have a neocortex, have a structure called the pallium which is believed to play the same role as the hippocampus and entorhinal cortex. My understanding that the pallium became the entorhinal cortex and hippocampus in mammals.
The entorhinal cortex represents where an animal is in the world and the hippocampus is short term episodic memory. These functions are essential for any animal that needs to remember places it has been and that needs to return to them. Every animal that moves about and needs to return to a nest at the end of the day must have some means of knowing where it is. This is a very basic function and will be old in evolutionary time. So it makes sense that the hippocampus and entorhinal cortex, or their equivalent, predate the neocortex.
Scott Purdy did some reading about these structures and might have a reference or two to suggest.
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Edit: I accidentally replied to @cogmission.
A bit old being 1998, but very comprehensive.
- Hypothesis on neocortical evolution
According to hypotheses of Rakic [232]and Marı́n-Padilla [175], the neocortex achieves a more organized and complex structure during the course of phylogeny by the lateral addition of new vertical functional units [232], and the creation of new (sub)layers in the neocortical plate, i.e., the `open system’ hypothesis [175]. In addition, comparing the allocortex (hippocampus and reptilian cortex) with the neocortex, we propose the general hypothesis that the distribution and route of the ingrowth of the main afferent systems (Fig. 4) guided by the pioneer neurons of the preplate define the developmental capacity of the cerebral cortex, and that changes in this plate zone have enabled the specific evolution of the mammalian neocortex. This hypothesis postulates that the preplate functions as a frame in cortical development and may help to guide cortical developmental phenomena, like the radial inside-out sequence of neocortical histogenesis, early influence of afferents on cortical development, and lateral and radial expansion of the neocortex, as described in more details below.
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In my informed opinion, this is more of an artificial question. Hippocampus and neocortex should be both viewed as organs of each other, thus we have to assume that they were co-evolved. If we need some boundaries within the brain, a more biologically relevent distinction might be made between internal and external sensation/action loops, therefore between the neocortex/hippocampus and the limpic system. In this perspective, the hippocampus and the neocortex would not be a kind of mathematical products of the -ostensibly “real”- world of all sensations, but they might rather be a dynamic representaiotn of the consciously triggered ones, as long as the limpic circuits respectively might be a representaiton of the subconsciously triggered ones.