A reader has written to me doubting what I wrote in my previous posting about birds retaining a fear of men from the time when they used to be caught and eaten in Medieval England.
It’s not strictly an actual memory — as we understand the term — that’s passed on from epigenetically from one generation to the next, but only an emotion, in this case fear, that’s looely associated with particular circumstances.
Go back to the original incident or incidents of 300 years ago. If a bird was caught it would experience intense fear. This would be experienced in a deep part of the brain. The bird would also have processed the perceptual circumstances that occurred at the same time — seeing a man — an unusual animal on two legs. This occurs in higher processing areas of the brain — in our case, the cortex.
If the fear and the perceptual memory become strongly associated in the bird and then the bird escapes, then it is possible for genes involved with the fear response to become ‘marked’ chemically. It is this emotional marking that has been proved in many experiments to be capable of being passed on from generation to generation.
The precise memory of the event in all its details, like all individual memories, cannot be passed on. The higher processing areas of the brain containing precise memories remain unique to an individual. The precise memory would have vanished when the bird eventually dies. However, if the bird had offspring, then its fear responses — to other general situations also — can, in fact,be passed on. This has been shown scientifically in the case of several species — not, as yet, in the case of birds, but I see no reason why not.
It’s not the precise memory of the fearful event that’s passed on. It’s that a fear response that’s associated with a collection of neurons in the higher processing part of the brain that were previously active can be passed on to an offspring. An offspring might have several different collections of neurons that were aroused on entirely different occasions. The same fear but several different general different situations. This sort of preparedness to several different sorts of fearful situations would be of great benefit to any species in which epigenesis occurs.
We possess a few epigenetic memories ourselves. If we see a coil of rope or a spring that moves, or looks as though it might do, then most people will instantly fear a snake even if they have never been close to one in their lives.
The markings that elicit epigenetic emotions can be inherited for two or more generations. They can die out if unprompted. If all English had died out 300 years ago, say, then an y Frenchmen visiting our shores would have found that birds would have been approachable. This is similar to the situation today when ornithologists visit uninhabited islands north of Scotland. The puffins and other birds there are completely friendly.
Whereas genetic mutations last for hundreds of years without change to adjust to long term changes in the envronments, epigenetic markings last for shorter periods and depend on more ephemeral changes.