Memory is as old as the brain itself. Ever since the first animal nervous system evolved over five hundred million years ago, creatures could learn from experience. But like the brain, human memory has evolved and become more complex.
Memory began in primitive organisms as a simple reflex system. When the body is affected by something nasty, such as a toxic chemical, the nervous system lays down a trace so that the next time the chemical is encountered, it is recognized and avoided. This capacity to learn from experience forms the basis of all memory. Even the microscopic roundworm has a memory of sorts in that it can learn to avoid certain chemicals. Fruit flies can remember a wider range of danger signals, including odors. More complex animals are able to store information besides danger signals. Bees remember the location of flowering plants, and rats can learn to navigate complex mazes.
Human memory is more complex still, reflecting the larger size of our brains. It is not simply the size of the brain in itself that matters, but the ratio of brain size to body size. In terms of its relation to body size, the human brain is bigger than that of any other animal. Elephants have larger brains than us in absolute terms, but their brains take up a smaller proportion of their body than ours.
Bigger brains for bigger groups
Scientists are still divided as to why the human brain evolved to be so big. Until the 1970s, most assumed that the expansion of the human brain was driven by adaptation to changes in the physical environment, such as climate change, varying ecological conditions, or changes in diet. However, in 1976, the psycho lo gist Nicholas Humphrey argued that human brains got bigger because of changes in the social environment – the most important being that we started to live in larger and larger groups. This theory is now known as the Machiavellian intelligence hypothesis, after the Italian statesman whose name became synonymous with cunning, manipulative behavior.
The Machiavellian intelligence hypothesis suggests a key role for memory. As group size increased, our ancestors needed to keep track of increasing members of people. They needed to remember what they looked like and what their character was like. Above all. They needed to keep a record of how everyone else in the group had treated them in the past. This, at least, is what is suggested by the theory of reciprocal altruism, which can be summed up as ‘ you scratch my back and I’ll scratch yours. The biologist Robert Trivers was the first to propose that social behavior can evolve if animals cooperate only with others who have cooperated with them in the past. Clearly, an animal would need to possess a sophisticated memory to keep track of cooperative animals and others that have not.
Humans and vampire bats
Biologists now think that among animals generally, reciprocal altruism is quite rare. Most altruism in nature seems to have evolved on the basis of kinship – the drive to bestow favors on family members rather than exchange them with strangers. In evolution, it really does seem that blood is thicker than water.
However, humans may be one of the few species in whose evolution reciprocal altruism has played a large role. Vampire bats are another: they form close friendships, and a bat will regurgitate food to feed a friend (but nobody else) should the friend be unsuccessful at hunting one night. Interestingly, the brains of vampire bats are much bigger, in relation to their bodies, than the brains of other bats. This expansion has occurred in the cortex (the brain’s outer layer), which is also the brain area that expanded most in human evolution. This provides some support for the view that increasing group size, and J.bove all the need for a good social memory, was vital in the evolution of the human brain.
Developing memory
As skills evolved, so did different forms of memory – from the procedural memory used by bipedal apes to find food and shelter up to 4.5 million years ago to the semantic and working forms of memory used by Homo sapiens.
Australopithecus afarensis
4.5- 2.75 million years ago. The bipedal apes, foraging on woodland savannahs in parts of Africa, probably had stored memories of food and other resources in the environment and procedural memory for skills to find food and shelter.
Homo habilis
2.5- 1.6 million years ago. These were the first species known to make stone tools, thus applying procedural memory to technical skills. They may have had more advanced knowledge of resources and increased use of vocalization.
Homo erectus
1.8 million- 300,000 years ago. Migrating into Asia and Europe, Homo erectus needed better memory for resources to survive in harsher climates. Episodic memory may have evolved to keep track of social debts and obligations in large social groups.
Homo sapiens
Two hundred thousand years ago onwards, Early modern humans applied knowledge broadly. Remembering facts linked with social information may have led to semantic memory. Finally, working memory evolved so different ideas could be held consciously in mind at the same time.
Memory Layers
In brain evolution, new capacities rarely replace old ones; the old ones usually remain, and the new ones are added on top. This is particularly clear in the evolution of memory.
Humans have evolved complex memories that can store all sorts of information that animals cannot, such as words and abstract concepts. But we also retain the older, primitive kinds of memory that our pre-human ancestors had. Emotional memories, for example, can exist alongside verbal memories, and sometimes the two can point in different directions. Emotional memories are mediated by very old brain structures, such as the amygdala, buried deep beneath the cortex (the outer layer of the brain). The amygdala, the hippocampus, and the thalamus are the main components of the limbic system, which is found in even the earliest mammals.
