Vertebrate voyages

In this chapter, a rat charts a safe route to cheese; a woman misplaces her car; and one of your ancestors swims for its dinner. But mainly we consider the kinds of memories that evolved in early vertebrates. The brain of these ancestors included the hippocampus, a brain area crucial for memory. The hippocampus evolved to guide navigation, which came with a bonus: support for other memories, such as the sequence and timing of smells and sights. By combining these memories in various ways, vertebrates can construct a map of their world, including spatial layouts and the proximity of items; the order of items in a sequence; and the appearance of landscapes as viewed from various angles. The memories of events and contexts arise from the same source.

Epilogue

In the epilogue, a couple of kids befriend a shy stegosaurus; a different stegosaurus worries about the rise of mammals; and a tyrannosaurus presents a situation report. But mainly we consider reptilian brains, the relationship of brain size to intelligence, and the evolutionary success of mammals. Contrary to an internet meme, no one has a “reptilian” or “lizard” brain lurking within. Our entire brain is human. Regarding intelligence, brain organization matters as much as brain size and maybe more. Once dinosaurs became extinct, the mammals that supplanted them had much smaller brains than large dinosaurs had. Instead, the success of mammals depended on the emergence of the neocortex, a new part of the brain. Eventually, this evolutionary innovation came to dominate both the brain and the memories that it contains.

Human heritage

In this chapter, a horse talks; pigs fly; and mice design a computer. But mainly we consider a form of memory that evolved in humans. Part of the temporal lobe expanded and added a generalization—representing the concepts and categories of cultural memory—to a specialization: representing distant signs of food. At about the same time, posterior parietal areas also expanded and added a different form of generalization—representing relations of many kinds—to a specialization: representing relations among quantities. Both the temporal and parietal lobes interact with the prefrontal cortex, especially the parts that first emerged in anthropoids. Together, these evolutionary innovations improved the ability of evolving hominins to cope with novel and challenging problems. In this sense, hominin evolution produced cortical areas with a specialization for generalization.

Anthropoid adaptations

In this chapter, a pig wins the Kentucky Derby; people remember bugs and beasts; and monkeys take flight. But mainly we consider memories that evolved in anthropoids. As these ancestors evolved from small animals into larger ones, they needed to travel long distances to obtain food. They used high-power vision and a new kind of color perception to detect distant sights indicating ripe fruit. The memory of these perceptions helped guide their choices toward the most valuable food items both near and far. The sounds made by boisterous, feeding birds provided similar information. Other new memories involved representations of quantities, such as the number of food items and their distance. Eventually, the new memories evolved by anthropoids also helped them recognize and communicate with members of their social group.

Primates of the past

In this chapter, a mummified monkey’s paw causes big trouble; a bushbaby eyes a wicked witch; and a monkey pigs out on fresh berries. But mainly we consider memories that evolved in early primates. As these ancestors adapted to a life in the trees, they became a lot like us. They developed forward-facing eyes and grasping hands; guided reaching movements with vision; moved themselves with their legs; and had a large brain. New cortical areas helped these ancestors survive by using new memories: of how to reach accurately while perched on swaying branches; of the precise amount of force needed to grasp valuable items; of the location and value of items hidden in a clutter of branches and leaves; and of objects and actions linked to a hidden food item’s desirability.

The humble heredity of humongous hemispheres

In this chapter, a distant ancestor makes a success of greed, laziness, and fear; a driver hits the jackpot; and Dorothy comes to a fork in the yellow brick road. But mainly we consider how the brain evolved. A succession of forks in our evolutionary road produced new brain areas that are crucial for memory. Each of these areas has its own kind of memory, which evolved in a now-extinct ancestral species long ago. In each case, new forms of memory helped these species survive and thrive in circumstances very different from our own. During human evolution, the brain began expanding about 3 million years ago. It reached its current size and shape about 400,000 years ago, long before the acceleration in cultural innovation that characterizes human societies.

A drive down memory lane

In this chapter, a man with amnesia sparks a new era in memory research; monkeys play a matching game; and a 19th-century politician explains how he seemed so clever. But mainly we consider the nature of memory and amnesia. The ‘man with amnesia’ was named Henry Molaison, better known as H. M., and he suffered from debilitating epilepsy. Brain surgery alleviated his seizures, but it had a shocking side effect: amnesia. His memory loss led scientists down two roads: one exploring the memories he lost; the other identifying the brain areas needed for normal memory. In this book, we consider memory from a different angle, examining how it evolved. We begin with a recent history of memory science and some background material on two important concepts: representation and homology.

Anthropoid augmentations

In this chapter, Dorothy braves lions (and tigers and bears); a dinosaur eats someone in Jurassic Park; and a proofreader gets a $7 raise. But mainly we consider how a new form of memory helped anthropoids avoid being eaten. When anthropoids faced a high level of volatility in their preferred foods, such as fruit, they had to make more foraging journeys to get enough to eat. Because they faced a severe threat of predation every time they did, these ancestors benefited from any reduction in the frequency of such excursions. So, they had to establish memories that limited foraging errors, and they had to do so quickly. New cortical areas provided this advantage, so anthropoids survived and thrived in a rapidly changing world teeming with predators.

The story of your life

In this chapter, a wizard reads Dorothy’s mind; a hurricane lashes the Florida coast; and Gatsby remembers his long-lost love. But mainly we discuss what makes memories our own. During human evolution, our brains developed a new form of memory that helped support our complex societies. New levels of “self” representation evolved in an expanding cortex, and they empowered people to infer the intentions and mental states of others. Eventually, these new memories became incorporated into older kinds. Embedding memories of “self” into memories of events established participatory memories; and embedding them into memories of facts or fictions established cultural memories. When these memories first evolved, the sense of participating in events and owning knowledge came into the world and made human memory what it is today.

Mammalian memories

In this chapter, Toto chases a cat, so the Wizard of Oz goes home alone; a woman is attacked by her own hand; and a mouse plunders a pantry. But mainly we consider memories that evolved in early mammals. These ancestors developed a new part of the brain called the neocortex, and it changed memory forever. The neocortex endowed early mammals with an ability to manage contradictory memories, each appropriate in certain contexts. Obsolete memories could be retained and used in case the ‘good old days’ returned, and new memories received a boost when it mattered most. The neocortex empowered early mammals to strike a balance among alternative ways of surviving: spending versus conserving energy; acting habitually versus making good choices; and acting with urgency versus patience.