Hanson China Editor: Karen Hurst Mentor: Thanos Siapas Word Count: approx. 3,350
Big Brains: Who needs em, anyway?
a
b
c Figure 1: Show me your monkey face! Two eyes, a nose, and a mouth are common features of all these primates but there is something indefinably more that separates them from other mere animals. Clockwise from top left: a. baby orangutan ( Dolphin Dreaming images index, 2002), b. young adult gorilla ( SOGHA index files, 2002), c. young adult orangutan ( Tosaw D. Evolution Happens, 2002) Locking eyes with a monkey is perhaps one of the strangest, yet most influential experiences a human can have. There is something in those eyes; something deeply and hauntingly familiar. That unspoken sense of familiarity quickly transforms into the realization that we are not so different from these gentle, and intelligent creatures. They do not have paws or claws, but hands with
separate fingers and skin that is just as creased and weathered as the skin on our own hands. They have fingerprints and cuticles to cover their rounded nails. They even have hangnails! The way primates hold their infants is also familiar a mother cradles an infant in the crook of her arm as the babys tiny hands grasp onto her. The mothers affection is pronounced and the baby is held this way constantly even while the mother sleeps. There seems to be a common depth beyond those eyes, a depth that hints at a sense of mutual understanding between us and the eyes we are looking into. It seems as though the owner of those eyes is thinking and feeling the same things we are; that he too feels a haunting sense of similarity an intangible closeness to us. Sometimes it seems that those eyes know more than we do; that he is fully aware of the similarities he shares with us and understands that humans are wrong to overlook them.
Similarities Among the Primates The modern scientific understanding of our similarity to apes and monkeys has lead to the creation of a category, the primates, which includes the prosimians such as tarsiers, lemurs, and bush babies; the monkeys, including gibbons and baboons; the great apes, including gorillas, chimpanzees, bonobos, and orangutans; and humans. As primates, we are similar in several major ways, the first being similarities of anatomical organization. Humans are the only creatures to walk primarily upright on two legs, but many apes have the ability to do this, although they do not often use it. The reason that apes have the ability to walk upright lies in the shape and organization of their hips, legs and back which
are very similar to that of a human. The anatomy of the primate hand is also remarkably uniform, which is particularly apparent in the fact that monkeys, apes, and humans are the only creatures that have opposable thumbs, allowing them to firmly grasp and manipulate objects. The second category of primate similarities includes behavioral features, such as primates characteristic displays of affection to their young, their use of body language and facial expressions to show emotion, and their social awareness in the context of correct social conduct. Just as humans do, apes and monkeys have a distinctive set of facial and body expressions that represent particular emotions. Primates are unique among mammals with regards to their expressive range because they have a particular set of facial muscles that allows for expression through varied facial distortions. Highly complex vocal chords also allow for a range of vocalized displays of emotion. Primates combine their abilities in facial and vocal expression with different body motions to communicate particular emotions, which is a kind of coordinated method for communication that is unique to primates.
Primate Intelligence The primates elaborate expressive capabilities are unique because of the great cognitive ability that is required to support and communicate so many behavioral moods and respond to those of others. This intelligence marks the third way in which all primates are similar. Intelligence, better described as brain capacity, cognitive ability, or neural capability, is related to the complexity of an organisms nervous system, particularly of its brain. Because the brain controls
all of an animals activities and abilities, an animal must have more neural connections for it to pursue complicated activities and abilities. For example, a human has a more complex brain than a dog because a person performs more activities that require a high level of cognitive ability. A simple task like remembering to pick up the keys before leaving the house in the morning requires a considerable amount of thinking and neural activity, even though it may feel like an automatic task. Although dogs are very intelligent creatures, such a task would be difficult for a dog because their neural structure is insufficient to allow such a task to become automatic. The more complex a brain is, the bigger it has to be to accommodate the necessary number of neural connections. The size of a brain is usually described by its mass, or by its mass as a proportion of total body mass (because brain size is also related to body size). However, for animals of roughly the same body weight, the more complicated the brain is as measured by the complexity of the animals activities, the bigger the brain must be in size and weight. The idea of encephalization, or brain weight relative to total body weight, is illustrated in the similarity of the body weight of both a mouse and a small fish. The mouse not only has a larger and heavier brain, it also exhibits greater encephalization and has a greater cognitive ability than the fish, even though their total body weights are on par. The remarkable degree of encephalization that all primate brains share enables them to perform many complex activities. Among mammals of the same body weight, the primate will have one of the largest brains in the group, if not the
largest. This principle holds true for primates of all sizes, ranging from the smallest prosimians to the huge gorillas. (Allman, 2000)
The Cost of Big Brains Primates large brain size is very unique among organisms and is shared only with the toothed whales (like the killer whale), and porpoises, which also have greatly encephalized brains. Few organisms have large brains, even though it would seem to benefit every organism to have a larger brain to enable them to perform more complicated tasks and thus survive more successfully in their environments. Large brains are rare, though, for reasons that are not entirely understood. The evolution of anatomical features or behaviors of a species occurs because it imparts a survival advantage to that species. Evolution enables a species to exploit its environment more successfully over time and is usually thought of as conferring a survival advantage, but many evolutionary changes also carry disadvantages. For example, the adult male mandrill (shown in Figure 2) has a very brightly colored face and rump, which is thought to be advantageous in that it acts as a display to attract females and signifies who the most dominant males are to the other members of the group. However, the disadvantage of this strange feature is that it minimizes camouflage, acting as a red flag to predators. This feature evolved despite the costs because of its reproductive and social advantages.
Evolution follows a series of simultaneous changes that occur until the overall advantages to a species exceed the disadvantages. For example, if the disadvantages of the mandrills colorful face did in fact outweigh the advantages, another evolutionary change might take place that would correct the problem of attracting predators. The mandrill might even develop a new skill that would allow him to better escape approaching predators. If the skill involved making a distinct call to other members of the group, their cooperative defense might intimidate the predator, but the development of such a skill would require the mandrill to evolve an increased cognitive ability, enabling it to make the call and work as a team with other group-members. As a result, the mandrill brain might evolve along with the bright skin coloration to ease its costs.
Figure 2: The Colorful Face of the Male Mandrill ( Phoenix zoo primate tour) These bright colors may attract female mandrills, but they also attract dangerous predators. Unfortunately, the development of a larger brain has consequent evolutionary expenses, too. The evolution of large brains is an exceedingly complex process and is based on a give-and-take relationship between costs and
advantages. One of the significant costs is that the larger a brain is, the longer it takes for the brain to develop into adulthood; humans remain dependent on their parents longer than any other land organism because their brains are greatly encephalized. Another limiting cost of encephalization is that big brains are energetically expensive. While every organ in the body needs some of the energy attained from food in order to function, brains are one of the biggest energy guzzlers in the entire body because they are constantly at work. Every action requires some neural activity, even if it does not seem to involve any thinking. The more encephalized a brain is, the more energy it must consume. The human brain uses about 20% of the entire supply of consumed energy, which is a great amount considering the many other complex tissues of the body such as the heart, muscles, and the organs of the digestive system. (Aiello and Wheeler, 1995) People often experience light-headedness or headache after not eating for a while because the brains energy requirements have not been met.
Evolutions Solution for the Development of Big Brains Somehow large brains evolved in primates in spite of the high-energy demands and long developmental time requirements that are necessary for their growth and maintenance. In order for encephalization to occur through evolution, two characteristics must have been present in the earliest primates environment. From the fossil evidence, precursor primates appear to have been much like modern day prosimians, such as the tarsier (shown in figure 3). Precursor primates were abundant in the thickly forested ecosystem that covered much of
the earth about 55 million years ago. (Allman, 2000, p.122)
Modern-day
prosimians and the larger monkeys and apes evolved from these precursor primates and continued to develop increasingly larger brains. But it was the first specializations of the precursor primates, namely their skill in hunting insects at night, which made these creatures successful and initiated the evolutionary giveand-take necessary for the further evolution of brain size. This process was dependent on both a strong adaptive pressure in favor of the development of a bigger brain and a way to overcome the corresponding expenses required by such a brain.
Figure 3: The Spectral Tarsier, a living fossil of a precursor primate. ( Cardinal, D. 2002)
The precursor primates, similar to this tarsier, presented the perfect foundation for the evolution of larger brained primates.
In the forests that existed 55 million years ago, the precursor primates probably faced strong competition for resources from other primate precursor species as well as from other species of mammals. The need, or adaptive pressure, for a species to be better at acquiring resources due to this competition was great. (Allman, 2000)
If a certain species of precursor primate was better at catching insects for food, it had an edge on its competition and hence had a survival advantage. One way to become better at catching food is to become smarter, i.e. to grow a bigger brain, and in this way, adaptive pressures gradually encouraged the development of a larger brain in the early primates. Natural selection, the process whereby species change in response to adaptive pressures, causes the individuals that are the most successful, or best fit to be the ones that pass on their genes to the next generation. It is possible that one member of a particular population of a precursor primate species acquired a new neural connection in his brain, via random genetic variation, that increased his reaction time after detecting an insect, which would make him a very successful individual in the population as he acquired more food than the others. His genes would thus be transferred to his offspring, some of which probably acquired the genes for the new neural connection. As the generations progressed, more and more individuals would have carried the genes this for new neural connection. In this way, bigger and better brains evolved very, very gradually within precursor primate populations. Finding a successful way to overcome the costs imparted by the increased size of the brain is another significant limitation on brain size. The easiest way to pay these costs can be for the species to sacrifice other features, by decreasing their importance to its survival. In other words, as the size of the brain increased through evolution in prosimians, they relied more heavily on their brains and hence it assumed an increasing relative importance to their survival. As a result, they could afford to trade off other less-important features.
The change to enhanced eyesight and reduced sense of smell that occurred in the early primates demonstrates how such a trade-off might occur evolutionarily. In the early prosimians, adaptive pressures emphasized eyesight for the detection of prey insects at night. As eyesight became better, the visual areas of the brain gained importance and became larger, the species ability to catch food increased, and the species became more successful. These early primates were no longer relying as greatly on their sense of smell to detect prey, and thus could afford a decrease in the part of the brain devoted to olfaction, which subsequently reduced the olfactory systems energy requirements, allowing for energy to be redirected to the growing needs of the visual system. Interestingly, this trade-off is further emphasized in humans, as humans have an even further reduction in olfactory ability. Robert A. Barton, head of the Evolutionary Anthropology Research Group at the University of Durham, England, and his colleagues have provided much evidence to support this tradeoff between olfactory and visual systems in primates, including humans. (Barton, Purvis, and Harvey,1995) Evolution often proceeds through this type of compensation, but requires that the relative importance for specific adaptations changes simultaneously. Thus evolution of large brains in primates depends on the relative adaptive importance of the various tissues and organ systems in the body.
The Expensive Tissue Hypothesis Leslie C. Aiello, a specialist in the evolution of human adaptation from University College London and Peter Wheeler, a researcher of the physiological
influences on human evolution at Liverpool John Moores University, have proposed a theory about how the energy costs of large primate brains were overcome. In their study entitled The Expensive Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution, they compared the metabolic energy requirements of various tissues in the human body to show that the large energy needs of the human brain are offset by a corresponding reduction in the digestive organs. Aiello and Wheeler showed that, although the brain requires a great deal of energy, it is definitely not the only part of the body to do so. Each of the major organs of the human body uses energy. The minimum amount of energy used by the brain at rest, as a proportion of the total body basal metabolic rate of a 65kilogram human male is 16.1 % (figure 4). (Aiello and Wheeler, 1995) The percent of the total body basal metabolic rate used by the liver and gastrointestinal tract however, is almost twice that of the brain.
Organ or Tissue Brain Heart Kidney Liver Gastro-intestinal tract Skeletal muscle Skin Total
Organ Mass (kg) 1.3 0.3 0.3 1.4 1.1 27.0 5.0 37.0
% of total body mass 2.0 0.5 0.5 2.2 1.7 41.5 7.7 56.9
% of total body basal metabolic rate 16.1 10.7 7.7 18.9 14.8 14.9 1.7 89.1
Figure 4: Mass and energy distribution of major human organs for a 65 kg human male ( Aiello and Wheeler, 1995) The brain, heart, liver, gastrointestinal tract, and muscles each have relatively high percent basal metabolic rate, and are the biggest energy users in the human body.
The fact that the gut uses so much energy made it a key area for evolutionary pressures to reduce in size or function in order to spare the energy necessary to increase brain size. Aiello and Wheeler thus suggested that, the metabolic requirements of the relatively large brains of primates are offset by a corresponding reduction of the gut.1
Changes in Diet While Aiello and Wheelers hypothesis suggests that primate encephalization occurred simultaneously with a reduction in gut size, the gut could not simply get smaller without its own set of costs. The gut uses a tremendous amount of energy (shown in figure 4) because digestion and the absorption of nutrients are energetically very expensive. In order to decrease the energy used for digestive processes, the gut must be reduced in physical size, which would reduce the total amount of energy a primate could consume. However, a change in diet allowed more energy to be obtained from smaller quantities of food so primates guts could be reduced without decreasing the total calorie intake. Since typical primate diets consist of insects, leaves, fruit, and sometimes meat, many primates can afford smaller guts than strictly herbivorous species. However, not all primates eat all of these items. In fact, those that eat mostly fruit rarely eat leaves and vice versa. Additionally, fruit-eaters tend to supplement their diet with meat and insects, while leaf-eaters do not. Compared to fruit and meat, leaves contain the smallest amount of usable nutrients and the greatest
1
Aiello LC, Wheeler P. April 1995. The Expensive Tissue Hypothesis. Current Anthropology 36: pg. 199.
concentrations of toxins. Thus leaf-eaters use a great amount of energy for digestion and must eat large quantities of leaves to meet their energy requirements. (Allman, 2000) On the other hand, fruit and protein-rich foods like meat, insects, and the seeds from some fruits are comparatively easier to digest and contain more usable nutrients than cellulose-based foods, so primates that vary their diets to include these items can afford to have smaller guts. (Aiello and Wheeler, 1995) Because certain foods like fruit and protein-rich items consist of a higherquality diet than leafy foods, changing to a higher-quality diet would result in less energy spent on digestion for evolving primates, while keeping their metabolic rates roughly constant. A higher-quality diet would therefore allow usable energy to be diverted from the gut toward the growing brain. As suggested by Aiello and Wheeler, A high-quality diet relaxes the metabolic constraints on encephalization by permitting a relatively smaller gut, thereby reducing the considerable metabolic cost of this tissue.2 Studies of the relationship between brain size, gut size, and diet type (see figures 5, 6, and 7 for summaries) show that fruit-eating primates have larger brains than similarly sized leaf-eaters, a finding that is consistent with Aiello and Wheelers data suggesting that primates with larger brains tend to have smaller guts. (Aiello and Wheeler, 1995)
2
Aiello LC, Wheeler P. April 1995. The Expensive Tissue Hypothesis. Current Anthropology 36: pg. 208.
Figure 5: The relationship between brain weight, body weight, and diet type for various primate species. The trend line shows that the relationship between body size and brain size varies depending on diet type. Data points corresponding to leaf-eaters are below the trend line, indicating that they have relatively smaller brains than fruit-eaters, whose data points tend to be positioned above the line. The data point for humans is significantly above the line, representing the extent o...