Humans have the largest brain of any living primate. Our brain is exceptionally large in relation to our body size. While other social mammals—whales and elephants, for example—possess big brains, their brain size tends to correspond to their overall size. Human beings, on the other hand, are disproportionately small relative to their brain size. Furthermore, the human brain is the most rapidly changing organ in mammalian history. Why did the human brain evolve to be so large, so quickly? Evidence suggests that this enlargement happened because of the social advantages it conferred on our evolutionary ancestors.
For a trait to evolve, it must be handed down to future generations through processes of natural selection. Under standard Darwinian theory, a trait must be advantageous in order to spread throughout the population of a species. This is what it means to be “selected for.” Big brains are believed to be advantageous because they store and process a great deal of information, enabling owners of big brains to be more socially successful. In other words, bigger brains improved our ability to relate to other members of our species—whether through cooperation or through competition.
Other evidence that our brains became big in order to improve our social skills can be found elsewhere in the animal kingdom. Other animals that have large brains tend to be the most social species on the planet. These include elephants, sperm whales, and dolphins—all highly social mammals. The sperm whale, with a brain that is the largest of any known current or extinct species, joins a social unit early in life and typically remains with it for a lifetime. Within social units, sperm whales spend a great deal of time devoted to socializing, emitting complex patterns of clicks called “codas.”
Another indication that big brains probably evolved for social reasons is that sexual selection, a form of natural selection in which mating partners “choose” a particular variation of a trait by choosing a partner that has that variation over other partners that do not, can progress much more quickly than nonsexual selection. The fact that the brain evolved over such a short period of time in evolutionary terms implies that sexual selection may have played a role in the rapid increase in human brain size. Given that mating and competition for mates are social phenomena, having an enhanced social skill set enabled by a larger brain may have made a member of a population more attractive for sexual selection.
Why else did humans evolve such a large brain despite our relatively petite body size? After all, having a large brain is costly. The human brain takes up approximately 2 percent of the human body but is responsible for a whopping 20 percent of the entire body’s energy use (the proportion of blood and oxygen directed to the brain). Tool use may provide another cause, in addition to the development of social skills: the brain increased in size more rapidly after our ancestors learned to make tools. Early tools included not only weapons used for hunting, but also equipment such as the hand ax, which allowed early humans to break down the meat before they ate it. This would have eased the burden on the digestive system, freeing up valuable metabolic resources to fuel the operations of a larger brain.
Bizarrely, over the past 10,000 to 20,000 years, the human brain has actually shown a reverse trend: it is shrinking. Across the globe, our brains are getting smaller. Some scientists predict that if the brain continues to shrink at its current rate, it will soon approach the size of the brain of Homo erectus, our ancestral relative from 500,000 years ago. While some fear that this means we are becoming less intelligent, others point to the warming climate. They argue that a body of smaller stature is more efficiently cooled, and a smaller brain follows a smaller stature. Critics of the climatic theory, however, point to the fact that over the 2 million years during which the brain rapidly evolved to be larger, there were also periods of global warming. So the recent shrinkage of the brain remains a mystery.
Now answer the questions.
| P1 | Paragraph 1 |
| S1 | Humans have the largest brain of any living primate. |
| 2 | Our brain is exceptionally large in relation to our body size. |
| 3 | While other social mammals—whales and elephants, for example—possess big brains, their brain size tends to correspond to their overall size. |
| 4 | Human beings, on the other hand, are disproportionately small relative to their brain size. |
| 5 | Furthermore, the human brain is the most rapidly changing organ in mammalian history. |
| 6 | Why did the human brain evolve to be so large, so quickly? |
| 7 | Evidence suggests that this enlargement happened because of the social advantages it conferred on our evolutionary ancestors. |
According to paragraph 1, for which of the following animals is the ratio of brain size to body size exceptionally high?
The phrase “conferred on” is closest in meaning to
| P2 | Paragraph 2 |
| S1 | For a trait to evolve, it must be handed down to future generations through processes of natural selection. |
| 2 | Under standard Darwinian theory, a trait must be advantageous in order to spread throughout the population of a species. |
| 3 | This is what it means to be “selected for.” |
| 4 | Big brains are believed to be advantageous because they store and process a great deal of information, enabling owners of big brains to be more socially successful. |
| 5 | In other words, bigger brains improved our ability to relate to other members of our species—whether through cooperation or through competition. |
According to paragraph 2, all of the following are true EXCEPT:
| P3 | Paragraph 3 |
| S1 | Other evidence that our brains became big in order to improve our social skills can be found elsewhere in the animal kingdom. |
| 2 | Other animals that have large brains tend to be the most social species on the planet. |
| 3 | These include elephants, sperm whales, and dolphins—all highly social mammals. |
| 4 | The sperm whale, with a brain that is the largest of any known current or extinct species, joins a social unit early in life and typically remains with it for a lifetime. |
| 5 | Within social units, sperm whales spend a great deal of time devoted to socializing, emitting complex patterns of clicks called “codas.” |
According to paragraph 3, further evidence that humans developed large brains to improve social skills comes from
| P4 | Paragraph 4 |
| S1 | Another indication that big brains probably evolved for social reasons is that sexual selection, a form of natural selection in which mating partners “choose” a particular variation of a trait by choosing a partner that has that variation over other partners that do not, can progress much more quickly than nonsexual selection. |
| 2 | The fact that the brain evolved over such a short period of time in evolutionary terms implies that sexual selection may have played a role in the rapid increase in human brain size. |
| 3 | Given that mating and competition for mates are social phenomena, having an enhanced social skill set enabled by a larger brain may have made a member of a population more attractive for sexual selection. |
According to paragraph 4, the fact that sexual selection may have helped the human brain to grow quickly in size is suggested by
Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect answer choices change the meaning in important ways or leave out essential information.
| P5 | Paragraph 5 |
| S1 | Why else did humans evolve such a large brain despite our relatively petite body size? |
| 2 | After all, having a large brain is costly. |
| 3 | The human brain takes up approximately 2 percent of the human body but is responsible for a whopping 20 percent of the entire body’s energy use (the proportion of blood and oxygen directed to the brain). |
| 4 | Tool use may provide another cause, in addition to the development of social skills: the brain increased in size more rapidly after our ancestors learned to make tools. |
| 5 | Early tools included not only weapons used for hunting, but also equipment such as the hand ax, which allowed early humans to break down the meat before they ate it. |
| 6 | This would have eased the burden on the digestive system, freeing up valuable metabolic resources to fuel the operations of a larger brain. |
The word “whopping” in paragraph 5 is closest in meaning to
All of the following are mentioned in paragraph 5 EXCEPT:
| P6 | Paragraph 6 |
| S1 | Bizarrely, over the past 10,000 to 20,000 years, the human brain has actually shown a reverse trend: it is shrinking. |
| 2 | Across the globe, our brains are getting smaller. |
| 3 | Some scientists predict that if the brain continues to shrink at its current rate, it will soon approach the size of the brain of Homo erectus, our ancestral relative from 500,000 years ago. |
| 4 | While some fear that this means we are becoming less intelligent, others point to the warming climate. |
| 5 | They argue that a body of smaller stature is more efficiently cooled, and a smaller brain follows a smaller stature. |
| 6 | Critics of the climatic theory, however, point to the fact that over the 2 million years during which the brain rapidly evolved to be larger, there were also periods of global warming. |
| 7 | So the recent shrinkage of the brain remains a mystery. |
According to paragraph 6, which of the following is true of the global climate during the time when the human brain was evolving to become larger?
The word “approach” in the passage is closest in meaning to
What is the author’s purpose in presenting the information in paragraph 6?
The word “stature” in the passage is closest in meaning to
| P6 | Paragraph 6 |
| S1 | Bizarrely, over the past 10,000 to 20,000 years, the human brain has actually shown a reverse trend: it is shrinking. |
| 2–3 | A Across the globe, our brains are getting smaller. Some scientists predict that if the brain continues to shrink at its current rate, it will soon approach the size of the brain of Homo erectus, our ancestral relative from 500,000 years ago. |
| 4–5 | B While some fear that this means we are becoming less intelligent, others point to the warming climate. They argue that a body of smaller stature is more efficiently cooled, and a smaller brain follows a smaller stature. |
| 6 | C Critics of the climatic theory, however, point to the fact that over the 2 million years during which the brain rapidly evolved to be larger, there were also periods of global warming. |
| 7 | D So the recent shrinkage of the brain remains a mystery. |
Look at the part of the passage that is displayed above. The letters [A], [B], [C], and [D] indicate where the following sentence could be added.
This is not restricted to a single region or hemisphere.
Where would the sentence best fit?
An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
The human brain is one of the most rapidly evolving organs in mammalian history.