Answers and Explanations—5.4 Why Humans Have Big Brains

P1	Paragraph 1	Comments
S1	Humans have the largest brain of any living primate.	Humans have big brains relative to their bodies.
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.	Human brains have also evolved rapidly.
6	Why did the human brain evolve to be so large, so quickly?	Why?
7	Evidence suggests that this enlargement happened because of the social advantages it conferred on our evolutionary ancestors.	Social advantages.

	According to paragraph 1, for which of the following animals is the ratio of brain size to body size exceptionally high?	Fact. “Ratio of brain size to body size [is] exceptionally high” = the brain is big, compared to the body.
✗	A Whales	S3 notes that whales have big brains, but the size of the brain corresponds to the size of the body.
✗	B Elephants	S3 mentions that Elephants are like whales in this regard.
✓	C Humans	Correct. S2: “Our brain is exceptionally large in relation to our body size.” Also S4: “Human beings… are disproportionately small relative to their brain size.”
✗	D Non-human primates	S1 suggests that other (living) primates have smaller brains. Nothing is directly said about the ratio of brain size to body size for these other primates, but humans are described as “exceptional” (S2), with “disproportionately small” bodies in comparison with their brains (S4). So you can safely conclude that other primates do not have the same kind of exceptionally large brains (relative to their bodies).

	The phrase “conferred on” is closest in meaning to	VOCABULARY. “Conferred on” means “given to” or “bestowed upon.” It is relatively neutral or slightly positive.
✗	A burdened with	“Burdened with” has a negative connotation.
✗	B displayed to	Something displayed or shown is not necessarily given.
✗	C discussed with	“Confer with” means “discuss with,” but “confer on” means “give to.”
✓	D given to	Correct. “The social advantages it [this brain enlargement] conferred on our evolutionary ancestors” = the advantages that the enlargement gave to our ancestors.

P2	Paragraph 2	Comments
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.	For a trait to be passed down by natural selection, the trait must be advantageous.
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.	Big brains are advantageous, especially for social relationships.
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:	Negative Fact. Three answers are true and supported by the passage. One answer is false or just unsupported. Facts could be anywhere in P2.
✗	A Advantageous traits are selected for.	S2–3: “a trait must be advantageous … to be ‘selected for.’”
✗	B Big brains can enhance one’s competitive skills in relation to other humans.	S5: “bigger brains improved our ability to relate… through competition.”
✓	C Big brains evolved so that human beings could compete with other species.	Correct. S5 states that bigger brains would have improved our ability to interact with our own species, not a different species. It may be true that big brains helped us to compete with other species, but P2 never says so.
✗	D For a trait to evolve, it must be inherited by future generations.	S1: “For a trait to evolve, it must be handed down to future generations.”

P3	Paragraph 3	Comments
S1	Other evidence that our brains became big in order to improve our social skills can be found elsewhere in the animal kingdom.	Other animals support this theory.
2	Other animals that have large brains tend to be the most social species on the planet.	Big-brained animals = very social.
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	Fact. This paragraph is about other animals with large brains that are also very social.
✓	A other animals that also have large brains and are highly social	Correct. S2: “Other animals that have large brains tend to be the most social species on the planet.”
✗	B the fact that sperm whales emit clicking sounds called “codas”	This fact in itself is not evidence of the claim that large brains improve social skills.
✗	C the socialization of dolphins with whales	The paragraph does not say that dolphins and whales socialize with one another.
✗	D the rate at which the sperm whale’s brain evolved over its history	The rate at which the sperm whale’s brain evolved is not discussed.

P4	Paragraph 4	Comments
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.	Sexual selection can progress more quickly than nonsexual.
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.	Rapid evolution of the brain may imply role of sexual selection.
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.	Mating is social, so if big brains help social skills, sexual selection could work here.

	According to paragraph 4, the fact that sexual selection may have helped the human brain to grow quickly in size is suggested by	Fact. S2 says that the quick evolution of the human brain (“the brain evolved over such a short period of time in evolutionary terms”) implies this fact (that “sexual selection may have played a role in the rapid increase in human brain size”).
✓	A the rapid evolution of the human brain	Correct. S2: “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.”
✗	B the difficulty in finding suitable mating partners	There was no discussion of finding suitable partners nor of the difficulty in doing so.
✗	C the role of sexual selection in organ development	This discussion is about the brain, not other organs.
✗	D the societal importance of natural selection	The importance of natural selection would not explain why sexual selection (a subset) might be responsible for brain size increases.

	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.	Simplify Sentence. S3 (highlighted) says that mating is social, so having good social skills (because of a big brain) may have made you a better candidate for mating and therefore sexual selection of your traits (such as having a big brain).
✗	A Because mating and competition aren’t social, a larger brain was necessary to improve social skills.	S3 says mating and competition for mates are social phenomena.
✗	B Improved social skills enabled a larger brain and therefore better mating and sexual selection prospects.	Improved social skills were enabled by a larger brain, not the other way around.
✓	C Having a larger brain allowed for better social skills, probably enabling better mating and thus sexual selection.	Correct. The logical relationships in S3 are captured here. A larger brain led to better social skills, which may have led to better mating (because it’s a social phenomenon), and ultimately sexual selection (which is natural selection of traits, such as having a big brain).
✗	D Attractiveness for sexual selection may have been driven by mating and competition, social phenomena that were enabled by larger brains.	This choice scrambles up the phrases from the original sentence, resulting in several unsupported assertions.

P5	Paragraph 5	Comments
S1	Why else did humans evolve such a large brain despite our relatively petite body size?	Why did humans evolve large brains?
2	After all, having a large brain is costly.	Large brains are 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.	Tool use is probably one reason.
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.	Tools allowed better digestion to fuel a big brain.

	The word “whopping” in paragraph 5 is closest in meaning to	VOCABULARY. “Whopping” is a somewhat informal word meaning “very large.”
✓	A enormous	Correct. S3: “The human brain… is responsible for a whopping 20 percent of the entire body’s energy use” = that 20 percent is a really big number.
✗	B estimated	Although the percent might be an estimate (rough calculation), this word is not related.
✗	C surprising	“Whopping” may include an element of surprise. You could even argue that it means “surprisingly large.” But “surprising” by itself isn’t a good substitute, because it lacks the meaning of “large.”
✗	D unjustified	Unrelated. “Unjustified” = not right or reasonable, not justified.

	All of the following are mentioned in paragraph 5 EXCEPT:	Negative Fact. Paragraph 5 focuses on how much energy the human brain consumes and how our ancestors used tools to free up energy.
✗	A the general use of early tools	S4 and S5 address early tool use.
✗	B the hunting function of weapons	S5 addresses early weapons used for hunting.
✗	C the operation of the digestive tract	S5–6 describe how breaking down meat “eased the burden on the digestive system.”
✓	D the use of the hand ax in social competition	Correct. S5 describes the use of equipment such as the hand ax, but in connection with cooking, not with social competition.

P6	Paragraph 6	Comments
S1	Bizarrely, over the past 10,000 to 20,000 years, the human brain has actually shown a reverse trend: it is shrinking.	Now brains are getting smaller!
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.	One hypothesis = getting dumber; another one has to do with 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.	Still 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?	Fact. S6 discusses the relevant time: “the 2 million years during which the brain rapidly evolved to be larger.”
✗	A It is responsible for the current shrinking of the human brain.	The question is about the climate in the past, not the climate in the present.
✓	B It was occasionally warm.	Correct. S6: “there were also periods of global warming.”
✗	C For the most part, it was not as warm as the climate is today.	This is not addressed in the passage.
✗	D It is shifting more now than it did historically.	This is not addressed in the passage.

	The word “approach” in the passage is closest in meaning to	VOCABULARY. The verb “approach” in this context means “become more like” or “become closer to being like.”
✓	A become like	Correct. The brain “will soon approach the size of the brain of Homo erectus” = with respect to size, the human brain is becoming more like the brain of Homo erectus.
✗	B make contact with	“Approach” can mean “make contact with” in a social setting (“she approached him about the issue” = “she came up to him and spoke to him about the issue.”) But it is unnatural to say that one brain “made contact with” another brain, unless you are talking about physical touch or communication.
✗	C measure up to	“Measure up to” implies a value comparison. “He tried to measure up to her” = “he tried to be as good as her in some way.” Here, there’s no value comparison.
✗	D infringe upon	To “infringe” is to limit, undermine, or even intrude. “By reading my diary, my sister had infringed upon my privacy.”

	What is the author’s purpose in presenting the information in paragraph 6?	Purpose. P6 presents a change in the trend (human brain sizes are now shrinking). The author concludes that the cause of the change remains a mystery.
✗	A To explain how the human brain is now evolving to become even larger	Opposite. P6 describes how the human brain is now shrinking.
✗	B To advocate for one hypothesis about current trends in brain size over another	The author does not advocate for either hypothesis. Rather, the author believes that the phenomenon remains a mystery.
✗	C To argue against a claim made elsewhere in the passage	The author is not arguing against any claim. The trend in human brain size (now shrinking) is different than it was before, but presenting this fact is not the same thing as arguing against any previous points.
✓	D To describe a phenomenon that opposes the pattern described in the rest of the passage	Correct. While the rest of the passage discusses how the brain evolved to become larger, this paragraph discusses a reversal of that trend.

	The word “stature” in the passage is closest in meaning to	VOCABULARY. “Stature” means “height of the body” in a physical sense, or “social standing” in a non-physical sense. The context here indicates the physical meaning.
✗	A intelligence	Someone’s mental capacity is not related to their height.
✓	B height	Correct. “A body of smaller stature” = a shorter person or body.
✗	C energy level	Someone’s energy level is not necessarily related to their height.
✗	D achievement	“Stature” can mean “achievement,” but only in a social context.

P6	Paragraph 6	Comments
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.	Correct. In the previous sentence you learn that human brains have been shrinking. The inserted sentence tells you that the phenomenon isn’t restricted to one region. The next sentence goes on to say that this shrinkage is indeed occurring globally.
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.	Here, Homo erectus has just been mentioned in comparison to modern day humans. So “this” would have to refer to that comparison, but it doesn’t make sense to say that this comparison is not restricted to a single region. The following sentence has nothing to do with the global nature of the phenomenon; that point was already made in S2, so it’s strange to bring it back.
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.	Here, the relationship between brain size and body size is discussed. Again, it’s strange to come back to the global nature of the phenomenon (human brain shrinkage) when it was mentioned in S2.
7	D So the recent shrinkage of the brain remains a mystery.	Once again, it’s strange to come back to the global nature of the phenomenon (human brain shrinkage) when it was mentioned in S2.

	This is not restricted to a single region or hemisphere. Where would the sentence best fit?	Insert Text. Although it is not perfectly clear what “this” is referring to, the sentence would make the most sense in a part of the paragraph where some phenomenon is being discussed as applying to the whole world (as opposed to just one “region or hemisphere.”) The following text would ideally emphasize the global nature of the phenomenon.
✓	A Choice A	Correct.
✗	B Choice B
✗	C Choice C
✗	D Choice D

	Whole Passage	Comments
P1	Humans have the largest brain…	Humans have big brains relative to their bodies. Human brains have also evolved rapidly. Why? Social advantages.
P2	For a trait to evolve, it must be handed down…	For a trait to be passed down by natural selection, the trait must be advantageous. Big brains are advantageous, especially for social relationships.
P3	Other evidence that our brains became big in order to improve our social skills can be found…	Other animals support this theory. Big-brained animals = very social.
P4	Another indication that big brains probably evolved for social reasons is…	Sexual selection can progress more quickly than nonsexual. Rapid evolution of the brain may imply role of sexual selection. Mating is social, so if big brains help social skills, sexual selection could work here.
P5	Why else did humans evolve such a large brain…	Why else did humans evolve large brains? Large brains are costly. Tool use is probably one reason. Tools allowed better digestion to fuel a big brain.
P6	Bizarrely, over the past 10,000 to 20,000 years, the human brain has actually shown a reverse trend…	Now brains are getting smaller! One hypothesis = getting dumber; another one has to do with the warming climate. Still a mystery.

	The human brain is one of the most rapidly evolving organs in mammalian history.	Summary. Correct answers must be clearly expressed in the passage. They must also be among the major points of the passage. They should tie as directly as possible to the summary given.
✗	a The most significant drain on energy in the human body is the brain.	This is too extreme of a statement. You don’t know that the brain is “the most significant drain” in this way. The statement is also too narrow in scope to capture a central idea of this passage.
✓	b Since a trait that successfully evolves must benefit the organism, large brains were probably advantageous to human ancestors.	Correct. This point is presented in P2 as the reason that large brains evolved. The passage as a whole explores the reasons for the evolution of large human brains.
✗	c Large brains are mostly restricted to marine life.	While dolphins and sperm whales are mentioned as having large brains, nowhere does the passage suggest that large brains are restricted to marine life.
✗	d Darwin’s theory of natural selection can explain many traits of animals on Earth.	This statement is too broad and is not a central idea in the passage, which is specifically about large brains among humans and their ancestors.
✓	e Social species tend to have the largest brains, suggesting that humans evolved large brains for social reasons.	Correct. This core reason for the evolution of large human brains is explored in P2 and P3.
✓	f The large human brain requires a significant amount of energy, which human ancestors accessed by using tools to aid in digestion.	Correct. P5 presents tool use as an additional reason for the evolution of large human brains.