This disquisition reviews another excellent article, "The Emergence of Intelligence", by William H. Calvin in the special 1998 issue, "Exploring Intelligence", Scientific American, pg. 44.

The Fourfold Expansion of Human Intelligence Began When the Ice Ages Began
    The author begins by observing that the fourfold expansion of hominid intelligence began 2,500,000 years ago at the same time the ice ages began. The author observes that, "warming and cooling episodes occurred every several thousand years, superimposed on the slower advances and retreats of the northern ice sheets. The vast rearrangements in ocean currents lasted for centuries, with sudden transitions that took less than a decade.
    "The abrupt coolings most likely devastated the ecosystems on which our ancestors depended. Because of lower temperatures and less rainfall, the forests in Africa dried up and animal populations began to crash. Lightning strikes ignited giant forest fires, denuding large areas even in the tropics. There was very little food after the fires. Once the grasses reemerged on the burnt landscape, however, the surviving grazing animals had a boom time. Within several centuries, a succession of forests came back in any places. featuring species more appropriate to the cooler climate.
    "Cool, crash, and burn. The progenitors of modern humans lived through hundreds of such episodes, but each was a population bottleneck that eliminated most of their relatives."
    It may be relevant to note here that unlike mountain gorillas, baboons and chimpanzees will kill and eat small animals if they're lucky enough to catch them. So this may explain why we're descended from chimpanzees rather than from the great apes.
    Dr. Calvin continues:
    "But when the cooling and the drought were abrupt, it was one unlucky generation that suddenly had to improvise amid crashing populations and burning ecosystems. We are the improbable descendants of those who survived—probably because they had ways of coping with these episodes that the other great apes did not exploit."
    "Improvising meant learning to eat grass—or managing to eat animals that ate grass. The trouble is that such are animals are fast and wary, whether rabbits or antelope. Small or big, they are best tackled by cooperative groups. But sharing a rabbit leaves everyone hungry, so the hunters would have tried for the biggest animals that cluster in herds. And this has an interesting consequence. If a single hunter killed a big animal, it was too much to eat; best to give most of the meat away and count on reciprocity when someone else succeeded. Sharing food also meant fewer fights and more time available to seek out scarce food.

The Evolutionary Advantages of Altruism and Cooperation
    "Each population bottleneck temporarily exaggerated the importance of such traits as cooperation, altruism, and hunting abilities. Even if each episode changed the inborn predilections of the hominids by only a small amount, the hundreds of repetitions of this scenario may explain some of the differences between human abilities and those of our closest relatives, the great apes. It is tempting to say that the abrupt coolings pumped up brain size, but what makes for better survival is something much more specific: hunting abilities and perhaps altruism. What might they have to do with intelligence?"

Other Primates Use a Different Area of Their Brains for Vocalizations
    Dr. Calvin notes that the development of the capacity for language occurred during the ice ages. "In most of us, the area critical to language is located just over the left ear. Monkeys lack this left lateral language area: their vocalizations (and simple emotional utterances in humans) employ a more primitive language area near the corpus callosum."

Wild Chimps Have a Vocabulary, Just Like Us, of About 30 "Phonemes"
    Dr. Calvin explains that wild chimpanzees have a phoneme vocabulary of about three dozen different vocalizations to convey about three dozen different meanings (about the same number of phonemes that we use). They may repeat a sound to emphasize it, but they don't string phonemes together to add a new word to their vocabulary. We do. We also string words together to make sentences. The author says,
    "For a glimpse of life without syntax, look to the case of Joseph, an 11-year-old deaf boy. Because he could not hear spoken language and had never been exposed to fluent sign language, Joseph did not have the opportunity to learn syntax during the critical years of early childhood. As neurologist Oliver Sacks described him,
    "'Joseph saw, distinguished, categorized, used; he had no problems with perceptual categorization or generalization, but he could not, it seemed, go much beyond this, hold abstract ideas in mind, reflect, play, plan. He seemed literal—unable to juggle ideas or hypotheses or possibilities, unable to enter an imaginative or figurative realm.... He seemed, like an animal or an infant, to be stuck in the present, to be confined to literal and immediate perception, though made aware of this by a consciousness that no infant could have.'"

Bonobos (Pygmy Chimpanzees) Understand Language at a 2.5-Year-Old Level
    Kanzi, the most accomplished bonobo (pygmy chimpanzee) can interpret sentences he has never heard before, such as "go to the office and bring back the red ball", about as well as a two-and-a-half-year-old child. Neither Kanzi nor the child constructs such sentences independently, but they can demonstrate by their actions that they understand them.

Primates Lack Foresight, and the Ability to String Words Together
    He states that with a year's experience, a child starts constructing sentences that nest one word phrase inside another. Syntax has treelike rules o inference that allow us to communicate concisely. He goes on to state that syntax seems related to the ability to plan ahead. Aside from instincts, animals exhibit surprisingly little deliberate planning. Chimpanzees use long twigs to pull termites from trees. Yet as Jacob Bronowski observed, none of the termite gathering chimps "spends the evening going around and tearing off a nice, tidy supply of a dozen probes for tomorrow".

Human Planning May Arise from the Ability to Spin Out "What-If" Scenarios
    Dr. Calvin says,
    "Human planning abilities may stem from our talent for building narratives. We can borrow the mental structures for syntax to judge combinations of possible actions." .... "But our thinking is not limited to language like constructs. Indeed, we may shout "Eureka!" when feeling a set of mental relationships that click into place yet have trouble expressing them verbally."

But:
   Language and intelligence are so powerful that we might think that evolution would naturally favor their increase. But Harvard biologist Ernst Mayr has observed that the fact that most species are not intelligent argues that intelligence is not at all favored by natural selection—or that it is very hard to achieve. "So we must consider indirect ways of achieving it, rather than general principles."

Intelligence and Speech May Be a By-Product of Some Core Capability Developed for a Different Purpose
    The author argues, as did Ian Tattersall, that evolution often proceeds through the side effects of other core features that served a different evolutionary purpose. He suggests that the ability to throw—viz., a spear—requires a great deal of cerebral machinery. When the distance to a target doubles, the launch window narrows by a factor of eight, and 64 times as many neurons are required to execute the maneuver. "As improbable as the idea initially seems, the brain's planning of ballistic movements may have once promoted language, music, and intelligence. Apes have only elementary forms of the ballistic arm movements at which humans are expert—hammering, clubbing, and throwing."
    He observes that "George A Ojemann of the University of Washington has also shown that at the center of the left lateral areas specialized for language lies a region involved in listening to sound sequences. This perisylvian region seems equally involved in producing oral-facial movement sequences—even non-language ones."
    "If mouth movements rely on the same core facility for sequencing as ballistic hand movements, then improvements in dexterity might improve language, and vice versa. The gift of speech would be an incidental benefit—a free lunch, as it were, because of the linkage."
    "These discoveries reveal that the" language cortex", as some people think of it, serves a far more generalized function than had been suspected. It is concerned with sequences of various kinds: both sensations and movements both of the hand and of the mouth. The big problem with fashioning new sequences and producing original behaviors is safety. Our capacity to make analogies and mental models gives us a measure of protection, however. Humans can simulate future courses of action and weed out the nonsense; as philosopher Karl Popper said, this, 'permits our hypotheses to die in our stead". Creativity—indeed, the high end of intelligence and consciousness involves playing mental games that improve the quality of our plans. What kind of mental machinery might do that?

Closing Remarks:
    Dr. Calvin sums up his article in these words,
    "In both phylogeny and ontogeny, human intelligence first solves movement problems and only later graduates to ponder more abstract ones. An artificial or extraterrestrial intelligence freed of the necessity of finding food might not need to move—and so might lack the "what happens next?" orientation of human intelligence. It is difficult to estimate how often high intelligence might emerge, given how little we know about the demands of long-term species survival and the courses evolution can follow. We can, however, evaluate the prospects of a species by asking how many elements of intelligence they have amassed. Chimps and bonobos may be lacking a few of the elements—the ability to construct nested sequences, for example—but they are doing better than the present generation of artificial-intelligence programs.
    "Why aren't there more species with such complex mental states? There might be a hump to get over: a little intelligence can be a dangerous thing. A beyond-the-apes intelligence must constantly navigate between the twin hazards of dangerous innovation and a conservatism that ignores what the Red Queen explained to Alice in Through the Looking Glass: 'It takes all the running you can do to keep in the same place.' Foresight is our special form of running, essential for the intelligent stewardship that Stephen Jay Gould of Harvard warns is needed for longer-term survival. "We have become, by the power of a glorious evolutionary accident called intelligence, the stewards of life's evolutionary accident called intelligence, the stewards of life's continuity on earth. We did not ask for this role, but we cannot abjure it. We may not be suited to it, but here we are."
 
    I presume that this is just one theory regarding the evolution of human intelligence. Gina and Paul Johns, how does this sound to you?