The previous BIOLOGICAL EPOCH treated the evolution of the central nervous system from the onset of multicellularity ~1 billion years ago to the increasingly talented primates living among the trees ~30 million years ago. We now resume that history of more recent times, tracing the dramatic rise in cranial capacity among our immediate ancestors of the past ~3 million years. Many of the rapid, sophisticated advances of the present CULTURAL EPOCH focus on the brain—either having caused the brain to increase, or conversely being caused by its increase.

Absolute Brain Size Human beings now have brain volumes of ~1300 cm³, about the size of a large grapefruit and a tissue consistency of Jello-O. In mass, that’s a little more than a kilogram, or a weight of ~3 pounds. Sizes of this cerebrum do vary from person to person, though no clear behavioral differences are known between people with brains as small as 1000 or as large as 2000 cm³.

On the other hand, most mental patients having reduced cognitive abilities do have distinctly smaller brains. Often measuring ~500 cm³, the brain size of these mentally retarded adults approximates that of a normal 1-year-old child. Apparently, the brain can be so tiny that its function is much impaired, meaning that a minimum brain volume is likely needed for “adequate” human intelligence as we know it. Once this threshold—probably around 1000 cm³—is surpassed, normal human behavior is possible.

What about our immediate ancestors? Does the fossil record allow estimates of the brain size of some of the prehumans that paved the way for our existence? The answer is yes, for anthropologists have been able to sketch a rough outline of the recent evolution of the brain. They do so by measuring cranial capacity of the hollow, fossilized skulls of our immediate ancestors, assuming that, as is now true for humans, apes, monkeys, and other modern mammals, the brain matter nearly fills the skull. Table 7-1 lists the results of these studies.

The partly bipedal, adult australopithecines of ~3 million years ago had brain volumes averaging not quite 500 cm³. This is just a bit larger than the brain of a modern chimpanzee and ~¹/₃ the size of today’s average human brain. Thus, fossil evidence supports the idea that our prehuman ancestors could walk on two feet before they evolved large brains.

The first true humans, perhaps Homo habilis of ~2 million years ago, had definitely larger brain volumes. Fossils studies show that this ancestor was fully bipedal and had an average cranial capacity of nearly 700 cm³. Not only that, but their fossilized skulls have a distinctly different shape from that of their forebears. Developed substantially were the frontal lobe behind the forehead and the temporal lobe above each ear, those brain regions regarded as sites of speech, foresight, and curiosity, among other useful behavioral traits. Coupled with these ancestors’ bipedal posture, the possibility that they may have also made primitive tools implies that at least two significant changes in behavior—toolmaking and bipedalism—were accompanied by significant changes in brain volume. Whether tool use led to bepedalism, or conversely, remains another of those chicken-or-the-egg conundrums, probably reinforced by positive feedback as noted earlier. At any rate, the fact that bipedalism freed the hands for tasks other than walking connotes a causal link among upright posture, toolmaking, and ultimately brain size.

Fossils also show that our closest relative, Homo erectus, had an average brain volume somewhat less (~1000 cm³) than those of our friends and neighbors alive today. What therefore amounts to a dramatic rate of cranial growth—50% increase in ~1 million years—coincides with the expansion of early humans to colder climates during the last ice age, an environmental strain that might have enhanced selection for larger brains to plan the use of seasonal resources, indeed to think innovatively about sheer survival. Large and small circular arrangements of stones found alongside the fossilized remains of this species furthermore imply that our ancestors of ~0.5-million years ago had domesticated fires and constructed homes outside of caves. Cut marks on animal bones further suggest that they had shifted from eating fruits and nuts to meat, giving them more energy per unit mouthful. Our recent human ancestors were beginning to challenge the environment—to change it for a change.

Comparisons of various cranial capacities in Table 7-1 then clearly imply that hominid advances—both biological and cultural—made in the last few million years are at least partly related to enlarged total brain size. During that time, our ancestors’ brains no less than tripled in volume. New behavioral functions, increased neural specialization, varied dietary preference, and improved cultural adaptations surely accompanied the steady evolution from Ardipithecus through Australopithecus, onward to Homo habilis and Homo erectus, currently culminating in Homo sapiens. It was not necessarily the fittest, nor even the strongest, who survived, but those best able to adapt to change.

Relative Brain Size Absolute brain size is important, but it can’t be the sole measure of intelligence. Small-bodied creatures such as birds have minute brains, especially compared to the much bigger ones of large-bodied creatures such as elephants. Yet in many respects birds act “smarter” than elephants, probably because the former have a lot less body to monitor and control. In fact, much of an elephant’s large brain consists of motor cortex—enormous numbers of dedicated neurons enabling those huge hulks to put one leg in front of the other without tripping. Hence the reason why most neurobiologists take as a better measure of intelligence a comparison of brain and body sizes.

Ratios of brain-to-body mass for many animals having similar overall stature show a clear separation of reptiles from mammals. Figure 7.13 shows this comparison, noting that a constant brain-to-body-mass ratio would display a diagonal line from lower left to upper right. For any given body mass, mammals consistently have higher brain mass, usually 10-100 times larger than those of modern reptiles of comparable size. Likewise, the brain masses of our prehuman ancestors (the early primates) also were greater, relative to body mass, than those of all other mammals.

FIGURE 7.13 — A plot of brain mass versus body mass for a variety of animals. The open circles represent reptiles (including some fish and dinosaurs), the filled circles represent mammals (including many birds), and the x's represent primates (including humans and their immediate ancestors). (Carl Sagan)

As noted in Table 7-1 the creature having the largest brain-to-body-mass ratio is Homo sapiens, namely, ~0.022. Dolphins come next (~0.016, which is also the value for H. habilis), followed by the apes, especially the chimpanzees (~0.006). The human brain is about as big as the genes can currently make it and still be safely delivered during childbirth—3 or 4 times bigger, relative to body weight, than the brains of our closest relatives, the great apes. These are data, not sociological sentiments.

Brain-to-body mass ratios then provide a useful index of the intellectual capacities among a range of animals. The systematically different ratios of Figure 7.13 virtually prove that the evolution of mammals from reptiles ~200 million years ago was accompanied by a major increase in relative brain size and intelligence. These ratios furthermore show that additional neural evolution paralleled the later emergence of human-like creatures from the rest of the mammals a few million years ago.

How Smart Are the Dolphins? More than any property, the brain most clearly distinguishes humans from other life on Earth. The development of speech, the invention of technology, and the rise of civilization are all products of the human brain’s rapid advancement. But what about other forms of life? Are there creatures on our planet today with comparable intelligence—animals having neural capacities enabling them to communicate, act socially, or make tools?

Brain-to-body mass ratios imply that, apart from humans, dolphins (Figure 7.14) are the smartest animals now on Earth. As a numerical measure of intelligence, their just-noted brain/body ratio (~0.016) matches that of archaic humans of ~2 million years ago and exceeds that of the australopithecines of 3-4 million years ago. Laboratory tests do imply that dolphin intelligence, to the extent that it can be realistically gauged, does lie somewhere between that of humans and chimpanzees. Biologically, dolphin evolution seems not too different from ours, yet culturally these remarkable creatures are far behind us, perhaps because they live in the water—and where there's water there would not likely be much tendency to discover the laws of applied physics (lever, pulley, inclined plane...), or therefore to invent technology.

FIGURE 7.14 – Dolphins, having made great strides while once on the land, retreated back to the sea with big advantages. Their high brain-to-body-mass ratio implies substantial intelligence. (Smithsonian)

Dolphins were not always aquatic creatures. Along with whales and porpoises, dolphins are members of a family of mammals whose ancestors were once land-dwelling. Owing to keen competition among many 4-legged amphibians ~50 million years ago, the dolphins’ ancestors returned to the sea, possibly either in search of food or because land niches were becoming too crowded. Some disadvantages would have undoubtedly accompanied such a seemingly backward move, but that ancestral decision—really an adaptation to change—probably saved them from extinction.

Dolphins, as we know them today, are well adapted to the sea. Their exceptionally strong bodies are streamlined for deep diving and speedy locomotion. They have extraordinary hearing beyond the range of humans, as well as an uncanny sonar system akin to underwater vision. This advanced system of echo location, now being studied by human naval officials for military purposes, may employ a kind of acoustical radar to map the position and movement of objects in their watery environment.

Interestingly enough, almost every year hundreds of dolphins (and whales too) beach themselves, especially along the outward-jutting Cape Cod off the New England seacoast. Most likely, their navigational beacons go awry, causing them to temporarily lose their way. Or, just perhaps, these dolphins are trying to make their way back onto the land. Are we sure ours is a humanitarian gesture when we so quickly “rescue” them and dump them back into the sea, or are we unwittingly keeping them out of our land-based niche?

Dolphins also have a well-organized social structure. They travel in schools or families and assist each other when in trouble; females often act as midwife for another dolphin. They’re not at all hostile, being extremely friendly to other dolphins as well as to humans. Dolphins seem to be the exception to the unwritten rule that all friendly species are inherently aggressive as well—though they certainly are known to ram sharks in a coordinated way if threatened, ganging up on the predator to protect their own.

In addition to their unparalleled ability to navigate underwater, dolphins communicate with one another by means of a series of whistles, quacks, squeaks, clicks, and other noises often resembling Bronx cheers. Although we can hope to communicate with them someday, the human range of generating and hearing noise is relatively limited (20 - 20,000 Hz) when compared to the dolphins’ much wider auditory range (2000 - 80,000 Hz). They are known to be able to produce and hear sounds within our audible range, but to do so requires them to grunt and groan at frequencies lower (bass) than normal. Most of the sounds normally made by dolphins are inaudible to humans, making it improbable that their way of expressing meaning overlaps ours at all. Not inconceivable, dolphins in captivity may have been trying to communicate with us for years. If so, they must be quite discouraged by our lack of response.

Interspecies communication will not be easy, whether among humans, dolphins, or chimps. Empirical findings to date nonetheless suggest that some common ground exists for future cultivation of, especially, dolphin-human links. At the least, it seems that both parties are interested in such a collaboration.