When it comes to brainpower, humans may owe their earliest ancestors for learning how to keep a cool head. How our brains evolved is a story of what it really means to be human.

by Robert Pool

It was Oct. 27, 2004, and FSU anthropologist Dean Falk was sitting in her study when the phone rang. “I’m David Hamlin,” the man on the other end of the line told her. “I’m with National Geographic, and I’m not selling magazines.” Hamlin then told Falk a story so fantastic that she wondered at first if it might be a hoax.

Anthropological researchers from Australia and Indonesia had discovered the fossilized bones of people so tiny that they would have barely come up to the waist of modern humans. One skeleton in particular, nicknamed “the hobbit” by the researchers, was of a female around 30 years old who was the same height as a 3-year-old girl today. What made the discovery particularly surprising was that the hobbit had lived only 18,000 years ago—meaning that this species of little people had lived at the same time as modern humans and may even have crossed paths with them.

Once Falk had been convinced that it wasn’t a hoax, Hamlin explained to her that National Geographic wanted her to examine the skull and offer her opinion about where the hobbit fit into the human family tree. How close a relative was it? And how, with such a small brain, was the hobbit able to create and use the tools that had been found with it? Could it really have been that intelligent?

Although Falk would be the first to admit that she is still a long way from answering that question, her research has provided a number of fascinating and important insights into the evolution of the human brain, and a conversation with her leaves one with a new and different appreciation of what it means to be human.

On the Dusty Hominid Trail

When Falk graduated from high school and entered college, it was not her goal to become an anthropologist. She actually started college as a math major. But as she was getting close to graduation at the University of Illinois, Chicago, she took a course on human evolution from zoologist Charles Reed, and she was hooked. Switching her major to anthropology, she studied the evolution of brain size. After receiving her bachelor’s degree in anthropology (with a minor in math), she took Reed’s advice and stayed at the university to get her master’s degree, learning as much about anatomy—and especially brain anatomy—as she could. Afterward she went to the University of Michigan to study for her Ph.D. in physical anthropology, and it was there she took her first steps on the road to understanding the evolution of the brain.

She focused first on Old World monkeys—those from Africa and Asia—because they are relatively easy to gather data on. With at least 78 living species, the Old World monkeys offer opportunities to compare and contrast that just aren’t possible with other primates. “I always knew I wanted to do hominids,” Falk says, but the hominids—modern humans and apes and their ancestors—seemed out of reach at the time. It’s easy enough to get data on modern humans and apes, but hominid fossils are rare, and it is difficult for a graduate student to get her hands on them. The fossils are generally locked away with access restricted to only a few. So Falk cut her teeth on the monkeys, all the while hoping she would get a chance to work with hominids.

Falk pauses briefly in the conversation to explain some terminology. Originally, she notes, the term “hominid” was used to refer to humans and their ancestors, but it has been expanded to include chimpanzees, gorillas, and their ancestors because the apes are so close to humans on the ancestral tree. So now anthropologists use the new term “hominin” to refer just to humans and their ancestors. Falk’s goal from the beginning of her anthropological career was to work with hominids, but with hominins in particular.

To study the brains of the Old World monkeys, Falk mastered a basic technique that would become one of her most important tools throughout her career. Take a skull, pour thin layers of silicon latex into it and allow it to cure, then remove the solidified latex. You are left with a molded form in the same shape as the brain that had earlier inhabited the skull. By studying this “endocast,” as it is called, you can see what the brain looked like, not just the general shape but also, in some cases, the individual folds of the cortex and even some of the blood vessels that run outside the brain. And the Old World monkeys were a good group to begin with, as endocasts from them are quite detailed and relatively easy to read. Hominid endocasts, on the other hand, generally do not contain as much detail because the brain does not leave as detailed an impression on the inside of the skull, and, as Falk would later discover, they can prove much more challenging to interpret.

In studying the brains of Old World monkeys, Falk uncovered some facts that had not been known before. There are two different types of Old World monkeys, the leaf eaters and the cheek-pouch monkeys, and Falk’s analysis showed that the two types differed in their “sulcal patterns,” that is, in the patterns of grooves on the surfaces of their brains. She also showed that monkey brains were asymmetric—something that had been known for humans but not for monkeys. In these discoveries Falk displayed the sort of careful observation and analysis that would become her hallmark. But still she was dreaming of working on hominins.

In the summer of 1978, after she had received her Ph.D. and was working at Southern Illinois University, Falk traveled to South Africa to visit the Transvaal Museum. The purpose of her visit was to study the museum’s collection of monkey fossils, but, as it happened, the museum also had an extensive collection of hominin fossils. Falk longed to get in to see them, but did not think that she, as a young and inexperienced scientist, had a chance. Then serendipity intervened.

Toward the end of her stay, Falk decided to look through some unidentified fossils that were kept in an out-of-the-way storage room. Some of the bits were fossilized skulls; others were “natural endocasts,” that is, fossils created when minerals replace the soft tissue of the brain and take on the shape of the brain. “In one dusty old box,” Falk remembers, “I found a fragment of a natural endocast that was too large to be a monkey, so it had to be a hominin.”

Thrilled, she spent a couple of days assembling evidence and arguments to prove to the director of the museum that this was indeed a hominin fossil and not some large monkey. But when she approached him with the fossil in hand, he took one glance at it and said, “Oh, I see you’ve found a bit of a hominin there.”

Just like that, the doors of the hominin fossil vault were opened to her. The director agreed to let Falk compare this new fossil with the ones already in the hominin collection so that she could write a paper describing her findings. And suddenly Falk was on her way to doing what she had longed to do: studying the evolution of the human brain.

Casting Light

Anthropology is a contentious field. The fossilized remains of human ancestors are relatively rare, and the ones that do exist are rarely in good condition or anywhere near complete. This means that interpretations are rarely clear cut. Furthermore, the field itself hits closer to home than many other areas of science—the issue of what our ancestors looked like is much more personal than, say, the age of the universe—and many of the scientists in the field are known for their large egos. Put it all together, and it’s a recipe for strife and controversy. And from her first paper about human evolution, Falk waded right into the middle of it.

With the natural endocast she had discovered plus six others she had then been allowed access to, Falk had seven fossilized endocasts to examine, all of them from hominins called australopithecines that had lived several million years ago. Comparing them with the brains of humans and apes, Falk could see clearly that the australopithecine (pronounced aw-STRAL-oh-PITH-uh-seen) brains were more ape-like than human-like, and she announced that conclusion in the paper that she published describing the seven endocasts. And suddenly she was thrown into the middle of her first anthropological controversy, for until that point everyone had believed just the opposite, that australopithecine brains were more human-like than ape-like.

To the outsider it may be difficult to understand how such controversies could arise. Why, for instance, can’t people just look at the brains and see whether they are more like those from apes or more like those from humans? Any why is it such a big deal anyway?

It is a big deal—to anthropologists, at least—because the answer says a great deal about how human ancestors gradually developed into modern humans over millions of years. Before Falk’s analysis, anthropologists had believed that australopithecine brains were very human-like, despite their relatively small size, which implied that our ancestors had been well along the path toward becoming human several million years ago. If Falk was right, those ancestors were not nearly as human-like as had been believed, and that challenged much that had been written and said by researchers in the field.

As for why it is so difficult to determine the right answer from the endocasts, reading the endocasts is not nearly as straightforward as it sounds. They are, after all, no more than brain-shaped rocks, and teasing out fine details from these rocks demands a well-trained and practiced eye. Sometimes the interpretation hinges on something as subtle as how one construes a particular bump or ridge on the rock. Different researchers can look at the same fossils and come up with very different opinions.

Dean Falk had no doubts about her interpretation of those seven endocasts, but other anthropologists did. One in particular, Ralph Holloway of Columbia University, would argue with Falk for more than two decades. Holloway insisted that the australopithecine brains were human-like rather than ape-like, and the two of them carried out a running debate in scientific journals. Falk would eventually write more than a dozen articles explaining why her interpretation, not Holloway’s, was correct. Today, most anthropologists would probably say that Falk has won the point, but there has still been no formal concession from Holloway.

Cool Heads Prevail

One of the things that quickly become apparent in talking to Falk about her research is just how much information can be gleaned from studying endocasts. At first glance one would guess that these fossilized brain impressions would provide very little information—details about the brain’s size and some general facts about its shape, but not much else. But through attention to detail and a healthy dose of cleverness, Falk has been able to wring some amazing stories from these brains.

Perhaps the contribution that she is most famous for is what she calls the “radiator hypothesis.” By pulling evidence together from a number of areas, including fossil skulls and endocasts from early human ancestors, Falk was able to shed a great deal of light on one of the central mysteries of human evolution: Why did human ancestors develop such a large brain?

Falk was studying the emissary veins, blood vessels that travel from the brain out through the skull through holes in the bone. Their presence is clearly visible even millions of years later because of these holes, so Falk could follow how the arrangement of these veins evolved over time.

When early hominids shifted from walking on four legs to an upright, two-legged posture, Falk explains, the blood flow had to adapt with it—or “re-plumb,” in Falk’s terminology—for it is a much different proposition to pump blood to and from a brain that sits above the rest of the body than when the brain is at about the same level as the heart. This is well known for snakes, for instance. Tree snakes, which spend much of their time in a vertical posture, have a vascular system that is very different from those of other snakes. The upshot is that it is possible to examine a fossilized skull and, by looking at the pattern of blood vessels that supplied the brain, tell a great deal about the posture of a creature that lived millions of years ago.

In particular, Falk was studying the blood vessels in two different types of australopithecines, the robust and the gracile. As the names imply, the robust species most likely was the bulkier of the two, with a heavier skull and larger teeth, while the skulls of gracile species were slighter. Falk saw from the endocast evidence that both species had re-plumbed as part of the transition to walking upright, but, she recalls, “I saw that the two had re-plumbed differently.” The robust australopithecines had one large sinus that served to carry blood away from the brain, while the gracile australopithecines had a network of smaller veins. It was an interesting observation, but it was nothing more than that until Falk realized the significance of what she was seeing.

In January 1987, Falk received a letter from Michel Cabanac, a French physiologist who had seen her published description of the different veins in the two australopithecine species. Cabanac told her that he had discovered that one purpose of the emissary veins in humans was to keep the brain cool. On a hot day or during intense exercise the brain will start to heat up, and if it gets too hot, the results can be brain damage or even death. So it is vital that the body has some way to cool off the brain in such circumstances, and Cabanac had shown that the system of emissary veins acted as a radiator. When the brain heated up, the blood flow in these veins actually reversed, going from the surface of the skin through the skull and into the brain. Since the blood in the skin was relatively cool—having been cooled by evaporation of sweat from the skin—it served to cool off the brain.

It took several days for Falk to put the pieces together, but finally it hit her in the middle of the night: The gracile australopithecines, with their system of many small emissary veins, had a much more efficient way of cooling their brain than did the robust australopithecines, which had just the single large vein exiting the skull. “They had different designs because they lived in different settings,” Falk explains. The robust australopithecines stayed mainly in forested areas and out of direct sun while their gracile cousins had moved out into the savannah where the sun beat down on them regularly, and so they had to have a more efficient means of keeping their brains cool.

And, Falk realized, this difference in brain-cooling would be crucial to the evolutionary destinies of the two species. The brain is an organ that generates a lot of heat, and the larger it gets, the more heat it generates and the more important it is that the brain has a good way of cooling itself off. Without an efficient system of cooling, the brain can get only so large before it runs into the problem of overheating. And that seems to be what happened to the robust australopithecines, Falk says. Without a good way to keep the brain cool, their brains never got larger than about the size of a chimpanzee’s brain.

The gracile australopithecines, on the other hand, did have a way to keep their brains cool, and so there was nothing to keep them from evolving larger brains. The radiator-like system of cooling did not cause their brains to get larger, Falk emphasizes—it merely allowed them to. But the development of that system was a crucial step, one that allowed the gracile australopithecines or some very close relative to evolve into modern humans. To test her idea, Falk looked at fossil skulls and compared brain size with the system of emissary veins. She found that in gracile australopithecines, as the brain size increased over time so did the frequency of emissary veins coming through the skull. It seemed clear that the growing number of emissary veins was allowing for increased cooling and a larger brain over time.

Falk labeled her insight “the radiator hypothesis,” and in 1990 she published a lengthy description of it along with the evidence she had found supporting it. Although the hypothesis seemed straightforward enough to her, once again she found herself embroiled in controversy, and once again the root of the controversy was the sensitive issue of just which species were the direct ancestors of modern humans. The famous fossil Lucy, discovered by the equally famous anthropologist Donald Johanson, was the first example found of a type of australopithecine called Australopithecus afarensis. The discovery of Lucy was one of the most highly publicized findings in the field of anthropology, and Johanson had always identified Lucy as a direct ancestor of modern humans. Yet Falk’s analysis implied that Lucy could not be a direct ancestor, because A. afarensis did not have a proper brain radiator.

“The article was more controversial than it should have been,” Falk recalls, “because it challenged Lucy’s importance.” Still, as was the case with her interpretation of the australopithecine skull being more ape-like than human-like, Falk’s analysis has stood up to scientific scrutiny quite well.

Cabanac, the physiologist whose work put Falk on the path to the radiator hypothesis, is effusive when asked about it. In coming up with the radiator hypothesis, he notes, Falk had to understand and weave together ideas from several different disciplines—something that very few scientists are able to do. “It takes genius to be able to see things that others do not see,” he says. “She has it, and she showed that with the radiator hypothesis.”

Today, that hypothesis is a vital piece of modern anthropology because it explains what allowed human brains to grow so much larger than ape brains. Most anthropologists now accept Falk’s contention that a radiator-like system of emissary veins in the skull was a vital prerequisite for the evolutionary growth of the human brain.

Hobbit-Sized Symmetry

Since the publication of her radiator hypothesis, Falk has researched and written extensively about a number of aspects of human evolution and the development of human behavior. She has, for example, studied the development of language in human ancestors and suspects that it has its roots in “motherese,” the simplified, singsong language that mothers (and some fathers) use to communicate with small children. (See "motherese") She has become an expert in brain asymmetries—how the left side of the brain differs from the right side—in different species and at different times in the past. And, more generally, she has established herself as one of the world’s experts on the evolution of the human brain. Thus, when National Geographic needed a well-respected and accomplished researcher to examine the hobbit, they came to her.

It was in October 2004 that National Geographic announced the discovery of the hobbit, whose formal name is Homo floresiensis. Researchers from Australia and Indonesia had found the most complete specimen in a cave on the island of Flores, which lies east of Bali and about halfway between Asia and Australia. The fossil skeleton they found was of a female hominid who lived 18,000 years ago and was about 30 years old when she died. The most remarkable thing about the skeleton was its size. The woman had been just 36 inches tall, the average height for a modern 3-year-old.

In addition to this skeleton the researchers found fossilized bones from seven other individuals, all of them of similar proportions. Although only the first skeleton included a skull, one other jawbone was uncovered, and it was only a bit smaller than the jawbone of the first skeleton. With all this evidence, the researchers felt comfortable saying that they had indeed discovered an entirely new species of hominin and not just an anomalous individual.

Evidence from tools found with these bones indicated that the hobbits lived on Flores from 95,000 years ago to as recently as 13,000 years ago, when they were apparently wiped out by a volcanic eruption. The hobbits seemed to have hunted the pygmy elephants that inhabited Flores at the time. While the elephants were small compared to today’s elephants, they still weighed half a ton, or about what a full-grown longhorn steer weighs. That would have made it quite a respectable prey for hunters the size of preschoolers.

That the hobbits used sophisticated tools and were such accomplished hunters implied to Mike Morwood (who led the team that discovered the species) that the hobbits were quite intelligent. But that posed a dilemma: How could a creature with a brain one-third the size of a modern human be that smart? Although brain size is by no means an absolute measure of intelligence, and creatures with larger bodies naturally have larger brains, scientists have known for quite some time that the rise in human intelligence over millions of years was accompanied by a sharp increase in brain size relative to body size. Perhaps, some scientists suggested, the fossilized skull was an anomaly. Perhaps, for instance, it came from an individual with microcephaly, a birth defect that causes the brain to be abnormally small.

To answer such questions for a film they were making, National Geographic called Falk, and she in turn called on colleagues from Washington University in St. Louis who had worked with her on earlier brain studies. They did not have the actual hobbit skull to work with, but they had extensive computerized tomography images—the same sort of images created by a hospital CAT scanner—made in Indonesia by the fossil’s discoverers. With those images they created a “virtual endocast,” that is, a three-dimensional computer image that showed all the external features of the hobbit’s brain that had left their mark in its skull. Falk also created a latex endocast from a model of the skull provided by Geographic, something that they could hold in their hands instead of look at on a computer screen.

Falk then compared the hobbit brain with brains from several others hominids: an adult female chimpanzee, a modern woman, an adult female pygmy, a microcephalic and an adult female H. erectus. The latter was the direct ancestor of modern humans that lived from about 1.6 million to 250,000 years ago; the hobbit’s discoverers had suggested that the hobbit was a diminutive descendant of H. erectus.

“The shape of the hobbit’s brain was very much like H. erectus,” Falk says. But there were surprises, particularly in the frontal lobe, where the brain does much of its logical thinking and planning. “The thing that most catches your eye about the hobbit endocast is two bulges sticking out at the front of the brain. They’re not like anything you see in a human or a modern primate brain.” The enlarged regions are in parts of the brain that in humans are known to be involved in initiative-taking and advanced planning. Falk’s team suggested they might explain how the hobbit could be such a sophisticated hunter and tool-maker while having a brain that was otherwise surprisingly small.

The hobbit’s brain also had some features that were similar to those of modern human brains, but it clearly was not a brain from either a pygmy or a microcephalic, Falk says. “The scaling of brain to body isn’t at all what we’d expect to find in pygmies, and the shape is all wrong to be a microcephalic. This is something new.”

More recently Falk has received additional funding from National Geographic to perform a study that might remove any lingering doubts that the hobbit might indeed have been microcephalic. She will study eight skulls from microcephalic individuals, both adults and juveniles, and subject them to the same analysis that she used on the hobbit skull and the comparison skulls. In this way she will have a range of microcephalic skulls to compare with the hobbit’s skull and she will be able to rule out the possibility that she was comparing the hobbit with a noncharacteristic microcephalic. She says she’s confident that the analysis will show the hobbit was not microcephalic because microcephalic skulls have certain characteristic traits—such as being flat in back—that the hobbit’s skull does not have. “I just want to put it to rest,” she says.

This still leaves the question of just where the hobbit came from. Peter Brown, a paleoanthropologist at the University of New England in New South Wales, Australia, and one of the discoverers of the hobbit, has suggested that it was a dwarf form of H. erectus. Just as the elephants of Flores had gotten smaller over hundreds of thousands of years on the island, perhaps a group of normal-sized H. erectus had made its way to the island and gradually become smaller as well. Falk, however, believes that while this is plausible, there is also another possibility: The hobbit’s ancestors may have been small when they reached Flores. In this case, both the hobbit and H. erectus may have evolved from a small—and as yet undiscovered—species of hominid that left Africa around 1.8 million years ago, or around the same time as H. erectus.

“Lucy was maybe 3 to 3 1/2 feet tall,” Falk notes, “which means that at 3.2 million years ago there was a small hominin. Also, at 1.8 million years ago we know there were still some small hominins in Africa, and at 1.75 million years ago there was a hominin in Eurasia with a small cranial capacity, 600-650 cc [about half the size of a modern human brain]. So there are hints that maybe there were little folks running around for a long time.”

It is an intriguing possibility. If Falk’s supposition is correct, then humans and their ancestors could have been sharing the planet with pint-sized cousins for a couple of million years, and those cousins would have eventually developed the characteristics that we tend to think of as distinctly human, such as the ability to use fire, the ability to make sophisticated tools, and the ability to cooperate in order to hunt large game.

Like so much of Falk’s work, it forces one to stop and think about what it really does mean to be human.