Tracing the Evolution of the Human Brain Through Casts of the Inner Skull
We may be biased, but we think the human brain is pretty special. All this week, mentalfloss.com is celebrating this miracle organ with a heap of brain[y] stories, lists, and videos. It all leads up to Brain Surgery Live With mental_floss, a two-hour television event that will feature—yes—live brain surgery. Hosted by Bryant Gumbel, the special airs Sunday, October 25 at 9 p.m. EST on the National Geographic Channel.
You might think of your skull as a hard case keeping your tender brain safe and sound. And that’s mostly true. But living bone is dynamic and responsive, and your brain is a “throbbing, vital, organic thing,” says Dean Falk, an evolutionary anthropologist at Florida State University and one of the world’s leading researchers on the evolution of the human brain. As she explains, “The pressure inside the skull in living individual animals makes impressions inside the walls of the braincase.”
These impressions can remain on the inside of the skull long after the brain itself has decomposed—in some cases, for many millions of years.
Some paleoanthropologists have capitalized on this tendency for the skull to retain phantom impressions of the organ that was once inside it by creating casts of the interior of the cranium. They're called endocasts.
An endocast is a cast of the hollow interior of an object, most commonly the skull of a vertebrate (then also known as an endocranial cast). Some are natural, the result of sediment filling the brain cavity; others are intentional, formed from clay, latex rubber, plaster of Paris, plasticine, or silicone. Still others are entirely digital, composed of high-tech scans that reveal the interior surface in closer detail.
Paleoneurologists, who study the evolution of the brain, use endocasts to study its size, shape, and surface morphology. By tracing how these characteristics have changed during our evolutionary history, they’ve gained deeper insight into the ways we’ve become the humans we are today, with a suite of characteristics we now consider essentially, singularly human.
mental_floss spoke to Falk and to Ralph Holloway, a Columbia University paleoanthropologist and another of the world's leading researchers on the evolution of the human brain, about what they have learned from decades of research on endocasts about brains both ancient and modern. We also spoke to Falk about her (sure to be controversial) theory that key milestones in our brain's evolution explain Asperger's syndrome.
FROM HORSE HEADS TO HUMAN BRAINS
The endocast emerged as a tool in paleoneurology in the first half of the 20th century thanks to the pioneering work of German paleontologist Ottelie “Tilly” Edinger. The daughter of the prominent 19th-century comparative anatomist (and University of Frankfurt co-founder) Ludwig Edinger, Tilly discovered that vertebrate brains leave imprints on the interior of the skull while studying the brain cavity of a Mesozoic marine reptile. After the animal’s death, its skull had filled with sediment that eventually hardened to stone, creating a sort of “fossil brain.” This natural endocast retained an imprint of the reptile brain exterior.
Intrigued, Edinger began looking into endocasts, which until then had generally been treated as curiosities by comparative anatomists like her father, who had focused on the flesh of recently deceased animals. Working mostly alone, Edinger organized taxonomically the endocasts she located in a variety of museum collections, and analyzed her findings. In 1929, she published Die fossilen Gehirne (Fossil Brains). This scholarly tome would prove to be highly influential in the use of endocasts as a way to study ancient brains that no longer existed in the flesh.
Her second seminal work, Horse Brains, in 1948, contained a key insight about the evolution of the mammalian brain that made as much of an impact as her first work. “She found that [brain] volume and organization were sort of in league with each other,” says Holloway. “There were periods of time in which the horse brain seemed to be reorganizing, and there were other times in which it seemed to be changing in size.”
That insight—that changing size and reorganization are both essential to brain evolution—would become key to our understanding of how our own brains developed. Though in earlier decades scientists had unearthed ancient hominids in various places—including Neanderthals in Europe, Homo erectus in Asia, and, crucially, a variety of hominids and ancient primates in Africa—more were emerging from the dirt and rocks by mid-century. This trend continued into the 1970s, when the use of endocasts became more common. (Of course, paleoanthropologists have continued to unearth hominids in the decades since. The most recent find is Homo naledi.)
One of the first endocasts Holloway made, in the late '60s, was of Taung child, who died around age 3 from an eagle attack in southern Africa between 2 and 3 million years ago. After death, the skull had filled with sediment, eventually forming a natural endocast. In 1925 Raymond Dart had assigned this child a new species, Australopithecus africanus, and claimed it was an intermediary between human and ape—an idea that was largely rejected for decades. Holloway's analysis helped cement Dart's case for Taung child as a legitimate link between apes and us.
Holloway used latex rubber early on (it's now largely degrading), plaster of Paris, and eventually plasticine. “I like to have something in my hand,” Holloway says. “I can take the clay and mold things around. I can sort of get a range of what I think is possible.” Today he also uses a silicone material.
Falk, meanwhile, initially chose liquid latex, which she'd pour inside, swirl around, and cure for hours; to expedite the process, she’d sometimes blow a hair dryer on it. Once the cast was set, she'd extract the hollow mold and pop it into shape. In 1980, Falk also made an endocast of Taung child and came to very different conclusions from Holloway; she thought then that its brain was more apelike than human. The two have argued in academic journals for decades about their differing interpretations of Taung child, especially about the location, size, and very existence of the lunate sulcus, a C-shaped furrow on the occipital lobe, the visual processing center of the brain.
Today digital endocasts are far more common; these are CAT scans that can be done even of sediment-filled natural endocasts like Taung's. A virtual endocast is now Falk's preferred method. Her virtual endocast of Homo floresiensis, the so-called Hobbit hominid discovered on the Indonesian island of Flores in 2003, bolstered its finders' argument that the small creature represents a new Homo species (which some still dispute).
The quality of an endocast depends on species, size, and age, Falk says. “Juveniles make really good endocasts. With old people, the brains start to shrink a little bit, and remodeling inside the skull will kind of erase some of the impressions.”
Hominid endocasts are measured for brain size and analyzed for visible features, and then compared to other brains. “We can follow these endocasts up to the present, when we actually have real brains,” Falk says. “And you can compare them to brain morphology from living apes, monkeys, and humans. You can also do endocasts of fossil primates.”
Endocasts are used by many paleoneurologists, in Europe, Africa, and the U.S. In America, two of the largest collections were created by Falk and Holloway; each has made hundreds of endocasts.
Endocasts have their limitations. The main drawback is that they only capture details on the surface of the brain, and the details they do preserve largely depend on the quality of cranial preservation. “In terms of the organization that you see on the outside surface of the brain, the endocasts can be murky,” Falk admits. “It's touch and go whether or not you're going to get much detail, or which part of the brain will show up [on the endocast]."
Nor can many brain changes that accompanied shifts in behavior show up on the outside surface of the brain, since many occurred internally. “Take bipedalism, for example,” Holloway says. “Bipedalism can’t be divorced from changes in the brain. Obviously a whole series of new motor cortex connections are being made. Something like bipedalism is extraordinarily complicated in terms of the neural anatomy involved. The problem is, when you have a skull that’s 3 million years old and you make an endocast of it, you can’t see anything, really, about those kinds of behaviors.”
WHAT HAVE ENDOCASTS TAUGHT US ABOUT THE HUMAN BRAIN?
The record of hominids begins about 6–7 million years ago. From the limited fossils we have, their brains appear to be ape-sized. Based on the scant few fossils from the next few million years, the brain seems to have plateaued in size until roughly 3.5 million years ago, around the time of the hominid genus Australopithecus, which includes the famous Lucy.
The fossil record gets much better around that time, Falk says. That’s how we know that after the long plateau, our brains began to grow—and they kept on growing for the next 3.5 million years, right on up through the Neanderthals—and then to us. (Our brains are smaller than Neanderthals' were.)
When you plot cranial capacity over time, the average brain size of living people is three to four times the size of Australopithecines like Lucy. Her brain was about the size of a large chimp’s (400–450 cubic cms, or ccs). By 2 million years ago, the hominid brain expands to 600–750 ccs, and by the time of Homo erectus, about 1.5 million years ago, brain size increased to 1000 ccs. Today our brains are roughly 1350 ccs.
Interestingly, that’s where the plotline of brain growth levels out. We seem to have plateaued in brain size once again, Falk says. “I suspect that has to do with the obstetrical limitations on the babies than we can bear. They just can't get bigger headed and have mother and child survive. I think that has capped the size of the brain.”
In fact, the modern brain appears to have shrunk by about 10 percent in the past 30,000 years.
But while many scientists view absolute brain size as the best measure for tracking the evolution of cognition in our early ancestors, as Falk writes in Frontiers in Human Neuroscience, size is not everything. The neurological organization of the brain is incredibly important too.
That’s where endocasts have also proven enlightening. Thought they can’t reveal the interior of the brain, they can reveal the brain's overall shape and size, and, crucially, the surface of the cerebral cortex. That’s important because the cerebral cortex is “where we do our highest thinking,” Falk says. Conscious thought, rational problem solving, planning, language, social skills, and scientific, artistic, and musical creativity are all facilitated by the cerebral cortex.
Paleoneurologists analyze features and patterns on the surface of the brain, which is covered in convolutions of gray matter called gyri that are separated by grooves called sulci. These patterns of sulci can reveal details about the organization of a specific brain at a point in time.
What they’ve found by looking at changes to the surface over time is that throughout our evolutionary history, once our brains got bigger, they reorganized too. While we're not sure whether changes in brain size and organization happened simultaneously, they've largely occurred in association over the past few million years.
When our hominid ancestors' brain changed, their behavior changed too. For instance, about 3 million years ago, the Australopithecus primary visual cortex gets smaller, and the parietal lobe expands; we can spot this on endocasts. Meanwhile, these creatures were walking upright. The reverse is likely also true: As behavior changed, the brain altered too.
When the hominid brain leaped in size about 2 million years ago, asymmetries developed, most notably in Broca’s area, a region on the left side of frontal lobe associated with language processing. “It has a very particular configuration,” Falk says. “In humans you've got a particular repeatable pattern of convolutions that you don't see in apes. That's a huge change.” Such asymmetries are characteristic of the modern human brain.
Another change, she says, appeared in the frontal lobe, in the prefrontal cortex. Neuroscientists have shown that one region, called Brodmann area 10, is greatly enlarged in humans compared to primates, and that the difference developed early on in our evolutionary history, perhaps 6 or 7 million years ago. This enlargement seems to have been related to the expansion of the prefrontal association cortices, which are parts of the brain that integrate information from other regions that are more specialized.
"What these changes have in common is that they're all related to the expansion of the association cortices," Falk says. "That's what makes humans humans: We have these brains with these networks where we can really integrate and compute information from multiple senses, including internal stimulation—just thinking on our own, for no reason at all."
CAN ENDOCASTS TEACH US ANYTHING ABOUT OUR BRAINS TODAY?
Perhaps. How did human brains get to be this way? How did we get to be this way? There are many theories. One old dominant theory gives credit to “Man the Hunter"; in this theory, the need to coordinate for the hunt gave rise to both speech and social cooperation. You may have also heard of "Woman the Gatherer," who is said to have been the catalyst for these same characteristics by cooperating with others, often multigenerationally, to gather food—the most reliable source of nutrition—and care for the young.
Falk argues for a third: Baby the Trendsetter. She posits that caring for our increasingly large brained, helpless young sparked a host of important evolutionary changes. One especially key development was the selection for language—witnessed in endocasts, for example, with the change in Broca's area—which Falk argues is the primary driver of our essential humanness. And we may have to thank babies for that. When we became bipedal, we lost the gripping toe that allows primate babies to hold onto their mothers as they go about their business. According to Falk's "putting the baby down" theory, to free up their hands, our upright early ancestors had to put the baby down to get things done.
Because they crave constant contact, babies don't like to be put down. To soothe them—a squalling, distressed young hominid was sure to attract opportunistic predators—Hominid mothers made vocalizations to their young. Today we call the seemingly universal tendency to coo at babies in a singsong tone "Motherese." Hominid proto-Motherese, Falk argues, was essential to the development of language. Hers is one of many ideas about how we developed this singular human characteristic.
The Baby the Trendsetter idea is the anchor for another theory Falk has, based on the idea that the evolutionary trends can be used to illuminate the modern brain. Specifically, she's looking at Asperger's syndrome from an evolutionary perspective.
Technically, Asperger's—a developmental disorder marked by high intelligence, low social skills, language facility, eccentric behavior, and obsessive tendencies—no longer exists; in 2013, it was folded into autism spectrum disorder, a new classification in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, or DSM-5. But Falk maintains that Asperger's is real; is not autism—not even high-functioning autism; and reflects a unique twist on the evolution of the human brain.
"I ask whether we should consider it pathological, or whether one should think of it in terms of natural human variation," Falk says.
She identifies three key trends in human evolutionary development that transformed the course of hominin neurological and cognitive evolution: a delay in locomotor development; the tendency to seek comfort from physical contact; and accelerated early brain growth. People with Asperger's, she says, express these three trends in a different way.
As for the first two trends, "Aspies" can be uncoordinated and clumsy, and their problems with social interactions are well known. And then there's the accelerated brain growth. The extraordinary spurt of brain growth that starts prenatally and continues through the first year is unique to humans among primates. "This was important in human evolution as human brain size increased over time," Falk says.
People with Asperger's have a first-year brain spurt that's on the extreme high end of the range of variation. "This is an advanced derived feature in human evolution," she says. This could be related to their tendency to be highly intelligent, especially in the computational and analytical realms. (See: Silicon Valley.) Falk is currently co-authoring a book on the topic with her 24-year-old granddaughter, who has Asperger's.
What does this have to do with endocasts? A few things. For one, there's still a lot we don't know about the brains of our early human ancestors, but we know a lot more than we used to, thanks to this somewhat old-school technique. For another, there's a lot we don't know about modern brains either. Falk's research into Asperger's is just one project out there among many attempting to connect the two. It's likely to be controversial. But that's fitting, in a way. What Falk, Holloway, and other paleoneurologists have documented with endocasts is physical evidence of some of the advanced cognitive characteristics that make us so different from our primate relatives—and from our own earliest ancestors. Debating the details, their larger importance, and whether they have any application to life today—well, that's essentially human too.