At 1 p.m. today I listened by phone to a press conference in Washington where scientists presented the first good look inside a Hobbit’s head. The view is fascinating. While it may help clear up some mysteries, it seems to throw others wide open.
Last October, a team of Australian scientists declared that they had found a new species of hominid that lived as recently as 12,000 years ago. It was short–maybe three and a half feet tall–and had a brain they estimated to be about the size of a chimp’s. Its bones were found along with stone tools, suggesting that it made good use of its scant grey matter. The fossils of this remarkable hominid were discovered in a cave on the island of Flores, which gave the hominid its name: Homo floresiensis.
As soon as the news broke of the discovery, some researchers expressed grave doubts. They suggested that H. floresiensis was actually just a group of human pygmies. The fossils discovered on Flores include only a single skull, and these skeptics suggested that its small size might be the result of a genetic defect known as microcephaly. If H. floresiensis’s discoverers had found another skull instead, it would likely have displayed the sort of shape you’d find on a living pygmy human.
This scientific debate quickly got eclipsed by an ugly tussle over the bones. A skeptical grand old man of Indonesian paleoanthropology, Teuku Jacob, wound up with the fossils for four months, during which time he arranged for other scientists to examine it and for some bone to be sent to Germany for DNA testing. Now he’s surrendered the fossils, and it looks as if we may be able to enjoy some actual scientific discoveries about these bones, rather than a yelling match. (Those interested in a more detailed chronicle and a fair number of links to more information may want to check out my previous Hobbit posts.)
Before the Hobbit bones wound up in Jacob’s safe, its discoverers had a chance to look inside its head. They enlisted the help of Dean Falk, a Florida State University paleoanthropologist who has spent years studying the interiors of hominid skulls to find clues to what their brains were like. While brains rot quickly, they leave behind marks where some of their folds and blood vessels were. And since the skull forms such a tight seal around the brain, it ends up with roughly the same shape. To get a good look at these details, Falk has helped pioneer the use of CT-scans to visualize the insides of hominid skulls.
The Hobbit skull was scanned in Jakarta at a 1-mm resolution, and the data was then processed at Washington University’s medical school. Falk and her colleagues then analyzed the interior of the skull to calculate the size and shape of the brain, and then produced a three-dimension model of it.
Falk and her colleagues made a careful study of the size and shape of the Hobbit brain, and then they created three-dimensional models of the brains of other hominids. They compared it to the brains of average female humans, a female pygmy, and a microcephalic girl. (They chose females because the Hobbit skull is believed to belong to a female.) The scientists also looked at endocasts of fossil hominids. As I mentioned in my earlier posts, the discoverers of H. floresiensis suggested that it might have evolved from Homo erectus, a tall, large-brained hominid that is believed to have left Africa about 2 million years ago and spread across Asia. Falk’s team looked not only at five Homo erectus skulls, but skulls of earlier hominids from Africa, such as Australopithecus africanus and Paranthropus aethiopicus.
The results are being published today on Science Express, the online arm of Science magazine (a printed version will come out in a future issue). The most straightforward results are the ones that address the skeptical suggestions about a small-brained human. The Hobbit brain doesn’t look anything like the brain of a microcephalic. Microcephalics have smooth brains, for example; the Hobbit has a normal convoluted surface. Microcephalic brains have a pointed top and a sloping forehead; the Hobbit brain is rounded on top and unsloped in front.
Nor does the Hobbit brain seem to belong to Homo sapiens. It is small (417 cc, which is less than a third the size of an average human brain), and lacks the distinctive shape of human brain. It is wider from ear to ear than it measures from the front to the back of the head, for example. The brains of human pygmies are indistinguishable from those of taller humans, both in size and shape.
Of all the brains that Falk and co. compared to Homo floresiensis, Homo erectus came the closest–in particular, Homo erectus skulls from Java and China. They are also unusually wide, for example. But the Hobbit brain also has some strange features that set it off from Homo erectus. In some ways it is relatively primitive. For example, at the back of the Hobbit brain there is a relatively small occipital lobe. On the other hand, Falk and her colleagues noticed some traits in Hobbit brain that are more human-like. It has more convolutions at the front, for example, than Homo erectus. The temporal lobe, where hearing, memory, and emotions are handled, is enlarged, as well as the parietal association cortex, where some sensory information is handled. Perhaps most intriguingly, a region of the frontal lobes known as Brodmann’s area 10 seems to be very large in the Hobbit. It is also large in living humans, and is known to be important in planning and other complex kinds of thought.
Now, a study of single skull cannot be the last word about an entire population of hominids. But it strengthen some possible explanations for Homo floresiensis, and weaken others.
1. A few ordinary pygmies and a microcephalic: It’s hard to imagine how its advocates will be able to continue promoting it. I can imagine a skeptic saying, “Well, this person suffered from an unusual form of microcephaly that the scientists didn’t look at.” But that would be a desperate reach.
2. An extraordinary group of Homo sapiens. Imagine that humans settled on Flores and then underwent a dramatic evolution that shrank their bodies and altered their brain structure. This explanation might account for the human-like features of the Hobbit brain.
But the time range of Hobbit fossils pretty much rules this one out. Modern populations of tall, big-brained humans are believed to have arrived in Southeast Asia about 50,000 years ago, and the oldest Hobbit bones are 95,000 years old. What’s more, tools on the island suggest that hominids have been on Flores for 800,000 years.
3. Descendants of Indonesian Homo erectus. It is less of a stretch to envision a population of Homo erectus evolving into the Hobbits. After all, its brain has the strongest overall resemblance to that of Homo floresiensis. And Homo erectus have been in southeast Asia for at least 1.8 million years. Perhaps a few Homo erectus individuals were swept onto Flores hundreds of thousands of years ago and gradually evolved smaller brains.
But this scenario is odd in its own way. It would require certain parts of the Homo floresiensis brain to have enlarged (relatively speaking), even as its overall brain size was shrinking drastically. And these enlarged regions allow us humans to do some of our most abstract thinking.
4. Something completely different. Before I explain what this might be, I have to explain a major problem with explanation 3.
Scientists have found a strong relationship between a person’s body weight and the percentage of their total weight made up by their brain. For a grown man of normal weight, the brain may make up just two percent of his weight. But a pygmy woman’s brain may be 3% of her body weight. These different percentages are the result of how the human brain and body grow. The brain grows rapidly during childhood and reaches nearly adult size around age ten. The body, on the other hand, grows more slowly and for many more years. Pygmies are smaller than average humans because their bodies stop growing earlier, but not before their brains have reached adult size. This relationship between brain and body follows a steady curve. It is so steady that you can predict what happens to the ratio of brain to body weight as humans evolve to smaller or bigger sizes.
Scientists have also drawn a similar curve for chimpanzees and other apes, but it’s noticeably different. That’s because their brain growth slows down dramatically after the first couple years, while their bodies can continue to grow to large size. For any given weight, a human’s brain is a higher percentage of his or her body weight than an ape’s brain.
It’s safe to assume that Homo erectus had its own curve. Drawing the curve isn’t easy, because there are precious few skeletons of Homo erectus thatinclude both a brain case and enough of a skeleton to estimate their body mass. Actually, there’s just one, a 1.5 million year old skeleton from Kenya. Its brain and body size suggest that Homo erectus had a curve midway between apes and Homo sapiens. That’s a nicely unsurprising result, because adult Homo erectus stood about as tall as living humans but only had a brain about 900 cc. (Ours are 1350.)
Now come the Hobbits. Because the scientists have both the skull and some parts of the skeleton, they can estimate both its body weight–about 50 pounds–and the percentage of its body weight made up of brain: 1.7% They plotted this value on their graph and found that it did not fall on the human curve, nor on the Homo erectus curve. Instead, it fell on the ape curve.
What this means is that if you scaled down Homo sapiens to the size of the Hobbit, its brain would be much bigger than the Hobbit’s brain. And it also means that if you scaled down Homo erectus, it would also have a much larger brain than a Hobbit.
This suggests that the Hobbits actually descend from some other hominid, one with an even smaller brain than Homo erectus. How could this have happened, given that Homo erectus and Homo sapiens are the only hominids known from southeast Asia? Perhaps the Hobbits represent a major branch of hominid evolution that’s been hidden from view till now.
About six million years ago, the first hominids branched off from other apes in Africa, and until about 2.5 million years ago they had brains that weren’t much bigger than a chimpanzee’s. One branch appears to have evolved big brain and tall bodies, and paleoanthropologists believe that this branch gave rise to Homo erectus, Homo sapiens, Neanderthals, and perhaps a few other species. Paleoanthropologists generally believed that these big-brained hominids were the only ones to leave Africa. Homo erectus moved out first, followed by successive waves of Homo, until our own species expanded out of Africa about 50,000 years ago.
But in 2002 scientists found a baffling hominid skull in the former Soviet republic of Georgia. It was 1.75 million years old, it had some resemblance to Homo erectus, but it had an amazingly small brain, measuring only 600 cc. Larger Homo erectus-like fossils have also been found at the same site, dating back to the same age, and scientists have been arguing whether they belong to the same species or to different ones. Some scientists have suggested that the tiny Georgian hominid represented a second migration out of Africa. These migrants were not Homo erectus, but perhaps belonged to an older lineage of hominids. That lineage might have included one or more of the small-brained hominids that are the oldest known species to have used stone tools (Homo habilis, or Australopithecus garhi). Michael Morwood, one of the Hobbit’s discoverers who was at the press conference this afternoon, mentioned that Homo floresiensis has some Australopithecus-like traits in the lower part of its skeleton.
So here is a fascinating scenario to consider: a small-brained African hominid species expands out of Africa by 2 million years ago, bringing with it stone tools. It spreads thousands of miles across Asia, reaching Indonesia and then getting swept to Flores. It may not have undergone any significant dwarfing, since they were already small. This would change the way we think about all hominids. Being big-brained and big-bodied could no longer be considered essential requirements for spreading out of Africa. And one would have to wonder why early lineages of hominids became extinct in Africa when one branch managed to get to Flores.
So explanations 3 and 4 seem to come out strongest at the moment. Either one would mean that the Hobbit represents an amazing experiment in hominid brain evolution. They suggest that some human-like features emerged in hominids that were separated from us by two or maybe three million years of evolution. Yet their brains were mosaics, sharing features with us and with other hominids, and also had features of their own. These strange brains, Dr. Morwood argues, allowed Hobbits to do things some pretty elaborate things, such as butcher dwarf elephants or make fires. It would be wonderful to know how these strange brains were wired together, but we have to be content with their shadows. But even shadows can sometimes reveal a lot.
Originally published March 3, 2005. Copyright 2005 Carl Zimmer.