Finally, more brains.
On Tuesday I wrote about how the second batch of Homo floresiensis bones had at last seen the scientific light of day. Today the critics who don’t think the Hobbit is a new species are making their way into scientific journals as well. They’re saying that the Hobbit brain looks an awful lot like a human brain.
Last year, as I described here, Dean Falk of Florida State University and her colleagues reported on a scan they had made of the braincase of Homo floresiensis. They compared it to the braincase of normal humans, of a human born with a congenital defect called microcephaly, and the braincases of other hominid species. Falk concluded that the brain did not belong to a human. While the Hobbit brain is small (about a third the size of a normal brain) it had several key differences in shape compared to the microcephalic brain.
In today’s issue of Science, a neursurgeon and two anthropologists from Germany published a “Technical Comment” on last year’s scan. The researchers had access to a collection of microcephalic brains, and they analyzed 19 of them. One brain drew their attention in particular, shown here in the top picture. The human microcephalic is on the left, and the Hobbit is on the right. They certainly look similar, and the German team report that they are almost identical in size (415 cc for the human, 417 cc for the Hobbit) and they have similar proportions, such as breadth to length (85% versus 86%).
In last year’s report, Falk’s team pointed out an area on the front of the Hobbit brain that was remarkably enlarged. Known as Brodmann’s area 10, it is also enlarged in humans, and is associated with planning and taking iniative. Given that the Hobbit bones were found alongside stone tools, one could imagine that this sort of mental equipment allowed them to make the tools despite having chimp-sized brains. But the German team found seven brains in their collection that had enlarged Brodmann’s area 10. Their medical records show that one of these individuals couldn’t even speak a few words, which suggests that one shouldn’t read too much in this particular bulge.
On a more general note, the German team points out that human microcephalic brains come in a wide range of sizes and shapes. The 19 brains they studied varied in size from 280 to 591 cc, for example, a range in which the Hobbit’s 417 cc fits comfortably. And so even if no single brain from their collection matches all of the details of the Hobbit brain, their variability show that it could plausibly be produced by microcephaly. Since only a single Hobbit braincase has yet been found, the German scientists conclude, “it is premature to exclude LB1 [the technical name of the fossil] from any pathological anatomy. Analysis of other skulls from the Indonesian island of Flores wil help address the correct taxonomy of the small-brained hominid.”
And now, for a word from Dean Falk and company…
The nice thing about Technical Comments is that they allow scientists to respond immediately to their critics. And the most powerful response Falk has is to publish a picture of her own, shown here.
Falk and her colleagues accuse the Germans of some basic mistakes–most significantly not measuring their brains in a conventional way, as Falk did. Falk tilted the human microcephalic braincase to the same orientation as the Hobbit brain. The second illustration here shows the human microcephalic braincase from Falk’s study to the left, the tilted microcephalic braincase in the middle column, and the Hobbit braincase to the right. Suddenly, the human braincase from the German report looks a lot more like Falk’s human than the Hobbit, particularly in the side view in the bottom row. As for Brodmann’s area 10, Falk’s team points out that it is smooth on the microcephalic brains shown in the German report and convoluted on the Hobbit’s.
Falk’s team make one particularly sharp accusation. They point out that one view of the human microcephalic braincase in the Germany report (the top middle braincase in Falk’s figure here) has a striking tip, but the side view doesn’t seem to show it (the bottom middle braincase). “We do not believe these images represent the same individual,” they write.
“If this is the best evidence that can be produced froma sample of 19 microcephalics,” they conclude, “we suggest that the authors reconsider their position on the microcephalic hypothesis regarding Homo floresiensis.”
It’s nice to see the debate move beyond sound bites on cable TV, but I found it odd that Science’s editors didn’t get more involved in this exchange. These sorts of arguments can only be settled if everyone plays by the same rules. So why didn’t both teams settle on exactly how to orient the braincases? And why didn’t the German team satisfy Falk’s group that they hadn’t mixed pictures of two braincases together? That seems like pretty simple procedural stuff.
More significantly, it seems odd that the German team wasn’t required to consider all of the evidence assembled so far about Homo floresiensis–including the various jaws, limb bones, and other remains from several individuals scattered across 80,000 years. It’s not just LB1’s braincase that’s weird–the jaw has no chin, some of its teeth have strange double roots, the arms are long, and some of the limb bones have peculiar twists. What’s more, all the evidence at hand suggests that they were all about three feet high. So if LB1’s braincase does belong to a microcephalic human, as the Germans suggest, then what’s their explanation for all the other bones? Could they have all belonged to humans too? Biologists learned a long time ago that they had to consider the total evidence when it comes to figuring out how individuals are related to one another, because of the complexities of evolution. In one particularly famous screw-up, scientists thought they had isolated DNA from a dinosaur fossil. Only when they had considered all the evidence (in other words, all the potential sources of the DNA) did it turn out to be a bit of contamination from a human. I know this new Hobbit paper was written as a technical comment on Falk’s study in particular, but it leaves lots of unanswered questions.
Admittedly, the total evidence is pretty sparse when it comes to Homo floresiensis. Just a single additional braincase would help enormously, and a tiny fragment of DNA could close the case. Which makes the ban on digging in the cave where the Homo floresiensis bones were found all the more incomprehensible. Fortunately, this new article from BBC reports that the Hobbit team is expanding their search to other sites, even on other islands in Indonesia. Let’s hope the braincases come rolling out like bowling balls.
Update, 12 pm: John Hawks has some questions too.
Originally published October 14, 2005. Copyright 2005 Carl Zimmer.