In October 2004 Australian and Indonesian announced they had discovered a three-foot tall species of hominid, Homo floresiensis, that was still alive no earlier than18,000 years ago. As I’ve detailed in previous posts, this claim has inspired a lot of debate, much which revolves around whether the fossils, found on the Indonesian island of Flores do in fact represent a new species, or whether they were human pygmies. This week a new study was published in the journal Biology Letters (link to come) that puts this debate in the proper evolutionary frame. The paper is not about hominids, however, but about bats.
Before I get to the bats, let me dwell a little longer on these Pleistocene hobbits. A great deal of the controversy has focused on the one Homo floresiensis skull found so far, which held a brain less than a third the size of a human’s and about the size of a chimpanzee’s. If Homo floresiensis really does represent a separate species, then its ancestors may have undergone a drastic evolution, which not only shrank their bodies but also their brains. One hypothesis for the origin of Homo floresiensis holds that it off from another species of hominid, Homo erectus, which arrived in southeast Asia 1.8 million years ago and may have been present there as recently as 30,000 years ago. Homo erectus was already about as tall as our own species is today, and had brains that were about three-quarters the size of ours.
Skeptics find this possibility implausible, arguing that it’s more likely this individual was just a pygmy human with some genetic defect. As far as I can tell, this skepticism about shrinking hominid brains flows from two sources.
One is the fact that digs on Flores have yielded some sophisticated stone tools and other clues that the hominids of Flores—human or otherwise—were able to hunt. Some people wonder whether it would be possible for a hominid with a chimp-size brain to use such tools, since the rise of tool use in hominids roughly coincides with the rise in brain size. It’s a fair question, since chimpanzees today can’t make the sorts of stone tools found on Flores. But it’s not any sort of slam-dunk refutation of the claim that Homo floresiensis were a separate species. First, consider the fact that the first signs of hominid tool use, 2.6 million years ago, came at a time when hominids still had brains barely bigger than a chimp’s. Second, size isn’t everything. Tool use may also depend on how a brain is wired, not just how much data-processing power it has. It doesn’t seem absurd to argue that as Homo floresiensis evolved a smaller brain, it retained the circuitry that made tool use possible. At least it’s a hypothesis worth testing.
The other source of skepticism, which I mentioned in my last post, is a vague sense that when it comes to hominid brains, evolution cannot run in reverse. It’s certainly true that if you draw a graph of hominid brain size over time, it has climbed to spectacular heights. Scientists prefer to chart brain evolution not simply by its raw increase in weight, but in how large the brain becomes in proportion to the rest of the body. For a mammal our size, we humans have a brain about seven times you’d predict. A great deal of research has gone into charting how brains get bigger over the course of evolution—not just in our immediate hominid ancestors, but over the past 200 million years of mammal evolution. The ability of our species to thrive so spectacularly seems to mainly depend on our extraordinary brains. Given that their size is one thing that makes them so extraordinary—and given that they’ve been increasing for so long—the notion of a shrinking hominid brain can seem absurd.
The discoverers of Homo floresiensis have pointed out reversals do happen. They point to how many species become dwarfs when they arrive on isolated islands. Elephants, deer, buffalo, and other species have shrunk over the course of just a few thousand years. It’s not entirely clear why this happens, but scientists suspect that being small is an advantage on an island with limited resources, and when animals arrive on an island without a lot of predators, there’s no longer a defensive advantage to being big. In some cases, these island dwarfs have evolved a simpler nervous system. So, the argument goes, Homo floresiensis is simply a hominid that happened to get washed up on a remote island and proceeded to evolve according to the rule of islands.
This argument may give you the impression that the evolution of smaller brains is just a digression from the main story of progress. Sure, a few hominids wind up on desert islands and evolve small brains, but back on the mainland, the hominid brain marches on towards our own spectacular size. In fact, it now appears that shrinking brains are a much more general feature of mammal evolution. And this is where we get to the bats.
Bats evolved about 50 million years ago. The first bats could fly and listen to the echoes of their shrieks to find prey, a radar-like technique called echolocation. These two adaptations allowed them to become efficient nocturnal airborne predators, taking advantage of a niche that may have been empty at that time. (Owls seem to have diversified at around the same point in history.) The result was a staggering evolutionary success, with bats now making up 20% of all mammal species on Earth. Bats obviously depend on their brains. They need to be able to process the complex information that they get from echolocation, and they need to be able to control their membranous wings. So you might think that bat evolution has been dominated by a steady expansion of their brains.
But as much as we may value the brain, it is just another organ. If the brain becomes bigger, an animal has to dedicate more energy to it and has less to supply to other parts of the body. This evolutionary trade-off has produced a lot of the diversity of life we see today—including even the size of beetle horns, a subject I blogged on a few days ago. And brains are particularly costly, requiring twelve times more calories ounce for ounce than muscle. It’s not easy to gauge the effect of this trade-off in our own lineage, because only 20 or so hominid species are known from the past six million years. But with so many bat species alive today, it is possible to see major trends in brain evolution by comparing them.
Kamran Safi, a biologist at Zurich University, and his colleagues compared 104 species of bats, noting their brain size, the shape of their body, and the ways in which they hunted. (Some bats specialize in hunting in open spaces, for example, while others can weave their way through forest foliage.) They then extrapolated back along the bat family tree to calculate how big the brain of the common ancestor of living bats was. And from their, they then moved forward through evolution, seeing whether there was a directional trend towards bigger brains.
They didn’t. It turns out that the first bats probably had brains that would be considered average for a living bat. Some bats have bigger brains, and some have smaller ones. Safi and his colleagues looked for other factors that had changed along with brain size in different lineages. They found that bats that had specialized for hunting in tight spaces evolved broad, large wings that provided them with agile maneuverability but also use up a lot of energy. They also tended to evolve bigger brains. By contrast, the bats that adapted to open spaces evolved narrow, small wings that didn’t demand much energy but also didn’t provide much maneuverability. These bats evolved smaller brains. This trend was especially strong in bats that hunt insects, as opposed to ones that have shifted to eating fruit or flowers. When bats evolved in ecological niches that demand a lot of brain power to control their wings, they evolved bigger brains. But when they could afford to slim their brains, they did—thus saving themselves the cost of fueling this hungry organ. These bats with shrunken brainsn were not defective, nor were they even rare flukes sequestered on some tiny island. They could still fly and hunt with perfectly respectable skill. They simply adapted to their surroundings.
Safi and his colleagues conclude that mammal brains may shrink thanks to many evolutionary forces, including a species’s diet, social system, or the length of its pregnancy. “A reduction in brain size should be a general property of evolution,” they write, adding that “The assumption that larger brains are derived [a new development in a lineage] is probably associated with the quest to explain why humans have large brains.”
The question of whether Homo floresiensis really did evolve a shrunken brain remains an open one. But if it does prove to be the case, we shouldn’t consider it a bizarre fluke. The bats are beginning to fly here in Connecticut, and when I see them flit across a twilight sky this summer, I’ll think of them as flying hobbits.