The Loom

bat.jpgIn 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.

Comments

  1. #1 Jeff Medkeff
    June 16, 2005

    As far as I can tell, the skeptics completely, and improperly, ignore the influence of culture on floresiensis. Is floresiensis’ brain so small that it can’t even learn to make and use tools when taught?

    It is one thing to invent a tool technology, another thing to learn to use what has already been invented. Cephalopathological Homo sapiens of many sorts can learn to use tools, and teach others how to use them. Chimps can learn tool use from humans, and then teach each other.

    The hypothesis is that floresiensis evolved from bigger-brained ancestors, who indisputably used tools and passed on their technology through generations. The skeptics need to explain why and how floresiensis’ brain evolution must have disrupted this process, before their case will sound rigorous to me.

  2. #2 Thomas Palm
    June 16, 2005

    The New Caledonian crow is adept at using tools despite its relatively tiny brain. Admittedly it is a brain much different from ours, but it does show that you don’t need a big brain just to use tools.

  3. #3 James
    June 16, 2005

    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.

    I see no absurdity in the argument but, just out of honest curiosity, how is a hypothesis of this nature tested?

  4. #4 Doug
    June 16, 2005

    I keep wondering why Flores is being described as ‘desert’ and ‘isolated’ when it’s neither. If these descriptions are relevant to the hypothesised reduction in hominid brain size then in this case they don’t apply.

    Also, I understood that there was no evidence linking the tools to Homo Floresiensis. Anatomically modern humans have been in the area for at least 60,000 years and there are traces of them (tools etc) everywhere.

  5. #5 Hungry Hyaena
    June 16, 2005

    An excellent post, regardless of whether or not it provides more fuel for either side of the H. floresiensis debate. The Safi bat study is enlightening and exciting. Thanks for bringing it to my attention.

  6. #6 Sean
    June 17, 2005

    This was a fascinating article.

    It raises one question for me.

    “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.”

    Could it be that moving into a new niche or adopting new behaviors puts a selective pressure on the population for larger brains but as their neural architecture re optimizes over generations to accommodate the new structure in more efficient ways, the selective pressure for smaller, more energy efficient brains becomes greater?

    Brain size might be the result of a tug of war between the needs of the metabolism and the demands of the environment.

    Maybe this is an obvious point to a biologist but it is a new thought for me so… hurray for me!

  7. #7 SteveF
    June 18, 2005

    The Neanderthals had larger brains than us.

  8. #8 david
    June 19, 2005

    Darwin’s Theory of Evolution – The Premise
    Darwin’s Theory of Evolution is the widely held notion that all life is related and has descended from a common ancestor: the birds and the bananas, the fishes and the flowers — all related. Darwin’s general theory presumes the development of life from non-life and stresses a purely naturalistic (undirected) “descent with modification”. That is, complex creatures evolve from more simplistic ancestors naturally over time. In a nutshell, as random genetic mutations occur within an organism’s genetic code, the beneficial mutations are preserved because they aid survival — a process known as “natural selection.” These beneficial mutations are passed on to the next generation. Over time, beneficial mutations accumulate and the result is an entirely different organism (not just a variation of the original, but an entirely different creature).

    Are we talking De-evolution now guys?

  9. #9 Aaron
    June 19, 2005

    “Are we talking De-evolution now guys?”

    No. The notion of evolutionary “progress” — from small to large, simple to complex, or stupid to clever — is almost entirely fallacious. We may be tempted to think that the transition from ur-worm to earthworm is progression, while the transition from ur-worm to tapeworm is regression, but in reality both transitions are examples of evolution — populations changing in response to the environment. Earthworms and tapeworms (and humans and hobbits, if hobbits are real) all exist because they are better at surviving and reproducing than their now-extinct ancestors, sisters, and cousins. If some of them have lost the need to live outside a host organism, or to lug around a bloated, power-hungry brain, it doesn’t mean that they’ve “devolved”; it just means that they’ve evolved in a different way than we have.

    Stephen Jay Gould can be a bit of a snob, but I recommend his book Full House as a wonderful argument against the concept of evolutionary progress.

  10. #10 Elliot Kennel
    June 22, 2005

    Carl, a very insightful article, thank you! I’ve been following the homo floresiensis story since it broke in the popular literature. One thing that has interested me is the oral tradition surrounding the existence of diminutive humans in Rampasa. There had been speculation from anthropologists that homo floresiensis might have survived until quite recently based on tales from Liang Bua. Now it turns out that people still live in Rampasa and tend to corroborate testimony from Liang Bua. The Rampasa villagers claim to be descended from diminutive hairy humans who used stone tools. However, since the current villagers are 4 feet tall instead of 3 feet tall, their stories tend to be dismissed. The implicit assumption is that homo floresiensis DNA is incompatible with homo sapiens, and therefore if there are pygmies (or descendents of pygmies) today, they can not be related to the skeletal remains of pygmies from thousands of years ago.
    There is also a long-standing tradition that a dimutive hairy bipedal creature (known variously as orang pendek, orang pendak or ebu gogo) still exists in the forest. Of course this is treated with great skepticism as well.
    This is not my field at all, but I think perhaps the skepticism is a bit parochial. Certainly chimpanzees and orangutangs can coexist with humans. Why not another biped?
    Also, is it really possible to ascertain that homo floresiensis was a separate species and could not mate with humans, based on bone structure and appearance? Other species (say, dogs) seem to have even greater variation.
    So I wonder why several of the anthropologists seem to be content to discount the oral traditions
    that suggest that a race of small hairy primitive 3 foot tall humans once lived on Flores and eventually interbred and assimilated with normal humans. What is so heretical about these stories? I hope that some competent anthropologist will take these claims seriously and investigate them.

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