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October 13, 2009

BASICS

In Mam mals, a Complex Journey to the Middle EarBy NATALIE ANGIER

magine what a dinner conversation would be like if you had decent table manners, but the ears of a liza

Not only would you have to stop eating whenever you wanted to speak, but, because parts of your ears a

now attached to your jaw, youd have to stop eating whenever you wanted to hear anybody else, as well.

With no fork action on your end, your waiter would soon conclude that you were obviously done workin

on that and would whisk your unbreached baked ziti away.

Sometimes its the little things in life that make all the difference in this case, the three littlest bones o

he human body. Tucked in our auditory canal, just on the inner side of the eardrum, are the musically

named malleus, incus and stapes, each minibone, each ossicle, about the size of a small freshwater pearl

and jointly the basis of one of evolutions greatest inventions, the mammalian middle ear. The middle ea

gives us our sound bite, our capacity to masticate without being forced to turn a momentarily deaf ear t

he world, as most other vertebrates are. Who can say whether we humans would have become so

voraciously verbal if not for the practice our ancestors had of jawboning around the wildebeest spit.

The middle ear also explains why mammals, as a group, have the sharpest hearing on Earth and thegreatest diversity of listening styles, from the bats and dolphins that can detect pressure waves bouncing

around at the spiky, ultrasonic end of the bandwidth, to elephants and humpbacked whales that can hea

nfrasonically, capturing the long, low sound prints muttered by their peers for miles around. All told, a

new study suggests, the middle ear was such a great invention, such an essential part of being a mamma

hat once evolution had seized upon it, no crude substitute or older model would do.

n the current issue of the journal Science, paleontologists report on the fossilized remains of a newly

discovered mammal from the Mesozoic era, some 123 million years ago. The snouty, 3-ounce, chipmunk

ize animal, named Maotherium asiaticus, lived in what is now northeastern China and darted around theet of the dominant dinosaur overlords, as mammals had been doing for some 100 million years.

What makes this proto-Alvin noteworthy are its ears: the crisply preserved fossil indicates that

Maotheriums middle ears were of a middling sort, half mammal and half reptile. Given the timing of th

ossil, and evidence from still earlier mammalian fossils, the researchers believe that Maotherium

epresents backsliding, a reversion toward a more ancient auditory design.

But not to worry. Evolution would rediscover its three-part invention at least once, if not again and agai
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or by the time the ancestors of todays mammals arose, the trait had been set. Every one of the 5,400 or

known species of mammal whether placental, pouched marsupial or weird outlying, egg-laying

platypussian monotreme has three ossicles freed from the tyranny of the mandible, freed to be all ear

all the time.

For those of us dealing with mammalian evolution, the evolution of the middle ear is a holy grail, said

Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh

and an author of the new report. Sensitive hearing made it possible for early mammals to coexist with tdinosaurs; it was really a matter of life and death. The new fossil, he said, gives us insight into the

omplex evolutionary process behind this central mammalian feature.

Nature walks in delirious ways, said Neil Shubin, author of Your Inner Fish and a professor of organis

biology and anatomy at the University of Chicago. This paper shows how the mammalian ear didnt

proceed in a linear progression, he said. Either it evolved multiple times independently, or it flipped b

and forth, but in any event were talking about a bush with many buds and twigs.

The finding also dovetails with recent work in molecular genetics and developmental biology. Amongmodern mammals, the middle ear of a fetus is one of the last structures to mature and migrate to its pro

position, and even after birth the little bones may retain a few lingering filaments of so-called Meckels

artilage, which connects the ear to the jaw in the early embryo. Dr. Luo and his colleagues suggest that

mutation to a developmental timing gene responsible for this late-stage disengagement might have

ssentially locked Maotheriums ears into a permanent embryonic state, just as can happen with rare

human craniofacial disorders like Treacher Collins syndrome. Fossil hunters, developmental biologists,

medical geneticists, were all meeting eye-to-eye, Dr. Luo said.

Researchers have long known that the middle ear bones of a mammal evolved from the jawbones of theieptilian forebears, and that the repurposing of the bones, as Dr. Shubin put it, for the sake of improve

hearing occurred in parallel with the refinement and elaboration of mammalian dentition. You go from

aw composed of many bones in reptiles to a jaw with a single bone in mammals, Dr. Shubin said. You

go from simple, generally conical reptilian teeth arranged willy-nilly in the mouth to matched sets of

molars, bicuspids and canines that come together in happy occlusion. You see mammals actually chewi

heir food, said Christian A. Sidor, an expert in mammalian evolution at the University of Washington.

n other words, where a mammals streamlined mandible might have lacked the snapping power of a

rocodile jaw, its complex teeth could pick up the feeding slack and free the extra jaw bones to take on

higher tasks.

Or rather, higher frequency tasks. Researchers propose that one selective pressure driving the evolution

he mammalian middle ear might have been the need to find a steady supply of insects, and even non-

nsectivorous humans remain exceptionally sensitive to high-pitched noises like a mosquitos whine.

The mammalian ear is also a master at detecting very quiet sounds: as an incoming pressure wave wiggl

he ear drum and the ossicles abutting it, the fulcrumed motions of the bones help amplify the waves
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nergy before sending it along. Early mammals were probably nocturnal, the better to avoid day-huntin

dinosaurs, and even today most mammals prefer to come out after dark. It was fundamental for mamm

hat their elaborated sensory system was one well-adapted for a nocturnal niche, Dr. Luo said.

Or for dinner by candlelight with a few silver-tongued friends.

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