Scientists discover ancient link to human ears
The new Cretaceous mammal Maotherium had terrestrial habits and scampered around on the ground. From its skeleton it is estimated to have weighed about 2 ounces, and was about 5 inches long. Scientists digging in northeastern China say they have discovered the fossil that could help explain how human hearing evolved. Zhe-Xi Luo, a curator at the Carnegie Museum of Natural History in Pittsburgh, was one of the study's authors.
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Around 123 million years ago, a small, chipmunk-like mammal that paleontologists are now calling Maotherium asiaticus roamed northeastern China, foraging for worms and other small insects until it and others in its lineage became extinct.
But a discovery of a fossil of the tiny creature -- unimpressive when compared to its dinosaur contemporaries -- has captivated a local paleontologist, who says the animal's microscopic ear bones may help those in his field figure out how the delicate and sophisticated middle ear of humans and other modern mammals developed.
Dr. Zhe-Xi Luo of the Carnegie Museum of Natural History was part of a team of scientists announcing the discovery of the prehistoric mammal. An article authored by Dr. Luo and other scientists appears in the latest issue of Science, released today.
Paleontologists believe that the mammalian middle ear bones evolved from a piece of the jawbone of the mammal-like reptiles that preceded them.
This theory is based, in part, on the way humans and other mammals develop. In reptile embryos, a piece of cartilage, called Meckel's cartilage, becomes hardened (ossified) and fuses with the jaw in reptiles, connecting structures in the ear to the jaw. But in humans and other mammals, this same structure exists in fetuses but disappears before they are born.
This most recent discovery of Maotherium is distinctly a mammal, but it has the ossified Meckel's cartilage of a reptile.
"If you use modern mammal as a standard, Maotherium is an important exception," Dr. Luo said.
It's the second time a mammal fossil has been found to have ossified Meckel's cartilage, but the last discovery, also made by a team of scientists that included Dr. Luo, was a far more distant relative of modern mammals. Dr. Luo likened this most recent discovery to a "cousin" of modern mammals.
Interestingly, Dr. Luo said the geneticists have been able to resurrect this particular feature -- ossified Meckel's cartilage -- by tinkering with the genes of lab mice. It also occurs pathologically in some mammals. In humans, it occurs with an extremely rare genetic condition called Treacher Collins syndrome.
The fact that what Dr. Luo observed in a mammal that roamed the earth 123 million years ago can be mimicked by altering a mammal's genetics demonstrates that evolution had a strong genetic component, Dr. Luo said.
"We have found the two ends of biology coming together," he said. "It basically shows that the same developmental program [was] also operating in the early mammal evolution."
Paleontologists are especially interested in how mammals developed such sophisticated hearing, because it's likely what helped them survive and diversify after the dinosaurs went extinct. Specifically, mammals have much more acute hearing than reptiles, allowing them to function at night, even when their vision fails them. And because their jawbones are separated from their ear bones, they are able to chew and hear simultaneously.
Dr. Luo called this ability "one of the most critical factors" that contributed to mammals' survival.
"We [mammals] managed to have a huge comeback after dinosaur extinction," he said. "All that is attributable to this origin of hearing that is so advanced in mammals."
First Published October 9, 2009 12:00 am