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W w w. s c ia m . c o m COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. SCIENTIFIC A MERIC A N 43 Beijing C H I N A Weng’an Chengjiang Guizhou Province Yunnan Province T WO DEPOSITS IN CHINA have preserved the remains of soft-bodied animals that provide new information about early evolution. In 2004 the author and his colleagues discovered the oldest known bilaterian animal in rocks collected from the 580-million- to 600-millionyear-old Doushantuo Formation, near Weng’an. Significantly younger fossils from the approximately 525-million-year-old deposits in the vicinity of Chengjiang have expanded understanding of the Cambrian explosion.

Pulp is visible inside the emerged tooth. Red stain colors dental hard tissues, highlighting enamel and dentin. Although lacking roots, the tooth is attached to surrounding jawbone by soft connective tissue. to induce the appropriate initiating signals for odontogenesis. Of the several potential cell sources, teeth themselves may be the most convenient. The Forsyth group’s results suggest that stem cells capable of forming tooth tissues, including enamel, could be present within teeth. Researchers elsewhere have also shown that dentin and other tooth tissues experience some natural regeneration after injury, which, too, suggests the presence of progenitor cells capable of generating a variety of tooth tissues.

Thus, the “explosion” of animal types was more accurately the exploitation of newly present conditions by animals that had already evolved the genetic tools to take advantage of these novel habitats rather than a fundamental change in the genetic makeup of the animals. B. AMADEO BACHAR The Cambrian explosion is generally thought of as a sudden increase in the types of bilaterian animals — those with a rightleft balance of limbs and organs. But the story is more complicated, and more interesting, than that.

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