A CURIOUS hiccup in the evolution of multicellular animals may never have happened. Radiometric dating of rocks in Namibia suggests there was no lull between the heyday of the Ediacara fauna in the Precambrian and the earliest representatives of the 鈥淐ambrian explosion鈥�, which gave rise to most of the modern animal phyla.
According to John Grotzinger of the Massachusetts Institute of Technology, writing in last week鈥檚 Science, the two groups may even have represented 鈥渁 single, protracted evolutionary radiation鈥� (vol 270, p 596).
Grotzinger鈥檚 conclusions would have helped Charles Darwin, who was puzzled by the sudden appearance of hard-shelled fossils at the start of the Cambrian about 543 million years ago and the lack of evidence of any evolutionary ancestors. In recent decades, scientists have discovered fossils of some of these missing links: the Ediacarans, and a previously overlooked group dubbed small shelly fauna, thought to be early representatives of the Cambrian explosion.
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However, geologists had thought that tens of millions of years separated the flattened and soft-bodied Edicarans from the shelled fossils that followed them. And because the two groups looked so different, some researchers suggested that the Ediacara fauna were a 鈥渇ailed experiment鈥�, with no connection to the Cambrian explosion.
Grotzinger, working with MIT colleague Sam Bowring, decided to date volcanic rocks from Namibia which lie between strata containing Ediacara fossils and found that they were much younger than they had expected. Most of the fossils 鈥� were younger than 549 million years old. Grotzinger also found two new 鈥渢ypical Ediacara fossils鈥� in Namibia 鈥� the first ever found virtually at the Precambrian-Cambrian boundary.鈥�
Grotzinger鈥檚 also discovered two 鈥渃ard-carrying members of the small shelly fauna鈥�, lying below the Precambrian/Cambrian boundary. 鈥淭he closer you look, the more it appears to be a gradational boundary, with nothing truly sudden happening there,鈥� he says. But there are residual doubts about whether the Ediacara fauna could represent merely the first stage of the Cambrian explosion. Even Grotzinger says there are other possibilities, including extinction of the Ediacara fauna at the boundary. However, the new study does remove the troublesome gap between two bursts of evolutionary innovation.