If you could travel 600 million years back in time, 1 one animal on Earth. In fact, you would be the only creature bigger than the head of a pin. 2 the Earth had existed for more than 4 billion years by that time, only the simplest, tiniest kinds of life 3 their home.
Then animals suddenly appeared. Ancient relatives of nearly every species that has ever 4 showed up between 543 million and 510 million years ago, during a time called the Cambrian period. The Earth’s animal population grew so 5 this period that it is called the Cambrian explosion.
(1) It’s hard to believe that the 6 are the ancestors of modern species. (2) 7 (3) It would then crush them in its circular jaw, which snapped shut like a camera’s shutter. (4) A fossil of Hallucigenia, another puzzling creature, had pointy spines. (5) Understandably, it took scientists years to figure out that they were looking at it upside down! (6) They jutted up from its back and resembled legs. (7) And Dr. Seuss could have invented Opabinia, a five-eyed creature with a hoselike snout. 
Fossils of Cambrian animals have been around for nearly a century. But until recently, fossils from the late Vendian period – the period immediately preceding the Cambrian 9 nowhere to be found. It was as if some pages were missing from life’s history book. Over the past few years, 10 fossils from this mysterious period have been found in ancient rocks in Africa and Siberia.
The fossils show that animals grew more complex in the Vendian period. Some were small and wormlike. Others 11 They all floated or crawled in the sea. These developments were important steps on the road to the Cambrian 12 earthquakes, ice ages and other big changes in climate also played a role in triggering the burst of new creatures.
 The view that it took hundreds of millions of years for new creatures to develop has been popular ever since Charles Darwin published his famed theory of evolution 14 Now, though, some scientists think there is another kind of evolution that happens in a geological flash: new research shows that the Cambrian explosion occurred in less than 10 million years. To find more evidence, they will keep studying the weird wonders of the Cambrian and the late-Vendian times. That’s where all the action is. 
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