Darwin's Predictions
Gaps in the fossil record will be filled in with key transitional fossils.
In his notebooks, Darwin worried about the many gaps in the fossil record that then existed, "like pages torn from a book." Perhaps the most glaring was the complete lack of fossils before the Cambrian Period (544-510 million years ago). "To the question why do we not find rich fossiliferous deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer," he wrote. If his theory of evolution was right, he knew such gaps would eventually be filled in. Only in the past half century have paleontologists unearthed fossils from the Precambrian, and it's a huge and ever-growing trove, with the oldest fossils dating to about 3.5 billion years ago. Moreover, they continue to unearth key fossils that bridge those gaps Darwin bemoaned, including Archaeopteryx, shown here. To see a range of such transitional fossils, go to Fossil Evidence.