In South American, Darwin found fossil species that were clearly related to modern armadillos, yet neither the fossils nor the living animals were found anywhere else in the world. In The Origin of Species, he explained that ''the inhabitants of each quarter of the world will obviously tend to leave in that quarter closely allied though modified descendants."

Before the start of the Cambrian period about 550 million years ago, multicellular organisms lacked hard parts like shells and bones and rarely left fossils. However, a few pre-Cambrian organisms left traces of their existence. Some ancient rocks contain stromatolites—the remnants of bacteria that grew in columns like stacked pancakes (right). top, a fossil just predating the Cambrian shows the outlines of a marine invertebrate that might have resembled a jellyfish.