Charles Darwin worried about a possible hole in his theory of evolution, but some American scientists may just have plugged it. For about a billion years after the dawn of life on Earth, organisms didn't evolve all that much. Then about 600 million years ago came the "Cambrian explosion." Everything changed relatively quickly, with all kinds of plants and animals emerging—which doesn't quite seem to fit with Darwin's theory of slow change, hence "Darwin's dilemma." Now, within a few days of each other, two new studies have appeared that could explain the shift, ABC News reports. One, by scientists at...