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 Yale and the Georgia Institute of Technology, suggests that oxygen levels may have been far less plentiful in the atmosphere prior to the Cambrian explosion than experts had thought.
The air may only have been .1% oxygen, which couldn't sustain today's complex organisms, indicating a shift had to happen before the "explosion" could take place. In a separate study, a University of Texas professor explains where that oxygen burst may have come from: a major tectonic shift. Based on geological evidence, Ian Dalziel believes what is now North America remained attached to the supercontinent Gondwanaland until the early Cambrian period, in contrast with current belief, which has the separation occurring earlier. That shift would have put more oxygen into the atmosphere, per a press release, and brought nutrient-rich water from the deep ocean to shallow waters, helping to foster new life forms, LiveScience reports. (Here's how cotton balls helped save Darwin's finches.)