A new PNAS publication describes challenging consequences of low oxygen to animal life. In the time leading up to the emergence of animals, from about 1.8 billion to 600 million years ago, the oxygen distribution in the oceans would have been extremely patchy, and even well-ventilated, shallow-water oases near the coasts would have been seasonally choked by anoxia or even toxic, sulfide-rich waters rising up from the deep. "So for part of the year they would be great places for animals to live, but for part of the year they would have been really inhospitable," Reinhard explained to 'Christian Science Monitor.' These results are now a centerpiece in the team’s assertion that low oxygen in the ocean and atmosphere may have challenged the rise of complex life for hundreds of millions of years.