Early in the Earth’s history, oceans and seas were relatively low in oxygen.... A huge pulse of the gas, around 800 million to 540 million years ago, brought atmospheric oxygen levels somewhere within 10 and 50 percent of current levels.... Ms Turner first discovered the network of tubes in 1992, but moved on, citing a lack of evidence to conclude they were sponges besides some general resemblance. She said: “ I found this thing that was totally out of place....