Despite being ripe for complex life, it took another 1.5 billion years for oxygen-hungry animals to evolve
Andrew Knoll/Harvard University
Earth was momentarily ripe for the evolution of animals hundreds of millions of years before they first appeared, researchers propose.
Chemical clues in ancient rocks suggest that 2.32 billion to 2.1 billion years ago, shallow coastal waters held enough oxygen to support oxygen-hungry life-forms including some animals, researchers report the week of January 16 in the Proceedings of the National Academy of Sciences. But the first animal fossils, sponges, don’t appear until around 650 million years ago, following a period of scant oxygen known as the boring billion (SN: 11/14/15, p. 18).
“As far as environmental conditions were concerned, things were favorable for this evolutionary step to happen,” says study coauthor Andrey Bekker, a sedimentary geologist at the University of