More than 600 million yearsa go, the Earth was mostly covered in ice RICHARD BIZLEY/SCIENCE PHOTO LIBRARY
Signs of a global catastrophe about 680 million years ago, known as Snowball Earth, have been found in the DNA of living bacteria in the oceans. Their genomes show that they nearly died out around this time, says Haiwei Luo at The Chinese University of Hong Kong.
“Their population size became hugely decreased,” says Luo.
Today, tiny photosynthetic bacteria called Prochlorococcus are incredibly abundant in the surface waters of oceans. A litre of seawater can
