Scientists have found fossil embryos that might be the oldest known remains of creatures with definite left and right sides--a group that includes people and the vast majority of modern-day animals.
The embryos are part of a spectacular trove of small but highly detailed fossils found in China. The fossils are about 570 million years old, give or take 20 million years, and come from an early and poorly understood stage in animal evolution.
The embryos are about the size of sand grains. They might be the earliest known remains of "bilaterians," which are animals that, unlike sponges or jellyfish, have definite left and right sides. That's "everything from worms to us," said Andrew Knoll of Harvard, one of the scientists reporting the findings in today's issue of the journal Nature. Another paper on the findings, from scientists in China and Taiwan, appears in Friday's issue of the journal Science.