Animal fossils are usually the remains of hard structures –
bones and shells that have been petrified through enormous
pressures acting over millions of years. But not all of them had
such hard beginnings. Some Chinese fossils were once the embryos
of animals that lived in the early Cambrian period, some 550
million years ago. Despite having the consistency and strength
of jelly, the embryos have been exceptionally well preserved and
the structure of their individual cells, and even the
compartments within them, have been conserved in all their
beautiful, minute detail. They are a boon to biologists. Ever
since the work of Ernst Haeckel in the 19th century, comparing
the development of animal embryos has been an important part of
evolutionary biology. Usually, scientists have to piece together
the development of ancient animals by comparing their living
descendants. But the preserved embryos give the field of
embryology its very own fossil record, allowing scientists to
peer back in time at the earliest days