Though the fossiliferous explosion known as the Cambrian period is often thought of as the beginning of animal life, animals actually appeared for the first time during a geological twilight called the Ediacaran, 635m-542m years ago, between the end of a great ice age and the arrival of all those well-preserved Cambrian fossils. The object on the left, less than a millimetre across in reality, is an example of what were once thought to be the encysted forms of Ediacaran algae. However, its similarity to the object on the right, which is the egg of a modern species of crustacean, has led Phoebe Cohen of Harvard University and her colleagues to suggest in the Proceedings of the National Academy of Sciences that it, too, is the egg of an animal. Which animal, exactly, is not clear. No crustaceans are known from the Ediacaran. Common adult Ediacarans included jellyfish-like critters and things that resemble flattened grapefruit segments. How these are related to the rest of animal-kind is not known. But somewhere among them must lurk one of humanity's ancient ancestors.

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