Carl Zimmer in the New York Times:
Lurking in the blood of tropical snails is a single-celled creature called Capsaspora owczarzaki. This tentacled, amoebalike species is so obscure that no one even noticed it until 2002. And yet, in just a few years it has moved from anonymity to the scientific spotlight. It turns out to be one of the closest relatives to animals. As improbable as it might seem, our ancestors a billion years ago probably were a lot like Capsaspora.
The origin of animals was one of the most astonishing and important transformations in the history of life. From single-celled ancestors, they evolved into a riot of complexity and diversity. An estimated seven million species of animals live on earth today, ranging from tubeworms at the bottom of the ocean to elephants lumbering across the African savanna. Their bodies can total trillions of cells, which can develop into muscles, bones and hundreds of other kinds of tissues and cell types.
The dawn of the animal kingdom about 800 million years ago was also an ecological revolution.
Animals devoured the microbial mats that had dominated the oceans for more than two billion years and created their own habitats, like coral reefs.