oldest fossilized evidence of animals

The oldest fossilized evidence of animals has been unearthed in Oman and reveals that tiny sea sponges were abundant 635 million years ago, long before most of the planet's other major animal groups evolved, according to a new analysis.

The ancient demosponges — probably measuring across no more than the width of a fork tine — were pinned down via fossilized steroids, called steranes, which are characteristic of the cell membranes of the sponges, rather than via direct fossils of the sponges themselves.

The Cryogenian is a geologic period dominated by glaciers and occurring prior to the Cambrian, when the fossil record shows a so-called explosion of most major groups of complex animals, including all the bilaterians (animals with a left and right side, unlike corals and sponges) — around 530 million years ago.

Prior to that, most organisms were simple. In fact, prior to the Cryogenian, the vast majority of marine organisms would have been single-celled bacteria and microalgae, some of which might have been multicellular as far back as 1.2 billion years ago, Love said.

Fossils of microbe colonies currently are evidence of the first known life on Earth, dating back to 3.5 billion years ago. Thereafter came a long period with little major development.

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