Becoming a body

The question

Animals are made of many cells that stick together, signal to each other and divide up the work. How did the ancestors of animals go from single cells to that kind of cooperation?

What the evidence shows

• The genome of a choanoflagellate, one of the closest living relatives of animals, contains genes for proteins that let cells adhere and signal. Much of the toolkit for building a body existed before animals did.
• Multicellular life of some kind is very old: large multicellular fossils are known from about 1.56 billion years ago, though what they were related to is uncertain.

What is still unknown

• No fossil records the step itself. Exactly when, where and how animal ancestors made it is unknown.
• The date shown on the atlas is a placement window between two eras, not an estimate of when animals began.

Sources

• King et al. 2008. The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans. Nature. (opens the publisher’s site)

  What it supports: Choanoflagellates, the closest living relatives of animals, carry genes for cell adhesion and signalling that were once thought to be unique to animals. They are living cousins, not unchanged ancestors.

• Zhu et al. 2016. Decimetre-scale multicellular eukaryotes from the 1.56-billion-year-old Gaoyuzhuang Formation in North China. Nature Communications. (opens the publisher’s site)

  What it supports: Large, regularly shaped multicellular fossils about 1.56 billion years old. Their place in the tree of life is uncertain, and they are not animals.

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