1 ene 630000000 año aC - animals evolve bilateral symmetry for the first time

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Around this time, some animals evolve bilateral symmetry for the first time: that is, they now have a defined top and bottom, as well as a front and back.

Little is known about how this happened. However, small worms called Acoela may be the closest surviving relatives of the first ever bilateral animal. It seems likely that the first bilateral animal was a kind of worm. Vernanimalcula guizhouena, which dates from around 600 million years ago, may be the earliest bilateral animal found in the fossil record.

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