One of the fundamental assumptions about what it takes to be an animal could well be wrong.
Possessing Hox genes has long been considered a defining characteristic of all animals: they control the development of body shape, instructing cells in an embryo to become an arm, eye or rib, for instance, based on where they are along the central body axis.
But now it seems that Cnidaria, such as the sea anemone Nematostella and a hydromedusa Eleutheria, have no Hox genes at all, says David Miller of James Cook University in Queensland, Australia. They still manage to shape themselves, but may use different genes, and a more laborious process (Current Biology, vol 16, p 4682).
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That suggests Hox genes evolved later than previously thought, and were not required to create the diverse forms of animal life we see today. 鈥淲e have shown that there is no absolute requirement of a Hox system to drive morphological diversity, because Cnidarians vary enormously in shape and size,鈥� says Miller. Instead, Hox genes probably evolved among bilateral animals, including humans, sometime after they diverged from the Cnidarians. That is thought to have taken place around 600 million years ago.
