AN UPDATED family tree of the animal kingdom could radically change the way we think about the evolution of species.

According to conventional thinking, simple animals, including sponges, jellyfish and corals, evolved step-by-step in a linear fashion into those with more complex bodies, such as mammals.

Now Rob DeSalle of the American Museum of Natural History in New York and his colleagues have challenged this way of thinking.

The team analysed DNA and other molecular evidence across the animal kingdom, including tiny sea creatures called placozoans. They have found that the placozoans are the closest living thing to the ancestor of all animals.

More intriguingly, these simple creatures belong to a group of organisms that, as DeSalle's team discovered, evolved in parallel to those that later developed into humans.

Since this deep division was forged half a billion years ago, before animals developed nerves, the finding implies that the nervous system developed twice - evolving independently in simple organisms like jellyfish and also in complex animals.