Widespread tetraradial symmetry among early fossil sponges
Sponges are usually considered to be the oldest living animals, having evolved before all other groups. The simplicity of their body structure and tissue organization has for many years made them candidates for the ancestral group of animals, and they have long been regarded as our best illustration of what the earliest animals would have looked like. This has been supported by genetic analyses, which suggest that sponges branched from other animals a very long time ago, deep in the Precambrian. Until recently, most zoologists believed that sponges were little more advanced than a colonial protist, with tissues and organs never fully developed. However, in recent years there has been an increasing appreciation of the complexity of their genetics and biochemistry, and this has cast doubt on the impression of simplicity. Now, new evidence from the fossil record confirms that early sponges were in some ways more complex than their...