后生动物 (metazoa) 是属于动物类，虽然在埃迪卡拉纪时期的水母 (刺细胞动物 Cnidaria)， 被列在最早期的化石记录里，但是海棉 (海棉动物 Porifera) 有可能比水母还要更早进化。 (依据化石的研究记录) 多数被矿物化的动物门类之骨骼，最早出现在寒武纪时期, 其年代已超过5亿年之久。也有另一种听似有理的说词是，一些没有留下任何化石证据的生物门，也有可能在寒武纪时的生物爆发 (Cambrian explosion) 中进化了。凯文・彼得森认为，最早期的后生动物大概出现在5亿7300-6亿5600万年前。
连结网页
- The Museums of Cape Town have a marvelous introduction to the kingdom of animals (Metazoa) with photographs and descriptions of the various phyla.
- A page from U Cal Berkeley about the Metazoa fossil record suggests it may be possible that nearly all the phyla of metazoa evolved in the Cambrian aside from
the Cnidaria, which evolved earlier, in the Ediacaran (Vendian).
- Were the Cnidaria present in the Ediacaran Era, or did they evolve later, in the Cambrian. This article from the New York Times discusses fossil and DNA evidence that seem to suggest conflicting dates for Cnidaria (600 MYA or 540 MYA).
- The Museum Victoria in Australia has a page about the oldest evidence of life, including the earliest animal life.
- Another possible ancestor of early metazoa would be the bilaterians who lived 580-600 million years ago.