Trends and Transitions in Early Animal Evolution: Towards a Palaeogenomic Synthesis  

The early history of animals on Earth is characterized by geologically long intervals with similar biotic trends punctuated by geologically short transitions to the next interval. 

Recent biomarker data places the origin of sponges in the Cryogenian before the end of the Marinoan glacial event (635 Ma). Microscopic fossils of metazoan affinity in the Doushantuo biota (580–600 Ma), and the lack of macroscopic animal fossils before the onset of the Ediacara biota, indicate that this first interval – which trends through the initial part of the Ediacaran – is one of microscopic animals, characterized by abundant sponges as well as the likely presence of animals with cnidarian and bilaterian affinities.  The transition to the Ediacara biota, which first appeared 575 Ma, marks the first evolution of macroscopic animals.  This interval, characterized by many intriguing and large soft-bodied animals such as the extinct rangeomorphs, also includes the first macroscopic bilaterians, such as Kimberella. The transition at the end of the Ediacaran into the Cambrian marks the initiation of the third interval, where the fossil record shows the appearance of biomineralized animals and a proliferation of crown group bilaterians.  Indeed, this interval should more properly be called the “Cambrian crown group explosion”.

These patterns of trend and transition from the fossil record of early animals are best understood within the context of changing environments, taphonomy and evolution.  Evolutionary processes determining size increase, and the proliferation of crown group bilaterians, each mark a major transition.  Their relative contributions need to be assessed against a backdrop where the various superclades into which modern metazoa fall were likely extant through much of the Ediacaran.  Ultimately, the combination of data from the fossil and stratigraphic record with modern studies in molecular biology will provide the paleogenomic synthesis necessary to most fully understand early animal evolution.