The Cambrian Explosion of Animalia is still perplexing to modern paleontology. The relatively sudden appearance of animal lifeforms that occurred at this point in Earth's history remains an evolutionary mystery. Various hypotheses have been put forward to try and account for rapid outburst of animal phyla at this time but no agreement has yet been reached. After billions of years of single-cellular evolution why did animals suddenly appear in the Cambrian fossil horizons? Where were the Cambrian ancestors?
Charles Darwin hoped that the problems of Cambrian ancestry would be resolved by the future identification of pre-Cambrian fossilized remains. This hope was realised in 1957 by the discovery of Charnia masoni, a multi-cellular, complex organism found within the Precambrian strata of Charnwood forest - this strata became known as the Ediacaran period (635 - 543 Mya). Subsequently many organisms from this period have been found around the world and some of those are nowthought to be specimens of the first animals.
It has been hard for paleontologists to trace any clear lineages between Cambrian and Ediacaran organisms and the differences in form and complexity remain stark over such a relatively short evolutionary period.
Molecular phylogenetic studies indicate the presence of ancestral organisms to the Cambrian lifeforms - reaching even further back in time than the Ediacaran period – essentially meaning that even if we can’t see their fossilized traces we can know that they were there. Nevertheless the question persists: Why no remains?
The relatively complex animals that must have preceded the Cambrian period may well have been soft bodied and difficult to preserve but to leave no trace is problematic. The stratigraphic record appears to have been almost wiped clean, so much so that it has been speculated that a massextinction event occurred between the Ediacaran and Cambrian periods wiping out all signs of earlier animal life.
One possible solution to this puzzle might be to assess where Precambrian animals might have existed on the planet. Is it possible that the first stages of the evolution of animal life might have been confined to deep ocean trenches?
Throughout its turbulent history the most stable environments on the planet have been found withindeep ocean trenches. It is hypothesised that life itself may have originated in or around deep sea vents. Even if life originated elsewhere it could still be the case that complex organisms were able to evolve in these relatively stable environments - feeding on the energies and minerals that are constantly emitted from the hydrothermal vents. From here such organisms would even be able to radiate around the planet along the network of deep ocean trenches. Over time these same organisms might have radiated onto the emerging continental shelves and once there havebe able to generate fossilized remains. Prior to that all remains would have fallen to the deep ocean floor.
The ocean floor is surprisingly young in geological terms (~125 Mya) due to the processes of the Deep Carbon Cycle – resulting in large portions of oceanic crust being subducted back down into the Earth’s mantle. Any organism that does become preserved within oceanic strata will correspondingly be subducted into the mantel. Speculatively the elusive pre-Cambrian organisms might have existed within these deep sea zones - evolving and radiating around the world - but have left no trace.
Over the last 150 years much headway has been made on these fundamental questions but they still remain unresolved. Determining how the Cambrian biota arose in such disparity and diversity remains pivotal in our understanding of the mechanics of evolution and consequently the Ediacaran to Cambrian transition continues to be a focal point for evolutionary biology.
