Out on the savannah, it is easy to find certain resources as they tend to be concentrated in limited areas. Trees, termite mounds, changes in terrain, all contribute to this concentration of resources. It is this sort of environment which has been hypothesised as resulting in our own bipedality which enabled human ancestors to move efficiently between resource hotspots. In a recent review, Budd and Jensen have proposed a ‘savannah’ hypothesis as an explanation for the evolution of bilaterian animals and consequently the Cambrian explosion.
Bilaterian animals are widely thought to have driven the Cambrian explosion, particularly as they function as ecosystem engineers altering the environment around them. The burrowing activities of bilateral organisms altered ocean chemistry and the nature of the sediment, opening up more resources to be exploited and resulting in a cascade of diversification. But why did bilaterians evolve in the first place? Ecological causes for the Cambrian explosion tend to presuppose features they should be explaining, such as the ability to burrow or the presence of predation (both likely contributed enormously to the diversification, but were also caused by it). Environmental causes tend to suggest limiting factors such as a lack of oxygen, which may actually be mistaking cause for effect.
The savannah hypothesis suggests that the Ediacaran biota also functioned as ecosystem engineers, causing carbon hotspots in the sediment and water around the organisms which were exploited by bilaterian animals which went on to diversify, eventually displacing their Ediacaran providers. Dissolved organic carbon in Ediacaran seas would not likely have clumped together, instead being spread out through the water column – not an economical resource for active organisms. Burrowing is highly energetic and would require dissolved carbon to be concentrated; without the Ediacaran organisms it would have been too diluted and sequestered away by the abundant microbial mats. Just like trees on the savannah, the Ediacaran biota concentrated dissolved organic carbon, providing sufficient resources for active burrowing and the need for motility.
In their thorough review, Budd and Jensen challenge the view that Ediacaran organisms went extinct by the start of the Cambrian period having been outcompeted and devoured by bilateral organisms. Instead, they survey putative evidence that shows that bilaterians first appeared towards the end of the Ediacaran period and that Ediacaran-type organisms persisted well into the Cambrian (and perhaps longer). At first, bilaterians would have been dependent on the Ediacaran ecosystem engineering, but went on to evolve their own sessile forms, such as crown-group sponges, and predatory habits which made them a threat to the Ediacaran biota – comparable perhaps to humans in the Terminator franchise creating Skynet and setting up their own demise.
They also reviewed the phylogeny of basal animals and take the view that sponges form a single clade which is the sister group to all other animals. They coined the term “Apoikozoa” which encompasses all animals and their sister group the choanoflagellates. And they made a case for Ediacaran organisms being early animals, albeit hugely problematic, whilst being highly critical of some of the optimistic interpretations. It is a paper which has provided a lot to mull over.
Budd, GE. and Jensen, S. 2015. The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews. doi: 10.1111/brv.12239