Bilaterians, the kind of creature we are most familiar with, have dominated the Animal Kingdom for well over half a billion years. But during the long period before their big breakthrough, they were living in the shadows. For tens of millions of years, they were just an odd collection of tiny bottom feeders, crawling over, in and through the muck. However, despite its diminutive origins, once bilateral symmetry evolved, it was poised and ready to seize the stage once the circumstances were right. This moment in time is known as “the Cambrian Explosion”.  But how exactly did bilaterians manage to take over in the end? In other words: What kickstarted the Cambrian Explosion here on Earth and can we expect the same thing to happen in alien biospheres? Was the Cambrian Explosion destined to happen at all or just a fortunate fluke?  Would bilaterally symmetrical animals always evolve on Earth-like worlds or were we just lucky? Let’s try to find out!

The principle of Convergent Evolution denotes the many general tendencies in the history of life on Earth leading to superficially similar solutions for the same challenges. Still, as far as we can see, there’s only a limited set of universal optimums to distinguish. One of the most fundamental of these is bilateral symmetry making a unidirectional creature. 

Bilateral symmetry of simple bilaterians

And bilateral symmetry is intimately linked to a single major event in life’s history: The Cambrian Explosion, which was when bilaterians took over the world. To get an idea of how and why bilaterians were suddenly so successful, we first need to know more about the environment of the period preceding the Cambrian. For about a 100 million years, the climate had finally been stable enough for life to thrive. Prior to that, the Earth had been in the grip of global ice ages, during a period known as the Cryogenian, which lasted 85 million years. At long last, the world’s oceans had thawed again hosting plenty of life, but not as we know it. During this early, stable period called the Ediacaran, Microbes dominated the Earth. The water column was filled with algae and the bottom was covered with a thick, living goo, referred to as microbial mats and consisting mostly of bacteria. If you ever had to clean pink slime from your shower sink, you may be familiar with it. 

Microbial Mats
Microbial Mats – Image credit: Penny Higgins @ Flickr.com

And in this slimey world of plenty, the first animals were becoming ever more visible. Or at least: A plethora of life forms that seemed animalian and which arose during an even earlier radiation called the Avalon Explosion, only to disappear again by the Cambrian. The fossil record of this period is littered with strange and often large shapes made of repetitive, fractal patterns resembling quilts and fronds

“Avalonian (Ediacaran) Rangeomorphs” by Charlotte G Kenchington | source

What these early creatures were exactly is still unknown, but an emerging view is that these Ediacarans were a kind of proto-animals lacking internal digestion. Perhaps only having a single tissue layer and thus no gut, these creatures were feeding through diffusion from the surrounding water instead. The frond-like forms stood up in the water column from which they may have picked out nutrition. The flat quilt-like forms covered bottom surfaces in close connection with the microbial mats. Here, they could have used external digestion to dissolve the mats and absorb the nutrients. If this view holds true, these were a kind of Alternate Animals living a bit like fungi, but with true tissue layers rather than a dense network of cells. 

There are also signs that the microbial mats were starting to be actively grazed by a very different kind of creatures; Creatures that moved forwards… in a single direction. And directional movement is a tell-tale sign of bilateral symmetry. So bilaterians already existed during the Ediacaran, yet -apart from many trace fossils- we only found very few fossil remains so far.  They seemed to only have played a minor role for the time being, so what eventually caused them to take center stage during the Cambrian Explosion? 

Some postulate that the Cambrian Explosion was kickstarted by the development of eyesight. A key innovation like that would then enable the emergence of macro-predators, that could hunt more actively for prey, and thus start an accelerating arms race. But that in itself doesn’t make a whole lot of sense. 

Many steps are required before you have eyes that are even remotely functional for predation. And for that you’d need a selection pressure sustained over a prolonged period of time. It’s more plausible that the selective feedback pressure pushing for the improvement of eyes, limbs, armour and more, came when the Cambrian Explosion was already well on its way. Besides, bilaterians had already diversified into different phyla long before the Cambrian, so something else must have kept them small and diminutive until then.  

Perhaps then, something did change in the global environment to trigger it in the first place. There is reason to believe that there was simply too little free oxygen in the sea water, and too few vital minerals in circulation, to support larger and more active animals. Minerals like calcium for instance are essential for both metabolic processes and for growing supportive and defensive structures like skeletons and shells. Global warming may have led to rising sea levels which exposed more rocks to water erosion and thus more minerals were released into the water. At the same time, this sea level rise would greatly enlarge the area of shallow waters for benthic life to inhabit and diversify in. This also coincides with the breakup of the ancient Rodinia continent adding more shoreline too. 

But the most plausible explanation may actually be a combination of both environment and evolution. As the climate stayed stable for long enough time, perhaps animals simply became progressively better at grazing algae and churning sediments, a process known as bioturbation.  Trace fossils showing the systematic grazing patterns of early bilaterians, actually increase in diversity and complexity in the period leading up the Cambrian. The nervous systems of bilaterians were apparently gradually being upgraded, perhaps in conjunction with other adaptations to the feeding apparatus and digestive system. 

Trace fossils and ichnotaxa from the Ediacaran and early Cambrian” by Muscente et al (2017)

The effect of this would be twofold: First of all, minerals and nutrients formerly locked in sediments and rocks covered by biomats would be released into the water, greatly affecting oceanic chemistry. Second of all, with the biomats slowly disappearing, this increased scarcity would lead to even more competition, driving natural selection towards the evolution of larger and more active animals. In fact, all these different factors may have been involved in a self-amplifying feedback-loop driving an exponential development that started out slow, but accelerated towards an explosive spike. 

The complex of factors initiating the Cambrian Explosion. Adapted from: Smith & Harper (2013)

So now that we have a better idea of what may have led to the Cambrian Explosion, let’s leave Earth for a moment and ponder the prospect of this happening elsewhere in the universe. 

Without going too much into detail right here and now, by the looks of it, all Earth-like biospheres will likely develop in a very similar way, assuming life will originate in water first and foremost. Naturally, these worlds will always have two primary types of aquatic environment. The so-called pelagic zone is the open water column inhabited by free-floating or actively swimming lifeforms. The benthic zone are those areas on, near or within bottom substrates like sediments and rocks. 

Like on Earth, alien biospheres too will have both pelagic and benthic zones in aquatic environments.

 There will also always be two primary ways for lifeforms to acquire energy, called trophic groups.  There are the autotrophs that harvest energy from abiotic sources like sunlight and there are the heterotrophs that feed on other life forms or their remains. And for heterotrophs, there will be two main strategies for collecting food: Passive and active. 

Now life on these worlds would presumably start out small, leading eventually to a global biosphere very much like the Precambrian Earth: The pelagic zone will likely also be dominated by small, free-floating, photosynthesizing creatures akin algae or phytoplankton, in turn fed upon by microscopic zooplankton. Shallow bottoms will be covered by assemblages of microbes which will inevitably cake together into microbial mats, giving the same stagnation of life as on Earth. 

Let’s call this presumed universal phase the Microbial Stage

A generalized biosphere in its initial “Microbial Phase”: Plankton dominates the pelagic and biomats cover benthic substrates.

Another clear trend that we’ll expect to see are attempts at increased size in some lineages. There are obvious advantages to evolving larger sized individual units, because a larger organism is harder to push around or ingest, and the other way around. However, there are natural limitations to the size of any basic living system and key adaptations are required to get around these, which will take time to come up with during natural evolution. 

Living creatures in such biospheres will continually try out different strategies for survival, be it small size or large size, specialist or generalist, passive or active. All different kinds of creatures may appear over the aeons, but will not automatically take over. For a long period of time, the original microbial life forms will inescapably put a temporary damper on any further developments. Typically, biomats will lock in minerals and suppress oxygen concentrations in the open water to levels lower than needed for larger, more active animals.  

Now, whether Earth’s Cambrian Explosion was triggered by evolutionary innovations, by environmental factors or a combination, may actually be irrelevant. No matter what, it’s just a question of time before some major upset enables larger and more active creatures to get the upper hand and break through the microbial hegemony. So, just like on Earth, life elsewhere may generally be expected go through these phases: 

  1. A Microbial Phase with microflora and -fauna 
  2. An Avalonian Phase with simple and mostly passive macro-fauna like during Earth’s Ediacaran. 
  3. A Cambrian Phase with advanced, active macro-fauna ending microbial domination. 

Whether a biosphere will always go through an Avalonian phase is uncertain, but active, bilaterally symmetrical animals will get the upper hand sooner or later, given enough time. And that is because -by their very nature- they are prone to develop the smarts and the tools needed to reach new potentials. As a most basic configuration, “Bilateralism” may after all be the only way forward.  😉 

But beyond that, are there any further overall layouts that we can expect to appear regularly? What is needed for living creatures to evolve larger sized individuals in the first place? And with increasing body size, what universal trends for animal body-plans can we apply to creating our own alternative designs? Let’s look at those questions in the next videos!

References

  • Bobrovskiy et al. (2019) “Simple sediment rheology explains the Ediacara biota preservation” Nature Ecology Evolution 3, 582-589 | link
  • Budd G (2013) “At the Origin of Animals: The Revolutionary Cambrian Fossil Record.” Current Genomics – 14, 344-354 | link
  • Chen et al (2019) “Death march of a segmented and trilobate bilaterian elucidates early animal evolution” Nature 573, 412–415 | link
  • Hoyal Cuthill JF & Morris SC (2017) “Nutrient-dependent growth underpinned the Ediacaran transition to large body size.” Nature Ecology and Evolution 1, 1201-1204 | link
  • Mángano MG & Buatois LA (2014) “Decoupling of body-plan diversification and ecological structuring during the Ediacaran-Cambrian transition: evolutionary and geobiological feedbacks” Proc. R. Soc. B 281 | link
  • Muscente et al (2017) “Environmental disturbance, resource availability, and biologic turnover at the dawn of animal life” Earth-Science Reviews 177 | link
  • Prieto-Barajas CM et al (2018) “Microbial mat ecosystems: Structure types, functional diversity, and biotechnological application” Electronic Journal of Biotechnology – 31, 48-56 | link
  • Smith MP & Harper DT (2013) “Causes of the Cambrian Explosion” Science – 341, 1355-1356 | link

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