‘Cambriform Ignitions’; does every planet with life have its own Cambrian Explosion?

 By Sigmund Nastrarruzzo and Biblaridion

This post was inspired by Biblaridion’s recent YouTube review of my Furaha book. Biblaridion focused on the numbers of limbs of various Furahan clades. This inspired many comments, touching on so many aspects of legs that Biblaridion and I felt that the subject deserved attention on this blog. The present post deals with the Cambrian Explosion and how body plans may affect leg counts. The second one will deal with functional aspects of the number of legs.

 

Opabinia, a Cambrian oddity; copyright Quade Paul;  source here

 A ‘Cambrian explosion’ usually does not refer to something blowing up in Wales. It could, because ‘Cambrian’ means ‘Welsh’, but we mean the radiation, diversification and quick evolution of life in the Cambrian period, around 500 million years ago. Before this explosion, in the Ediacaran period, the ocean floor was covered in bacterial mats. There were also odd tubular or flat quilt-like organisms; they were slow, soft, and had eyes nor teeth. Compare that to the radically different world at the end of the Cambrian: almost all modern phyla had evolved and animals had hard parts to help them crawl, burrow and swim. They used eyes, teeth and armour to find lunch or avoid becoming dinner.
Although all this took place half a billion years ago, it is relevant for speculative biology: should we expect events similar to the Cambrian explosion on other planets too? Are the consequences in terms of phyla and building plans similar too?

Self-reinforcing evolution
Let’s start with an Ediacaran sea floor, covered in microbial mats with some slow, soft and mostly sessile organisms here and there. If you enter a new trait that allows animals to form hard materials, they can, for the first time ever, tunnel beneath the mats. The early Cambrian saw a huge proliferation of burrowing animals (sometimes called the ‘Cambrian substrate revolution’). All this burrowing helped to mix and churn the seabed, releasing the nutrients that had until then been sequestered in the sediment under the microbial mats. And these extra nutrients allowed for the evolution of larger and more complex burrowers, which further increased the circulation of nutrients, and so on.
    Those hard tissues provided anchor points for muscles, allowing skeletons and crawling toothed animals. Once the owners of those teeth developed a taste for other grazers, the race to grow defensive armour began. All this activity became more efficient when the animals’ light-sensitive pits developed into proper eyes. In turn, those eyes needed a nervous system to process signals. Teeth, speed, vision and brains all helped one another develop in a mutual reinforcement.
    Environmental changes such as a rise in oxygen levels would accelerate such a self-reinforcing evolution, so both environmental and evolutionary factors may have been involved in a positive feedback loop.  
    If some of these elements were absent on another planet, the process might be much slower, resembling a slow fire rather than Earth’s explosion. In fact, it’s now believed that the Ediacaran fauna that preceded the Cambrian itself first appeared following the Avalon explosion, which occurred about 30 million years before the Cambrian. The path to complex active multicellular organisms may see several spurts of increased complexity rather than a single sudden jump.
However, whether over a short or long term, a self-reinforcing tendency towards more activity seems inevitable, provided there is energy to spend. We therefore think that an evolutionary spurt in early life is likely elsewhere. Mind you, as other planets through no fault of their own cannot be expected to have a Wales on them, we hereby introduce the general term ‘Cambriform Ignition’ to describe this early phase of evolution.

Click to enlarge. The scheme show that most phyla originated early. Source: Zhang et al 2013.

 

From fast and fluid evolution to a fixed Bauplan
We haven’t mentioned a very important aspect of the Cambrian explosion yet, one that sets it apart from other periods of large evolutionary change. This is the emergence of many phyla, each with its own Bauplan (a German word literally meaning ‘building plan’). A body plan describes anatomical aspects including symmetry, segmentation, and, yes, it can include the number of limbs.
    A phylum’s body plan is genetically determined and is produced in every animal of that phylum by a precise genetic control over the formation of an embryo (notably the famous Hox-genes). The products of these genes diffuse through the embryo and tissues respond to their concentrations, for example by forming a limb bud.
At present, body plans are basically immutable, which means that you should not expect a simple mutation to result in a fundamentally different body plan with, for instance, a different number of limbs. And yet those immutable body plans all came about in a short time, so at the time those body plans must have been remarkably fluid, the opposite of their current fixed nature. Some plans disappeared again, such as the one producing Opabinia, with a midline eye and jaws at the end of a tentacle. That genetic fluidity later froze the body plan in all those phyla that were already genetically quite distinct. This parallel trend to fixate body plans in separate lineages only makes sense if fixating a body plan makes good evolutionary sense, in each and every surviving phylum.
What was the advantage of fixating the body plan? Well, remember that a body plan reflects instructions on how to grow an embryo. If that process is not tightly controlled, many embryos will be malformed and die because the changes are detrimental. Making embryogenesis more reliable would definitely be worth passing on. So-called ‘complex regulatory gene networks’ evolved that make embryogenesis more reliable. Once this protective embryogenesis system was in place, there was no turning back and the body plan stayed what it was.

Click to enlarge; levels of protection of bady characters; Source He & Deen 2010 

There are different levels of gene fixation. The core level of protection is a ‘kernel’, and it defines genetic traits that correspond to characteristics that define a phylum. Slightly less well protected genetic units are more open to genetic and evolutionary change, defining traits that correspond to orders and families. At the bottom rung of this classification are very mutable traits, conforming to genus and species levels.
Animal breeders can easily select for the most mutable traits, such as a shorter nose or a longer body. But other traits, such as having four limbs in a tetrapod, are fixed, and no dog breeder will succeed in getting a functional six-legged dog.    

Cambriform Ignitions elsewhere
As said, we think that evolution is likely to produce Cambriform Ignitions on other planets. But must that process always include a fixation of body plans? Probably: if such a fixation is beneficial, then body plan fixation is very likely to happen elsewhere too. But we can still speculate about this scheme and play with it. Here are a few thoughts:

• If the fluid phase of forming body plans lasts a short time before the plans are fixated, the result might be a planet with just a few different body plans. If those plans all include respiratory or circulatory systems that are not suited for large size, that planet may never develop large animals.
• In reverse, a long fluid phase might result in hundreds of different body plans, many more that the thirty-odd we have on Earth. Those worlds would be astonishing!
• Even if an alien biology involved a radically different mode of inheritance, such as horizontal gene transfer or a coding molecule that allows for a greater degree of genetic flexibility, there would still be a benefit in ‘locking’ certain features that couldn’t afford to be altered. Generally, evolution favours genetic diversity (hence the evolution of sexual reproduction), but species that can ensure that no inherently maladaptive traits come about will still have a sizeable advantage. This boils down to a degree of shape consistency, meaning that shapeshifters and their ilk do not seem very likely.              
• And finally, if the leg number happens to be stored in the most protected kernels of the genome, the number of legs will be fixed. If, however, that number is stored less securely, the number may be open to mutation! Such a ‘leg number fluidity’ would only work if the resulting legs are fully functional, including the neural machinery to provide sensory and motor integration of the additional legs. (Such integration doesn’t have to take place in the brain. Remember that the primate brain is a very centralised control freak, and control of a leg can also be delegated to a local brain -octopuses!- or to a spinal cord analogue -cats!-.)

    
Mind you, the number of legs can vary considerably between Furahan rusp species and can even vary within rusp species (that’s in The Book!). If you are not convinced, please consider Earth’s millipedes or velvet worms. In millipedes, the number of legs can vary throughout life and between individuals. In some millipedes the number varies in steps of 11 segments, which again has to do with genes and embryogenesis. Velvet worms can have anywhere from 13 to 43 pairs of legs depending on the species, and females tend to have more legs than males. This strongly suggests that the leg count in these animals is not immutably locked in the best-protected part of their body plan but is stored in a less protected part. They are fluid in this respect.

 

Reading material

The Cambrian Explosion: The Construction of Animal Biodiversity. Erwin DH, Valentine JW. Roberts and company Publishers 2013.

Zhang X L, Shu D G. Current understanding on the Cambrian Explosion: questions and answers. PalZ (2021) 95:641–660 https://doi.org/10.1007/s12542-021-00568-5

He J, Deem MW. Hierarchical evolution of body plans. Developmental Biology 337 (2010) 157–161

Willmore KE. The Body Plan Concept and Its Centrality in Evo-Devo . Evo Edu Outreach (2012) 5:219–230 DOI 10.1007/s12052-012-0424-z

Enghoff H. The Size of a Millipede. Berichte der naturhistorisch-medizinischen Verein Innsbruck 1992; suppl 10, 47-56

Minelli A, Edgecombe GD. Zoology: The view from 1,000 feet. Current Biology 2022; 32, R213–R236 doi.org/10.1016/j.cub.2022.01.072