‘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 an odd proboscis. 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

How well can connected boxes learn to swim? The ecosystem game

Sometimes I play a computer game, mostly of the simulation type. While looking for something different from steering a European nation through history, or building a sprawling city somewhere, I came across a game promising to show biological evolution.

It would not be the first time a game tried to do that; Spore promised that too, and there are some others. I even wrote a post about Spore for this blog, because I was curious how the game designers dealt with the number of legs a creature might have. That number did not evolve by itself, but was chosen by the player. While the anatomy and movement of these legs were cleverly arranged, they turned out to be completely predetermined. In other words, the gradual changes in shape and capabilities of the resulting beasties had nothing to do with random variability followed by the environment pruning the stragglers, which is how real biological evolution works. Instead, Spore relied on Intelligent Design by the developers, and to a lesser extent by the player, acting as a minor deity.  

Click to enlarge; copyright Tom Johnson

But this ecosystem game promises the opposite. You get to play with a barren stretch of sea floor and have to turn it into a thriving ecosystem. The game will create swimming animals completely on its own, at first anyway. There is random genetic variability, and the unfit are weeded out, leaving their more successful brethren to forge on. You may wonder whether a full evolution simulation handled in this way would be any fun to play. After all, the premise would firstly be that genetic, anatomic and functional variability are all left to chance, and secondly that the environment provides all the selection pressures. What is left for the player to do?

In the demo, the player can indeed not control the characteristics of the beasts at all, but can guide evolution by altering the environment. The player has to place new plants or simple animals as food, and will also have to provide spawning areas and cordon off some pleasant mating grounds. Then you watch to see whether your Chosen Species rises to your challenge.

In the final game, there will also be a possibility to tinker with the anatomy of the beasts directly. An example of how that may look is shown above. Much as I like the idea of a fully independent evolution model, I also look forward to take up my duties as Minor Deity and start tinkering. In the full game you do not need content yourself with one Chosen Species; there will be various species, and herbivores as well as predators. At present, the game is in an early stage, so do not think you can lord it over a complex ecosystem just yet. That will be later. But a large part of the true evolutionary part is already in place. Let’s discuss the mechanics and the nervous systems of these beasts.

Tom Johnson, the creator of the game, provided some explanations. The animals consist of connected rectangular boxes of varying width, length and breadth. That’s it: they are boxes. That is what is shown in the video above. In the game, the animals look much nicer, as the boxes are depicted as smoothed forms with some nice fishy textures. At one end of one box there is supposed to be a mouth. To help the player, this part is shown as having a distinctively fishy head, with two eyes and two jaws. In a way this is a pity, as otherwise the animals have nothing that reminds you of a vertebrate. They are wholly and spectacularly asymmetrical! The boxes move at the connecting points, sometimes around one axis, sometimes around more than one. If one box moves with respect to another, this creates forces acting on the water around our hopeful monster. There is drag, there is angular momentum, and the creatures moves. Well, if you play the demo you will find that the earliest forms flop rather than swim and can be so painfully clumsy that they die before they even make it to any food.

The animals have nervous systems with an input layer, a layer for connection and integrationr (the Brain!) and a layer of output neurons controlling the muscles of each body part, which is, unsurprisingly, a box. You can actually see the neuronal connections in action, with impulses speeding along the axons (although at present the number of visible impulses does not reflect the true impulse frequency yet – for that you have to look at the number next to the neuron-). In successive generations, both the anatomy of the boxes and their nervous systems evolve. It is not quite clear to me yet whether the neural system develops at random or in response to mechanical needs. At any rate, I think it is an impressive feat to have both an evolving anatomy as well as an evolving brain linked to that anatomy.

Tom told me that the capacity of the evolutionary fitness principle was well born out by the game. At one point it turned out that some features of the animal worked only with a specific frame rate. In other words, the modelled animals had taken full advantage of one characteristic of their particular world, even though that was a completely unintentional one.

Another example is the animation above this paragraph: the animal seems to wave one limb in such a way that it acts as sort of propeller. In my searches for original means of locomotion to use on Furaha, I had tried to find a way to have a limb do just that. The problem is of course that in biology you cannot have a body part going through a complete 360 degree turn, as that would of be incompatible with blood vessels, nerves and muscles running to that limb. I could not see well enough how the movement worked, so I asked Tom. His reply was that I needn’t bother, as that was an early design, and he had not specifically stopped the animals from having continuous circular motions. So, again, blind evolution had used what it could and found a way...

Tom was kind enough to send me a unique illustration for this blog, shown above. The wrote: "This creature lives in the deep sea with a rocky, spiked terrain and evolved a nice downward glide to stick to the seafloor and eat the sessile creatures attached to it." I like it.

As I said, at present this game is available as a free demo, here or here. I will be keeping my eye on it, to see what strange forms can and will evolve. Charles Darwin once wrote the following about evolution:

 “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

Isn’t it fascinating that we can now see these principles in action, right before our eyes, on our computer screens?