‘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

Biblaridion's review of 'Wildlife on the planet Furaha'

 Yesterday, Biblaridion published a YouTube review of The Book, in part based on an interview with me. I guess most readers will know about Biblaridion because of his large and long-running video series called 'Alien biospheres', in which he methodically discusses the subtype of speculative biology that deals with life elsewhere than Earth. He has a whopping 155,000 followers, so I was very happy that he wanted to pay attention to the Furaha project. We have met several times in person at TetZooCons and DinoCon. 

I supplied Biblaridion with various images, most of which had not been published before. However, these unpublshed images are not new ones in The Book, because people should have a reason to buy that. I made a few new animations for Biblaridion, mostly of rusp skeletons. Rusps do not have a fixed length because of how the 'rings' in each segment are connected to neighbouring rings. Those connections functionally resemble springs at four connection points, so the rusp skeleton as a whole resembles a harmonica a bit. 

Here is a video showing 'torsion' of the rusp skeleton. Biblaridion shows another one depicting lengthening of the rusp skeleton. But before that one causes any misunderstandings I hasten to add that rusps do not become significantly longer or shorter. The mean distance between neighbouring segments stays the same, so if the left margins of two segments approach one another, the right ones will tend to move apart. The reason for this is that the overall volume of the beastie does not change and neither does the circumference of the segments. Think of an earthworm that becomes thinner when it becomes longer. If hat worm, for some reason, could not become thinner, it couldn't lengthen either. I exaggerated the harmonica effect in the animation for Biblaridion's video to clarify how the skeleton works, not as an illustration of how rusps would actually move. Rusp lengthening is however large enough to make it difficult for students at the Institute of Furahan Biology to measure rusp length... 

As an aside, human vertebral colmns do not have fixed lengths either. Our intervertebral discs become flatter during running or during the day and longer at night, adding up to a one or two cm difference.     

For those new to my blog, please have a look. The search function allows you to find items on many SpecBio subjects. 

Furthermore, there's my YouTube channel and an Instagram channel. If you wish to know how to order The Book, look here. If you order directly from Crowood and mention 'TCP10', you get a 10% discount. I'll also ask the publisher when the book becomes directly availble in the USA and elsewhere. 

 

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Addition 29 January: I asked the publisher; the UK distributor may not deliver to Canada and the USA; the distributor to do that may only do so after six months (the book was published on 3 December, so that would be early June then. Blackwell's apparently has free shipping for a while, but I don't know for how long.. 

https://blackwells.co.uk/bookshop/product/Wildlife-on-the-Planet-Furaha-by-Gert-Van-Dijk/9780719845710

New speculative biology book about future insects

One main playing field of speculative biology concerns life somewhere else than Earth, such as my Furaha project. Another describes how life on Earth might have evolved if some event in the past had taken a different turn, such as Dougal Dixon’s book The New Dinosaurs, and the third, the subject of this post, deals with life on Earth in the future.

Well-known examples are Dixon’s After Man and the French book Demain, les animaux du futur (see posts A, B, C, D and E). While these projects discuss various animal clades, one group of animals receives less attention than it deserves, based on ecological clout, numbers of species and of individuals: insects!
  
That changed with a new speculative biology book about future insects. Before you run to the nearest bookshop, you should realise that it is in French. At the time of writing the authors do not yet know whether there will be versions in other languages. The book is called Les insectes du futur, a title that should be understandable without knowing French. The subtitle is Petite entomologie post-effondrement, which means ‘small entomology after the collapse’. The book was published last September by Belin in France. The first author is Lucas Etienne, a researcher who designed the arthropods and made the illustrations. You may already know the second author, Jean-Sébastien Steyer, as he was one of the two authors of Demain, les animaux du futur ; he is a palaeontologist (see his book Earth before the dinosaurs) and has published various books popularising science. 

The book counts 163 pages and describes a large variety of insects and other arthropods, as told by two human researchers who make their way from Paris to Monaco in the year 2499. The protagonists make this journey after leaving the underground collection rooms of the Muséum d’Histoire naturelle where they sat out a nuclear war. They encounter many species of arthropods that are as new to them as they are to the reader. The book is divided into chapters that describe ecosystems and various biological principles, such as camouflage, symbiosis, parasitism, flight and adaptations to aquatic life. Let’s have a look at a few organisms.

Click to enlarge; © Belin Éditeur/Humensis, 2025

Nepa dendrobates
This animal may look like a tropical frog, but it is in fact an insect of the clade Hemiptera. The bright dazzling colours of the frog it mimics are a warning to would-be predators that these frogs are poisonous. The insect mimics the frog in colour and shape, so predators should mistake the tasty insect for a poisonous frog and stay clear. This is a nice example of Batesian mimicry, something than can be described as a sheep in wolf’s clothing. I wondered what happens if some predator that never eats frogs comes across such an insect. That predator would not be warned off by the colours because it never eats frogs, poisonous or otherwise. It would still not eat the insect as it wouldn’t recognise the insect as an insect, but as something inedible. Does that turn the effect into simple camouflage rather then Batesian mimicry? That sounds like a nice subject for a biology examination.

Click to enlarge; © Belin Éditeur/Humensis, 2025

Aquatic ants
Quite a few insects adapted to a life underwater, but never ants, or so I thought. I was wrong: there is an ant that dives into the fluid of pitcher plants to steal animals that fell in, and there is a mangrove species that allows its nest to be inundated by the tide. Etienne and Steyer developed their own aquatic ants. They developed gills, have grown considerably larger than any terrestrial ant and have become good swimmers. These ants are still colonial and still build their own housing, which in this case resulting in underwater ant cities. These marine ants developed symbiosis with corals, which is a very interesting idea, so altogether they paint a very intriguing picture.

Click to enlarge; © Belin Éditeur/Humensis, 2025

Abyssal wasp
This wasp descendant can grow to an astonishing length of 60 centimetres. It is bioluminescent thanks to symbiosis with luminescent bacteria. Unfortunately, the book provides little detail how it lives, although it is clear that it has lost its sting.

The story takes place in the year 2499, almost 500 years from now. That may be long enough for mankind to further change the climate and to damage or destroy many ecosystems, bringing about an overall collapse. But how much biological evolution can take place in that time? In other words, what is the maximum speed of evolution? That is probably complex. Factors that must play a role are how large the pressure to change is, such as due to a quickly changing environment and mixing species that were previously separated. The amount of genetic variability and the mutation rate would also be important modifiers of change. The morphological changes in the insect book are so large in such a short time that evolution here has jumped, something known as ‘saltation’.

One theory of saltational evolution by Goldschmidt described ‘hopeful monsters’ that came about through very large mutations, large enough to explain the advent of new species. This concept of saltational evolution was never wholly discarded and new papers keep on appearing on the subject. I do not know enough about it and cannot therefore afford a strong opinion on the matter. Readers of this blog will know about ‘punctuated equilibrium’, a theory stating that species do not change for a long time but may then suddenly undergo a major change. At present, there is evidence for both long periods of stasis as well as for evolutionary jumps. In short, the speed of evolution can vary. 

But the future insect book still forces a closer look at that speed. The fossil record does not exactly have a nice temporal resolution, with a complete fossil every hundred generations or so.

 

Click to enlarge; source: wikipedia

This Wikipedia graph neatly explains that gradual change over 10,000 years will show up as a qualitative jump if you only sample the record once every 10,000 years. But the graph is quite hypothetical. Very well, let’s hypothesise a bit ourselves. Making an insect look like a frog must involve changes to a great many genes. It seems extremely unlikely for all genes involved to mutate in the correct direction at the same time (but if this does happen, you might get an evolutionary jump). It seems more likely that weeding out any variant that reduces ‘frogginess’ requires untold encounters between predators, the insect and the frog. How many generations would that take? Likewise, adapting insects to water is likely to go through successive stages, perhaps starting with short dips to get food. From there, you can expect changes allowing the animal to last longer under water. I would not be surprised that breeding under water would be one of the last changes to take place. Again, it seems unlikely that all features would change in the correct direction at the same time.          
  
I understand the style figure of using human guides to show these new animals. The alternative would have been to move forward a few million years and to depict the animals as they are, without human interest. Would that have been better? That depends on what you want from such a book. The authors must have had great fun using their new arthropods to illustrate various biological principles such as Batesian mimicry. They did not in fact use the term Batesian mimicry; instead, they chose to teach by example, making their book highly accessible. That is an advantage and does not harm the fun. Come to think of it, showing how biology works is, for me, the essence of why speculative biology is fun.