The Cambrian explosion was when evolution GOT REAL! Often called the “Big Bang” of biology, it marks the period when the earliest animals diversified into a remarkably wide range of different life forms, leading to the major groups known today. Famed paleontologist Stephen Jay Gould made the case that the Early Cambrian was exceptional, stating:

Wind back the tape of life […] and the chance becomes vanishingly small that anything like human intelligence would grace the replay.

Gould’s idea of the magnitude of Cambrian diversity did not stand the test of time, but his greater point of historical contingency still holds true. This also goes for the developmental constraints that are set by these early biological experiments for their respective descendants. At the same time, basic physics and environmental factors work the opposite way by making creatures superficially more similar or convergent. So when speculating about alien biospheres how can we apply the interplay of contingency, constraints and convergence? Let’s find out!

Something had stirred up the primordial oceans… A handful of worm-like creatures, which had been scraping a living off the seafloor for millions of years prior, quite literally, were suddenly empowered to grow limbs, fins, eyes, claws, armour, and more! But each in their own unique ways… The particular solutions each lineage of these early worms came up with determined the kind of creature their respective ancestors eventually would become. But it may have been a risky business with far reaching consequences!

At the time, it seemed the Early Cambrian exhibited an extraordinary array of bizarre body plans that could not be placed in conventional groups. So many, in fact, that the precursors to vertebrates, and thus humans, disappeared into the crowd. Gould saw it as a biological lottery of body designs with profound significance for later life on Earth. He believed these were all radically different experiments with only a handful surviving. Like the Butterfly Effect, if one of these species had been snuffed out, we may have missed out on butterflies themselves, and also other bugs… Well, OK, maybe we could live with that. But for all we know, the strange worms that evolved into vertebrates may have perished, along with the potential for us, human beings ever appearing.

However, Gould had overstated the uniqueness of the different lineages of the Early Cambrian. Most of its peculiar creatures have now found a home as basal branches of the existing main groupings called phyla.

The highly unusual animals of the Early Cambrian turned out to be transitional forms leading to familiar phyla.

Gould’s exaggeration clearly stems from his habit to poke at common misconceptions, especially our conceit that human beings are supposed to be an inevitable outcome of evolution.

Yet the myth of human exceptionalism persists. We are sitting here on one branch of the evolutionary tree and looking back down on the path that led to us, it feels like an inevitable outcome. It gives us the idea that there is a direction to biological evolution and it has always been heading toward us, human beings. What we tend to forget is that all the other lineages have survived just as long as ours and many are successful in their own ways, even though they don’t build computers or rockets. Nevertheless, the thought of evolution moving up a ladder is commonplace and goes way back. Before Darwin’s theory of natural selection, earlier ideas on evolution like those by Jean-Baptiste Lamarck, imagined evolution as a steady progression through predetermined phases.

Before Darwin, Jean-Baptiste Lamarck assumed evolution to be driven by an intrinsic force working upwards through predetermined phases.

And remarkably, this is still ingrained in our way of thinking as it pervades popular culture. We still think of evolution as being in stages, gradually moving up the stairs one step at a time. In science fiction, humanoids are always expected to evolve on alien planets as a general rule. And for instance in the X-men franchise, the mutants that turn into superheroes or supervillains respectively, are presented as the “next step in human evolution” that spontaneously start appearing. But there are no next steps in evolution. Evolution doesn’t go in a long term direction towards a specific goal. All it does is respond to the immediate conditions that a population of creatures lives under. It feels its way ahead and fans out in all favourable directions. Like a slime mold spreading.

Take for instance early fish: It’s not like they knew they’d eventually have to go on land. There was just a group of peculiar ones for which crawling rather than swimming was a nifty solution to a challenge they were facing in their local environment. Early tetrapod legs projected sideways and could thus not carry the body over land already. The latest theory suggests legs first evolved as a solution for crawling through thick, underwater vegetation in marshes or mangroves. Among these new swamp-clambering fish, there was then another peculiar species that could drag itself briefly onto land, if need be, to get a snack or just into safety, and it went on from there. These early ancestors of ours went through all the trouble, so to speak, to become amphibians, only for some species to completely turn their backs to land and become fully aquatic again!

Although it seems “staged”, the evolution of land vertebrates was the result of a chain of incidental adaptations to local environments without the eventual end-goal in mind.

The early amphibian Eogyrinus is one of those; and many later lineages of reptiles, birds and mammals also went back to the water again, as if the conquest of land was just an old fad. Examples like these very clearly illustrate that evolution is not unidirectional. It just goes with the flow for every single species in very limited time frames, no matter what happened before and no matter what may lie further ahead. The direction of evolution is only centered around populations directly dependent on their local environment during a small window of time. This also means that evolution could have gone entirely different directions, had some key factors in the environment been tweaked one way or the other. These are the small contingencies that end up making huge differences in the long run for evolution.

Now, we’re still not sure what factors may have triggered the Cambrian Explosion, but we do know that it was when one group of creatures in particular rose up to dominate the Animal Kingdom. Aptly called Bilateria this group covers a huge range of different creatures many of which have attained high degrees of complexity. We’ll go into more detail about this issue in the next video, but in the meantime, I can recommend an excellent video series by YouTuber Jackson Wheat on the Cambrian Explosion.

In short, the more we learn about the Cambrian Explosion, the clearer it becomes it was just a regular diversification event, which aren’t actually that unusual in the history of life on Earth. Also known as radiations, these often follow some radical change in the environment leading to mass extinctions. A whole range of ecological niches then opens up ready to be taken over by less specialised and thus more adaptive species. And it’s often some evolutionary innovation that then enables an unremarkable group of creatures, looming in the background, to take advantage of the ecological vacuum, and take center stage.

This lends credence to the idea, that had these events not taken place or not at that time, evolutionary history would have gone down a completely different path. Like how a minor tuck of gravity hurled an asteroid towards the Earth ending the dinosaur dynasty thus enabling mammals to take control of the global wildlife. Despite long being assumed to be “superior” to the reptilian dinosaurs, a major upheaval was actually needed for mammals to be able to take over. This flipping of the table, rather than a steadfast progression of the “best designs” towards victory, seems to be the basic pattern.

This is important to keep in mind: Evolutionary contingencies undermine the idea of evolution supposedly being geared towards any particular life-form; Especially humanoid life forms! But if you want to design an alien biosphere, it’s in principle impossible to not have the goal in mind of the complex creatures you want to populate it with. So what to do?

Well, we can start with acquainting ourselves with the different evolutionary contingencies that we know played out in our own planet’s past. Apart from serving as inspiration, we can consider similar events to happen on other planets that are like Earth. Are these events bound to happen elsewhere no matter what or not? Are they universal or are they unique? Devising your own series of evolutionary contingencies would add a lot of depth to your alien world.

In the next video we’ll continue our quest starting with the Cambrian Explosion itself. Would Earth-like exoplanets perhaps each experience their own early “Cambrian” Explosion? Let’s look into it!

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