Evolution has no long term direction and no end-goal. It is governed by chance events, both genetic mutations within the organisms themselves and the ever wandering environment they try to survive in. And the same will hold true on other planets as well. With every contingency follows the establishment of novel developmental constraints as creatures become dependent on the very adaptations that were key to its success. But what does this mean in practice? In other words: How do developmental constraints work and how can we apply this principle when thinking about life evolving on other Earth-like planets? Let’s find out!
It was a long and winding evolutionary history that led to the specific body designs of humans and other animals. Random mutations create the raw material for evolution: The genetic variation within each species. The relatively stable environment on Earth is affected by unpredictable events occurring within its four spheres of atmosphere, hydrosphere, geosphere and biosphere, as well as from outer space. This has been going on for billions of years, but not at an even pace. Life’s history has been punctuated by many diversification events, also called radiations, where new kinds of creatures end up dominating the relatively stable periods that follow.
As natural selection pulls and nudges each lineage down to individual creatures, each will respond accordingly and in different ways, depending on their genetic toolbox. Local events during very short time-frames can therefore determine the long term fate of evolutionary lineages. This is called contingency, also known as the Butterfly effect, and it’s a very important principle to keep in mind.
For starters: Let’s go back to the beginning: The Cambrian Explosion. Calling it an explosion is a bit over the top, because it spanned tens of millions of years. But it did reveal the basic blueprints for each of the major phyla that we are familiar with today: Arthropods, Molluscs, Annelid worms, Echinoderms, Chordates including Vertebrates, and many more.
And these very blueprints, once arrived at, turned out to be remarkably conservative! Most of the diversity within phyla, are more or less variations on a theme, be it increasingly convoluted ones. Why is that?
Well, it may be because it’s much safer and easier to adjust an existing design that already works, rather than to rework things from the ground up again. Creating a hopeful monster is much riskier for survival than to tweak an already well-adjusted layout. If it ain’t broke, don’t fix it! Or in other words: If it works fine, “fixing” something as fundamental as a body plan will almost inevitably break it, most of the time in any case. And this dependency is manifested as developmental constraints that keep creatures on a distinct path and limit their evolutionary potential to some degree.
Many temporary solutions crucial for survival during some stage in the past, cannot always be easily altered in the long run to open up for new possibilities. What made sense for a species in their environment back in the day, oftens puts unexpected constraints for future descendants. Arthropods like insects, for instance, are doomed to remain under a certain body size, due to physical restrictions imposed by their exoskeleton and breathing apparatus. In proto-arthropods, the exoskeleton made a tougher and more versatile creature able to survive the primordial seas. But now, both gravity and oxygen pressure aren’t working out well for any potential mega-bugs. Being on the outside of an organism instead of the inside, an exoskeletal system of plates and tubes is proportionally much heavier, and structurally weaker, than an endoskeletal system of rods. Scaling this up would eventually run into an upper boundary where the internal musculature would run out of space and strength to keep up.
An exoskeleton is also typically impervious to gas exchange: Oxygen can’t enter and carbon dioxide can’t get out. Insects solved this problem by an intricate network of gas tubes, called trachea, connecting internal tissues with the outside environment. However, there are limits to the distance the gases essential for can traverse. It’s clear that radical changes to the body design of an insect are required to enable them to become larger. And evolution prefers the easy road.
To give an example that is closer to home. (Well, if you consider Australia close to home.) This is also why there are no marsupial ungulates like “pouched horses” or antelopes. Kangaroos are the closest thing to ungulates, but chose a different approach to a similar problem. The explanation is that marsupials opted for a reproduction system, where their young are born very early and have to clamber up to the pouch to develop further. For that they need to have well-developed front paws from very early on in life. And it’s not so easy to convert clawed front paws like that into hoofed front legs. It’s not impossible, but the threshold is much lower for only specializing the hind legs for speed. And kangaroos are doing just fine, matching ungulates in speed and performance.
Marsupials and placentals each represent different approaches to bearing life young, as opposed to laying eggs, but the reason for this split remains uncertain. One popular hypothesis revolves around marsupials originally being primarily tree-living, making their particular reproductive mode an advantage.
This is how contingencies in early stages of evolutionary adaptations can set constraints for further developments!
Now with this knowledge in mind, let’s take a closer look at the Cambrian Explosion, which, as we discussed in the previous video, really was just another adaptive radiation. However, there was one thing that was special about the Cambrian Explosion! And that was that bilaterians didn’t really play a major role before then. They were still goofing around with different configurations of organs in order to survive, when -all of a sudden- THEY WERE ON and had to do with whatever they had developed so far! And they share at least one feature that made them quite special: Bilateral symmetry But despite this cool new trick the early bilaterians stayed in the shadows of the supposedly primitive life forms of the late Precambrian. So whatever the reason was for their eventual predominance, it’s not being bilateral in itself.
Had the Cambrian Explosion been triggered by some “invention” like eyes, carnivory or bilateral symmetry itself, all bilaterians would have had the same basic body plan. But because they had diversified long before then, there was this wide array of forms to depart from. However, only some of these rose to predominance while others stayed in obscurity or even disappeared forever. And this is where it gets interesting:
If you want to imagine an alien looking fauna, you could simply pick one of the other permutations as starting points. Or even make a custom one by combining traits from different lineages. Examples of alternative starting points like these actually do exist in real life, even if these never took off in the same way as arthropods or vertebrates did. Take for example polychaete worms, which show a mix of arthropod, vertebrate and squid features. What if these evolved further into land animals instead? I will cover that in another future video.
Still, you have to keep in mind that there are historic reasons that a creature looks the way it does. Therefore it’s important to understand what led to the different developments within each respective lineage. And I hate to keep hammering this point, but each choice has to make sense for the specific way of life of an ancestral species within a very limited time frame and local environment.
More on this and also on what exactly triggered the Cambrian Explosion in the next video. Would convergence in the end save the day and lead to similar outcomes no matter what? We’ll see next time around.
References
- Doube et al (2018) “Limb bone scaling in hopping macropods and quadrupedal artiodactyls” Royal Society Open Science 5| link
- Erwin D et al (2011) “The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals” Science 334:6059, 1091-1097| link
- Erwin D & Valentine J (2013) “The Cambrian Explosion: The Construction of Animal Biodiversity”
- Keyte et al (2010) “Developmental origins of precocial forelimbs in marsupial neonates” Development 137 | link