Random Rambling: Biology
We have... let's say an eclectic mix of interests, and we feel our patrons do too. With that thought in mind, we present a feature we call "Random Rambling." It isn't quite random, but it's close: we made a little spinner with some of our favorite subjects. Every Friday, we give the thing a spin, and then you, lucky people that you are, get to learn a fact related to the subject the spinner landed on.
This week's topic is:
Biology!
So let's talk about coelacanths.
These guys.
If you've heard of these guys, you're either a marine biologist or someone described them to you as "living fossils." This is because this particular type of critter (there are two living species that we know of) first evolved somewhere around 400 million years ago, before animal life on land developed. Now, it's a bit of a misunderstanding to think that the modern coelacanth and the original coelacanth are the same species, but they are certainly the same shape: They've got these thick, stubby fins, rather than the normal spiny fins that most fish have. In motion, they look kind of like a fish that replaced all of its fins with tails of various sizes. They're pretty trippy.
We bring up those fins because they're the reason you have four limbs.
Theory Time
Obviously, what we're about to discuss as though it were fact is instead a theory. We feel compelled to explain to you find people that a theory is not a fact. It is instead an explanation that best fits the facts we have right now. For example, fossil theory says that the reason we find less complicated fossils the deeper we go and more complicated fossils near the surface is that, over time, layers of rock form and bury the fossils. This theory best fits the facts because we can literally do the same thing right now, so we know how the process works; we're just assuming that nothing changed in the fundamental process since we started writing things down. If that's the case, and we know how long the process takes, we can use that and some other dating methods to figure out how old the fossils we dig up are.
Are there other explanations for fossils? Sure. They just aren't as good. So we use the best explanation we have until we find a better one, and we use the word "theory" because saying "the best explanation we have until someone else comes up with something better" is a mouthful.
With all that said, back to the coelacanth. Remember how we said that coelacanths predate life on land? Well, because of how those knobby little fins work, and because of the fossils we've found that are both younger and more complicated than those first coelacanths, we think that land-based vertebrates got their start as fish, like the coelacanth, with knobby little fins that were sturdy enough to stand on. The theory goes that coelacanth-like fish discovered they could use their multiple knobs to move from puddle to puddle, and because they could survive drying conditions and sudden changes to the environment better than the fish without knobs, they grew to dominate the almost-dry areas. Over time, certain varieties of kinda-coelacanth transitioned from spending minimal time outside of the water to goodly chunks of it, and once that happened, they could start eating things they found on land- a whole smorgasbord of food not available to the permanent aquatic creatures. These guys became the first amphibians, and from there some evolved to only be on land.
We'd say that from there the rest is history, but, well ALL of it is history.
Tetrapods 4 Life
So what does this have to do with you probably having two arms and two legs? Well, if you look at that coelacanth picture up there, you'll note that it has a lot of fins. In point of fact, it has eight fins, if we don't count the tail. Of them, six work similarly enough to legs that they could have been used as such. You might think that would mean that two of them were superfluous and evolution dropped them over time... and, as far as we can tell, you'd be right! It looks like the back two fins (one top, one bottom) weren't all that useful on land, and eventually disappeared, leaving the land-fish with four limbs. One of the closest relatives of the coelacanth are the tetrapods, which means "four feet." That'd be us, as well as almost every other land-based vertebrate.
However, not all coelacanth-like creatures back in the day had the same number of fins. More importantly, there's nothing in the build of a coelacanth-like creature that says "only four limbs will work." If a fish like the coelacanth had two anal fins instead of one, its descendants would have had six legs without any real issue.
So why don't we have any six-legged animals? Well, two big reasons. First, it turns out that four limbs are pretty ideal for low-energy movement on land. This means that the fellows with four legs wouldn't have to compete as hard for resources to be as successful as the six-legged fellows. Second, when animals started to transition completely to land, they discovered running. This might be tetrapod bias talking, but so far as we can tell, running is best done by creatures with four limbs.
There is also a third reason, but we don't know how much of an impact it had.
Playing Dice With Evolution
Evolution, so far as we can tell, is unplanned. Some critter that looks a little different than its friends happens to do better than them and reproduces. Other critters try out something new, it doesn't work, and they die out. Over time, these two possibilities play out so many times and in so many ways that we have every currently living variety of animal, and even more varieties of animals that died out.
The thing is, the reason for some varieties of animals not working out can often be "bad luck." We've had quite a few mass extinctions happen on Earth (so far as we can tell), and the creatures that survive those extinctions usually aren't the ones that were best-suited to survive; they were the ones who got lucky.
Probably the most famous instance of evolution changing course because one group got lucky is the "extinction" of the dinosaurs. We put "extinction" in quotation marks because it wasn't dinosaurs that died out, and it wasn't mammals that survived the extinction. What actually happened was that basically every species over a certain size was wiped out. Smaller species lived, evolved to fill in those spots where the big guys used to be, and life continued. One set of smaller species was the ancestors of all mammals. Another set? Small, feathered dinosaurs. We call them birds nowadays.
We mention this because, around the same time climbing around on land became the big thing, another mass extinction happened. We aren't 100% on what caused the extinction; the current consensus is that a series of massive volcanic eruptions ruined breathing for basically everyone. There was a 50-50 chance your entire species got wiped, and the extinctions were indiscriminate.
One type of creature that survived, more or less by chance, was the tetrapod... although not all of them did. A variety of proto-mammals got wiped out, which led to the dominance of dinosaurs for almost 200 million years before mammals got a second chance.
One type of creature that didn't survive was the hexapod. And there's no real way of telling if what did them in was chance, competition, or a tendency to break their not-quite hips when trying to run.
Conclusion
Who knows? If Mother Nature, when it flipped its coin, had landed on heads, maybe we'd see six-limbed animals today. They likely wouldn't dominate, but there are several niches where having six limbs is more useful than having four. Climbing would be easier, for example, and six limbs provide a more stable platform for larger animals as well. As demonstrated by the sheer number of invertebrates with more than four limbs, it could work out for them.
Maybe all they need is another chance, like the one mammals got after the fall of the big dinosaurs. Maybe all they need is an environment in which the status quo was flipped, and six legs were better for running than four. Were that the case, given enough time...
Well, anything is possible.