Adrian Currie writes...
Us humans really, really like stories. And not just for entertainment. Some stories, most obviously personal narratives, human history, and so on, are meant to be true, and moreover explanatory. We often use such stories to frame our lives, our self-perception, and explain ourselves to others.
One thing that makes a story seem really satisfying (as an explanation!) is simplicity. How often, when we’re telling stories about ourselves, do we pick out special, privileged moments? We highlight particular bits of our history and say this was what made all the difference. I have trait x because of my genes. I have trait y because of [foundational experience in my youth]. I have trait z because my parents raised me in a particular fashion. In scientific investigation of the deep past, as well, such simple narratives have their appeal. Here’s an example.
This is a Fossa:
You might recognize fossa as the baddies from the kid’s film Madagascar. They live on the aforementioned island, and are really weird. They are, in effect, mongooses: their closest relatives are the mongooses, meerkats, and other little critters from the African mainland. Fossa, though, are way bigger (approaching a meter, excluding tail). They also climb trees and eat primates. They are, in short, giant tree-climbing monkey-(well, okay, lemur)-eating death-mongooses. How did that happen?
Around 20 million years ago the fossa’s weasel-sized ancestors arrived on Madagascar. Madagascar is a large island, and islands often generate strange evolutionary effects. As opposed to continents, islands are isolated; it is difficult to get to them. When lineages do make it to an island, they often take advantage of the lower competition, and the ‘empty’ niche space. The isolation of islands leads to all kinds of strangeness: enormous flightless birds, tiny elephants, predatory death-mongooses. And the reason is roughly the same each time: the isolation of islands leads to empty niche space, often less competition, and often different resources than larger, more connected geographical zones. This leads animals to evolve in weird and wonderful ways.
On Madagascar, no one was eating those delicious lemurs. Enter the (proto) fossa. Natural selection moulded her to a new lifestyle: her size increased as she developed a taste for lemur.
Notice the shape of the explanation I just provided: I told you about a general set of dynamics—how islands work evolutionarily—and then showed how the fossa’s case fits into those dynamics. I unified the fossa with a bunch of other island-dwelling weirdos: they, and pygmy elephants, and so forth, are all instances of the evolutionary effects of island life. This is what I call a simple narrative. We don’t need to know much to follow it: we just need some general facts about islands, and then be told that the fossa’s ancestors turned up on an island.
Such simple explanations are great when you can get them—that is, when they are true. However, often simple narratives just aren't available. Let’s look at another example.
Here is the image which Mike Skrepnick kindly allowed us to use for the website:
It is a Diplodocus, one of the sauropods. Most dinosaurs weren’t really so large, but sauropods were. The biggest sauropods rivalled Baleen Whales in terms of brute length. But whales have it easy: the sea provides buoyancy and has much higher biomass, sufficient to support all of that blubber. The biggest land-based mammals have managed is a paltry 12 meters (Paraceratherium), compared to the 30 meters or so for the longest sauropods. Of course, we musn’t overplay it: much sauropod length consists in the long neck and tail, and its barrel of a body is more comparable to the biggest mammals. But still, the biggest sauropods are consistently put much higher in both weight and length to mammals. How did they manage this?
Now, if you want a simple narrative—one in which I explain some basic dynamics and then show how they apply to sauropods—I have bad news. Sauropod gigantism is not like fossa gigantism: sauropods evolved to the sizes they did because of a slightly bewildering, unique set of circumstances. Let’s dip our toes into the detail…
Even basal sauropods of the early Jurassic had some distinctive sauropody features. They were quadrupedal (or near-quadrupedal) and had that distinctive long-neck, long-tail, barrel-shaped body. Unlike mammals, they didn’t chew and they laid eggs. They weren’t enormous yet, but there’s reason to believe that this collection of features was needed for gigantism to evolve.
Let’s start with that long neck. There’s a lot of debate here: what it was used for, whether it could be lifted high above the shoulder, etc… regardless, it’s pretty plausible that the sauropod neck allowed it to take in more food with less effort than us puny-necked mammals. In a wonderful paper, Ruxton & Wilkinson compare the sauropod neck to old-school vacuum cleaners. Back in the day, the machinery needed to get good suction required a massive vacuum body, making dragging it around exhausting. In response, designers made the vacuum’s hose really long, allowing the user to deposit the vacuum in the centre of the room and wander around with the hose. The same principle, they suggest, could work for sauropods. Roughly, then, the long neck allowed sauropods to take in more food with less effort.
But if long necks are so great, why don’t mammals have them? (And giraffes don’t count: their necks are significantly less manoeuvrable than sauropods’ likely were!). Well, because of our fancy teeth. We mammals really take chewing seriously, and we have a whole bunch of complex jaw and dental features to show for it (this is why palaeontologists can often identify a new species of mammal from a single tooth!). This puts a limit on how small our heads can get relative to our bodies. We need big heads to accommodate our fancy teeth, and this makes long necks an encumbrance. Oh, also, all that boring chewing really limits the speed at which we can guzzle down food, as opposed to the sauropods who could get a lot food inside them quickly.
Okay, what about egg-laying? Well, it’s plausible that one of the constraints on mammal size comes from a trade-off between the size of your population, and how big the individuals in your population are. On the face of it, the bigger each individual, the smaller the number of individuals in your population. Given the same carrying capacity, a landscape could support quite a few little guys, but only a couple of big guys. Small populations are often bad news: they are less resilient to shocks. A big population can more easily bounce back from, say, a bad season, natural disaster, or epidemic, which would take out a small population. Eggs, though, provide a solution: sauropods could lay a bunch of eggs each year (and each little sauropod requires minimal care), meaning that even if the small adult population took a big hit, the next generation could more than refill those numbers (sauropods weighed around 5kg at birth, growing to 10,000 times that size!).
Okay, so there are differences between sauropods and mammals that explain how they mitigated the costs of increasing size—they were the right lineage for gigantism. But we still haven’t explained why they got so big. Let’s try to do that now. There’s reason to believe that, during the Jurassic, the sophistication of predators increased: big, possibly pack hunting, critters like Allosaurus and the iconic raptors turned up around this time, and it’s likely there were arms races between these new predators and their prey, who started evolving fancy armour and other defences. The sauropods, it seems, responded by getting bigger—too big to eat! But this getting bigger also required a whole bunch of other evolutionary changes, they needed to solve problems with respiration, blood circulation, body heat, and much more (I’ll leave those for another day!).
Even in this basic sketch we get nothing like the simplicity of the fossa case. Call this a complex narrative. For sauropods, a lot of diverse information is required to provide an explanation. And there is no nice, single, unified set of dynamics I can appeal to. Such narratives might not be as satisfying as simpler ones—they might not give us that ‘understanding feeling’—but the truth doesn’t care about your psychological states.
Sometimes history is simple—as in the fossa case—but sometimes, I suspect very often, history is not simple, and our explanations must be complex to meet them. I have a suspicion this is true of our personal histories as well. Perhaps it’s true that occasionally, for some people, simple narratives suffice: perhaps you ended up where you are, or being the kind of person you are, for simple reasons. I doubt it though. I’m guessing that we’re more like sauropods than fossa in this regard. If that’s right, then (assuming we care at least a bit about truth!) we should be extremely cautious of stories which are too simple. I have trait x because of my genes. I have trait y because of [foundational experience in my youth]. I have trait z because my parents raised me in a particular fashion... Perhaps we should be careful of accepting such explanations too quickly…
Bibliography
The basic ideas of this post are drawn from:
Currie, A.(2014). Narratives, mechanisms and progress in historical science. Synthese, 191(6), 1163-1183.
More details about Sauropods can be found in:
Biology of the Sauropod Dinosaurs, Understanding the Life of Giants. Klein, Remes, Gee & Sander (eds). Indiana University Press, pp57-82.
Ruxton, G. Wilkinson, D. (2011). The energetics of low browsing in sauropods. Biol. Lett. 7(5) 779-781.
Sander, P. M., and M. Clauss.(2008). Sauropod gigantism. Science 322:200-201.