October 2018

Where's the evolution?

Scientists recovered only a handful of (very large) bones from L. mafube. To determine whether this dinosaur walked on two legs (bipedally, as Tyrannosaurus rex did) or four (quadrupedally, as Brontosaurus did), researchers relied on a simple measurement: the circumference of the humerus (the equivalent of your upper arm) in comparison to that of the femur (the thigh bone). Unsurprisingly, in all living land animals (and in extinct ones for which stance is well documented in other ways), there is a consistent relationship between these figures. Quadrupeds have a thicker “arm” bone in comparison to their thigh bone than do bipeds. This makes sense. Quadrupeds would only be able to support their weight with their front legs if those bones were sturdy. And L. mafube had a sturdy “arm” bone, putting it squarely in the range of four-legged walkers. However, reconstructions suggest that its forelegs had a flexed position, splaying to the sides of its body, unlike the four-legged sauropod Brontosaurus, which stood on four legs positioned like columns beneath its enormous body as an elephant does. L. mafube walked on four legs, but it could probably also have reared up on its hind legs and used its front legs for foraging.

L. mafube seems to have been a transitional dinosaur — one that illustrates an intermediate step in the transition from being bipedal to being a committed quadruped like Brontosaurus. But this transition was not particularly smooth. In fact, when scientists reconstructed the evolutionary history of this group, the sauropodomorphs, they found evolutionary changes in walking mode had occurred several times. The ancestral sauropodomorph walked on two legs. A few different two-legged groups split off from that ancient lineage before it transitioned to four-legs with a flexed stance. Some more groups split off. Then two-legged walking arose again, and some more groups split, and then back to flexed, four-legged walking. A few more groups (including L. mafube) arose from this ancestor, as did, eventually, a lineage that evolved an elephant-like stance and spawned dinosaurs like Brontosaurus. If this all seems a bit helter skelter, it’s because it is! Evolution is a messy process that does not follow a preordained trajectory. From random variation, natural selection favors whatever happens to work better in a lineage’s particular environment at that particular time. There is no grand plan. Evolutionary switchbacks happen all the time.

The overall picture of dinosaur diversity also contradicts common misconceptions about evolution. It’s now well established that birds are actually dinosaurs (since the attachment point for the avian branch of the Tree of Life nests within the dinosaur clade). It might be tempting to envision an evolutionary transition from lumbering four-legged titan, to a nimble bipedal velociraptor, to an actual bird running on two legs until it eventually takes flight. But as we’ve seen time and time again, evolution doesn’t follow a linear path — and rarely does that path conform to our idea of what might constitute “advancement.” In fact, the ancestor of all dinosaurs was almost certainly already bipedal — and several different quadrupedal dinosaur groups evolved from this ancestor, going from two legs to four. There’s nothing more “advanced” about walking on two legs.

Further, birds were not the “goal” of dinosaur evolution. Non-bird dinosaurs lived on Earth for nearly 200 million years, and the first birds arose about halfway through that span and evolved flight shortly after. Birds were flapping around alongside other earthbound dinosaurs for around 100 million years before the rest of their clade was rendered extinct. It is easy to misconstrue birds as the capstone achievement of dinosaur evolution, but in fact they are merely the lucky survivors.

As scientists discover more fossils and new species like L. mafube, the dinosaur branch of the Tree of Life is being filled out. And as it is, we find more and more examples reinforcing the idea that evolution is not a march of progress. Instead it is a blossoming of exuberant diversity generated in response to the assorted forces, accidents, and vagaries of a complex environment and Earth’s unique history.