Paleontologists have long questioned why Tyrannosaurus Rex – a giant killing machine with 9-inch-long

daggers for teeth – had such puny arms.

Were short arms particularly useful when grappling with prey, or when Sexy Rexy was in mating

mode?

Did they provide just the right leverage when getting up after a nap?

Maybe.

Like some evolutionary traits, from the gecko's camouflage, to the human's opposable thumb,

to the orchid's deceiving shape, perhaps those little limbs gave T. Rex some real advantage.

But it’s also quite possible that they didn’t.

For one thing, T. Rex might have ended up with small arms simply because big - or even

normal-sized - arms were a DISadvantage.

Perhaps once T-rex’s ancestors got big enough, they could hunt and eat with their giant jaws

alone – and their arms weren't worth the energy they took to haul around.

But evolution can’t just swap out not-so-useful traits for completely new ones.

Instead, over generations, T. rex’s arms may have simply gotten smaller and smaller

until there was no longer a significant cost to keeping them.

Then they just...stuck around - a lot like how we humans ended up with our tiny tailbones.

Another possibility is that evolution gave T. Rex tiny arms simply by chance, the same

way it gave Ireland lots and lots of redheads.

In a relatively small, isolated population, a trait that doesn’t hurt OR help an individual’s

chances of surviving and reproducing can, just by the luck of the draw, become more

and more prevalent from one generation to the next.

It’s possible that the size of T-rex’s arms didn’t matter much either, and all

the dinos just happened to end up with short arms.

On the other hand, weirdly-small arms have also evolved in other large T-rex relatives,

suggesting that the trait might not be totally random.

Right now, scientists are studying the microscopic wear and tear on the arm bones of the most

famous T-rex of all to determine how she used them.

This work might help us figure out if T. rex’s arms were useful like our thumbs, useless

like our tailbones, or random like our redheadedness.

Right now, the answer is still out of reach.

This video was sponsored by the University of Minnesota, where students, faculty and

staff across all fields of study are working to solve the Grand Challenges facing society.

University of Minnesota professors Suzanne McGaugh and Emma Goldberg, who advised us

on the science behind this video, investigate how several different mechanisms of evolution,

from adaptation to negative selection to genetic drift, affect traits -- like T. rex’s tiny

arms -- in a variety of species.

Professor Goldberg uses math to explore how a plant's reproductive traits affect its chances

at becoming new species or going extinct, and professor McGaugh rappels into Mexican

caves to better understand how blindness in cavefishes evolves independently in different

populations.

Thanks, University of Minnesota!