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Why don’t we see certain traits in nature?
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If evolution is so innovative, if it’s powerful
enough to create this and this and this and
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whatever this is, etc… then…
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Why can’t humans grow wings and take to
the sky?
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Or why don’t fish have propellers?
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Why are there no 5-legged cats or giraffe-sized
chickens?
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Why do no animals have wheels?
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And why haven’t zebras evolved machine guns
to fight off lions?
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Today, instead of talking about incredible,
mind-blowing traits that evolution has created,
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like we’ve done so many times before, we’re
going to talk about the limits of evolution’s
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creativity, and why certain traits are impossible
to evolve.
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There is a myth in evolution that nature is
infinitely creative.
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It isn’t.
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Probably.
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By considering the reasons why certain things
can’t evolve we can learn a lot about how
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evolution actually does work.
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So yes, while these would all be awesome…
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I don’t make the rules, evolution does.
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… stupid evolution.
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I want a tank zebra.
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We begin in my childhood nightmares.
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The 1985 film Return To Oz was one of the
most terrifying films I ever accidentally
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flipped the channel to as a young nerd.
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This unofficial sequel to the 1939 film The
Wizard of Oz provided the fodder for more
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than one nightmare thanks in large part to
these: The Wheelers.
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There is something deeply unnatural about
a creature with wheels instead of hands or
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feet.
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Both because it’s extremely creepy, and
because it’s biologically impossible.
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The wheeled animal question is a canonical
case study when it comes to impossible things
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in evolution, or “forbidden phenotypes”.
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And remember, a phenotype is the observable
physical properties of an organism.
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There are organisms that roll up their entire
bodies to enable wheel-like rolling movement,
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like pangolins, spiders, tumbleweeds, and
roly-polies… or whatever you call these
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things where you’re from.
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Or maybe we could envision an animal rolling
around like the mulefa, a fictional species
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in the fantasy series His Dark Materials,
which hooks its feet into round seed pods
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and uses them as wheels.
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But acting like a wheel or using a found object
as a wheel is not the same as having wheels
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as body parts.
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No animal uses a rolling wheel body part to
move its body.
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And some of the reasons why are pretty obvious:
A structure capable of rolling continuously
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around an axle would be physically separated
from the body and therefore impossible to
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build in a developing animal.
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This would also make it impossible to deliver
nutrients and blood or nerve impulses to the
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wheeled appendage as well.
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But there are other less-obvious reasons animals
don’t have wheels.
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Which brings us here.
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To a Bronze Age foot soldier, the most terrifying
sight imaginable would’ve been enemy chariots
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rolling onto the battlefield.
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Manned by a driver and either an archer or
javelin thrower, the ability of these horse-drawn
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wheeled vehicles to move quickly across the
field of battle made the chariot the dominant
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shock and awe weapon of its time from Mesopotamia
to the Mediterranean.
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But by the 6th century AD, the chariot, along
with almost every other wheeled form of transportation,
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had basically disappeared between North Africa
to Central Asia.
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How could such a seemingly dominant technology
vanish?
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Because wheels had been replaced by camels.
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This happened for several reasons: The roads
originally laid down across the Roman empire
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had deteriorated.
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The skill and craftsmanship required to make
efficient wagons and carts had been slowly
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forgotten.
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But most simply, in this particular region
camels were just better and more efficient
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than wheels when it came to carrying stuff.
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The camel can travel farther, with less food
and water than a horse or ox.
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They can cross rivers and rough terrain easier
than a wheeled cart, and where a wagon requires
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a person to tend every two animals or so,
half a dozen fully loaded camels could be
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managed by one person.
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Likewise, Europeans were stunned to find no
wheeled vehicles used by the Aztecs, Incas,
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and other American indigenous cultures, even
in places where llamas were used as pack animals.
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Archaeological discoveries show us early American
cultures definitely invented wheels
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of their own, but beyond water wheels for
milling, or toys, they didn’t find wheels
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all that useful or necessary for their particular
terrain and environment.
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And this is how chariots and camels and creepy
Wizard of Oz sequels relate to evolution:
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Biological or cultural adaptations depend
on the environment in which they arise.
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The best solution to a problem depends on
the problem.
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While propellers and spinning blades are an
optimal way to move human-made craft through
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the water, the fins of fish are actually more
efficient at providing propulsion in most
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cases.
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So, there’s no fish with propellers, and
because we haven’t been able to match evolution’s
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aquatic creativity, we don’t have boats
powered by big tail fins.
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Likewise, the wheel, as a technology, isn’t
intrinsically better or more advanced than
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other ways of moving.
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The wheel only dominated in certain environments,
under certain conditions.
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And even if animals were capable of growing
wheels as body parts, in most environments
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and terrains they would probably work worse
than feet or hooves.
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Which brings us to a different kind of terrain
altogether… the fitness landscape.
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Here I don’t mean fitness like your ability
to run a mile or do pushups.
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In biology, fitness is essentially a score
that represents a trait’s ability to survive
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and reproduce.
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The wings of birds, bats and even pterosaurs,
are all modified structures of the arm, hand,
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and fingers.
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They are an example of convergent evolution,
where organisms that aren’t that closely
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related evolve similar features or phenotypes.
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You have to go pretty far back to find the
common ancestor between these winged creatures,
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but the wings of birds, bats, and pterosaurs
are all descended from an arm that is built
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essentially the same as yours or mine.
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One bone up here, two bones here, lots of
little bones here and long bones here.
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So theoretically, humans could evolve wings,
right?
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Actually, no.
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When a trait evolves, every stage of its evolution
pretty much has to provide an advantage, or
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at least not be harmful.
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Even if the final trait, like humans flying
with wings, would be super cool and give us
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lots of new advantages, we would have to be
able to get from this to wings in a way that
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every step is beneficial.
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You can’t evolve anything that reduces your
fitness.
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A fitness landscape is a way to look at a
lot of different variations and how they score
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versus one another.
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Each square represents a variation or genetic
possibility.
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The closer the squares, the more similar two
variations are, and the further the squares,
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the more different they are.
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The fitness of each genetic possibility is
represented by its height on the landscape.
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Here’s the problem.
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You can only ever move uphill, toward higher
fitness.
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There may be a highest peak, with the best
trait, on your landscape but you can’t get
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there because you’d have to travel down
into a valley first.
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Evolution is walking around this landscape
blind, it doesn’t plan or have foresight,
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so organisms often get stuck on top of these
little hills, called local optima.
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They’re as good as they can be without getting
worse.
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The human eye is a perfect example of a local
optimum.
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If the optic nerve and the eye’s blood vessels
ran behind the eye instead of through it,
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we wouldn’t have a blind spot in our vision.
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This is how octopus eyes are built and it’s
a much better way to make an eye.
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But we can’t jump all the way to that higher
fitness peak, or travel through a valley where
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we’d make our eyes worse in the meantime.
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Likewise, if we wanted to have wings like
birds, each stage of our evolution from arms
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to wings would have to provide a benefit.
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When the first birds were evolving from raptor-like
dinosaurs, they already had feathers for attracting
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mates.
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So they could use them to glide like flying
squirrels—that’s a fitness advantage.
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And each tweak and variation would help them
glide farther—more fitness improvements—all
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the way to powered flight… at the top of
a fitness hill.
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For humans, variations in our hands or arms
that gave us a small amount of gliding ability
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wouldn’t really improve our fitness right
away.
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But having dumb wing hands would have a lot
of costs when it came to things like using
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tools.
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So even if being able to truly fly would be
a huge improvement overall, those intermediate
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steps can’t evolve if they mean moving down
the fitness hill.
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And this is why zebras don’t have laser
turrets.
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Zebras would clearly benefit from evolving
defensive laser weapons to keep lions away,
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but they can’t because the intermediate
steps of evolution have to be helpful or at
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least not harmful.
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A laser turret or machine gun or bazooka is
only useful when it is complete.
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The intermediate, functionless laser organ
would just be hogging vital nutrients, a fitness
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loss.
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And sure, projectile weapons have evolved
in other animals.
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Like archerfish that hunt by spitting water,
or antlions that use sand as a projectile
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weapon.
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But if you’re prey, running away might just
work well enough, so you’re stuck at a local
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optimum without laser turrets.
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Evolution works like trying to make improvements
to an engine while the car is in the middle
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of a race.
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You can’t break your engine in order to
make it work better.
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Of course, I’m not willing to give up my
dream of flying so easily.
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Maybe there’s another way humans could grow
wings.
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Why can’t we just grow a new set of limbs?
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You can imagine this strategy working for
some bugs like millipedes.
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Entire body segments could be duplicated thanks
to mutations in genes controlling how the
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body develops, and poof: you’ve got a new
pair of legs to add to your 48 other pairs.
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Mutations in a class of body-patterning genes
called Hox genes have been linked to misshapen
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feet, hands, skulls, and even extra fingers
or toes.
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In very rare cases humans are even born with
extra limbs, like this baby who was born with
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three arms.
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But almost always, these duplicated limbs
don’t function, because the new limb also
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needs bones, joints, its own muscles and nerves,
and each of those duplications would require
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countless other mutations in other genes.
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That’s beyond unlikely.
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Evolving wheels instead of feet, or finding
a fish with a propeller, or modifying our
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hands into wings like a bat, is difficult
enough to be impossible.
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But at the end of the day there also has to
be a need to evolve - you don’t just get
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something because it’s cool.
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This is perhaps the worst car ever made in
the history of driving.
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The Trabant had a 25 hp two-stroke engine
closer to what’s in a lawnmower than a car,
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a plastic body, no fuel door, no rear seatbelts,
not even a turn signal indicator.
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Although their design changed little from
the late 1950s through 1990, when production
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ceased, more than 3 million were sold.
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That’s because this was one of the only
automobiles available in Communist East Germany.
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Before the fall of the Berlin Wall in 1989,
East Germany was a closed economy, meaning
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there was no competition from other car manufacturers
and no pressure to improve this vehicle.
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For natural selection to happen an organism
has to run into some challenge that impacts
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its survival, what we call a selection pressure.
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The East German lawnmower sedan never had
selection pressure from other automobiles
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forcing it to improve, so it continued to
sell well despite being very bad.
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Sauschlecht!
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Until humans experience a selection pressure
for gliding, we won’t start to evolve adaptations
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that lead towards flight.
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And sometimes selection pressures can be so
sudden or different, that there is no adaptation
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for evolution to even act on.
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The dodo had no adaptations for defending
itself, because it never encountered humans
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or other predators, and well, we all know
how that ended.
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But beyond all these principles of evolutionary
biology, one of the biggest limitations we
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face is physics.
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PHYYYYYYYSICCCCSSSSSS!!!
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The reason we don’t see gigantic land animals
like Mr. Longneck here any more, is because
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of two pesky bits of physics.
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One, gravity.
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You’re probably familiar with that one.
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Personally it’s how I stay so down to Earth.
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Heh.
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Two, the square-cube law.
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As an organism gets bigger, its volume increases
much faster than its surface area.
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So as something grows and gets more and more
volume, any process that depends on the amount
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of surface that you have will become less
and less efficient unless you change your
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shape to make more surface.
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This is the reason big complex organisms like
us are multicellular instead of 6-foot-tall
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single-celled amoeba blobs.
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That wouldn’t provide enough surface area
to exchange nutrients and waste and make energy
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for all of our big blobby volume.
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So instead our bodies are made of 37 trillion
cells, give or take.
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It’s the only way to have enough surface
area for all… thissss.
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Gravity and the square-cube law combined is
why here on Earth it’s unlikely we’d ever
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get land animals weighing more than 100 tons.
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It’s also the reason if you’re ever given
the choice of fighting one horse-sized duck
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or 100 duck-sized horses, you should pick
the fight versus a horse-sized duck because
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duck legs would snap like toothpicks under
the weight.
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And the limitations of physics are not just
something animals have to worry about.
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Calculations of how gravity affects the flow
of water in trees estimate the maximum height
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a tree could ever reach on Earth is 130 meters.
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And sure enough, the tallest tree we know
of… is just under 116 meters tall.
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However, a different physical environment
comes with different physical restrictions.
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Blue whales, the largest animals to ever live
on Earth, weigh up to 173 tons because the
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buoyancy provided by their watery environment
counteracts the gravity.
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Therefore, on different planets with lower
gravity or different atmospheric composition,
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we might be able to see bigger animals.
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For instance, 300 million years ago the concentration
of oxygen in Earth’s atmosphere was much
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higher than it is today (reaching as high
as 30%).
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And insects which don’t have lungs and circulatory
systems like we do—they breathe by gases
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just diffusing into their bodies—were able
to grow much larger than insects today.
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Dragonflies the size of birds?
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Not thankyou sir.
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For all of nature’s creativity, a few empty
spots remain on the tree of life where branches
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seemingly can’t grow.
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What we call impossible phenotypes, that as
far as we know have never arisen in the history
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of Earth.
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Like freshwater coral reefs, or birds that
give birth to live young.
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Or plant-eating snakes.
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I mean, there are thousands of species of
snakes!
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You’d think one would take advantage of
the food source that all other snakes are
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ignoring and go vegetarian!
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Snakes’ closest relatives, lizards, many
of them eat plants.
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I mean, making the switch from a meat eating
to plant diet worked for pandas.
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Well, barely.
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But maybe there’s something about how a
snake is built that prevents it from getting
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enough energy from plants.
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Of course maybe we just don’t know.
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For a long time it was believed that all spiders
were carnivorous, until we found one that
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eats plants.
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It’s difficult to say things are totally
impossible.
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Given more time, evolution may well produce
some of the things we’ve talked about.
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Imagine if alien evolutionary biologists studied
life on Earth 450 million years ago.
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Flowering plants, flying insects, animals
able to walk and breathe on land… these
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may have seemed like impossible phenotypes
back then, but all of them happened.
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So while there are limits to what evolution
can do in single lineages, like humans growing
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wings or animals with wheels, we don’t really
know where the boundaries of evolution are
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as a whole.
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Like… what about animals with an odd number
of limbs?
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I don’t mean three limbs like Simone’s
super cute dog Scraps.
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A three legged dog is a four legged animal
that lost a limb due to an accident or medical
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issue.
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Or a 5-limbed animal like a starfish—radial
symmetry removes many of the limitations that
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would prevent odd numbers of limbs.
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I mean a three-limbed animal like a dolphin.
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Think about it.
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How many “legs” does a dolphin or whale
have?
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Their fins have two lobes, and their prehistoric
ancestors walked on all fours, but for all
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intents and purposes couldn’t we consider
a dolphin an animal that evolved to move with
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three limbs?
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Does that mean kangaroos and their powerful
tails, or monkeys with prehensile tails are,
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functionally speaking, 5-limbed animals?
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Are snakes one-limbed animals?
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In these cases, although they are rare, there
was an evolutionary path that allowed an odd
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number of limbs or limb-like appendages to
arise from even-numbered ancestors… that’s
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a pretty remarkable leap.
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For all the rules and impossibilities we’ve
talked about, the things evolution has been
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able to mold and create are pretty impressive.
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Instead of looking at what nature hasn’t
built, maybe we should marvel at all that
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has been.
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Stay curious.
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We’re still not gonna grow
wings though.
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