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So you know that cheesy old picture of human evolution,
with an ape getting more and more upright until it
proudly steps forth as a human being?
You probably know that’s not how evolution works.
It doesn’t have a plan; organisms aren’t all headed on a set trajectory
toward some imagined biological “perfection”.
Evolution just throws everything at the wall and sees what sticks.
Well, it can’t even plan ahead that much.
A bunch of different things exist, the circumstances of life slam them against the wall,
and the ones that survive wall-banging are the ones that stick.
Or more technically, traits arise through random mutation,
and natural selection means the more advantageous ones are passed on.
And sometimes, as these adaptations arise,
parts and functions change in bizarre ways over time.
After all, natural selection also means that as long as there is no lethal disadvantage,
non-optimal traits can still get passed down.
With the bar always set to “good enough”, the traits that evolve can be pretty clunky.
So it’s worth taking the time to appreciate
just how inefficient evolution can be sometimes.
Spoiler warning: No, we humans are not exempt.
Photosynthesis: the method of making food most highly recommended
by cyanobacteria, algae, and plants the world over.
Photosynthesis relies on an enzyme called RuBisCO.
The first and still most common form of this process is called C3 photosynthesis.
Using water, carbon dioxide, and solar energy,
RuBisCO strings together a molecule with three carbon atoms, hence “C3”.
That molecule subsequently is converted into a sugar.
However, this enzyme is notoriously inefficient.
Most enzymes can process thousands of chemical reactions a second.
RuBisCO can handle about three molecules of CO2.
Worse still, it’s pretty indiscriminate with what molecules it processes.
It readily wastes energy and water by breaking down oxygen
when it gets too hot and dry.
And the resulting byproducts can cause damage to the cell.
So why hasn’t evolution improved RuBisCO?
Being able to photosynthesize at all is kind of impressive.
After all, the C3 pathway allowed ancient microbes
to make their own food from practically nothing.
Just some common resources plus the actual sun.
So once these early cyanobacteria cracked it,
that was the way of photosynthesis for the next couple billion years.
But it’s not the only way.
Much, much later in the history of life on Earth —
we’re talking only a few tens of millions of years ago —
plants evolved two new photosynthesis pathways: C4 and CAM.
C4, as the name suggests, involves a molecule with four carbons,
produced by a CO2-specific enzyme in a different physical location than RuBisCO.
That molecule is then moved to where RuBisCO is,
converted back to CO2, and handled as normal.
Waste oxygen doesn’t actually get anywhere near RuBisCO,
and the whole process works more efficiently.
CAM, meanwhile, keeps oxygen and RuBisCO apart in time instead of space.
It exchanges oxygen for carbon dioxide at night, stores the CO2 as malic acid,
and then converts that into sugars by day.
This is seen mostly in succulents that live in dry conditions –
the nighttime gas exchange means more efficient water use, too!
But in spite of how effective these forms of photosynthesis are
at solving C3’s deficiencies, they haven’t replaced it.
Only about 3% of land plant species are C4 photosynthesizers,
while about 6% of plants use CAM.
Those C4 plants have had a lot of success, including as food crops.
But their abundance doesn’t mean C3 just goes away.
Because evolution doesn’t have a delete button!
C3 still works well enough that the genes for it still get passed on,
so it… keeps being a thing.
Marsupials are famous for having pouches to carry their offspring.
But much more than a fashionable accessory, these structures allow marsupials
to give birth way earlier in development than other mammals do.
We’re talking only a few weeks of gestation.
But the really weird part, at least from the perspective of us placental mammals,
is that as soon as marsupials are born, they have to climb from the birth canal
all the way up to a teat in the pouch and latch on.
That’s all while they’re still basically just an embryo.
They then finish developing in the pouch over the coming months.
So what’s the evolutionary reason to make your offspring
parkour their way up your body?
We don’t know for sure.
The split between placental mammals and marsupials mostly involves differences in soft tissues.
Those don’t get preserved well in the fossil record —
so we don’t totally understand how the split happened.
The best evidence we have are differences in embryonic forelimbs
and maybe in teeth.
These features are thought to be important to the
immediate post-birth climb-and-clamp that marsupial young go through.
The main hypothesis is that giving birth super early
lets marsupials respond to harsh or changing conditions.
A placental mammal’s pregnancy is a significant investment of time and energy.
But it’s thought that a marsupial can more easily stop
when things get bad, and try again later.
But ultimately, to the question of “Why did some mammals start giving birth
to helpless little blobs that have to free solo their way up to their first meal or die?”
the answer is… we don’t know.
Next, the recurrent laryngeal nerve, which controls most movement
in the vertebrate voice box, is wrapped around part of the aorta.
Put another way, a nerve from your throat makes a detour
down to your heart before heading up to your brain.
This is true even in the longest of necks in the vertebrate world.
A giraffe’s RLN theoretically only needs to travel about 10 centimeters
from the larynx to the brain.
Instead, it reaches all the way down to the aorta and back –
a distance of about 4 meters.
Now picture a long-necked dinosaur.
Yeah, scientists think that situation was even worse.
So how did this engineering screw-up happen?
It’s down to how an embryo slowly develops from a ball of cells
to an animal with all its distinct parts.
A small handful of the same genes control where and when parts of the body form in
all animals, and there’s not much room to mess around with that pattern.
And as it happens, the RLN forms next to, and at the same time as, the aorta.
This results in the two looping through one another.
The neck only starts to form later on in the process,
gradually separating the brain and the heart as it grows during the rest of gestation,
and thus making the RLN way longer.
Imagine an ancestor with no neck to speak of, like an ancient fish.
And instead of a larynx, it has gills.
In a body like that, the distance from the heart to those gills
would have been negligible.
Thus, the random looping of these parts would have made no difference to survival.
Consequently, that developmental pattern would have been passed on,
even as fish gave rise to descendants that would one day become giraffes.
So because evolution can’t predict what changes will occur in the future
and can only use the tools that it already has,
we’re stuck with those fishy blueprints for the RLN and the heart.
Now, our eyes are the main way that we detect
certain frequencies of electromagnetic radiation.
You know, how we see stuff.
And we vertebrates do it backwards.
Our photoreceptors, the structures in our eyes that receive light,
are behind the nerves that actually send that light information to our brain.
In other words, light has to pass through these nerves to reach the photoreceptors.
Then that information is transmitted by the nerves,
which converge in a hole in the retina to get to the brain.
Which means in addition to being inefficient,
our method of vision leaves a subtle blind spot.
The eyes of cephalopods — octopuses, squid, and the like —
evolved independently of vertebrate eyes.
And they actually have all their parts arranged in an order that makes sense.
So why are our eyes so convoluted, if nature has demonstrated
that it’s capable of a more efficient solution?
This question has made the vertebrate eye
the poster child for evolutionary inefficiency.
See, eyes didn’t evolve all at once.
Like most features,
they arose as a result of gradual changes.
And those changes didn’t know they were making an eye,
so they didn’t have an ideal end point to work towards.
Our early fishlike ancestors evolved primitive light detection organs
from tissue that extended out from the brain.
That tissue effectively turned inside out
as it developed into an eyeball across evolutionary time.
It would have started out as neural tissue with a little bit of light sensitivity,
located on the top side of a transparent body.
Eventually, that gave rise to slightly more light-sensitive nervous tissue
in specific spots under patches of transparent skin,
which would slowly develop into the eyeballs we watch
YouTube science videos with today.
Cephalopod eyes instead evolved from light receptors on their skin —
so from the outside in instead of the inside out.
In both cases, they evolved a concave shape,
which catches light more efficiently than a flat patch.
Eventually, the focusing ability of eyeballs as we know them would prove
advantageous enough to convergently evolve in these two totally different lineages.
It’s just that the cephalopod ones turned out way more streamlined than ours.
So sure, we can look at our eyes now and go geez, that’s dumb.
But it was kind of a logical result, given where they started.
Though given the option, I do recommend being more like cephalopods.
In general.
They’re pretty great.
Bipedalism is pretty unique to humans, at least as far as mammals go.
But going from horizontal to vertical over a relatively short stretch of
evolutionary time means that our bodily architecture just plain isn’t up to code.
Spines have been around for half a billion years, and for most of that time,
they weren’t meant to support weight upright.
They evolved underwater, where buoyancy mostly took care of the weight problem.
Initially they functioned simply as protection for what we now call the spinal cord,
and as an anchor for stronger and stronger swimming muscles.
Gradually, spines developed a nice bridge-like arch shape
as animals moved onto land and got bigger and heavier.
This arch was great for supporting the weight of a bunch of guts
all slung underneath a horizontal vertebral column.
However, making the switch to bipedalism a few million years ago
meant trying to basically use that bridge as a skyscraper.
Individual vertebrae needed to be cushioned for the weight
of a whole body smooshing them together.
And that spinal arch had to change shape, giving us the S-curve we have now;
our forward-curving lumbar region helps keep our center of gravity
directly over our legs for balance.
Switching to full-time bipedalism allowed our ancestors to travel across land
further than any other species of primate ever had before.
Plus, it freed up half their limbs for stuff other than locomotion.
You know, hands?
But the fairly rapid structural changes involved mean a whole host
of fun aches and pains that we still get to enjoy.
And that’s not all, because our feet are a mess too.
The bones in our feet evolved to handle entirely different stresses.
Now they just get slammed into the ground and have to support an entire body.
Another quirk of bipedalism is that birth is much harder with our narrow pelvises,
at least compared to most other mammals.
Yet as inconvenient and difficult as standing upright can be, enough human babies
survive that our genes for being like this will keep getting passed on.
Coming down from the trees may have had many advantages,
but because nature makes it all up as it goes,
we ended up with some pretty cobbled-together solutions to the resulting problems.
In conclusion, evolution works with what it’s got, and only well enough to get by.
It’s not efficient – it’s just… sufficient.
There’s no rhyme or reason to what traits may change in organisms over time.
But how and why those changes end up sticking around
can be downright fascinating.
Thanks for watching this episode of SciShow,
which was brought to you by this month’s President of Space, Matthew Brant.
Your generous support means a lot to us, so thank you.
And that goes for all our other patrons, too — you guys are the best.
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