[♪ INTRO] 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. If you’d like to get involved, check out
patreon.com/scishow. [♪ OUTRO]
