Kids are weird
and not just because they do a lot of weird things,
like sticking crayons up their noses or eating dirt.
Kids are weird because humans go through a lot of changes between infancy and adulthood,
like that whole baby teeth thing.
There's a lot going on in your body
in the first couple of years that you're alive,
and some of those things are pretty frickin' bizzare.
Many babies are born with blue eyes
and they can change colour until they settle on a more permanent hue
Eye colour is influenced by pigmentation of the outermost region of the iris called the stroma
In this case it's a pigment called melanin which also affects your hair and skin colour
The specialized cells that produce and store melanin are called melanocytes
and melanin reflects and absorbs light in different ways depending on the amount of pigment
and the distribution of melanocytes in the iris, which leads to different eye colours
Blue eyes have almost not pigment
Green or hazel eyes have some yellow and some brown pigment
and brown eyes—you guessed it—have lots of brown pigment
Many newborn babies
especially those of Caucasian decent
who generally have lower levels of melanin throughout their life
don't have much pigment in their eyes, making them look blue.
Over time, in some people, the melanocytes will begin to produce more pigment
leading to a slow shift in eye color throughout childhood.
This usually slows down by around age five
but some people may change eye colour later in life as well
There's a lot of variations because
scientists estimate that around 16 genes may play a role in determining eye color
In utero, your brain actually over-produces brain cells called neurons
through a process called neurogenesis
Before you're born, nearly half of those brand new neurons will die off naturally, through a process called apoptosis
Basically, programmed cell-death
Scientists think that apoptosis is a really important part of helping your brain make the right neuronal connections to function
All cells have the program, and your brains use protein signals to keep it turned off or turn it on.
For example, when a developing neuron grows toward the right cell and makes a connection,
called a synapse, it receives some proteins called neurotrophins, which protect against apoptosis
Neurons that don't make the right connections don't get these neurotrophins, and die off instead
By the time you're born, your brain is mostly done with neurogenesis and apoptosis.
But the wiring isn't finished yet;
your brain continues to fine-tune these synaptic connections throughout your childhood
as a kid, you have way more neuronal connections than you need.
Connections that are used the most will be stabilized and protected so
your brain can keep signaling with them and the extra connections that go unused
will be pruned back by the brains immune cells called microglia.
The microglia wrap around these extra connections and break them off of the neuron, gobbling up
the cell debris. This helps the neurons focus on maintaining their stable, highly
used synapses and makes way for new synapses to connect and develop.
When you're born, your head is enormous compared to the rest of your body.
An infant's brain is already a quarter of the size of an adult brain, but their
body mass is much, much smaller than the mass of an adult, and by the time you're 2
your brain is already about 75% the size of a fully-grown adult brain
Now this might seem strange since we talked about how your brain doesn't grow many new
neurons after you're born. Turns out, your brain keeps producing other kinds of
brain cells, collectively called glia, to help support and maintain the neurons
but how can your brain grow so much when your skull is surrounding and protecting
all that tissue? Well, we've evolved these soft spots or fontanelles on our infant
skulls, places where the skull hasn't yet fused together, leaving it flexible
so a baby's head can be compressed a little bit to pass through the birth
canal. Compared to other primates our soft spots fuse relatively late in
childhood at around two years old instead of just a couple months, allowing
space for the brain to gradually grow and develop. So your giant baby head can
expand to hold all of that growing brain mass and the rest of your body can catch
up to it later. Human babies grow really slowly compared to other mammals we have
a prolonged childhood and little kids require a lot of attention and
supervision it's kinda hard to imagine why staying little for so long could
have an evolutionary advantage. Some scientists think they figured it out, and
it's because our brains use a lot of energy using brain imaging data a team
of researchers from multiple universities estimated how much energy
our brains need at different points in our lives. They were specifically looking
at how much glucose
the brain was consuming in 29 kids and compared it to the body's overall daily
energy requirement and the glucose consumption of seven young adult brains
as a control. Surprisingly they found that even though our brains are
proportionately largest right after we're born
that's not when they seem to be using the most energy. When you're around five
years old, right when your childhood growth rate is slowest, your brain uses
energy at a rate of about two-thirds of your body's resting metabolism. This is
around the time when your brain is really stabilizing and pruning all of
those neuronal connections. So scientists think your brain could be using so much
energy doing this construction work, that your body has to slow down other
processes like growing up and, once your brains energy consumption starts to
decrease again, you'll eventually hit the last big growth spurt, that fun friend we
call puberty. Everyone's familiar with the sensation of shivering basically
when you get cold your brain starts signaling your muscles to make them
quickly contract and relax. The process produces heat and warms up your body
But babies under the age of around three months haven't developed the systems and
muscle tissue needed to shiver yet, so they have to keep warm some other way
They have more of a special kind of fat called brown fat with lots of blood
vessels and energy producing cell organelles called mitochondria. So when
babies get cold, they burn brown fat located in their chest, shoulders and
backs This releases heat energy to warm them up and this whole process is called non
shivering thermogenesis. It's essentially the same thing that helps keep some
animals warm during extended hibernation. This is really important because babies
are particularly susceptible to cold. They can't dress themselves or move away
from cold things, so they needed a good way to keep warm
The larynx is the technical term for the voice box. It helps you breathe without
inhaling your food by closing the epiglottis the flap of cartilage behind
the tongue so food can slide right on past to the stomach. Plus the combination
of vocal cords inside the larynx and air coming from the lungs allows us to
produce speech. The human larynx is lower down in the throat then in non-human
primates and scientists think this anatomy helps us articulate so many
different sounds. It's pretty important for producing language which I will talk
more about in a second. In infants the larynx is positioned really high up
pretty much inside of the skull which is similar to non-human
primates like chimpanzees. And this positioning is really useful. It allows
babies to breathe through their noses while eating. Since babies aren't very
coordinated this is really important for protecting their lungs from any liquids
or food bits so they don't choke. But by two years of age the larynx has begun to
drop into the neck so the pathways to the lungs and stomach begin to intersect.
As an adult it's up to you to figure out how to safely breathe and eat at the
same time. Now this doesn't sound that interesting after all we use language
every day and of course you learn it as a kid. But that's what makes it so cool
we learn language without any formal teaching. Linguists think the basics of
language: speaking signing and linguistic comprehension are biologically innate
basically all humans can use these skills. There are lots of different
theories about how we learn language and none of them are a really complete
explanation. But scientists do think it has something to do with experiencing
language in your environment. Jean Piaget, legendary child psychologist, was a fan
of the cognitive theory. He believed that children first developed the ability to
understand the idea of a thing like the concept of a ball or the feeling of love
and then they learn how to express it in words.
Another popular theory is learning through imitation and positive
reinforcement. Children learn by imitating adults and getting feedback on
what's right and what's not but kids of course can make up completely new
sentences and words that they'd ever heard before, so this theory can't
explain it all. So there's another possibility: the innateness of certain
linguistic features. The theories that all humans are born with some innate
grammatical knowledge and just need exposure to a primary language to start
putting it into action. This could maybe explain why all languages have things
like vowels and consonants and nouns and verbs. If we're somehow hardwired to
understand and use them, even the very first languages could contain these
basic kinds of tools. So parents always say that kids grow up fast, but it turns
out there are a lot more weirder, cooler things to childhood development than you
might have expected. Thanks for watching this episode of scishow brought to you
by our president of space
Dillon Barth who would like to remind everyone that if you want something
you've never had, you have to do something you've never done. If you want
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another patreon.com / scishow and don't forget to go to youtube.com slash
scishow and subscribe. ...learned it in picture books and preschool cat's meow
dogs bark and ducks quack, but not all animals use such obvious methods of
communication and some of them are downright weird