Evolution from Single Cells to Humans

Today I’m going to show the step by step
evolutionary journey from the earliest single celled organisms all the way to modern humans.
I’ll be using my Evolution and Classification of Life chart, which is available as a poster
from my website UsefulCharts.com. As the title suggests, this is both an evolution
chart and a classification chart. So it not only shows how all living things are connected
from an evolutionary point of view, it also shows the categories that are used to classify
living things. So you’ll be hearing familiar terms such as kingdom, phylum, class, and
order. This video is part of a two-channel collaboration
with fellow YouTuber Stefan Milo. Whereas I will describing the full journey of human
evolution, Stefan will be talking about some of the quirks in that process, which demonstrate
that our bodies were not designed from scratch but rather bear traces of a long history of
adaptations. So make sure you check his video after this one. You’ll find the link on
screen at the end of the video as well as in the description. So we’re going to start at the bottom of
the chart with the emergence of life about 4 billion years ago. That life would have
been in the form of single celled organisms, similar to the bacteria of today. Now, I’m
not going to discuss how that first cell came to exist because that’s something that is
beyond the scope of evolutionary theory. However, there are several ideas out there about how
that happened so if you’re interested in learning about that, you’ll want to search
for the term abiogenesis. I’m also not going to talk about viruses – you can see I’ve
placed them off to side here – because there is currently some disagreement over how exactly
they relate to life and whether they evolved before or after single celled life. But what we do know is that life eventually
proceeded in three different directions. These are called domains and they are the top classification
level, coming before the more well-known kingdom level. So we have bacteria, which includes
single-celled organisms that often cause infections in humans such as cholera or tetanus. Then
we have archaea, which are kind of like bacteria and can sometimes be found in really extreme
environments such as super hot or super salty conditions. And then finally, and most important to us,
is the eukaryote domain. Eukaryote means “true cell” and is so labelled because eukaryotes
have cells with a nucleus and other inner parts.
This originally occurred because of a process known as endosymbiosis in which several different
types of cells came together to form one big cell that was more complex. This likely occurred
about two billion years ago. From this point, life again proceeded in three
different directions. First, there were different types of algae that eventually evolved into
the plant kingdom, Second, there were various types of mold that evolved into the fungus
kingdom and then third, there were several amoeba-like organisms that evolved into the
animal kingdom. All of these simple forms of life are called protists and in fact they
can actually be divided into a bunch of mini kingdoms. The Fungus kingdom, of course, includes
the familiar mushroom but also things like yeast and the mold that grows on bread. But this chart is mostly concerned with the
two really big kingdoms: plants and animals. And since we’re mostly going to be looking
at the animal side, let’s quickly look at the plant side before we do so. So from red
algae and brown algae eventually came green algae. Plants are green because they have
a special part of their cell that other living things do not. This is called a chloroplast
and its the part that captures energy from the sun through a process called photosynthesis. From algae came the earliest types of land
plants, such as moss. That occurred about 450 million years ago. From there, we then
get the simplest types of plants with branches and leaves, like ferns. Next, we get a really
important development. We get plants that bear seeds and thus are able to replant themselves
over longer distances. And it is at this point that the planet kingdom splits into two main
divisions. We get the conifers, which are trees with cones such as pine trees or cypress
trees and then we get angiosperms, which are trees and plants with flowers. Now, interestingly, flowering plants actually
evolved relatively late in earth’s history – only about 150 million years ago. That’s
half way through the age of the dinosaurs. And when I say flowering plants, I’m not
just talking about things like tulips, roses, and daises. I’m also talking about most
of the food that we eat – everything from wheat and rice to fruits, vegetables, and
even chocolate. All of those things come from flowering plants. Okay, what we really want to focus on though
is the animal kingdom. Most people will be surprised to learn that a sea sponge is actually
an animal. It might look like a plant but it’s actually
comprised of a bunch of simple animal cells bunched together. From this point, what happened
is that different cells evolved to perform different types of jobs, such as those involved
in movement. So for example, we eventually get jellyfish. A jellyfish is still a very
simple creature but unlike a sea sponge, it can move around. This is because it has nerve
cells that send electrical signals to certain parts of its body, causing those body parts
to jerk and thus causing itself to swim. Now keep in mind, as we go step by step, that
each new major development along the evolutionary journey took millions, if not tens of millions
of years to occur. So none of these changes happened fast. A million years is a really
long time and it’s really hard for our brains to think using numbers that are that big.
So just remember that evolution occurs because the mechanisms involved such as mutation and
natural selection have really, really long time periods to work with. Okay, so around 550 million years ago, we
get what’s called the Cambrian explosion. That’s when we get animal life started to
go in a bunch of different directions. And it all started with something called bilateria.
If you look over at this flat worm here, you’ll notice that one side of it mirrors the other
side. Well, this is basically true for all animals from this point forward, including
humans. That’s why we have two eyes, two arms, two legs, and so forth. From here, animals can be divided into two
categories: protostomes and deuterostomes. Protostomes include the various worm phyla,
as well as the phyla arthropoda and mollusca. Phylum arthropoda is comprised of all those
creepy crawly things that most humans tend to hate, like spiders and insects but also
larger creatures like lobsters. Phylum mollusca is comprised of creatures with hard shells
like snails and clams but also some really intelligent sea creatures such as octopi and
squid. It is from the early deuterostomes that the
vertebrates eventually evolved. All vertebrates belong to the phylum chordata and they differ
from the invertebrates (shown in purple) in that they possess a back bone, or spine – that
long column of bones that runs down the back and protects the spinal cord. The earliest vertebrates were all fish, first
jawless fish and then fish with jaws and teeth. After this point, we then get cartilaginous
creatures like sharks but also, for the first time, creatures that have what we consider
to be a full skeleton: the bony fish. Bony fish can be divided into two main categories.
Most fish are ray-finned fish. These include all the familiar types of aquarium fish such
as tetras, guppies, and goldfish and also all the fish that humans tend to eat like
cod, halibut, and salmon. The lobe-finned fish are quite a bit different. They have
downward facing fins with more muscles in them. Eventually this feature led to the evolution
of certain fish that could prop themselves up in shallow water, like the now extinct
tiktaalik. From a creature similar to tiktaalik came
the tetrapods, tetra meaning four, and pod meaning feet. So tetrapods are animals with
four limbs, which basically includes all of the animals that we tend to think of as being
animals. First off, there’s the amphibians, like frogs, who live both in the water and
on land. But then something evolved call the amniotic sac which allows some animals to
lay their eggs on the land instead of in the water. From this point, animal life proceeded in
two very different directions. We get the early synapsids, that eventually evolved into
mammals; and we get the sauropsids, that eventually evolved into reptiles and birds. Since we’ll
be focusing mostly on mammals from this point forward, let’s quickly look at the sauropsids
before we do so. The three main reptile orders that still exist today are the crocodiles,
the lizards and the turtles. Most of the other orders are now extinct, such as the fish-like
plesiosaurs and ichthyosaurs and the bird-like pterosaurs. Although those groups are sometimes called
dinosaurs, the term “dinosaur” technically only refers to these two orders here: Saurischia
and Ornithischia. Ornithischia, which means bird-hipped, includes dinosaurs like triceratops
and stegasaurus, whereas Saurischia, which means lizard-hipped, includes the huge long-necked
herbivores (called sauropods) as well as the carnivores (called therapods). In the therapod
group we get the infamous T. Rex as well as the raptors. The therapods are also important
because it is from that group that birds eventually evolved, one of the earliest being the Archaeopteryx. You might have thought that birds would have
evolved from the bird-hipped dinosaurs but take note that they actually evolved from
the lizard-hipped dinosaurs, and in particular, from the therapods. The technical term for
birds is aves so the bird class is called the Avian class. Note that the ancestors of
game birds like chickens and ducks evolved first and then we get a large group known
as the Neoaves, where we find all sorts of familiar birds like eagles, pigeons, and crows. Okay, let’s now go back to the synapsid
line. The earliest synapsids looked a lot like reptiles, sort of like this pelycosaur
here. But eventually their skeletal and organ structure evolved along a very different path.
Initially, this had a lot to do with movement. If you watch a lizard move, you’ll notice
that they tend to waddle from side, whereas a dog has the ability run in more of a galloping
style. This is a very rough way to explain the initial difference. The first big group of synapsids were the
therapsids and from them evolved the cynodonts. Eventually, the cynodonts developed one of
the key features of the mammal class – they became warm blooded, which meant they also
evolved hair or fur in order to regulate their temperature. Of course, the other thing that
sets mammals apart is that they possess mammary glands, which they use to feed their young.
In fact, this is where the word mammal comes from. One of the earliest types of mammals that
still exists today is the duck-billed platypus. They belong to the monotreme order and most
people think they look awfully strange. But most mammals belong to a group called Theria,
which simply means beasts. There are two major categories of beasts – marsupials and placentals
– and the fact that they evolved differently had a lot to do with geography. For example,
today, we only find marsupials in the Americas and Australia. Marsupials, such as the kangaroo or opossum
grow their babies in a pouch whereas placentals have a placenta and are thus able to grow
their babies inside and give birth to them when they are more well developed. I should
note that up until the time that modern humans arrived in Australia, there were all sorts
of other types of marsupials – types that we don’t see today – including marsupials
that looked like cats and marsupials that looked like bears. But let’s continue with the placental line.
Some of the oldest branches of this group eventually lead to elephants as well as sloths
and armadillos. But another branch leads to the Boreoeutheria group, which has two main
categories: Laurasiatheria and the Supra-primates. Humans are supra-primates so let’s look
at Laurasiatheria first. Lots of well-known animal species belong to
Laurasiatheria, including all of those that are carnivores. The carnivore order includes
bears, dogs, and cats but also sea creatures such as seals and walruses. Laurasiatheria also includes the Ungulates,
which are the animals with hoofs. Those with an odd-number of toes are called perissodactyls
and include horses and rhinos. Those with an even-number of toes are called artiodactyls
and include farm animals like pigs, cows, and sheep but also animals with antlers, like
deer. Interestingly, dolphins and whales are also closely related to the artiodactyls.
They evolved from land animals that eventually returned to the sea. Okay, let’s look at the Supraprimates. From
the original supra-primates evolved rabbits and rodents but also the earliest types of
actual primates. We tend to associate primates with monkeys but the primate order also includes
animals such as tarsiers and lemurs. Monkeys, as well as apes, belong to a special group
of primates called simians. A lot of people use the words monkey and ape interchangeably
but really they are quite different. Apes do not have tails and are generally quite
a bit larger and smarter. It is of course, from the early ape species
that humans evolved. Currently, there are only four kinds of great apes: orangutans,
gorillas, chimpanzees, and humans. But over the last 15 million years, there have been
many other kinds as well. It’s important to remember that the great apes of today all
evolved from these now extinct species, not from each other. So, although you may sometimes
hear it said that humans evolved from chimpanzees, this is actually not true. Both modern humans
and modern chimps evolved from a now extinct common ancestor who lived about seven million
years ago. To us, that creature probably looked more like a chimpanzee than a human. But really,
they were equally both. From that common ancestor evolved the now
extinct genus known as Australopithecus, which included the female known as Lucy, who’s
skeleton was found in Africa in the 1970s and dated to about 3 million years ago. Australopithecus
is thought to be one of the earliest apes to walk primarily on two feet. From Australopithecus, evolved the genus homo,
which is the genus that modern humans, called homo sapiens, belongs to. Currently, homo
sapiens is the only species in this genus. But over the last 2 and a half million years,
many other homo species have come and gone. We know this from the archaeological record,
which gets larger and better every year. One of the earliest homo species was Homo habilis.
They were among the first humans to start extensively using tools. Then there was Homo
erectus, the African version of which is now called Homo ergaster.
They are thought to be the direct ancestor of Homo heidelbergensis, from which came homo
sapiens, our close cousins the Neandertals, and a recently discovered group known as the
Denisovans. You only have to go back around 50,000 years in history to find more than
one human species still existing on our planet. At that time, the Neandertals and the Denisovans
were still around, as was Homo Florensiensis, the hobbit-like species that lived on an island
in what is today Indonesia. But of course, nowadays, homo sapiens is the
only human species left. I’ve chosen a member of the South African San tribe to represent
our species since genetically, they are closer to the ancestor of all living humans than
any other group on Earth. So, there you have it. A step by step look
at how modern humans evolved all the way from single celled organisms. Of course, there’s
a lot more that could be said on the topic and in this video, I wasn’t able to go into
all the various lines of evidence that support the theory of evolution. However, fellow YouTuber
Stefan Milo has made a video that starts to scratch the surface of that subtopic. Be sure
to click the prompt at the end of this video or click on the link in the description.


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