- [Instructor] About 3.5 billion years ago,
single-celled organisms were the only life forms
that existed on Earth.
These organisms passed on their genetic material
with slight changes to their descendants.
And over long periods of time,
these genetic changes resulted in new species.
And eventually, the vast diversity
of modern organisms evolves.
Some organisms share a more recent common ancestor
than others do.
Scientists have found evidence
that we humans are more closely related to chimpanzees
than we are to rabbits.
And we're more closely related to rabbits
than we are to sharks.
And you might be wondering how can scientists
determine how closely two species are related
to each other.
Asking a shark,
"Hey, do you think we shared a common ancestor
440 million years ago?" doesn't help,
because even if they spoke English,
they probably wouldn't know the answer either.
Scientists analyze the similarities and differences
between species to help figure out
how they might be related in evolutionary history.
And they found significant evidence for evolution
by using a variety of methods,
including examining the fossil record,
analyzing the DNA of different organisms
and comparing the development of embryos,
which are what organisms are called before they're born.
However, you don't need a microscope or a fossil
to find evidence for evolution.
You can find it by looking at the anatomical
or physical features of organisms alive today.
One way that scientists use information from the present day
to make sense of past evolutionary relationships
is by searching for clues called homologous features.
When two species share a structurally similar
anatomical feature that they inherited
from a common ancestor,
we say that the feature is homologous.
The word homologous begins
with a Latin prefix, meaning the same.
And this makes sense,
because when two species have homologous features,
it means they share the same ancestor.
Have you ever looked at an x-ray of a human's arm?
If you have, you might have noticed that we have
humerus, ulna, radius, carpals and metacarpal bones.
If you looked at the skeleton of a bird's wing,
you could see that birds have
a similar bone structure to ours.
In fact, this similar bone structure appears
in the forearms of many other animals,
including dogs, cats, whales, elephants, and bats.
These animals can use these features in different ways
to fly, to run, to swim or to wave hello.
But despite these differences in function,
the similarities of the bone structure
indicate that these species inherited these features
from a common ancestor.
This means that the bone structure is a homologous feature.
Compared to species with few similarities,
two species that share many homologous features
are likely to be more closely related,
which means that they're likely to share
a more recent common ancestor.
Species with fewer homologous features
are likely to be less closely related,
which means that they are likely to share
a more distant common ancestor.
For example, the wing of this bird
has more structural similarities
to the leg of this lizard
than to the wing of this bat.
This indicates that the bird is more closely related
to the lizard than it is to the bat,
which means that the bird and the lizard
share a more recent common ancestor
than the bird and the bat do.
When scientists believe they have found species
with homologous features,
they can use other methods to find out more
about how the species evolved over time.
They can compare the anatomical features of modern organisms
to those found in fossils
to see how much they have in common.
Sometimes, scientists find features
that seem to be homologous at first glance,
but the features actually evolved independently
along different species' lineages.
These features are called analogous features.
For example, let's take a look
at the wings of two flying creatures:
a bird and a butterfly.
In order to figure out whether the wings
are homologous features,
we need to examine the physical structure of the wings.
The bird's wing is made up of hollow bones
while the butterfly's wing is made up of membranes
made out of a protein called chitin.
So even though birds and butterflies can both fly,
their wings have very different structures.
This means that bird wings and butterfly wings
are analogous features.
This indicates to scientists
that birds and insects did not inherit that feature
from a common ancestor.
Instead, the ability to fly using wings
evolved independently in each lineage.
Comparing the anatomical features of organisms
has given scientists a better understanding
of how life on Earth has evolved from common ancestors.
So next time you meet a friendly shark,
perhaps the two of you can bond over your shared ancestry.