Hi. It's Paul Andersen and welcome to disciplinary core idea LS4A. It's on evidence
of common ancestry and diversity. When Darwin published his ideas on common descent he really
didn't have molecular evidence to back him up. He didn't know how genetics really worked.
And he didn't have clearly a picture of what DNA looked like or how it operated. And so
he didn't have a lot of evidence. And when you're looking at life the one thing that
jumps out is the diversity of life. Look at all the different forms of life we have from
the archaea to protozoans, animals, fungi, plants and bacteria. This is a branching tree
of life. And the one thing that jumps out is how different it all is. And so the unity
is missing. But there's going to be clear unity within life as well. So all life uses
the same genetic material. It all uses DNA. It uses the same machinery. All cells are
essentially put together in the same way. And so there are two concepts that are important
that your students understand. That is the unity. It's what ties us all together. And
then it's the diversity. It's all the differences in life. And when Darwin came up with this
idea of common descent, and I love this picture. It's right out of his notebook, it was an
amazing idea. He believed that everything on our planet, all life came from one common
ancestor. And it branched off. And I like how he wrote here, I think this is what occurred.
And if you compare that to the current day phylogenetic tree of life with all of the
different groups coming off of that one single strand, they look very very similar. And so
what evidence did Darwin use to back it up? Well he started with fossil evidence. And
so if you look at these four types of animals, they all have the same bone structure. And
so even though they look different, like a human arm, a dog leg, a bird wing and a whale
flipper, they probably had an ancestor that had similar bone structure. And you could
also see it in the fossil record, in the rock itself. You could see fossils changing over
time. So horses for example. As they moved out on to the plains got larger and larger
and larger. And you could see the change in their fossils. A big piece of evidence we
have today is molecular evidence. So DNA evidence. And if you look at the organisms on our planet
that are most related to us, we can line up our DNA and we can compare it to their DNA.
And if you line up human DNA and chimpanzee DNA, you'll find that there's only 1.2 percent
difference in all of their DNA. If you look at a gorilla that jumps to 1.6. And in a baboon
it jumps to 6.6 percent. And so we can compare how much our DNA is alike. And I mean exactly
alike. And so we can see who's related to whom. There are new molecular tools that we're
discovering as well. And so we can look at the evolution of organisms through their development.
And so there's a set of genes called the hox genes that essentially tell an organism where
to put its body parts. And if we look at the box genes in a fruit fly and compare it to
a vertebrate we're going to find that they're very very similar. And so we can see who's
related to whom based on these simple sets of genes. And this idea of evolutionary development
or sometimes scientists refer to it Evo Devo is very promising when we're trying to build
this phylogenetic tree. We can also look at fossils and compare those to living things.
So this is the glyptodont what was a massive armadillo like creature that weighed about
as much as a car. And we can compare its fossils with living organisms like an armadillo today.
So what's the teaching progression? Well in the lower elementary grades you want your
students to understand that extinct species, be it dinosaurs or plants that aren't around,
or fish that aren't around are going to look very similar to living species like these
lizards that we have today. As you move into the upper elementary grades we want to start
talking about fossils. Both fossils of large organisms, like dinosaurs. And fossils of
microscopic organisms, like these stromatolites right here. And they tell us a little bit
about what the history on our planet used to look like. Not only what the life looked
like but what the environment was like at that time. And also telling them that we can
compare organisms that we have today to organisms that once were alive. As we move into middle
school we want to talk about the importance of rocks. And especially sedimentary rocks.
And sedimentary rocks are formed when other rocks are broken down and then they're compressed
usually with water over a long period of time. And we have thousands of different layers
of sedimentary rocks on our planet. And what they tell is is not only the history of the
earth, because we can radiocarbon date this rock and we know how old it is. But it also
tells us about the history of life. Because we can find fossils contained within the sedimentary
rocks. And so the fossil records are really documenting, right here we're looking at these
fossil beds which are in the John Day strata. And what you'll find is as we look through
it we can look at different times. So that gives us time. And then we can look at their
relative locations to one another. And so we can learn about how life came to be. How
life ended. How species went extinct. The great diversity of life. And then how species
have changed over time. Other things we want to talk about in middle is this molecular
evidence that we're using to piece together history. And then this idea that we can compare
living material or living organisms to organisms that were once on our planet. And so when
we build this phylogenetic tree, in other words this tree of life, what evidence are
we using to put it together? Well you should talk about that in high school. This idea
that DNA is on of the biggest pieces of evidence that we have. Every living thing on our planet
uses DNA. But it doesn't have to be DNA. We could look at the amino acid structures within
proteins. We can also look at the anatomy and how they've changes over time. And then
we can look at Evo Devo. We can look at embryology and see what that tells us about development.
And so that's common descent. And I hope that was helpful.