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Course: Middle school biology > Unit 8
Lesson 1: Evolution and common ancestryCommon ancestry and evolutionary trees
Birds are direct descendants of dinosaurs from millions of years ago. Lineages like these can be modeled using evolutionary trees, which show relationships between different species and their common ancestors. These trees are built using evidence like fossils, body structures, and DNA. Created by Khan Academy.
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- "The dinosaurs are still Among Us"(17 votes)
- So, wouldn’t a normal pigeon from New York be a killing machine 165 million years ago?(10 votes)
- what did the TreX evolve into?(3 votes)
- how do you know that birds are dinos(1 vote)
- Birds arent technically "dinos" they are descendants of dinosaurs and we know this because we look at earlier dinosaurs that look similar to our birds today.(9 votes)
- how are birds related to trexs(4 votes)
- what did the Meglodon evolve into?(2 votes)
- they died of like the t-rex during ice age period and scientists suspect it is because of sudden change in temperature(3 votes)
- So basically dinosaurs turned into a "Two legged feathered common ancestor" Wow...(3 votes)
- Well, dinos were the common ancestors, but sure(1 vote)
- that's a pigeon not a dove, right?(3 votes)
- Pigeons ARE doves.(1 vote)
- I thought the dinos died because an asteroid?(3 votes)
- how come they didn't go extinct when the meteor hit?(2 votes)
- The meteor mostly killed plant life, and so it limited the overall food on the planet. The larger dinosaurs and reptiles had to eat more, and the limited resources couldn't satisfy their diets, so they died out, but the smaller animals, like the Maniraptora, didn't die out due to their smaller needs.(2 votes)
Video transcript
- [Instructor] Have you ever heard someone call birds living dinosaurs? You might find that hard to believe. After all, the city pigeons
that you see wandering around town don't look
particularly ferocious like a Tyrannosaurus rex. But it turns out that
our feathered friends are actually the direct descendants of a group of dinosaurs that
lived millions of years ago. Birds, like every modern species, have a series of ancestors
stretching back through time. You can think of these as
the species' grandparents, great-grandparents,
great-great-grandparents, and so on, going back
thousands to millions of years. This series of ancestors is called a species evolutionary lineage. And, also like every modern species, birds have related species that share a part of their lineage but then branch off on
their own evolutionary path. The modern bird lineage stretches way back to 165 million years ago
to the Jurassic Period. During this time, iconic dinosaurs, like the stegosaurus and
brachiosaurus, roamed the Earth, as well as distant relatives
of our modern-day birds, like archaeopteryx and anchiornis. Both archaeopteryx and anchiornis, among other bird-like dinosaurs, are considered some of the first birds based on their long
arms, feathered bodies, wings and other physical characteristics that are shared by all birds. But all birds, both modern-day species and these earlier bird relatives that lived in the distant past, all traced their evolutionary lineage back to a common ancestor, which is an ancestral group of organisms that is shared by one or more lineages. In other words, any of the
modern birds that you see today, whether it's a dove or a penguin, they all evolved from the same
bird-like dinosaur ancestor. Evolutionary trees can be
used to show the relationships between different lineages,
like different bird species, and their common ancestors. Scientists build evolutionary trees using various lines of evidence, such as examining fossils
and body structures or using DNA to identify shared traits and other similarities. The trees serve as models
for studying evolutionary relationships over time. So let's take a look at a tree that scientists built for city pigeons, also called rock doves,
and their relatives. What can the tree tell us? How are doves and these
other species related? Evolutionary trees are
called trees for a reason. The evolutionary
relationships between lineages are represented by branches
showing who is related to whom and to what degree. The tips of the branches
of an evolutionary tree show the modern groups of organisms. In this case, we have doves,
mesites, and sandgrouse. They all look different from one another, but you can see some physical traits, like their small bills, that are similar between
all of these birds. Let's take a look at
this evolutionary tree. What do you think that
this branch point means? The branch point, or the
place where branches split, represents common ancestors
that existed in the past, among these bird lineages, from which these modern-day birds evolved. Mesites and sandgrouse shared
a common ancestor in the past, about 60 million years ago, which is represented by a branch point. But what does this branch
point mean exactly? Basically, this branch point represents that at some time in the distant past, at least 60 million years ago, a bird species existed, that
wasn't a mesite or sandgrouse, but it did have some traits found in both mesites and sandgrouse. This species was the common ancestor. At some point, this common
ancestor species split and gave rise to two
different evolutionary paths or lineages. One of those paths gave rise to mesites and the other led to sandgrouse. And if we go even further back, to about 65 million years ago, we see yet another branch point representing another common ancestor. This common ancestor would later
split to form two lineages, one giving rise to doves and the other leading
to the common ancestor of mesites and sandgrouse. Looking at this evolutionary tree, we can also learn about
how closely related different lineages are. Organisms that share a common ancestor in the more recent past are more closely related to one another. But organisms that share a common ancestor in the more distant past
are less closely related. The common ancestor of
mesites and sandgrouse existed in the more recent past than the common ancestor of mesites, sandgrouse and doves. This means that mesites and sandgrouse are more closely related to each other than they are to doves. Dinosaurs are far in the past now, but their relatives are still among us. All the birds that you see every day evolved from a two-legged
feathered common ancestor that roamed the Earth over 165 million years ago with dinosaurs. So the next time you see
a rock dove flying by or picking up scraps in a city square, you're really looking at a dinosaur, or at least a distant
relative of a dinosaur.