Common ancestry and evolutionary trees | Evolution | Middle school biology | Khan Academy

• [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.