Cellular evidence of common ancestry | High school biology | Khan Academy

Perhaps the most mind-blowing idea in all of biology is the concept that all living things we know of, based on current evidence that we have, all originated from a common ancestor. So it doesn't matter whether we're talking about a simple bacterial cell, which actually, in reality, isn't so simple after all; a tree made up of trillions and trillions of cells; a hairy primate made up of trillions and trillions of cells; or seemingly well-dressed agricultural kittens, which are also made of trillions and trillions of cells, that they all share a common ancestor.

You might have seen things like these evolutionary trees; this is an example right over here. This is saying the same thing: that everything that we see in the world today, all living things, regardless of what domain they're in—and we would be a subset of animals right over here—there's so many animal species that they all share a common ancestor several billions of years ago. But you should be skeptical; we are scientists here. How do we believe this? What is the evidence for that?

One piece of evidence is by looking at the cellular level and looking at commonalities amongst different groups and realizing that it would be unlikely for them to develop independently of each other. For example, all life forms that we know of have DNA; they all have RNA. And it isn't just how they encode information; it's also the biochemical processes that occur in the cells. They all have some form of glycolysis. But this seems—and these are just these aren't the only things that we've observed are common to all life forms—they're all based on cells as the basic units, which are bound by a membrane.

In theory, these things, I guess, could have developed independently of each other without having a common ancestor, but having a common ancestor is the best explanation of why we see these different processes, some of these are quite complex, or these different structures throughout life as we know it. And so you're saying, "All right, I can maybe buy that there's this common ancestor right over here," but how do we construct this tree? How do we know when things branched off?

Because some of these branches off of these trees, once again, these would have occurred hundreds of millions or billions of years ago, and none of us were around to observe that happening. And once again, that goes to more structural evidence. For example, amongst what we now classify as eukaryotes—so everything in this brown color, this branch of the tree right over here—we see that all of them have membrane-bound organelles. Membrane-bound organelles are things like a nucleus or mitochondria that we study in many other videos.

They all have linear chromosomes. So in other groups in this tree of life, in this evolutionary tree, you might have circular chromosomes, but common to all eukaryotes are the linear chromosomes. They all have chromosomes that contain introns. Introns are sequences of DNA that don't code for genes that will then code into proteins, and we're still exploring what the point of introns are.

But the reason why all of these have been classified together is that they have these similarities, so we believe that they would have formed their own branch. Based on how similar things are, that's where we theorize when things might have branched off. Now that we have more sophisticated tools of sequencing DNA and RNA, we can look at how different those sequences are to construct more and more precise trees like this.