[♪ INTRO] The title of this video does not make sense.

There are no single-celled dogs. Dogs aren’t just multi-cellular… they’re vertebrates.

Mammals even. A single celled organism is by definition not a dog. And yet… And yet.

Between six and 11,000 years ago, there was a dog. I do not know the dog’s name, I do not even know if it was male or female. But we do know quite a bit about it.

It was black or brown with more than a bit of coyote ancestry, and it came from an inbred population, and also, we know it had cancer. And to understand what happened next, and how truly bizarre it is, you have to understand what cancer actually is. And I know what you’re saying, “Hank, I know what cancer is.” And maybe you think you do!

You think that cancer is a disease caused by an organism’s cells malfunctioning and reproducing uncontrollably and moving to different parts of that organism’s body. And that is true, but also, it’s kinda not true. The question is: are the cells malfunctioning, or are they succeeding?

And to answer that question, you have to ask another one: why do malfunctioning cells so often reproduce uncontrollably?! Now it’s convenient to talk about evolution in terms of what it “wants,” but evolution doesn’t “want” anything. One school of thought proposes that natural selection is simply successful traits, and their associated genes, succeeding and being passed onto future generations.

And nearly every cell in a dog is trying to help the dog survive. This is what evolution is selecting for at the scale of the dog. Cells work together to keep the animal functioning, so that the genetic material can persist in the current dog, but, more than that, in the offspring of the dog.

But sexual reproduction isn’t the only kind of reproduction. Single celled organisms often reproduce asexually, and natural selection nonetheless still results in evolution in those species, just more slowly. And we can imagine that, inside of a dog, natural selection isn’t just selecting on the scale of the dog; it is also selecting on the scale of the cell.

Natural selection is not a complicated idea. If something can make more of itself, then there will be more of it. If a rat has a gene that allows it to have 1000 babies, then that gene will spread throughout the rat population pretty dang fast.

But if a cell inside an organism has a gene that allows it to have 100,000 or a hundred million copies of itself, that gene will ALSO spread through the organism very very rapidly unless something stops it. And, when that happens… when individual cells inside an organism evolve ways to replicate themselves unchecked, we call that cancer. One way of thinking about cancer is that it is simply the result of evolution acting on the cells of a multi-cellular organism rather than the organism itself.

And this is kind of a hot take, but it’s one that makes sense according to what we know. Cancer cells are evolutionarily successful. Not for the organism, of course, but for the cell.

Seen through this lens, cancer is not an organism’s cells malfunctioning. It is kinda a return to a single celled lifestyle. A step, at least.

They still require you as a host organism, and since they usually hurt the host (you) in the process, it’s not a great step. Still, your cells have become more like a pathogen. They have human DNA, but they have started living for themselves instead of for you… metabolizing, replicating, and evolving.

So one way to look at cancer is that it is a single-celled organism, with a multi-cellular organism’s DNA. Now evolutionary pressure on the organism level encourages LOTS of systems for preventing individual cells from growing unchecked because, when that happens, the organism can no longer pass on its genes… because of how it is dead. But evolution at the level of the cell is hard to overcome.

Natural selection is always selecting; genes are always mutating. It only takes one cell being really good at evading the immune system and selfishly replicating itself perpetually for the whole organism to crash. Great, Hank, thanks!

Wonderful. That’s all very terrifying and upsetting… cancer is the most natural thing ever, and despite the fact that our bodies have evolved a tremendous arsenal of tools to prevent it from killing us, it nonetheless remains an extremely common cause of death. But, for the most part, all of this is just semantic line shifting.

Is cancer a kind of single-celled organism? Is a hot dog a sandwich? It doesn’t really matter, it’s a deadly disease and it’s not like it’s going to jump off of the sick person and go live on its own.

Unless it does exactly that. Remember that dog that died thousands of years ago? Well, before it died, its cancer managed to spread to at least one other dog, potentially through sexual contact.

These cancerous cells were now a contagious pathogen that, for thousands of years, have been successfully living the single-celled lifestyle. This disease is called Canine Transmissible Venereal Tumor or CTVT. And you really cannot say that the cells that cause it aren’t…alive.

Because they grow, they reproduce, they metabolize, and they evolve. A dog with CTVT does not have a cancer made of its own cells, it has a disease made of cells that are descended from a dog that has been dead for thousands of years. As the authors of a 2014 paper put it, “It is remarkable that a somatic genome whose DNA would normally have survived for no more than 15 years during the life of one dog has continued to exist for several millennia as a parasitic life form.

CTVT’s survival and global dominance is a testament to the ability of the mammalian somatic cell genome to adapt and persist in a new ecological niche.” In other words, this cancer, if you look at it a certain way… is a single-celled dog. Now the success of CTVT is very weird and unusual. Cancer cells evolve to succeed inside a very particular environment: the body they are a part of.

And they evolve to evade the immune system of that one organism. That’s why cancer jumping from one organism to another is extremely rare. Now, we’re not entirely sure how CTVT did this, but other transmissible cancers have occurred in species that were in population bottlenecks, which just means that their population shrank a lot, and the individuals in that group wound up being severely inbred.

It isn’t difficult to imagine this happening to a population of early dogs. In a situation like this, CTVT would be able to jump from dog to dog because their low genetic diversity might have made it less likely that their immune systems to notice that those cells belonged to somebody else. Given the new evolutionary pressure not just to grow in one organism, but to be transmissible and survive in other organisms, CTVT continued to evolve, and it can now infect not just most dogs, but also related animals like jackals and coyotes!

The fact that there is a line of pathogenic cells, reproducing asexually, descended from an ancient dog that has persisted for millennia is one of the weirdest things that I have ever found out about biology, and it only makes sense to me if you view cancer not as a malfunction, but as a kind of terrible evolutionary success. Cancer through this lens, is a kind of biological inevitability and so we need to see it as a thing that our bodies have developed systems to control and that we can help our bodies control. Evolution allows these selfish cells to evade both our bodies systems for destroying them, and our technologies for destroying them.

We are in a race, but it is one that we are winning more now than we ever have before. And if you want to learn more about this, let me suggest “The Cheating Cell” a fantastic book, which is how I found out about this single celled dog… at once the smallest, and the oldest dog on Earth. And if you wanna learn more about how you can help SciShow continue making free science content for everyone, let me also suggest patreon.com/scishow, where you can learn more about becoming a SciShow patron and all of the cool perks that go along with that. [♪ OUTRO]