Humans sort of think like engineers. We think of what we want as an outcome and how we can get there sort of in the shortest distance. And we may use some trial and error, but we really try to minimize the error.

Whereas really these evolutionary processes start with error. They start with a random change and then try those things out.

And it's really hard to get our heads around how can all the diversity and sort of seeming order that's out there in the world emerge from a process dependent upon chance.

I'm Sean B. Carroll, evolutionary biologist and author of several books, including my most recent, "A series of Fortunate Events: Chance and the Making of the Planet, Life, and You."

[Narrator] How life works, the staircase of evolution.

So evolution, what it really means is change over time. So we wanna know how that change occurs over time.

And there's two dimensions to this process and it kind of works like a staircase. And one process is mutation and that's the rise in the staircase. Those occur by chance.

If mutation didn't happen, all things would be identical. So you need mutation to make individuals different from one another. Those mutations are genetic changes, changes in their DNA.

If by chance, that changes the property that favors reproduction, survival of the individuals, it will spread, and that's the selection process and that's sort of the run in the staircase.

Chance invents and natural selection propagates that chance invention.

And once that process has happened, new mutations can then be added on top of that, maybe even changing the character a little bit further, and that's another rise in the staircase. And if that's working better, that's gonna spread.

And so it's the cumulative set of mutations and the cumulative process of selection that takes us up that staircase.

So, that staircase could have goodness knows how many stairs, but it has to go through this stepwise process of individual mutations arising, sweeping through the population, new ones arising, sweeping through the populations. It doesn't go from the base of the stairs to the top of the stairs in one jump.

And this is hard for people to get their heads around because they may think about that first step, and it's sort of hard to imagine, you know, how do you get something as complicated, as individual organs like an eye or a wing of a bird or things like that, but, time, immense time.

The speed with which some new adaptation spreads through a population depends upon the magnitude of the advantage it conveys.

If it's about a 3% advantage, meaning that about 103 individuals survive for every 100 that don't have it, well that will take about 1,000 generations to spread through the population.

Now I'm saying generations because it depends upon the generation time of the creature. So if the generation time, for example, if humans is 25 years, it'll take 25,000 years for that to spread through the human population. But if the generation time is 20 minutes, you can work it out. It'll take, you know, 20,000 minutes to spread through, which is much shorter.

It's a matter of generations because it's reproduction is a generation by generation process.

It's really important to underscore that these mutations occur at random without any consideration of whether they're good or bad for the organism. It's the external conditions that are gonna sort this out.

So some things can be good for one creature and bad for another, okay? So for example, a color change might make a creature more invisible in some settings, more visible in another.

Some mutation may make it better adapted to warmer climates, less well adapted to colder climates, right? So it depends on these external circumstances. So the mutations arise at random.

Well what about those external circumstances? What we'd say the abiotic conditions, whether the physical world that the creature is living in, well that's also generated a lot at random, tectonic plates moving across the earth. You know, vulcanism, all the things that shape the conditions of life are often due to physical processes of the earth.

Of all the many thousands and millions of things that creatures have come up with, I have a few of my favorites that I think really exemplify this process of adaptation. And one of my favorite sets of creatures, there's some fish called Icefish.

They live in the southern ocean around the Antarctic, and they live in water that is actually below the freezing temperature of fresh water. They're right around about 29 degrees Fahrenheit, is the ocean around the Antarctic. And that's a challenging environment.

So challenging for example, that you may know that there's ice flows around there, and if little ice crystals just get into the bodies of these fish, they'll nucleate freezing, they'll freeze like fish sticks.

So they need to have a mechanism that prevents them from freezing at that cold temperature. And what they've done is they've evolved antifreeze. Certain proteins in their bloodstream made in very large quantities suppress the ability of ice crystals to grow inside their bodies.

And so they're able to tolerate that subfreezing water of the Antarctic and other fish aren't.

So what's happened over the last 30 or 40 million years is that a lot of fish that were once swam in those oceans, sharks and rays and all that, they're all gone. They're extinct from those waters, we find them in more temperate waters.

But it's the Icefish and the antifreeze bearing relatives that exploit those waters. And the Icefish have also come up with something that's incredibly nifty and shocked naturalists when they first discovered it,

which is if you open, which any fish, you know, slice it open, you're gonna see red blood because that's something that animals with backbones have and have had for almost 500 million years on this planet.

But you slice open an Icefish, their blood is colorless 'cause they've gotten rid of red blood cells. So they don't even have a mechanism for carrying oxygen actively in their bloodstream like we do. They've ditched red blood cells.

And the reason they've ditched red blood cells is that those low temperatures, it makes their blood too viscous. And that's a disadvantage. To the Icefish, it was an advantage to get rid of red blood cells.

To the rest of us, it's instantly fatal. So it's a good illustration of just how conditional these adaptations are, that to exploit the resources of the southern ocean, you gotta make antifreeze and get rid of your red blood cells, where the other parts of the world antifreeze would be irrelevant and your red blood cells are necessary for every second of life.

Let's talk about speciation. That process of variations helping to adapt, that's the process of adaptation. But speciation is the generation of two species from one.

What that take for there to be two species to form? And we know, and this is where in the island biology of Wallace and Darwin help, is that they were seeing species on different islands.

Well, islands provide some isolation, and that isolation means that those populations aren't exchanging genes. Over time, each of those populations will accumulate mutations that exist in one population and not the other vice versa.

Well, those could become genetically distinct enough that if those things were ever to come back again in contact, they may not be compatible with one another, and they'll be species.

So how long does that take? Well, it's been estimated for big animals like mammals and birds, that works out to roughly be about 2 million years. But 2 million years is still a long time.

And you've probably heard, for example, that there's now very strong evidence that homo sapiens are species mated with Neanderthals a distinct species.

And what's really happened in the last few decades is that for biologists, that species barrier has become much more porous than we first thought it was.

In the old days, in Darwin's day, species were characterized as really distinct things and we didn't think there was any kinda shenanigans going on between them.

But we now understand that it's a much more porous situation that for some period of time as populations are diverging, they can get back together. And things that we humans might call distinct species can't often interbreed.

Another common question or idea is that, you know, for example, if humans evolve from apes, why are apes still around?

Well, the important thing is to understand that evolution is a splitting process. So the family trees keep splitting.

So the human part of that tree has had now its own separate history from the eight part of that tree. We shared a common ancestor about 6 million years ago.

But apes have gone on living as they do, for example, in the old world. And there's a great diversity of apes, of course still here. Baboons, orangutans, gorillas, chimpanzees, et cetera.

While the human branch has gone on, and has its own evolutionary trajectory, it's not that evolution is linear and that everything new replaces the old, it's a splitting process.

And so that's why the Tree of Life has just split into enormous numbers of branches from common ancestors.

Two of our main records of evolution are the fossil record and the DNA record.

The DNA record is largely only accessible to us for living creatures and maybe some creatures that have lived over the past million years. You know, do we have every brick? Do we have every intermediate?

No, 'cause you know, extinction takes away 99.9% of all species. So if there was no extinction, we'd have a perfect record of evolution, right?

We don't have the DNA record of dinosaurs, for example, but we've got the fossil record of dinosaurs. So we use these two records to sort of reconstruct the history of life.

That allows us to reconstruct the evolution of things that are fairly complex. Let's take something like a walking limb from a fish fin.

This evolutionary process transpired over 20, 30 million years, about 380, 390 million years ago. To do that, we have to get fossils that represent the various stages of evolution and see how did all the bones change?

So you went from a swimming fin to a walking limb, and the fossil record by now is pretty darn good.

Then we can go to creatures that have fins, fish, and we go to creatures like amphibians that have limbs and we can figure out how do those genetic programs work and where are the differences.

Now we don't have every detail sorted out and that would be incredibly time consuming and expensive to do, but we certainly have the general picture that we understand how the bones changed in history and we understand how the development of program changed to generate a limb in the place of a fin.

And that's not something Darwin ever had. That's something that we really only had for the last 20 to 25 years.

So evolutionary science keeps building on this huge foundation and what we get is an ever more detailed and ever more confident record of what's happened.

But what's at no doubt is that this process, mutation and selection, mutation and selection is the universal, going on in every population of every living thing every day.

In fact, this process of mutation and selection seems so universal that we think it must have played a vital role at the origin of life. And that anywhere where life exists in the universe it's operating.

So evolution is this vast and rich process and you know, we've been trying to understand it for 160 years. And in the course of that, there's some misunderstanding in just how it's communicated, where I think there's some conflict between the terms we scientists use and sort of their common understanding.

And one of those is theory. We talk of scientific theories. Theory is much higher on the hierarchy than, for example, a fact or just an observation.

A theory is assembled from lots and lots of facts and independent lines of evidence that sort of cohere. So a theory is really sort of the top category of scientific idea.

Where of course, you know, in the street theory might mean that's my best guess or that's just something that I'm conjecturing

The way we talk about those kind of conjectures, we call those hypotheses. And that when hypotheses have been rigorously tested, that's what can contribute to making a theory.

So folks, when we speak of theory, think that we're still very tentative about the truth of evolution. That's not all the case.

The theory of evolution, which has grown enormously in the last 160 years is a huge body of observations, evidence, and facts that are consistent with one another, that come from completely different sources of science.

And that's what gives it its power. Of course wish that maybe that connotation of theory that's, you know, more of the everyday connotation would be better understood.

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