Quirks & Quarks·Q&A

Do you feel lucky? A biologist explains we exist because of 'A Series of Fortunate Events'

Sean B. Carroll argues that understanding the randomness of the geological and chemical processes that gave rise to life as we know it can be perspective-changing

Understanding the randomness of life can be perspective-changing

Biologist Sean B. Carroll argues in his new book that we are only here due to pure chance. In fact, he says there were 70 trillion possible ways your parents could have genetically shuffled their chromosomes to make you unique. (Ethan Miller / Getty Images)

Originally published on December 19, 2020.

What are the odds that led you — a sentient being capable of processing abstract concepts such as "probability" — to read this story about how random chance is at the heart of biology and evolution? 

In a new book, biologist Sean B. Carroll argues that our world, our biological history and genetic diversity, and even our lives are shaped by random events.

Carroll is vice president for science education at the Howard Hughes Medical Institute and professor of biology at the University of Maryland.

He spoke with Quirks & Quarks host Bob McDonald about his new book A Series of Fortunate Events: Chance and the Making of the Planet, Life and You.

Here's part of their conversation.

In your book, you start with the planet-shattering event 66 million years ago when an asteroid hit the Earth, which eventually gave rise to mammals, and to us. But you also talk about another kind of collision — a little slower — that shaped us and created what you call, "an animal for all seasons." What happened there? 

We live in a really unusual time for the planet. We've been in an ice age for more than a couple million years and ice ages are rare — the last one was about 300 million years ago. 

So 50 million years ago, if you visited the planet, it was largely ice free from pole to pole, but it's cooled over time. And one of the events that really accelerated that cooling was the movement of a small plate. It's the Indian subcontinent which, as you may know, that the continents move around as part of tectonic plates. 

That Indian subcontinent was down by Madagascar about 65 million years ago, and it moved rapidly north until it slammed into Asia. And that collision is what built the Himalaya and the building of the Himalaya — by drawing carbon dioxide into the atmosphere — accelerated the cooling of the planet and eventually the freezing over, for example, of Antarctica and of the northern hemisphere, which sets up the ice ages. 

Well, you might say, how is this all connected to us? Well, in Africa, it's not so much about hot and cold as it is about warm and dry. And those very rapid cycles, geologically speaking, of warm and dry, are really challenging to the persistence of plants and animals over longer periods of time. 

Members of a research team are pictured conducting excavations at the Vleesbaai archaeological site on the south coast of South Africa where humans made stone tools about 74,000 years ago in this 2016 photo released in 2018. (Curtis W. Marean/Arizona State University/Reuters)

And when we look in East Africa, we realize that hominids, our ancestors have fared pretty well relative to other animals through this two million year period of the ice ages. And then you think about why that might be? 

Well, it's during this time that our brains got three-fold larger in volume, and it's during this time that we expanded our use of tools. It's when we took command of fire. So we're an animal that can fashion our own habitat, that we became sort of an animal for all seasons. 

And that's probably the geological event that set in motion the evolution of a large-brained ape on the African continent. 

So those are the mega events that got us here by chance. Let's get down to the microscopic scale of life where genetic mutations drive evolution. How does that work? 

The DNA of every species is different and the DNA of every individual is different. And those differences in DNA give us individual variation — why you and I and everybody else on the planet is different — but it also gives us the differences between species. 

The big question is, how does DNA get those differences? And we understand now that those differences arise at random. They're mistakes essentially made in the copying of DNA. 

Illustration of damaged DNA, which could lead mutations and eventually, genetic diversity. (Shutterstock)

So in you and I, Bob, we each carry probably 40 or 50 mutations that weren't present in either of our parents. Now, that's among 3 billion base pairs that have been copied to make us. So it's a very small number relative to the overall number of letters. But nonetheless, it's this small contribution of new mutations in every individual and every generation that gives us sort of the raw stuff of diversity that makes up the world. 

And it's those mutations that makes me look sort of like my parents, but not quite. I'm still an individual, right? 

Well, also, they've shuffled the chromosomes, so there were lots and lots of different possible children for your parents as well. 

Well, tell me about that.

Let's have a little fun with it. So how many genetically different brothers and sisters could you have had from one set of parents? And the math goes like this: each parent contributes 23 chromosomes and they have two alternative versions of each chromosome, so 23 pairs of chromosomes. 

But mom only contributes one of each and dad only contributes one of each. So when you put them together, it's 70 trillion from just one couple! So that tells you that no two fertilized human eggs on the planet will ever be the same. 

You say the mutations have more than evolutionary implications about life on Earth. You write about the typo that killed 35 million people…

Yeah. The scientists studying the AIDS virus who were trying to figure out where the virus came from realized it was incredibly similar to a virus called Simian Immunodeficiency Virus, or SIV. And detailed work led to the discovery that a change in one place in the viral genome of SIV enabled the virus to get into human cells. And that's the root of the AIDS epidemic. 

I was writing the book before COVID happened. And we're all living through the same event again, which is randomly mutating virus in some wild species, somehow made contact with humans, jumped species from whatever that host was into humans, and as we know, has changed the world throughout 2020. This just underscores how much we live in a chance-driven world.

In his new book, biologist Sean B. Carroll lays out: "A Series of Fortunate Events: Chance and the Making of the Planet, Life and You." (Princeton University Press)

Produced by Jim Lebans. Written by Sonya Buyting. This Q&A has been edited for length and clarity.


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