A Series of Fortunate Events

A new book by evolutionary biologist Sean Carroll, called A Series of Fortunate Events, takes a look at the whole train of unlikely coincidences in our history that led to all of us being here today. Sean joined Chris Smith...

Sean - Well, it's a series of cosmological, geological and biological accidents that really explain both collectively how we all got here, and individually. Some of these events are as great as the asteroid impact 66 million years ago, without which probably mammals would still be an obscure group of animals on the planet, and we certainly wouldn't be having this conversation; right down to the sorting of chromosomes - if I may say - in our parents' gonads, and the unique genetic combinations that come from that. We're each a one in 70 trillion event with respect to our parents.

Chris - Okay, well let's wind the clock back then. We are here today - and we also have with us Lee Berger, palaeoanthropologist, who can also help us out with some of the human evolution - but we've been here for really what's the blink of an eye. So where do we, as anatomically modern humans, stop, and where does the chain of events that actually make it possible for animals that turn into us kick in? Can you just give us some sort of timeline on what these events are and how they fit into our evolutionary history?

Sean - Sure. I start with the asteroid impact - which I think most people have heard of - 66 million years ago, because the more we understand about life before and after the impact, in fact from the fossil record, the more we appreciate that had this event not happened, the fate of mammals is very unclear. Mammals had been around for probably a hundred million years at that time, but life on land was dominated by the great dinosaurs. After the dinosaurs were wiped out because of the ecological catastrophe that that asteroid triggered, mammals that were small really took off. And in fact, only last year there was a treasure trove of fossils unearthed in Colorado here in North America, to really show how rapidly mammals took off once dinosaurs were out of the picture. And eventually those mammals branch into the modern forms we know, and all the different groups including primates.

So we understand without that cosmological event, a six mile wide space rock that was probably circling the solar system for 4 billion years... without that accident, we're not having this conversation. And the margins of that accident are fascinating. It's one thing to say, "we're all here by accident". It's kind of glib, but when you get to the specificity of these events, I think that's where some of the power comes from. And that asteroid: had it for example entered the Earth's atmosphere maybe 30 minutes sooner and landed in the Atlantic, or 30 minutes later and landed in the Pacific, it probably wouldn't trigger a mass extinction. Where it hit matters. And so you're looking at a one in 500 million year event in terms of an asteroid of that size, and it just happened to hit a piece of the Earth that could trigger a reset of life on Earth.

Chris - What about things that have come since then? Because we know that the Earth's climate has been a very changeable thing; over tens of thousands to millions of years it's changed a lot, hasn't it? We've had ice ages, we've had warm periods, we've had the Earth having the poles completely melted at certain points in our evolutionary and geological history. So how does that overlay on this?

Sean - Great point. I want to tee this up to Lee as well. The last 2 million years, we've been in one of the most volatile cycles of the last 300 million. The ice ages, the onset of which was a couple of million years ago... this is an incredibly volatile cycle where you not only have ice sheets advancing and retreating, but really in places like East Africa, it's not so much about temperature as it is about wet/dry. And from the palaeontological record we understand that our ancestors lived through - speaking a little bit longer term, over the centuries or millennia - incredibly dynamic climatic cycles. And it's a widespread view that our large brains are really the result of selection for our ability to craft our own habitats. The ice age has had a lot to do with the pace and direction of human evolution.

Chris - Lee?

李-很有趣,因为我们一直运行our understanding from ancient DNA, and how we're having to recraft everything we know. And I think it's very exciting to think of these chance events; and they also include now the idea that what was a simple story of human evolution ten years ago... we had a very pat idea of how we actually evolved in almost a ladder like phenomenon. Maybe it was a tree, but it wasn't a very bushy tree. Now, a tree isn't even a good model. We're looking at hybridisation. We're seeing that at any one moment in the past, that we have all of these different species, and really different species, like Homo naledi, Neanderthals, the hobbits, and large brained Homo sapiens all existing at the same time. And it may be just chance encounters that are occurring at any moment between two of these that allows a hybridisation event. Ten years ago we thought Homo sapiens was just this purebred race horse out of Africa, and we were destined to dominate this world because of our large brains. Now we know we're this mongrel, full of all kinds of other DNA, and messed up; and likely almost all of that was chance, through chance meeting of species. Some of it driven by climate, some of it driven by catastrophe, and some of it just in our genes.