Life on a Young Planet: The First Three Billion Years of Evolution on Earth

"It was not so long ago that many believed that life on Earth began in the Cambrian, perhaps around 540 million years ago. This is when the age of the earliest known fossils. It was only after the Second World War that evidence of microscopic life on Earth was recognized in the geological record. Even when I started my own studies the number of fossil sites was in the single figures. What geologists found once they knew where to look and what to look for were microscopic evidence of bacteria and algae – a record of more than 3 billion years. However what Andrew Knoll describes is not just an unexpected fossil record but also a record where the evolution of life has shaped our planet. Even today there is debate as to where life originated – many now believing that it was in the bottom of the ocean near black smokers where the bacteria could get energy from the centre of the Earth rather than from the sun. However a key change took place at around 2700 million years ago. This is the evolution of oxygen-generating cyanobacteria I referred to earlier. At first this oxygenation changed the chemistry of the oceans but later as oxygen leaked out into the atmosphere then an ozone layer could form, there was protection from the harmful ultraviolet radiation from the Sun and life could then venture onto land. This is a rich detective story involving all the sciences but nonetheless is written in an easy to read and engaging style by a researcher that has done so much to develop the field. His passion comes through. Knoll shows the intimate relationship between the evolution of life and the evolution of the planet. Our understanding of the earliest origin and evolution of life has made rapid progress over the last few years meaning that this book will continue to be updated by one of the world’s leading authorities."

"I’m not an astrobiologist and because we don’t have any other examples of life yet, I like to learn about the Earth. One of the most impressive popular science books I’ve ever read was Stephen Jay Gould’s Wonderful Life . That was a lovely book about the development of complex life, the Cambrian explosion. Knoll’s book is in the same category. It’s very well written and has a broad sweep of 3 billion years of history. He’s a very preeminent geologist and paleontologist and his expertise covers a number of fields. He’s at the top of his game and he lays out the subject beautifully. It’s just a nice book setting out a subject that I’m not trained in, in a way that I can understand it. It also addresses the issue of how inevitable life is, if you have all the ingredients. How does it progress? Again, with a sample of one, you can’t generalize. You can’t inductively draw a conclusion. But he raises all the issues. He’s a good writer, and I like the fact he’s not hyperbolic. He avoids dramatic statements. He doesn’t draw a conclusion when there’s no conclusion to be drawn. He’s quite cautious. I appreciate that, because in his field we’re dealing with slender evidence. Sometimes it’s hard to find an old rock. If you just go anywhere out on the surface of the Earth, an average rock you might pick up is very unlikely to be more than 100 million years old. Finding a three billion-year-old rock is hard. The evidence we have of life that far back is very sparse, and the rocks have all been altered by geology, heating, pressure, and so on. Knoll is suitably cautious about drawing too firm conclusions about this long geological record that we have. Knoll is not spending a lot of time back there because then the speculation level gets extreme. There are argumentative academic debates about when the first evidence for life is. It’s quite solid at 3.7 billion years ago, but there are people who are pushing 3.9 or 4 billion years old. He’s smart enough to stay away from that debate, because it’s hard to be sure when you go that far back. But what is true from the sequencing of RNA is that the Archaean branch of the tree of life—that extra branch that was discovered 50 years ago by Carl Woese—does seem to indicate a warmer-than-tepid environment. That’s what led people to the deep-sea hydrothermal vents. It’s not Darwin’s warm little pond, it could be quite an intense high pressure, high temperature, chemically toxic environment where life started. That’s quite possible. The interesting thing about those environments is that you don’t have any sunlight. When people say, ‘for biology, you need a star,’ you don’t really. You can have life without one. I covered that a bit in my book on the issue of rogue planets—exoplanets that have been booted from their star systems early on, when it was all chaotic. Some of those planets are heavy enough to hold atmosphere and have geological heating. They are just little self-contained ecosystems. You don’t need a star to have biology and a planet can also just float through interstellar space without one. I’ve dipped into this episodically. I wrote a book with Random House called The Living Cosmos 12 years ago which was about the broad sweep of astrobiology (not just exoplanets). That was my first approach. Then I had a couple of slightly more academic books, including Talking About Life: Conversations on Astrobiology , published by Cambridge University Press. That’s how I learned a lot. Exoplanets were just booming so much, I felt I had to come back into the subject. It is moving fast, but we’re at a tricky phase now. We have 5,300 exoplanets. I mean, you feel sorry for a young research student in the field. It used to be if you found a cool planet or an Earth-ish planet, you could get a paper and a headline. Now you can find 100 planets, and nobody cares that much because there are so many. Just increasing the body count won’t move the field forward much—except in an esoteric research-y sense. Everyone’s just waiting for the next step, which is to find biology. That’s what people, including scientists, want to happen. It’s just a lot harder than finding a planet in the first place. We don’t know how long it’ll take. The people I know who are working on this say five to seven years. But they’re also cautious. They point out that the evidence is not necessarily going to be compelling. It’s going to be ambiguous. If you find oxygen in the atmosphere of a planet, you think, ‘Oh, there must be microbes, because that’s where the oxygen we breathe came from!’ Then geologists will say, ‘No. Wait a minute. We have very simple geological ways to get 10% of oxygen in an atmosphere.’ It’s not 18%, like we have, but it’s quite high. Oxygen could be a false positive. Methane is a false positive. All the things that we think might be traces of biology could be just traces of geology. So proving you’ve found something is hard. And then the null is even harder. Proving there’s no life on these planets is hard too. We think these are going to be nice binary experiments, we just inspect these planets up close, and we say, ‘That one’s dead. That one’s living. That one’s dead.’ It’s not going to be like that. It’s going to be an arduous process without a lot of screaming front page headlines. I’ll be very excited when the Mars samples come back, because they can be scrutinized molecule by molecule in a lab. If there’s stuff in there that represents fossilized microbes, we’ll find it. That’s a good experiment. The exoplanet atmosphere experiment is hard. It’s a very tough field because it’s very competitive. Just like with the history we started by talking about—where careers were destroyed by claiming exoplanets and being wrong—there’s going to be people claiming life and being wrong. It’s going to be tricky. I don’t know how it’s going to go. Those claims are going to start happening in four, five, seven years, because the technology to do the experiment is maturing. James Webb will do some of it, but it was never designed for this experiment. Its design predates the discovery of exoplanets, so it was not optimized for that. It’ll do it in a very painful way, taking a lot of time and not looking at that many targets. Three huge, ground-based telescopes that are under construction will all take their first light in the next five to six years. They are all looking to do this experiment. They’re the killer app of a big telescope that blots out the star that’s a billion times brighter than the planet, smears the planet light into a spectrum, and looks for biosignatures or traces of life. These are 20- to 30-meter telescopes, so will be very good at doing it. Everyone’s racing to do that experiment, so I think they’re going to be pushing the envelope quite soon."

"The whole story of the first three billion years of life on Earth is that there were no plants or animals. In terms of living organisms, everything was microbial. Exactly. Andy Knoll is a world expert in an area of micropalaeontology, which is the study of fossils of microbes. He was a student of the late Elso Barghoorn, a pioneer of micropaleontology at Harvard. Together they developed the field of study of the earliest fossils of single-celled organisms. What Andy has done is really exposed us to the world before animals and plants, when there was strong evidence of life but the world was totally controlled by single celled organisms, the protists. We don’t have a good fossil record for that period. The fossils we do have are largely organisms that had cell walls and were fairly large. The protists are a lot bigger than bacteria, but their fossil record goes back almost 2 billion years. Andy has actually worked on older fossils. Some of these are very controversial. In the UK there was a very famous paleontologist, Martin Brasier. He was at Oxford University, but tragically died last December in a car accident. “For the first three billion years of life on Earth, there were no plants or animals. In terms of living organisms, everything was microbial” There was another palaeontologist who was a student in the same lab, but a little bit before Andy Knoll, Bill Schopf. Schopf had discovered fossils in Australia that went back to about 3.6 billion years. Brasier claimed these were artifacts, but later in life, re discovered, I would say, fossils from Australia that also date back to about 3.4 billion years. The field is still trying to disentangle what these microbial structures that are poorly preserved in the rocks mean. Andy Knoll tells the story of the earliest life forms that he can detect, and the earliest evolution and co-evolution of life and geological processes. That book was very influential for me. Exactly. Lovelock would call this a Gaia type of system. That term doesn’t appeal to me, but I understand what Jim Lovelock is trying to say. It’s about the co-evolution of life and geological processes: They influence each other. It’s not like geology is the driver and biology is the slave, biology also has a feedback on geological processes. Andy brings that up quite well in his book."

"This is a book about the beginning of things. So we are now going right back in time, not just before the dinosaurs, not just before the trilobites, but to the very, very early days on the planet. It’s broadly called the Precambrian, going back beyond 3,500 million years ago, but the point is that this was in some ways the most mysterious part of the history of life on the planet. When Charles Darwin was around, no fossils were around from this period. What’s happened over the last 30 or so years is that more and more discoveries have been made in these Precambrian rocks. Because of new techniques mostly, but also because people have taken the trouble to look properly. A lot of the fossils are things you might expect, like algae, bacteria, so they’re very small and not easy to find. Knoll tells very well the story of the development of the early years of our planet and how life actually shaped it, because not a lot of people know that it was life itself that made the planet habitable for animals. In the early days there was little oxygen, which of course all animals, including us, need to breathe. In fact, early life evolved and appeared in the absence of oxygen. It was only when organisms appeared that were able to carry out photosynthesis – mostly bacteria to start with, followed by algae – that oxygen was released. So, in other words, we would have died of suffocation in the early days of the planet. It took two billion years of work by these organisms to oxygenate the planet sufficiently for our kind of life, including the trilobites, dinosaurs and ourselves to evolve. Two billion. Two thousand million years, yes. So you can’t overestimate the importance of this early period in making the planet what it is today. It’s very important, when we muck around with the atmosphere as we’re doing, to realise that what we have is actually a product of this ineffable and long period of planetary evolution. I don’t think people realise how long the journey has been to get to where we are now."