Planetary Context in the Origin of Life
Any search for life beyond our planet ought to begin here on Earth, says Andy Knoll, paleontologist and co-founder of the Origins Of Life Initiative at Harvard. As an earth scientist, Knoll's fundamental interest, he says, is "the planetary context of evolution."
Life and Earth are intimately intertwined: as life evolved, its course was shaped by Earth's history. The same will be true of life on any other planet, says Knoll, the Fisher Professor of Natural History at Harvard, who also serves in the departments of Organismic and Evolutionary Biology and Earth and Planetary Sciences. When Earth's atmosphere provided the right combination of elements -- carbon, nitrogen, a few more essentials-- life was able to be born. When the recipe of elements in the atmosphere changed -- when plants started photosynthesizing, for example, and filling the air with oxygen -- evolution was affected: Bigger animals requiring more oxygen could evolve. As plants evolved photosynthesis, taking up carbon dioxide to produce energy and oxygen, they affected Earth's environment. When we humans developed carbon-based energy sources -- burning wood and coal and petroleum, pouring more CO2 into the air-- our activity started changing the planet, too. So the planet-life relationship runs both ways, as Knoll's geological research shows: Earth influences life, and life shapes Earth.
3.8 billion years, as far back as we can see in the Earth's history, life is there (microbial life, that is). Earth is about 4.567 billion years old, but the period before 3.8 billion is a Dark Age, life-wise: "Plate tectonics erases Earth's history as it goes along," Knoll explains. "Tectonic sheets of rock grinding together delete the past Earth. There's no sedimentary rock left older than 3.8 billion years -- hence, no fossil record." So as far back in Earth's history as we can see, we find life. But the life we see, of course, changes: it has evolved. And it is this record of evolution that Knoll aligns with Earth's history, in order to see how the two interact.
Fish, 400 million years ago, went from being the size of minnows to sharks. What caused the shift in size? You might not expect to find the answer by analyzing rocks, but this is just what Andy Knoll and colleagues did. They analyzed rocks of different ages for their isotopic composition of the element molybdenum, which is sensitive to the oxygen level of the oceans. What the researchers found is that at two points in Earth's history -- 550 million years ago, the other 400 million -- there were huge spikes in the oxygen level of the oceans, and hence, of the atmosphere. When Knoll and colleagues compared this "oxygen history" to the "animal history" in the fossil record, they saw correlated changes: At points where oxygen spiked, evolution created larger animals. The second oxygen explosion led to those minnow-size fishes evolving into shark-size beasts. Predatory fish of all kinds evolved in response to the higher oxygen content in the air. As predatory fish evolved, this affected the population of prey fishes, which in turn altered the balance of the plants that produced oxygen in the first place. So, as Knoll explains, we can see the interplay between Planet and Life.
How salty was the water on Mars? How acidic? Was it too chemically harsh for life as we know it on Earth? Or might things have lived in Martian seas? Andy Knoll's search for "the planetary context of evolution" extends beyond our planet to others, too.
"The same tools we use to ask our own planet about its history, we can use to ask other planets about theirs," he says. In a recent paper in Science, for example, Knoll and colleagues analyzed the prospects of Martian water: If water was ever on Mars, they asked, what kind of water was it? What they concluded is that water on Mars must have been much more salty than Earth's oceans, too salty to sustain any life we're aware of on Earth. If life ever did exist on Mars, the team concludes by studying Martian dirt, it must have been more robust to salt than any life we've seen on Earth.
To envision life on Mars, we will have to expand our biological imaginations. And to understand life anywhere, Andy Knoll reminds us, we must understand planets.
"Planetary history is the context for thinking about the history of life on a planet," Knoll explains. "When we explore Mars, it's our experience on Earth that informs that exploration. When Dimitar Sasselov, Dave Charbonneau and Dave Latham give us a sense of planetary atmospheres from Kepler, it will be our understanding of relationships between life and environment that will inform our interpretation of those atmospheres. It's hard for me to imagine efforts to understand either the origin of life on this planet or the distribution of life through the universe that don't revolve around the nature of planets."
Why does it matter, when life began, or how old the Earth is?
Asking why Origins of Life matters, Knoll says, is like asking why should anyone study Shakespeare. "And part of the answer is, we're better off as humans if some people study Shakespeare. I think that's the case here." Origins of Life matters, Knoll says, "in the sense that questions of how did life arise are some of the oldest and deepest questions you can ask."
When asked what motivates him personally, "Curiosity" is Knoll's answer. "That's why anyone becomes a scientist. I feel fortunate that for the last forty years I've woken up every morning A) wanting to understand how life has evolved on this planet, and B) knowing that I could go into work and make inroads into that question."
"Curiosity is a strong motivating force."