Earth’s Earliest Life Written in Stone
Reconstructing Earth Life's Deep History and What It Suggests About Life Elsewhere in the Universe
Andrew H. Knoll
Fisher Professor of Natural History and
Professor of Earth and Planetary Sciences
About the Lecture
Earth records its own history, encrypted in sedimentary rocks laid down one bed upon another through time. For nearly two hundred years, paleontologists have mined this archive for the fossils of plants and animals. We now know, however, that this record of shells and bones and leaves and wood captures only the last fifteen percent of life’s evolutionary history. In fact, life has been part of the Earth’s surface for most of the planet’s history, and for most of this time life was exclusively microbial.
How do we discover and interpret geological signatures of early microorganisms and the world they inhabited? Laboratory study of samples from as far afield as the Arctic, Siberia and Australia reveal microfossils, sometimes remarkably well preserved, that document the deep history of photosynthetic bacteria and, later, eukaryotic microorganisms. Chemical signatures in the same rocks record other bacteria and archaeons that cycled carbon, nitrogen and sulfur through ancient ecosystems. Other chemical details help us to reconstruct Earth’s environmental history, showing that life existed for a billion years or more before oxygen began to accumulate in the atmosphere and ocean.
Everything we know about life in the universe is from what can be found on the Earth in the geological record and in the living biota. At a time when perennial musings about life elsewhere are being reframed in terms of telescopes and planetary spacecraft, how does Earth’s record guide – and possibly limit – exploration for life in other parts of our solar system or beyond? Earth’s early biological record has informed the exploration of sedimentary rocks on Mars, guiding the search for ancient life on a planetary neighbor that probably does not harbor living organisms today. What aspects of life as we know it are likely to be universal, and in what ways might alien life depart from our terrestrial experience? In the absence of a second example of life, we cannot answer such questions, but continuing discussion will play an important role in guiding exploration on the search for life in our solar system and on the planets orbiting distant stars.
About the Speaker
Andrew H. Knoll is the Fisher Professor of Natural History at Harvard University. Previously he has served Harvard as Associate Dean of the Faculty of Arts and Sciences; Chair of the Department of Organismic and Evolutionary Biology, Professor of Biology and Professor of Earth and Planetary Sciences. Andy came to Harvard as Associate Professor of Biology in 1982, after five years on the faculty of Oberlin College. Andy also serves as a member of the science team for NASA’s MER mission to Mars.
Andy’s research focuses on the early evolution of life, Earth’s environmental history, and, especially, the interconnections between the two. His research combines extensive fieldwork, paleontological discovery and geochemical analysis aimed at understanding the history of oxygen in the atmosphere and oceans.
Andy’s honors include the Walcott and Thompson medals of the National Academy of Sciences, the Oparin Medal of the International Society for the Study of the Origins of Life, the Phi Beta Kappa Book Award in Science (for his 2003 book Life on a Young Planet), the Moore Medal of the Society for Sedimentary Geology, the Paleontological Society Medal, and the Wollaston Medal of the Geological Society of London.
He is a member of the National Academy of Sciences, the American Academy of Arts and Sciences, the American Philosophical Society, a Foreign Member of the Royal Society of London, and has an Honorary Fellowship in the European Union of Geosciences.
Andy earned a BA in Geology from Lehigh University and a PhD in Geology from Harvard in 1977.
President Larry Millstein called the 2,390th meeting of the Society to order at 8:05 p.m. in the John Wesley Powell Auditorium of the Cosmos Club in Washington, D.C. He announced the order of business, that the evening’s lecture would be livestreamed on the internet, and welcomed new members to the Society. The minutes of the previous meeting and the lecture by Thomas Zurbuchen were approved without correction.
President Millstein then introduced the speaker for the evening, Andrew Knoll, Fisher Professor of Natural History and Professor of Earth and Planetary Sciences at Harvard University. His lecture was titled, “Earth’s Earliest Life Written in Stone: Reconstructing Earth Life’s Deep History and What It Suggests About Life Elsewhere in the Universe.”
Knoll began by stating that the present is not a representative moment of life on Earth, but rather an end memo. Scientists know this because Earth records its history in rocks.
Scientists have examined modern Earth life and inferred a common microbial ancestry. Knoll said this deep microbial history is captured in Earth’s fossil record.
For example, a large sedimentary rock formation in Spitsbergen, Norway, includes wavy laminations which Knoll said were created by sediment deposits trapped by ancient microbial mats. Knoll said that 750-800-million-year-old cyanobacteria encased in diagenetic chert nodules preserved in the formation confirm the ancient microbial mat theory. In fact, Hawai’i is home to comparable modern formations.
In Northwestern Australia, there is a group of 3.5 billion-year-old sedimentary rocks that outcrop at the surface. While molecular markers are unavailable in these rocks, scientists have found biogeochemical signatures in them, strongly suggesting there was a working biological carbon cycle when the rocks formed. The rocks also include laminations like those found in Spitsbergen, and preserved organic microspheres.
But, Knoll said, oxygen gas has only been physiologically important to the atmosphere and surface oceans for 2.4 billion years. To reach this conclusion, Knoll addressed banded iron formations, the source of most modern industrial iron.
Banded iron cannot, in principle, form in today’s oceans because to form, iron must be able to move in solution through sea water which cannot happen when oxygen is present. Incidentally, banded iron formations are prevalent in the geological record up until approximately 2.4 billion years ago, when it almost entirely stops.
Taken together, Knoll said the available evidence suggests life originated early in Earth history – but that it did so in the absence of oxygen. In sum, early life on Earth developed as though on an alien planet.
Knoll said we can make use of this understanding to approach the examination of life on other worlds.
Knoll then turned to the rover Opportunity, which NASA landed on Mars in 2004. Opportunity’s in a flat plain with strong hematite signatures, suggestive of once-flowing water. Opportunity’s observation of laminated sedimentary rocks, dissolved salts, and flow patterns corroborate the flowing water hypothesis. The rover also observed deposits of jarosite, which is formed under oxidizing and acidic conditions. An Earth analog for this type of environment is Rio Tinto, Spain, in which there is an abundance of fossilized life.
However, Knoll said the one-time existence of water does not mean Mars once had a life-friendly environment. Opportunity observed a pattern of salt deposits that indicate the water evaporated to the point that no Earth-like life could tolerate, and never returned.
But Knoll concluded, while farther away than Mars, he believes we will find carbon-based life on other planets.
President Millstein invited questions from the audience.
A guest asked whether any other bodies in our solar system have potential for life. Knoll said that while improbable, the geysers on Enceladus and the oceans of Titan could theoretically be home to microbial life.
A member asked whether Knoll has considered viruses on other worlds. Knoll said the last ten years of virus research have opened the door to such consideration. But, he said it is unlikely to find viruses on planets without other life because, on Earth, viruses are completely dependent on cellular life.
After the question and answer period, President Millstein thanked the speaker, made the usual housekeeping announcements, and invited guests to join the Society. At 9:50 p.m., President Millstein adjourned the 2,390th meeting of the Society to the social hour.