At the Cosmos Club, Washington, DC
March 23, 2018
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.
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