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  • The Great Filter

    I say that the “Great Filter” is behind us. That’s a big deal on many levels.

    What is the Great Filter, you might ask? You would have to understand the context which starts with Enrico Fermi at the head of a table with other Nobel Prize winners sitting down to lunch in Los Alamos in the late 1940s. They were there as part of the Manhattan Project and had already built the atomic bomb.

    I first heard of this legendary (some would say apocryphal) conversation from my physics mentor at MIT, Philip Morrison. Morrison had done a post-doctoral fellowship in physics with Robert Oppenheimer and then followed him to Los Alamos to direct the famous Manhattan Project. Some of you may have seen the movie, “Oppenheimer,” so you know the setting.

    Morrison was sort of Oppenheimer’s chief of staff and privy to most of the important decisions, discussions and diversions there. He was the proverbial fly on the wall. I loved hearing his stories at MIT. A few years later, I found myself at the California Institute of Technology where I got to know the Nobel laureate Richard Feynman, a great legend in physics and the youngest group leader at the Manhattan Project. So, every chance I got with Feynman, I eagerly tried to confirm some of Morrison’s stories, including the famous Fermi luncheon story. I learned the trick of getting Feynman to talk by giving him the Morrison version to which he would usually say, “Let me tell you the real story.”

    But this time, their two versions were very similar. And it went something like this (with a bit of paraphrasing): Fermi would visit from Chicago every few months and stay at Los Alamos for a couple of weeks. There, he was most welcome for his contributions to the theory of atomic fission, for his generally supple and brilliant mind, and for his good cheer. He was a charming raconteur and enjoyed challenging mind tricks which he often set up as bets. He always won the bets as he could mentally calculate probabilities faster than anyone else.

    As an example, a few years earlier he watched the first atomic explosion, Trinity, from a distance of about 10 miles away. By letting go a handful of newspaper strips, Fermi interpreted the size of the shock wave as it passed and then pronounced the power of the atomic explosion which turned out to be quite accurate.

    But at this luncheon, Fermi was quiet, which made an impression. Eventually, Edward Teller, another Nobel winner, asked Fermi if he was ill. Fermi denied being sick but admitted to being rather depressed. And then he explained why, beginning his monologue with the question, “Where are they?”

    “Who?” someone responded.

    “The aliens. The visitors from an advanced technological civilization from another planet,” Fermi said.

    “Why this nonsense, Enrico?” Teller replied.

    “In this galaxy alone, we have 200 billion stars with likely many planets circling each sun. Some of these planets must harbor conditions not too hot, not too cold, etc., so as to be suitable for life,” Fermi said. (We now know this to be true and nickname them “Goldilocks” planets).

    “Even if the odds of life on any given planet are low, and even if the odds of evolution taking one species to intelligence, to civilization and to technology are poor, it still computes to a very high chance that there are thousands of technologically sophisticated civilizations out there,” Fermi said. (This is now known as the Drake Equation.) “Why haven’t they visited us?”

    The Great Filter 1

    Deep sea hydrothermal vent. Also called “black smokers” and likely the venue for the origin of life on earth. They provided energy in the form of proton gradients and the minerals for compartments and electron transfers. Image courtesy of: OAR/National Undersea Research Program (NURP); NOAA; P. Rona.

    “Come on, you know how vast the distances are. If they left from the other end of the galaxy, it would take them at least 100,000 years to make it to Earth,” Teller said.

    “So what? 100,000 years; a million years; 10 million years. Who cares? It’s all just a brief instant in time compared to the 14 billion years of our universe. And if they didn’t feel like a long trip, they would send robots. And to make the search of the galaxy more systematic and complete, their robots would mine planets along the way and self-assemble many more copies of themselves. Hence, they would spread exponentially. We’ll be doing that ourselves within 500 years. You don’t think we’re the only ones, do you?” Fermi asked.

    Teller and the others began to see that Fermi wasn’t just being silly. After a long silence one asked Fermi, “So what is your explanation?”

    “There are two and only two possibilities. I will call them the ‘Great Filters.’ The first Great Filter would be behind us. That one says that there is one critical step in the evolution of an intelligent creature that is basically impossible. The probability of that is zero and so even with the 200 billion suns to start with, it comes to nothing,” Fermi said.

    “Well, we know that’s wrong. We’re here, so that step can’t be impossible,” Teller said.

    “Fortunately, Edward, you are wrong. We’re having lunch. So, we’re here, and so we are totally biased,” Fermi said. (This is an example of what we now call the Anthropic Principle, an extreme version of ascertainment bias). “We are not a test of that hypothesis. Something crazy may have happened that led us to that which could never happen again. But having happened, we are here to discuss it with sandwiches.”

    “What is the second Great Filter, Enrico?” another scientist asked.

    “Ah,” he said, “You are. You, my friends, gathered around this table have launched us into a world with weaponry so great as to not only win a war, but to lead to another final war where extensive use of these nuclear bombs will destroy all life, or at least all civilization and technology. So, then we won’t be sending robots to self-replicate and visit the galaxy in 500 years. It’s quite likely that all great technological civilizations inevitably blow themselves up. The second Great Filter is in front of us.”

    This is now called the Fermi Paradox. And it still terrifies many physicists today. But there is a point of view that saves us from that. And I want to finish this editorial with that idea.

    So, what have we learned in the last few decades that gives me reason to think there has been a miscalculation in the Fermi and Drake computations? On planet earth, life began as early as 4 billion years ago. We have recently learned that single cells began by harnessing energy from alkali vents under the ocean and then later changed their cell biology to become independent for their energy and carbon metabolism needs. But for about 2 billion years, nothing too dramatic happened. And then it did.

    Two billion years ago there were prokaryotes that came in only two flavors: Archaea, that were good with the weird chemistry and high temperature of these undersea volcanic vents, and bacteria that were marvels of biochemical wizardry that could go almost anywhere on earth. But both were going to be limited to being just one-celled organisms, forever, it would seem. And one-celled organisms are limited in many respects from the sort of thing we find interesting, such as the development of size, specialized organs, motility and amazing organs, such as the eye and brain.

    All these things only became possible when the great symbiosis occurred. An Archaea engulfed a bacterium and not only this maximized their mutual assets, but allowed for the accumulation of size and energy that would lead to amazing things. And the resulting eukaryote was a miracle in ways that included taming the toxicity of oxygen and then using oxygen as an asset. In 1966, Lynn Margulis controversially proposed that this event happened and twenty years later the penny dropped as multiple technologies allowed us to peer backwards in time.

    By about the year 2000, the verdict was clear that she was right. But before this symbiosis could happen, several dominoes had to get lined up just so. How did the archaea phagocytose the bacterium? No archaea can do it now. And before that, a marvelous system called chemiosmosis had to be available for energy. As Nick Lane has shown us, this is the proton gradient that got invented in the context of hot hydrogen shooting out of the undersea vents with such a high pH that essentially the atoms got stripped of their electrons and became protons in the physical setting of mineral compartments. Add membranes and this provided free power for the first cells, but this gradient could not easily be sustained and regenerated without something else. That became possible when evolution, in all its cleverness, harnessed quantum mechanics for quantum electron tunneling (QET). This allows electrons to tunnel under an energy barrier using a series of iron-sulfur clusters (also conveniently available in the minerals of the undersea vents).

    Eventually, these iron-sulfur complexes were aligned by a marvelous molecule that I have been studying for 30 years — Complex I which goes awry in a mutation that causes blindness (LHON). Complex I later got flipped around by evolution to run backwards in another amazing trick called photosynthesis. Some of the bacteria harnessed the process of photosynthesis to use water and sunlight for their energy leaving oxygen as a byproduct (the oxygen in our atmosphere comes exclusively from that).

    So, with this QET trick, Complex I could maintain the proton gradient. And to exploit this proton gradient came another great invention of biology — ATP synthase. This is a mechanical turbine that uses the proton gradient to mechanically spin, and with each revolution, to pump out an ATP, the universal currency for energy in biological systems. Watch a movie of this marvel.

    So, you had to have five stars aligned for the eukaryote to come into existence: 1) Chemiosmosis for the proton gradient, 2) phagocytosis to make symbiosis, 3) symbiosis to make proto-mitochondria that use 4) quantum electron tunneling to fuel more proton gradients, and 5) ATP synthase to exploit the gradient to make ATP. And with enough ATP the eukaryote, your forebearer 2 billion years ago, could grow to sizes 10,000 times larger than bacteria because they had 100,000 times more ATP. Prokaryotes are brilliant at making all sorts of organic molecules, but eukaryotes are great at producing energy, and energy fuels size and differentiation. Eukaryotes could now do something even more fun; change into multicellular organisms and that would lead to amazing animals with hearts, and spines and muscles and eyes and … brains, the energetically most expensive tissue of all. The brain is high maintenance, but so worth it!

    The transition from prokaryotic to eukaryotic life forms represents the major evolutionary threshold in the history of life on this planet. It set the stage for a vast expansion in biological complexity, enabling the rise of everything from towering trees to advanced animals with intricate organ systems. Research, including phylogenetics, in the last 20 years has shown the origin and early evolution of eukaryotic cells to be one of the most exciting and challenging puzzles in biology.

    Some people like to point to the eye and say it’s proof that evolution can’t happen. They are ignorant for not understanding that you don’t have to go from half an eye to get to a whole eye while they argue that half an eye won’t work at all. In evolution you always need to start with an advantageous arrangement from which you can make it even better with new genes and natural selection. In the case of the eye you can always keep moving from good to better as you move from a useful pigment spot to the wonders of a vertebrate eye. But in the case of symbiosis, I’m in much greater wonder and amazement.

    For if all five elements weren’t there, all at the same time, you couldn’t make the jump from prokaryotes (archaea and bacteria) to eukaryotes. And without that jump you couldn’t have the size and complexity that allowed evolution to power up and go crazy with innovative organisms. To be clear, bacteria could never have grown or developed the structural elements that gave rise to big moving parts and specialized systems. And it’s never come close to happening again on this planet.

    Bacteria were destined to stay small and simple trapped by problems such as surface area to volume problems. Almost 4 billion years have proven that to be true. Eukaryotes were the breakthrough to complexity. And what were the odds of all five stars aligning up first? That this was close to zero is exactly what had to happen to make and get us past the first Great Filter. This only happened once in 4 billion years and maybe it is close to impossible to happen at all. But it did, and we enjoy a hindsight view of an ascertainment bias that makes us almost blasé that it happened at all.

    But it was not inevitable; if there are a trillion universes, we might be the only one in which it happened. This is the Great Filter behind us. This also should give us hope that maybe there needn’t be a second Great Filter in front of us. And maybe, just maybe, if we as a civilization have enough wisdom and political will, we can avoid the second Great Filter and make it to the future. If Fermi were still alive, I think he’d be surprised, but also, very happy.

    I have a slide I sometimes used when lecturing medical students, and it reads: “Over billions of years, every one of your ancestors was a success. Don’t mess it up now.”