The history of life on Earth is largely microbial. “You can certainly run a world without dinosaurs and humans, but you can’t do it without microbes.’ says Harvard Fisher Professor of Natural History and Professor of Earth and Planetary Sciences, Andrew H. Knoll. “For vast stretches of time, bacteria and other single-celled organisms were the only life on Earth. The age of the dinosaurs to the present day,” Knoll said, “represents roughly 5 percent of the history of life.”
“When we think about the history of life, we tend to think about dinosaurs or, if you’re really informed, trilobites. That’s just the tip of the iceberg,” Knoll said. “Not only were microbes the first living things on Earth, they were critical to the Earth’s transformation. The rise of photosynthetic bacteria called cyanobacteria was a crucial step because these bacteria ingested carbon dioxide and released oxygen.”
Earth’s Oldest Living Individuals -Dead but Not Dead
In December of 2018, the Deep Carbon Observatory announced the astounding fact hat the mass of the microbes living beneath Earth’s surface amounts to 15 to 23 billion tons of carbon, a sum some 245 to 385 times greater than the carbon mass of all humans. The world’s oldest living individuals may not be gnarled bristlecone pines or shimmering aspen clones, but microbes locked in rock miles beneath the surface whose goal is to not to grow or reproduce, but “simply” to achieve immortality.
“We do not know whether the microbes of these subsurface environments reproduce at these slow rates of biomass turnover,” wrote Frederick Colwell and Steven D’Hondt in a review called Nature and Extent of the Deep Biosphere in 2013, “or live without dividing for millions to tens of millions of years.” Dead, but not dead.
Microbes buried deep beneath Earth’s surface are protected from cosmic radiation – a frequent killer of the preternaturally aged – by thick overburdens of water, sediment, and/or rock (Muons, the form in which cosmic radiation reaches Earth’s surface, can only penetrate tens of meters into rock), reports Jennifer Frazer for Scientific American. Whereas Such radiation steadily mutates the DNA of organisms living on Earth’s surface.
Problem of Alien Life–How Would We Even Know It was Alive
An amazing meeting took place in February of 2015 in San Jose, California when eminent scientists met to discuss and debate a bold question: “If we came across alien life, would we even know it was alive?” That was the question posed at the annual meeting of AAAS based on the observable fact that all known life on Earth fits a particular mold, reported Science.
But “if we have other organisms out there that do things just slightly differently, we might miss the boat,” said geobiologist Victoria Orphan of the California Institute of Technology to the attendees, referring to Earth’s standard protocol, known as the “central dogma,” using DNA and RNA to store genetic information, and translating that into proteins. And all living things, said Orphan, “rely on the same handful of chemical elements. So, when searching for life in remote or extreme environments scientists typically look for signs of the kind of life we’re familiar with.”
Biologists have proposed the existence of a “shadow biosphere”—an undiscovered group of living things with biochemistry different from what we’re used to. Most of life’s diversity on our planet is too small to see, making microbes the most likely place to look for these new types of life. Already, new discoveries are shaking our beliefs about what life is. Recently discovered giant, amoeba-infecting viruses blur the line between life and nonlife—although they rely on their hosts for essential biological functions, the bacteria-sized viruses have complex genomes. Such unexpected discoveries suggest that we shouldn’t define what we are searching for by what we know is already out there, Orphan said.
Alternative paths to the Evolution of Life
NASA planetary scientist Carolyn Porco of the Space Science Institute in Boulder, Colorado, suggested that we look for a system that is out of equilibrium, a sign of an environment out of whack—and perhaps life. Life takes in and uses energy, Porco observes, altering its environment in the process as on Earth, where. without life, our planet would not have an oxygen-rich atmosphere.
Chemist David Lynn of Emory University suggested that “misfolded proteins—like the those implicated in neurodegenerative diseases such as Alzheimer’s—show some similarities to life, namely that they can generate diversity in the different ways that they fold, and can undergo chemical evolution, in which those folded proteins are selected not genetically, but chemically. Such precursors could form complex chemical networks, which might be the foundation of radically different life elsewhere in the universe.”
Biochemist John Chaput then with Arizona State University, suggested that “Life did not choose DNA or RNA out of chemical necessity. There may have been many alternative paths to the evolution of life.”
Enter Arizona State University cosmologist, theoretical physicist, and astrobiologist, Paul Davies, who suggests in 2018 in his book, The Eerie Silence, that “During my career, opinion has shifted from life’s origin being a bizarre fluke unique in the universe (‘almost a miracle’ in the words of Francis Crick), to the belief that the universe is teeming with life (‘a cosmic imperative’ in the words of Christian de Duve),” Davies said. “How can we settle the matter? For several decades astronomers have been sweeping the skies with radio telescopes hoping to stumble across a message from ET. So far they have been met by an ‘eerie silence.'”
“Meanwhile, astrobiologists have considered how signatures of microbial life might be detectable in the solar system or in the atmospheres of extra-solar planets,” Davies added more recently in The Demon in the Machine. “If life really does form readily in Earth-like conditions, it should have started many times right here on Earth, so we should look for a ‘shadow biosphere’ of life, but not as we know it, under our very noses.'”
In my view, the most promising place to search for a second genesis is right here on our own planet. If life does indeed get going easily, as so many scientists fervently believe, then surely it should have started many times on Earth. Well, how do we know it didn’t? Has anybody actually looked?
Hidden 1st, 2nd, 3rd Genesis?
When the epoch of early bombardment on Earth finally abated there may have been dozens of independently formed organisms cohabiting our planet. The fascinating question Davies asks, “is might at least one of these examples of life-as-we-don’ t-know-it have survived to the present day? Almost all life on Earth is microbial, and you can’t tell by looking what makes a microbe tick. You have to delve into its molecular innards. So might there be, intermingled with the microbes representing ‘our’ form of life, representatives of this ‘other’ life – it would be truly alien life, in the sense of being descended from an independent genesis. The existence of an alien microbial population has been dubbed a ‘shadow biosphere’, and it carries the intriguing possibility that there might be alien life right under our noses – or even in our noses – overlooked so far by microbiologists.”
“it’s possible to bracket the range of chemical complexity, but where along the line of molecules from amino acids and sugars to ribosomes and proteins could one say that life was definitely involved?” Davies asks. “Is it even possible to identify life purely from its chemical fingerprint? Many scientists prefer to think of life as a process rather than a thing, perhaps as a process that makes sense only on a planetary scale.”
Davies suggests that chance played only a subordinate role in incubating life and that the process was more ‘law-like’, more of an imperative, “as de Duve expressed it. Is it possible that the blueprint for life is somehow embedded in the laws of physics, he asks, and is an expected product of an intrinsically bio-friendly universe? Perhaps.
But the problem, Davies writes is that “these musings are philosophical, not scientific. What sort of law would imply that life arises more or less automatically wherever conditions permit? There is nothing in the laws of physics that singles out ‘life’ as a favored state or destination. All the laws of physics and chemistry discovered so far are ‘life blind’ – they are universal laws that care nothing for biological states of matter, as opposed to non-biological states. If there is a ‘life principle’ at work in nature, then it has yet to be discovered.”
Just a Single Microbe of Life as We Don’t Know It
All it would take to settle the “question are we alone” Davies concludes, “is the discovery of a single microbe – just one – which represents life, but not as we know it. If we had in our hands (or rather under our microscopes) an organism whose biochemistry was sufficiently unlike our own that an independent genesis was unavoidable, the case for a fecund universe would be made.”
If life can happen twice, Davies suggests, it can happen a zillion times. “And that single alien microbe doesn’t have to be on some far-flung planet; it could be here on Earth. It could be discovered tomorrow, upending our vision of the cosmos and mankind’s place within it and greatly boosting the prospect that intelligent life may be out there somewhere.”
The Last Word
“Since our paper in 2013 there’s been more work on how life lasts over long periods, especially from studies performed in seafloor sediments,” wrote Frederick Colwell in an email to The Daily Galaxy. “Marine sediments – and the microbes therein – are ideal for such studies because we understand sediment ages from geological records, and because the sediments typically are uniformly buried and undisturbed. Here’s a possible quote:
“Both slow rates of biomass turnover or cell division and the complete absence of cell division may play a role in long-term survival,” Colwell explained, “Active cells exist in some seafloor sediments that have been buried and undisturbed for up to a hundred million years (Morono et al. 2020), though their rates of grow and cell division are likely very slow under in situ conditions. On the other hand, endospores which are dormant and do not divide, appear to become dominant over vegetative (dividing) cells in some marine sediments buried after only a few tens of meters (Wormer et al. 2019). There is probably some spectrum of cellular responses to long-term survival that spans from rare cell division to no cell division and the response is likely determined by the environmental conditions and the ability of cells to respond to deprivation.”
Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via Frederick Colwell, Science, Scientific American and Davies, Paul. The Demon in the Machine (pp. 182-183). University of Chicago Press. Kindle Edition.
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