This is knowing the universe, the history, and philosophy of Astronomy. I'm Chris Impey, Distinguished Professor of Astronomy at the University of Arizona in the Astronomy department. In this module, we'll talk about life beyond Earth, the subject of Astrobiology. What is Astrobiology? It's the subject where we speculate, given that we only know of one place in the universe at the moment with life on it and it's the planet that we're standing on. We speculate about the probability of life elsewhere and how we might go about finding it and characterizing it. It's an important subject that's part of Astronomy, but also highly disciplinary. In fact, we're extending the Copernican Revolution. As we've already seen, the progression from Copernicus through the 20th century displaced us insignificance and centrality in the universe, such that now we know we're not even made of the stuff the universe is made up, which is primarily dark energy and dark matter. A final step in the Copernican revolution, apart from the discovery of other universes that we've already talked about, would be the discovery that biology is not unique to the Earth. In a sense, we might expect that given that humans are not special in every other aspect. Why are astronomers generally very optimistic that life will be discovered even though we still only know of one planet with life? Why do they even go beyond that to speculate the biology may be common in the universe? It's a convergence of arguments. None are convincingly conclusive, but they're all suggestive. First of all, the organic molecules that form life's basic chemicals naturally and easily form in lab conditions or on the early Earth. We also know that life appeared early in the history of the Earth after perhaps a couple of hundred million years when the Earth was a very inhospitable place. We know the ingredients for life, carbon, and related elements are widely available in time and space. As we'll see now, there's evidence that planet and moon habitable locations are abundant. Also, the robustness of life on Earth, extremophiles that can occupy bizarre evolutionary niches, and extremes of physical conditions of acidity, salinity, temperature, pressure, and so on, indicate that life can survive a wide range of conditions and so might be found under situations that are not quite as hospitable as the Earth. We've also seen that the chemistry of the universe is hospitable to life. Here's the element abundance that we've already talked about. This came from the life story of stars. The life story of stars has been about making biogenic elements, carbon, nitrogen, and oxygen, those first peak after hydrogen and helium. That's the basis for biochemistry. We know those elements are created in stars and ejected into space to become part of other stars and then solar systems and planets. We also know that the cycling of elements has progressed through the history of the universe such that there's more carbon and carbon related elements now than there was a billion years after the Big Bang. In a sense, the Universe is getting more hospitable for life. After 14 billion years, there's plenty of carbon, nitrogen, and oxygen out there with which to make the biogenic elements and molecules. But we must remember that we're in a state of high uncertainty. This is a young field, Astrobiology is still a great state of ignorance. It starts at home. The most part of our biosphere is still unexplored. Over 99 percent of the microbes that we've discovered have not yet been cultured to see their full biological nature, because that's quite difficult to do. Within the solar system, we've only had remote sentience by robotic probes and we haven't sent people to any of these destinations yet. It's very indirect information. Although we are discovering large numbers of habitable planets now, after centuries of speculation, we really don't know how to characterize most of them very well. Finally, in the search for intelligent life in the universe, abbreviated as SETI, only a tiny fraction of the possible parameter space of SETI has been explored so far. As Buckminster Fuller put it in a brilliant dichotomy of thought, we are either alone in the universe or we're not. Either way, the implications are staggering. We should also remember that biology is an example of one thing. Species as diverse as fungal spores and woolly mammoths and blue whales and elephants are all based on the same biological template, the same genetic material. This is the tree of life as expressed in the RNA or DNA of organisms and creatures. This is mostly a microbial tree of life. The root of the tree is about four billion years ago, last common ancestor of all life on earth. Most of the radiating branch points involve microbes with or without nuclei. Eukaryotes, the cells with nuclei are seen on the right. If we were to zoom in on the branch or the twig of animals, then apes, primates, all our common ancestors and close relatives in the animal kingdom are smaller than the dot on the eye of animals. Humans are the tiniest little twiglet on the tree of life. Most of the tree of life is microbial. We also expect the universe to be abundant with microbial life and perhaps large creatures with brains and sentience are rare. That's speculation. But this is the genetic and biological diversity of our planet represented as a tree. Also, it's one thing in the sense that all biology uses the same genetic code, the same delicate interlaced double helix of DNA to hold the information roughly equivalent to an encyclopedia. Biology uses a four-letter alphabet. We don't know if that alphabet is unique to the Earth or if other situations might have led to biology with a different alphabet or a different genetic code. The genetic code is brilliantly designed, but there was no intelligent designer because it encodes information quite efficiently in fairly simple molecules, mostly only have a dozen or two dozen atoms with some redundancy allowing for error checking of the code. If we analogize the DNA and the information storage in biology to a book, then the base pair level is like a letter, a codon is like a word, a gene is perhaps a sentence saying something about the organism. The organism has blue eyes or likes coffee, or is prone to lung cancer. The simplest organisms we know of, bacteria, are a short book with about 10 million atoms or one million bits of information. The human genome is of course large. It's equivalent to an encyclopedia with nine billion atoms and six gigabits of information, sort like a CDs worth or DVDs worth of information. If we turned this into a tape and unraveled the DNA, then we can make a scale model, and a letter would be 300ths of a millimeter. The human DNA in that context is 20 kilometers long, and you can imagine the words written out on a 20 kilometer strip of tape, which would be like an encyclopedia. In fact the actual DNA of a human cell, if it were fully unraveled end-to-end, is two meters long, which is amazing given that the cells are just a couple of microns across. When we think about life in the universe, we also have to go out of the box because we don't know that natural selection, the way Darwin planned it, or devised it, or explained it, is going to apply in other situations. It might, but it might not. We have to think more broadly about the situation, and that's tricky to do because how do we generalize beyond the single example we know and have been studying all the time? When I want to give biologists a hard time, I tell them they're just studying one example of a general case and we have to find life beyond Earth, and then they'll be looking at the general phenomenon. What kind of extrapolations of conventional biology might we consider? Here are some examples of weird life at the microscopic level. This is just speculation. We can have life that's a little bit weird where it uses different amino acids or bases for the nucleic acids. It might be a little more weird where we don't actually use the triad of DNA, RNA, and proteins for coding and expressing information, which is called the central dogma in biology, or it could be very weird where we actually don't have a carbon basis or a water basis as the solvent. There are certainly other chemicals like ethane and methane that could possibly suffice for a solvent or a medium in which molecules meet. Silicon life has been considered. It's not as sturdy as carbon life when you do computer modeling, but it could exist. Then the totally weird level, we might have some high density forms of biochemistry or organisms literally operating at the scale of molecules. There's nothing fundamental about a limit of the human cell for the package of genetic information. It could be much smaller. At the large scale, other possibilities exist too. Life exists based on about 50 different cell types. There's a distinction between prokaryotes, or cells without nuclei, and eukaryotes, cells with nuclei, like all the plants and all the animals. But you could have more cell types than we have in another circumstance. You could have weird operations at the higher level of symbiosis. Life on Earth was able to not just change its DNA slowly and steadily by sexual reproduction or by mutation, but also in packages by swapping genes. If this were extended to the level of gene and organisms swapping, then you might have a symbiosis that would accelerate the rate of evolution and the types of organisms that result. If you want to get weirder than that, you could go away from the idea of a cell as a container, as the fundamental basis for life. You can operationalize biology of conceiving it as a network as opposed to a container. That's a radical re-imagining of biology. Then if you want to get totally weird, you can consider biology that operates at the scale of planets. Geoengineering a planet so that an organism or a set of organisms can exist rather than the individual organisms surviving and dying. There are plenty of possibilities there for imagining biology beyond what we've seen. Essentially we're asking a hypothetical question. Would we be surprised if life beyond earth, differing from what we've seen in a four billion-year history, had other possibilities and those possibilities might be that it doesn't exist at all, of course, or is identical to our basis of life? Would that surprise us? Would we be surprised if it didn't use DNA, but used another information storing molecule? Biologists acknowledge that DNA works very well, but it's not unique as an information-storing molecule. Do we think life actually need a star to exist? That's not clear, because there's life on earth underground, deep in the oceans, that doesn't exist based on starlight or the photosynthetic food chain. Does it not use cells as the container? Does it not use carbon? Or is it even unrecognizably different from us? These are all things we must consider if we're thinking out of the box about biology elsewhere. It doesn't have to be just the way it is on this planet. We recognize also that some of the simplest definitions are difficult. Biologists don't always agree on a definition of life. See what you think about the boundary between living and nonliving. What you're looking at, of course, was a bass speaker and on top of it a steel tray, flour, and water. That simple. We define life in terms of complexity or behavior or patterns that change? How do we actually define life? It's not trivial at all. How will we know it when we see it elsewhere? The definition of intelligence is even trickier and the waters even murkier. Doctor, what is the definition of life? That is a big question. Why do you ask? I'm searching for a definition that will allow me to test an hypothesis. Well, the broadest scientific definition might be that life is what enables plants and animals to consume food, derive energy from it, grow, adapt themselves to their surroundings, and reproduce. You suggest that anything that exhibits these characteristics is considered alive. In general, yes. What about fire? Fire? Yes. It consumes fuel to produce energy that grows. It creates offspring. By your definition, is it alive? Fire is a chemical reaction. You could use the same argument for growing crystals, but obviously we don't consider them alive. What about me? I do not grow, I do not reproduce, yet I am considered to be alive. It's a good question Commander Data, we'll have to return to that issue. Exobiology, biology beyond earth or astrobiology, also the term used. We're now considering whether it uses planets as a basis. Is life always found on a planet as it is with the earth? What is it that makes recognizable life? Would we recognize if it weren't on a planet? Would we recognize it if it didn't use carbon as a basis? What would we even say about life that was unrecognizable? How could that be the case? So we have to ask some very big and profound and perhaps unanswerable questions as we consider biology beyond this planet.
