Two natural things to think about astrobiology are the fact that there's a sweet spot for life. We remember the idea of Darwin's warm little pond, a gentle temperate environment where life first emerged. Well, this turns out to be wrong. Life almost certainly started under extreme conditions near hydrothermal vents, superheated water, toxic chemical environments far from Darwin's warm little pond. Also, we think in terms of the Goldilocks zone the region from a star where water on the surface of a planet can be in liquid form, and we think of that as being the habitable zone. Life on Earth disavows us of our notion that the habitable zone is so limited. Because microbial life on earth has radiated until almost every possible ecological niche, and a few that are almost unimaginable. Extreme creatures are called extremophiles as a category. Most of these are microbes quite unfamiliar to us. In fact, most of these microbes have not yet been cultured in the lab, and so we understand them quite poorly at a biochemical level. In that sense, it's humans that are unusual because we and the other large mammals are unable to tolerate physical extremes or as microbes have no such problems. The bounds of life on Earth are illustrative because if we venture beyond the Earth, we will find different physical environments. It's natural to believe that life will adapt and mold to the physical environments available to it, and so we shouldn't expect the same form of biological life in different physical environments. The last few decades have given us an extraordinary sense of the diversity and robustness of microbial organisms. We say that life needs water, but life has been found in the high Atacama desert in places where it rains barely once a century. So while it needs water, it certainly doesn't need much. Under these conditions, evolution's rate slows down by factors of thousands or 10,000's but life persists. Life has been found that can survive below the freezing point of water. Because water expands when it freezes, normal cells will burst and of course kill the organism. But some organisms have evolved the capability to develop antifreeze which stops them from freezing at temperatures of minus 13 or even minus 18 degrees C. Similarly, we imagine that life couldn't really survive above the boiling point of water, but it does. Life has been found at temperatures of 130 to 135 degrees C near hydrothermal vents and in deep terrestrial environments where the radiative heating from radioactive material is intense. At some point, going up in temperature, the structure of DNA would unravel, denaturing as it's called. So we think life cannot exist beyond a certain temperature. But life certainly violates the traditional habitable zone even on Earth. Some forms of life on Earth are extremely tolerant of radiation. About 30 years ago, microbes were discovered in a can of meat that had been irradiated to attempt to sterilize it. This microbes were subsequently found to be able to tolerate radiation tens of thousands of times the dose that would kill URI in a few seconds. To do this, the organisms Deinococcus radiodurans is the prime example, keep stacked copies of DNA in short segments so they can repair their DNA hundreds or thousands of times faster than we can. It's important to note that these organisms don't just tolerate these conditions, they thrive on them. Some microorganisms can tolerate toxicity also that would be fatal to humans. There are toxic EPA sites in Northern California that used to be the side of minds where there's mercury, cadmium, and superheated water from hydrothermal vents underground and springs. Life has been found in these situations and places where humans can barely go. Not just single microorganisms but entire ecosystems doing quite well. Even greater surprise came about six years ago when the microorganism were discovered deep in a mine shaft in South Africa. It survived on dim radioactive glow from the natural material in the rock cave. There was no sunlight there. Its source of energy was geothermal. Also, extraordinarily, it seems to be an ecosystem of one. Bacteria have been found existing in rock. Sometimes, they're near the surface of the rock using crystals to let light in in a miniature solarium. But life can also be found deeper within rock or even below the crust of the earth. By some estimates, the biomass underground and in the interior of the Earth exceeds the biomass on the surface by an order of magnitude. Most extreme of files are bacterial or microbial small microscopic in size, but not all. Familiar to many school kids is the tardigrade or water bear. This small organism has its own phylum with hundreds of different species. It has four pairs of legs, a digestive system, a single gonad, and an central nervous system albeit of a primitive kind. Biologists learn about tardigrades to understand how cryptobiosis works. Because tardigrades can curl into a ball and go into a desiccated state where the water content is less than one percent of normal, and survive apparently for thousands of years in that state being carried on the wind or in the fur of animals. NASA biologists are quite seriously looking at tardigrade mechanisms to understand how we might one day travel to the stars. Perhaps the biggest misconception about life on the Earth is that it needs a sweet spot or a habitable zone, a gentle environment where water can be a liquid. In fact, microbial organisms on Earth have adapted to an extraordinary range of environments. We imagine that the even more extreme environments in space might not preclude biology. There are microbes on Earth that can live below the freezing point of water and above its boiling point, they can live deep within rock, they can withstand enormous radiation doses and toxic heavy elements. There are even larger biological creatures that can go into wait states. Cryptobiosis, lowering their metabolic rates or subsisting on tiny amounts of water. All of this is illustrative for how diverse the environments might be that host biology beyond the Earth.
