With only one example of biology to study and understand, we're left speculating how strange life might be on other Earth-like planets, which we know exist in abundance. How weird can life be? We can answer this question in two different ways or at two different levels. One is at the macroscopic level of the organism itself, and we can take it in stages. We can imagine life being just slightly weird, where the cells that form the basic chemical structure of an organism are different. Instead of the 50 or so cell types that humans have, perhaps a larger and more diverse suite of cell types. Perhaps there are the same prokaryotes and eukaryotes cells with and without nuclei, but the organization of those cells into an organism is done differently. The next level of weirdness might involve evolutionary time scales and mechanisms that are somewhat different. It was a surprise a few decades ago, when it was found that evolution does not simply occur by the gradual change of the DNA or RNA genetic structure. Sometimes organisms accelerate their evolution by swapping entire genes. Sometimes they create symbiotic relationships with other organisms, or even subsume them. It's speculated that the evolution of cells with nuclei involve such a process of symbiosis. When genes or entire organisms are co-opted, or swapped, or incorporated into a superior organism, genetics can move at a rapid rate. The next level of weirdness would violate the fundamental substrate of biology, where everything is essentially based on the cell. We and all organisms are made of cells, down to the smallest bacteria. But the function of biology might be possible without the cell as an organizing unit. Perhaps biology can work in terms of a network. A chemical network operating in a liquid medium without the cell as a container is both mathematically, topologically, and biologically a different organizing principle. Finally, the weirdest of all might be biology taken to a planetary scale. Already we know of organisms which can spread over a square mile or more. These are typically plants or some found in the oceans. But what if organisms could cooperate on much larger scales and essentially do geoengineering, harnessing the resources of an entire planet by cooperative behavior? And this doesn't have to be higher organisms, it could be bacteria. Bacteria use communal sensing to cooperate with other species of bacteria and even co-opt higher organisms into their survival. Simply, we need to use our imaginations to understand the possibilities of life elsewhere in advance of finding it. If we look at the evolutionary history of life on Earth, we notice that it seems to have gone through some surges. It was simple for 3 billion years, nothing larger than the nail of your thumb. And in fact, higher levels of organization really only happened with life moved onto the Earth or proliferated into the oceans about 500 million years ago in the Cambrian era. But if life could evolve more rapidly using gene swapping or symbiosis of cooperation amongst organisms, perhaps evolution need not have taken 3 or 4 billion years to produce us. Perhaps on an Earth-like planet, with different mechanisms of evolution, you could develop intelligence at our level much quicker. The other level of speculation about weird biology is at the microlevel, at the level of the individual molecular components and how they function. Already on the Earth, we've experimented with the toolkit of biology. All life is based on a single genetic code, but Steve Benner and other chemists have experimented with altering the amino acid suite that life uses, or even the base pair suite, and have had some success. Life is robust enough that some tinkering of the toolkit is possible without violating the idea of natural selection or evolution. We can imagine, therefore, rather different nucleic acids or bases that go towards making a genetic code. Biochemists will say there's no guarantee that RNA or DNA are the only replicating molecules that can provide a solid basis for complexity and life. The next level of weirdness would be a violation of what biologists call the central dogma. The triad of RNA as the underlying storage medium, DNA as its robust long-term storage, and proteins for the expression of structure and function in a cell. Perhaps life could exist with a different organizational schema amongst its complex molecules. The next level of weirdness would be taking away one of the substrate chemicals that we think life depends on, water or carbon. Water is by no means the only solvent for which it's possible to have complex biochemistry. In fact, to a biochemist, water is a double-edged sword. It provides a liquid medium and solvent for the facilitation of chemical reactions. But because it's such a good solvent, it actually destroys some chemicals faster than it makes them. It's a corrosive substance in some situations. It's possible to imagine other chemicals, like glycol, which might actually be better mediums for life to exist, although not as abundant as water. With carbon, it's more difficult to imagine a variation. We know carbon is abundant in the universe. Silicon occupies the same column in the periodic table, and so science fiction writers have speculated for half a century on silicon-based life. And biochemists have actually done some simple experimentation. It is possible to have complex chemistry with silicon as the substrate. But the molecules formed tend not to be as stable, or as complex in general, as can be done with carbon. So even if silicon biochemistry is possible, carbon biochemistry would probably be favored in a cosmic sense. Finally, and at the totally weird level, there's the possibility of even higher levels of information density. The information density of DNA and the single cell of a human is very impressive, an encyclopedia's worth of information in something just ten microns across. But if you look at the possibilities of information storage using biological molecules, which are currently being explored as computer scientists try to develop quantum computing. There is, as physicist Richard Feynman once said, room at the bottom. It's possible, in principle and in theory, to have much higher levels of biological information storage, much higher densities than in current DNA-based material, perhaps by 9 or 12 orders of magnitude. We have no idea if nature has produced such high densities of information storage elsewhere. But if it had, life could actually be microscopic sized, and you could have the functional equivalent of a large mammal existing in a package the size of a cell on the Earth. An extraordinary thought. In general, we have to open our minds to all of these possibilities and explore as many of them as we can with computers, and theories, and hopefully with some lab experiments. If these higher levels of information density were possible in terms of biological storage of genetic code, then we could have much more rapid evolutionary time scales. Once again, the 3 or 4 billion years that it took life to develop high intelligence on the Earth might be a long time scale rather than the shortest possible. We can speculate usefully about weird life because there's no guarantee that biology elsewhere will be the same as life on Earth. At the organism level, it may involve different mechanisms for evolution based on gene swapping or subsuming one organism within another. And these could accelerate the time scale of evolution compared to gradual genetic deviations of the DNA or RNA. At the microscopic level, there's no guarantee that RNA or DNA are universal. Other replicating and information-storing molecules are certainly possible. It's even possible to have much higher storage densities of genetic information than current biology by orders of magnitude. We could, for example, have life without water as the liquid medium, or perhaps not even using carbon as the basis for macromolecules. All of these things are possible in principle. We just don't know yet whether they happen in the real universe.
