Thank you so much to Doctor Kershenbaum for his helpful reply!
For my part I certainly agree that competition would have a major impact on the morphology of alien life forms, especially very advanced ones. Here are the points on which I differ with the good doctor.
1. I can easily imagine non-bilateral organisms performing at higher levels of speed, endurance, and maneuverability than bilateral competitors. I can picture marine habitats where apex predators look like giant paramecia with big sensor clusters and routinely sweep up swarms of little jawless fish-like bilaterals in their maws. On land I can picture giant amoeboid creatures running on numerous pseudopods and grabbing slow amphibian-like bilaterals. And in Carl Sagan’s famous book COSMOS the artist Adolf Shaller depicted life in the clouds of Jupiter. It all appears to be radial organisms, buoyed by hydrogen gas balloons. In the lower right is a falcon-like predator that has shed its balloon and is now gliding in to attack a much larger prey organism. It is plausibly radial.
2. It is true that, in the history of very large organisms on land and in the air on earth, bilaterals were clear favorites. That could possibly be inevitable but it could also be a thing that happened just by chance, as a series of contingent events in evolution. It didn’t happen at all – there were no land or flying animals at all – for the first 90% of life’s history on earth, with the first animals inhabiting the land 415 million years ago or so. To many that suggests it wasn’t inevitable. Moreover, there is a much bigger radiation of much more diverse organisms, where competition was super intense and no bilaterals ever arose at all. Protists. Camera like eyes did arise in protists, lending support to Simon Conway Morris’ conjecture that eye structures with lenses and retinas will be universally favored by natural selection. But those eyes are found on asymmetrical organisms. The evidence from protist evolution seems to me to suggest that bilateralness is not at all the automatic winner in evolutionary competition. I know that the doctor is thinking mostly about alien life that could give rise to intelligent species but I think that protists could have generated intelligence. Many protist lineages gave rise to colonial forms, including giant kelp (and also, one more step removed, us). I can imagine making contact one day with an intelligent species something like kelp, that have their nervous systems distributed over a huge network of cell-like units covering an area as large as a kelp forest. There has even been theoretical speculation that very exotic life forms could achieve technological intelligence while remaining at the microbial scale (or so it was claimed in the novel The Andromeda Strain).
3. This and the doctor’s mention of complex ecosystems brings up the issue of complexity, which I find very vexing. Though it is often stated that life on earth evolved toward greater complexity, the events that seem to have marked sudden increases in biological complexity; first the origin of eukaryotes (protists) and then the origin of multicellular life (plants, animals, and fungi) both seemed to happen AFTER the majority of the history of life on earth, and all of a sudden, without long gradual increases in complexity. This makes both seem rather more like chance breakthroughs than inevitable end points of lengthy and robust processes. Moreover I am not even comfortable saying that humans are the most complex life forms on earth. Many animals have superpowers that humans do not. Peregrine falcons can survive and proliferate in the Arctic without need for fire, clothing or shelter, they can fly at 300 kph (186 mph), visually lock on to target birds at a range of 2 km, and strike ducks on the wing, killing them with one blow. Moreover, both peregrines and humans are simple in one respect: they must have a steady supply of molecular oxygen or they will rapidly die. The only way vertebrates can generate the energy required to keep their cells alive is to constantly react that oxygen with organic molecules (food) that they take from other organisms. They can’t store enough energy to survive even brief interruptions of their metabolisms, and they can’t go dormant like so many other organisms can. We have only one way of keeping our metabolisms running. Species of bacteria and archaea, on the other hand, may have dozens of means of generating energy from substrates like formate, methanol, and formaldehyde. And some can produce all the organic compounds they need from very simple substrates like carbon dioxide and water, thus never needing to eat, and using only minerals as energy sources. Plants, multicellular photosynthetic organisms, not only routinely conduct the miracle of turning sunlight into food, they also generate thousands of secondary and tertiary chemical compounds that animals do not. Therefore, organisms we call simple may have chemistries that are much more complex than ours. And the biochemistry going on inside a cell is what determines its complexity, isn’t it? With the discovery of DNA and the development of practical ways of sequencing whole genomes, there was finally an objective, empirical way to measure the complexity of organisms. The genome, after all, is information, and shouldn’t the total amount of information required to build an organism be equal to its complexity? When sequences were compared there were some problems for those that wished to find that humans were the most complex. Many organisms, such as amphibians and lungfish, had bigger genomes than humans, and those of birds such as the peregrine falcon seemed very small, apparently having undergone reduction. It was discovered that much of the data in these sequences was silent or junk DNA, so that was subtracted, and then the numbers made humans look better. However there were still problems; it still took much more information to build a Paris japonica, a flowering herb for example, with a genome 50 times longer than that of a human being. This is called C-value paradox. There is also not a correlation between number of genes and complexity, this is called the G-value paradox. So now another proxy for complexity is used, which is the total number of different cell types, where humans do come out on top. I don’t feel like that is necessarily logical, because I can imagine an organism with the most cell types that is in fact simpler than one with fewer cell types, where each cell is doing much more complex biochemistry. In general this kind of thing shows the philosophical problem of anthropocentrism; humans think humans are the pinnacle of evolution, but we are biased. That basic issue contaminates almost any speculation about alien life and especially alien intelligence. There might be other pinnacles if we measured objectively and mathematically, but they might just not seem as impressive to us because they aren’t very human.
4. I think that fundamental differences in life on different planets could have huge effects on the outcomes of evolution there. Every living thing on earth must keep water inside its cells and our multicellular bodies, for example, making us much denser and and squishier than we would need to be if we were made out of, say, just organic polymers. If alien organisms do not need to be water balloons, they could be just 40% as massive for the same volume. Imagine how fast a cheetah would be if it could produce comparable thrust but massed 60% less! And, again, I often think of life forms that could be complex aggregates of crystalline chambers of different chemical compositions. I also think of organisms that might use radiation as a source of food, perhaps harvesting the alpha radiation from tritium, rather than reacting different molecules, to obtain energy. It turns out that radiotrophic fungi have even evolved on earth! NASA often says that they focus their search for life in places with conditions that we know can produce life: namely, places where there is liquid water. Lacking that, we also look in places where there may have been liquid water long ago, and places that are relatively easy to scan from orbit, or access with robotic probes, like the surface of Mars. We tend to think of the rocky surfaces of other planets, with warm little pools, as the natural habitat of alien life. Then in the last few decades we opened up the types of habitats we consider promising to include deep sea hydrothermal vents and liquid oceans beneath ice sheets, though we have never yet determined for certain that these habitats exist anywhere else but earth. However there is a place we have determined, for certain, that there is ice and formerly was flowing liquid water, and is also encrusted with complex organic matter like Polycyclic Aromatic Hydrocarbons, and many other molecules, collectively called Tholins. That is in asteroids. Our recent expeditions to asteroids and comets have shown them to contain clay (which forms when water breaks down rocky minerals), carbonates (which form in water), and huge deposits of tholins. Despite this new evidence I have not yet heard anyone propose asteroid belts as a new target for the search for alien life, but it seems to me this is a glaring oversight. If asteroid belts turn out to be a more common cradle for life to arise than are the surfaces of large planets, then all bets are off on body plans, This is because, in microgravity, the basic problem of lifting an organism’s mass off the ground is now achieved with next to no force. Legs are no longer strictly necessary, nor even wings. And, if living things adapt to leave the watery environments of asteroids and enter very tenuous atmospheres or near vacuums, they need not be aerodynamic or streamlined at all. They could look like a bunch of grapes and still jet around just fine with little puffs of gas.
Carl Sagan made good point about the search for intelligent alien life. At present the only practical way we have of contacting another intelligent species is radio communications. So if we ever do find an alien intelligence we can be confident we will have at least one thing to talk about: radio astronomy. But even then I don’t think that requires a bunch of alien radio technicians sitting around oscilloscopes under a steel radio antenna. They could be crystalline organisms that transmit radio from their bodies and whose minds work in ways that are incomprehensibly alien to us. I am sure that Dr Arik Kershenbaum and I would be equally and utterly thrilled to get to see what, if any, alien life forms exist on other planets, moons and asteroid belts, and we both know that is highly unlikely in our lifetimes or the near future of the human race. One day, though, humans may get their minds blown by something we find out there.