Alien Life: Convergence vs. Phylogenetic Signal

I am reading a great new book by Dr. Arik Kershenbaum, a Zoologist at Cambridge University. His career is in top-quality scientific investigation of animal communication. But this new book is titled The Zoologist’s Guide To The Galaxy.


Please do buy it and read it. Kershenbaum was a student of Dr. Simon Conway Morris and, like him, he has concluded that the history of life on earth shows that, over and over, separate lineages of organisms have evolved to find the same solutions to problems like how to see, how to move in the ocean, how to move on land, and how to fly. He points out that the laws of physics and chemistry have absolute limits and, this, combined with natural selection, should constrain the range of possible solutions that come to predominate to just the few that work best. And that should go for alien life just as much as that on earth. I broadly call this school of thought “Convergence-ism“.

Dr. Kershenbaum is very good about using qualified language, and always notes the possibility that he could be wrong. He also makes a lot of other excellent points. For example, that, although there is no way that a wing covered with feathers made of keratin could be of any use in, say, the atmosphere of Jupiter, where the clouds would be made of ammonia and temperatures would range from cryogenic to furnace-like, some other wing structure WOULD. If there was life in the atmosphere of Jupiter it would be quite likely to have wings that, though they have no common inheritance with the wings of earth animals (phylogenetic similarity) would have functional similarity (wings) and therefore would be recognizable to us earthlings. Going one step further, he argues that there will likely be actively moving organisms among alien life forms, and these would functionally qualify as animals, even if they share less ancestry with earth animals than seemingly unmoving and passive plants or bacterial colonies do. I think that is an original idea, although I am by no means well-read on the subject. If so it should be called the Kershenbaum Functional Equivalence Principle.

I would like to have a gentlemanly debate with Dr. Kershenbaum, because I admire him but I still disagree. Not a formal debate, where he would destroy me in seconds with his vastly superior education, knowledge, and intellect, but an open letter, internet, blog-level kind of debate. I already tweeted to him and he graciously replied that he is glad he triggered some debate; an exemplary scientific attitude. Still, I remain of the opposite persuasion.

I can put my objection succinctly. Every living thing on earth shares a common ancestor that lived a mere 4 billion years ago (in a universe that is 13.8 billion years old!). Life on another planet, or moon, or asteroid belt, or accretion disk, would share NO ancestor with us. It would quite likely not have arisen from RNA nor use DNA as its genetic material, and it might exist in a very different temperature, pressure, radiation, gravity, and chemical environment and, maybe, on a different timescale as well (crystalline alien life might act on the timescales at which crystals grow, perhaps taking centuries to move and hundreds of millennia to reproduce). Therefore, every alien organism should share less with, and thus be MORE different from, us than ANY earthly life form. That means aliens should be MORE different from humans than pandoraviruses, endolithic chemosynthetic bacteria, extremophile archaea, lichens, euglenids, tardigrades, an 80,000-year-old clonal patch of aspen trees, basket stars (Phrynophiurida), or hagfish. Over 4 billion years, through global cataclysms, and despite invading wildly disparate earth environments like the interstices of solid seafloor gabbros, boiling springs of sulfuric acid, and the frigid altitudes over the Himalaya mountains, every earthly organism retained highly conserved features: we retain water as our solvent, nucleic acids as or genetic material, lipids (hydrocarbon derivatives) for our cell membranes, and proteins for our enzymes and cytoskeletons. To me this suggests that life on earth CAN’T replace the traits it started out with, it must stay within constraints that it inherited from the last common ancestor of all life on earth. Let’s call my position “Phylogenetic Signal-ism.

The question, at one level, is simple. Which matters more; convergence, which is undeniably ubiquitous on earth, or phylogenetic signal which, also, is? We have, at present, exactly zero real evidence about the existence or nature of alien life. So, on my facebook post about this, I summed it up this way:

“when one is writing a book on a subject about which there is zero data, all of the content is, by definition, speculation. In that case the only thing a writer can do wrong is fail to speculate. That is what I think the good doctor has done here. And I fear that this disregard for speculative possibilities fundamentally misunderstands the job of pondering the possibilities of alien life. This exercise MUST be imagination.”

That was the zing I selected for Dr. Kershenbaum because he does not go on and on coming up with crazy possibilities or citing the thousands of different concepts of alien life from science fiction. But that’s not entirely fair because he does do a few, and some pretty good ones, including Hoyle’s super-intelligent interstellar black cloud, and his own ideas about buoyant organisms living on the benthos formed on the underside of Europa’s sea ice. But, basically, he is saying that, if we meet alien beings some day, they will probably have a face, at least one left and one right leg, and two camera-style eyes, so who cares if they are made out of polyaromatic sulfonyl halides instead of DNA, or have spinel eye lenses instead of our crystallin proteins? I take the opposite position, thinking that, if we start out with a different chemistry, in a different astronomical environment, there is no reason to think that natural selection will arrive at the same end states. Eyes and legs are not inevitable, just because they arose repeatedly on earth.

The reason that I am inserting myself into this is that, 31 years ago, when I was in college studying comparative morphology and ecology, my hobby was to think up ways that biology could be other. I spent my weekends, and some of the time when I should have been studying, in those pre-internet days, in the science library, with textbooks open on the tables, taking notes and doing doodles and sketches. I was trying to work out plausible alien habitats and organisms that strictly followed the rules of chemistry and physics but diverged from ALL of the biology on earth. All life on earth is based on nucleic acids, proteins, lipids, and carbohydrates, so I researched other possible chemistries. Most life on earth is based on cells, so I considered alternatives to that. Large organisms on earth are multicellular, so I considered different ways that living matter could be organized and increase in complexity. Many earthly life forms are attuned to sensing electromagnetic wavelengths from infrared to ultraviolet, the photon frequencies we call light. I imagined life forms living in different radiation environments around giant planets, maybe without luminous stars, that might use active or passive radio and microwave sensory systems.

I hope to do a few blog posts about this subject in the near future. The next one will be about bilateral symmetry in animals. This is the fact that, if you consider the body plan of most (but by no means all) animals, the right side is the mirror image of the left side. The anatomy of the dorsal (back) surface is different from the ventral (belly) surface. This is a defining and fundamental characteristic of the Bilaterians: including the super-diverse and successful arthropods, chordates, mollusks, annelids, and many other phyla. Dr. Kershenbaum argues that we can be confident that most alien organisms will be bilaterally symmetrical. But does that stand to reason? Simply put, if aliens are bilateral, I’ll eat my hat. And I’ll tell you why next time.

A sketch from 1989: a biota on a hypothetical moon orbiting a gas giant planet. Here we see a rift in the global ice sheet, with flowing ammoniated water and a sort of reef, or tangled riverbanks, of sessile organisms that catch organic matter raining from the dense atmosphere. Note the two, brachiating, intelligent, and non-bilateral alien organisms to the lower left.

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