Life converges

One of the most fascinating features of biological evolution -- particularly as it applies to the possibility of life on other planets -- has to do with the concept of constraint.

Which features of life on Earth are, in some sense, inevitable?  Are there characteristics of terrestrial organisms that we might expect to find on any inhabitable world?  Stephen Jay Gould looked at this question in his essay "Replaying the Tape," from his brilliant book on the Cambrian Explosion, Wonderful Life:

You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past -– say, to the seas of the Burgess Shale.  Then let the tape run again and see if the repetition looks at all like the original.  If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur.  But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life?  What could we then say about the predictability of self-conscious intelligence?  or of mammals?

Some features that have been suggested as evolutionarily constrained, with arguments of varying levels of persuasiveness, are:

• a genetic code based on some kind of nucleic acid (DNA or RNA, or some chemical analogue)
• internal cell membranes made of phospholipids, to segregate competing chemical reactions from each other 
• multicellularity, with some level of tissue specialization
• in more complex organisms, some form of symmetry, with symmetrically-placed organs
• some kind of rapid-transit system for messages, analogous to our nervous system (but perhaps not structured the same way)
• cephalization -- concentration of the central processing centers and sensory organs near the head end

It's interesting when science fiction tackles this issue -- and sometimes comes up with possible pathways for evolution that don't result in humanoids with strangely-shaped ears and odd facial protuberances.  A few that come to mind are Star Trek's silicon-based Horta from the episode"Devil in the Dark," the blood-drinking fog creature from "Obsession," the giant single-celled neural parasites from "Operation Annihilate," and Doctor Who's Vashta Nerada, Not-Things, Gelth, and Midnight Entity.

So the search for extraterrestrial life requires we consider looking not only for "life as we know it, Jim," but life as we don't know it.  Or, more accurately, to consider to what extent our terrestrial biases might be blinding us to the possibility of what evolution could create.

It's worth considering, however, how often evolution here on Earth ends up landing on the same solutions to the problems of survival and reproduction over and over again, a phenomenon called convergent evolution.  Eyes, or analogous light receptor organs, have evolved multiple times -- some biologists have suggested as many as fifty different independent lineages that evolved some form of eye.  Wings occurred separately in four groups of animals -- birds, pterosaurs, insects, and bats.  (If you include structures for gliding, add flying squirrels, sugar gliders, colugos, flying fish, and flying lizards.)

Even biochemical pathways can reappear, something I find astonishing.  Take, for example, the research that came out this week in Nature Chemical Biology, which found that two only distantly-related plants -- ipecac (Carapichea ipecacuanha), in the gentian family, and sage-leaved alangium (Alangium salviifolium), in the dogwood family, have both come up with complex biochemical pathways to generate the same set of bitter, emetic compounds -- ipecacuanha alkaloids.

The last common ancestor of these two species was over a hundred million years ago, so there's a strong argument that they evolved this capacity independently.  And indeed, when the biochemists looked at the enzymatic pathways, they're different -- they found entirely different chemical synthesis methods for producing the same set of end products.  Weirdest of all, they both evolved an enzyme that cleaves a sugar molecule from the alkaloid precursor, and that's what activates it (i.e., makes it toxic).  In the living plant's tissues, the enzyme and the precursor are segregated from each other.  It's only when they're brought together -- such as when a herbivore chomps on the leaves -- that the sugar is split away from the precursor, the alkaloid is activated, and the herbivore starts puking its guts up.

Clever strategy.  So clever, in fact, that it was stumbled upon by two entirely separate lineages of plants.  The rules organisms play by are the same, so perhaps not surprising there are similar outcomes sometimes.

The whole thing highlights the fact that there is a limited range of solutions for the fundamental difficulties of existence.  It has to make you wonder if, when we do find life elsewhere in the universe, it might look a lot more familiar that we're expecting.  I don't think it's likely we'll bump into Romulans or Ice Warriors or Krillitane, but maybe there are features of life on Earth that will re-evolve in just about any conceivable habitable planet.

But hopefully there won't be any Vashta Nerada.  Those things are terrifying.

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