At Bil­ly Dem­b­ski’s blog, Granville Sewell won­ders What if we DID find irre­ducibly com­plex bio­log­i­cal fea­tures?:

In any debate on Intel­li­gent Design, there is a ques­tion I have long wished to see posed to ID oppo­nents: “If we DID dis­cov­er some bio­log­i­cal fea­ture that was irre­ducibly com­plex, to your sat­is­fi­ca­tion [sic] and to the sat­is­fac­tion of all rea­son­able observers, would that jus­ti­fy the design inference?”

No, at least not nec­es­sar­i­ly. It isn’t clear why we should make that imme­di­ate leap.

There are two relat­ed prob­lems with that jump. First, the fact that some­thing meets the def­i­n­i­tion of Behe’s irre­ducible com­plex­i­ty does­n’t mean it could­n’t have evolved (Behe has acknowl­edged this point). Sec­ond, the fact that some­thing is unevolv­able by known means (which is what Sewell and Behe usu­al­ly seem to mean by IC) does­n’t mean that the only avail­able expla­na­tion is design.

A sim­ple exam­ple comes from nat­ur­al arch­es in the desert South­west. Just like human made arch­es, they are struc­tures which are cur­rent­ly irre­ducible (remove the wrong bit of rock and they’ll tum­ble) but which unlike the Gold­en Gate Bridge, no one invokes a design­er to explain struc­tures like Rain­bow Bridge.

Why not? One rea­son is that, when Rain­bow Bridge formed a few mil­lion years ago, there was­n’t any­one around capa­ble of con­ceiv­ing of or con­struct­ing such a thing. If there was no design­er and no con­struc­tion crew, there was no design. There’s also no par­tic­u­lar evi­dence that any­one would want to have built a bridge at that site, nor any evi­dence of mechan­i­cal scrap­ing or oth­er exter­nal inter­fer­ence with the rock, so there’s no pos­i­tive case to be made that the design and con­struc­tion actu­al­ly hap­pened. Per­haps more impor­tant­ly, there are good expla­na­tions for nat­ur­al arch­es already. The geo­log­i­cal the­o­ry behind those expla­na­tions makes testable pre­dic­tions, and we can ver­i­fy those pre­dic­tions against mea­sure­ments in the field. “Irre­ducible” does­n’t mean some­thing isn’t acces­si­ble by grad­ual process­es, just that, hav­ing reached a cer­tain point, you can’t remove parts. 

On top of that, there already are objects which biol­o­gists would glad­ly agree seem to meet Michael Behe’s strict def­i­n­i­tion of irre­ducible com­plex­i­ty, but which no one (not even Behe, so far as I know) thinks require any sort of “design inference.”

There are tiny things called mito­chon­dria in each of our cells. Those organelles process oxy­gen, oxy­gen which would oth­er­wise kill the cells by inter­act­ing with impor­tant mol­e­cules. Each of those mito­chon­dria in each of our cells con­sists of var­i­ous struc­tures, includ­ing two mem­branes sur­round­ing a cir­cu­lar chro­mo­some and the nec­es­sary mol­e­c­u­lar entourage to pro­duce the pro­teins with which it process­es oxy­gen and releas­es mol­e­c­u­lar ener­gy (ATP) into the cell.

Remove the mito­chon­dria, and the cell dies. Remove parts of the mito­chon­dria, and they die and take the cells with them. Half a mito­chon­dri­on would be unques­tion­ably use­less. So where did it come from, and why don’t we invoke design to explain it?

Part of the rea­son is that there’s no evi­dence that there was any­thing at all capa­ble of design­ing, let alone man­u­fac­tur­ing, cells with mito­chon­dria in them when the first mito­chon­dr­i­al cells exist­ed. The exact date is still a bit fuzzy, but those first cells date back at least 1.2 bil­lion years, and prob­a­bly fur­ther back. All mul­ti­cel­lu­lar life has mito­chon­dria, and the equip­ment and knowl­edge nec­es­sary to man­u­fac­ture such a thing from scratch in a lab­o­ra­to­ry did­n’t exist a decade ago, and may not exist today. Invok­ing design seems a lit­tle sil­ly under the circumstances.

But there’s a bet­ter rea­son not to fuss with such an implau­si­ble idea. That cir­cu­lar chro­mo­some found in mito­chon­dria is very sim­i­lar to the cir­cu­lar chro­mo­some found in some bac­te­ria. And those two mem­branes are exact­ly what you’d find if one cell engulfed anoth­er into a vac­uole (as they do before digest­ing food par­ti­cles). What if some ancient cell engulfed a bac­teri­um which could process oxy­gen, but did­n’t man­age to digest it? That engulfed cell would soak up oxy­gen from the engulfer, and the engulfee would digest par­tial­ly processed mol­e­cules from the cell, and emit excess ATP for the engulfer to use. Win-win. It’s a testable hypoth­e­sis that dates back at least to the 1950s, and which was cham­pi­oned most promi­nent­ly by Lynn Margulis. 

Mar­gulis cit­ed sim­i­lar evi­dence to argue that var­i­ous oth­er organelles in the cell, and oth­er forms of bio­log­i­cal nov­el­ty above the cel­lu­lar lev­el, can be attrib­uted to the effects of sym­bi­ot­ic coop­er­a­tion. The response to this pro­pos­al has been mixed. With mito­chon­dria (and chloro­plas­ts) there’s good rea­son to think that endosym­bio­sis can explain how you’d get what looks like one cell liv­ing inside anoth­er. With struc­tures like per­ox­i­somes or cil­ia, the evi­dence is less compelling.

Mar­gulis right­ly points out that her mech­a­nism dif­fers from those which form the core of the neo-Dar­win­ian syn­the­sis: muta­tion, selec­tion, genet­ic drift and gene flow. Because she pro­posed a new evo­lu­tion­ary mech­a­nism, she does not describe her­self as a neo-Dar­win­ist. This isn’t a chal­lenge to the basic prac­tice of sci­ence nor to com­mon descent. She sim­ply rec­og­nized that exist­ing evo­lu­tion­ary mech­a­nisms were not ade­quate to explain cer­tain bio­log­i­cal phe­nom­e­na, and devel­oped a new idea to help explain that.

As I’ve point out before, the sort of cel­lu­lar endosym­bio­sis we infer took place with mito­chon­dria can be seen in tran­si­tion­al form in the world around us. Researchers at the Uni­ver­si­ty of Rochester and the Craig Ven­ter Insti­tute pro­duced yet anoth­er exam­ple. Fruit flies infect­ed with a bac­teri­um which enters the host cells – Wol­bachia – were found to have import­ed part of the bac­te­r­i­al genome into their own. Mito­chon­dr­i­al genes have also been found grad­u­al­ly migrat­ing into the genome of host cells, and one of the authors of the Wol­bachia paper explained “In a way, Wol­bachia could be the next mito­chon­dria. A hun­dred mil­lion years from now, every­one may have a Wol­bachia organelle.” Again, the dis­cov­ery of large swaths of DNA sud­den­ly appear­ing in the fruit fly genome required an expla­na­tion beyond those devel­oped for the Mod­ern Syn­the­sis (it is, as I put it above, “unevolv­able by known means”). Sci­en­tists need­ed an expla­na­tion beyond those which exist­ed in tra­di­tion­al biol­o­gy. But there was no rush to invoke invis­i­ble intel­li­gent genet­ic design­ers or engi­neers. A nov­el nat­ur­al process­es was more than ade­quate, and yields new testable results and impor­tant new avenues of research (which a design infer­ence would­n’t do). 

(I’ve writ­ten about Wol­bachia before, if you’re interested).