Evolution's Arrow

The evolution of life on earth has no direction and no predetermined end; what is adaptive today might not be tomorrow, and the scores of extinct creatures preserved in the rocks of this planet attest to an ongoing process that results in what Charles Darwin rightly called "endless forms most beautiful and most wonderful." The man hailed as the co-discover of natural selection, Alfred Russel Wallace, would not agree with my opening remarks, though. While Wallace contributed much to biological science in his own right, the incorporation of anthropocentric spiritualism into his hypotheses caused a good deal of controversy amongst naturalists concerned with evolution, and at least in terms of our own species Wallace favored a teleological view of evolution that involved some sort of supernatural intervention several times in the history of life on earth. Such beliefs are likely a large part of the reason why Darwin is most closely associated with evolution and Wallace often remains a footnote, and while this might not be entirely just in light of Wallace's other contributions to areas like biogeography his allowances for the supernatural to intervene in the process of evolution got under the skin of other scientists of the time. Even so, ides of a teleological or orthogenic process of evolution survived and even thrived for a time, and even today debates over whether evolution is "directed" or not remain.

Before proceeding further, though, it would perhaps be wise to take a moment to define what I mean by direction in evolution. The term "direction" alone is ambiguous, so for the purposes of this essay I'm going to take it to mean that direction means that a process is imbued with some sort of purpose or progresses towards an end point, even if that end point is transitory and leads to the continuation of the process in a new direction. Such a view was probably best expressed in the somewhat strange views of the Jesuit preist and paleontologist Teilhard de Chardin. In the last chapter of The Phenomenon of Man, de Chardin reflects a progression of humans towards an "Omega Point" which would be an intellectual and spiritual leap forward to a new level. He writes;

These and other assembled pointers seem to me to constitute a serious scientific proof that (in conformity with the universal law of centro-complexity) the zoological group of mankind - far from drifting biologically, under the influence of exaggerated individualism, towards a state of growing granulation; far from turning (through space-travel) to an escape from death by sidereal expansion; or yet again far from simply declining towards a catastrophe or senility - the human group is in fact turning, by planetary arrangement and convergence of all elemental terrestrial reflections, towards a second critical pole of reflection of a collective and higher order; towards a point beyond which (precisely because it is critical) we can see nothing directly, but a point through which we can nevertheless prognosticate the contact between thought, born of involution upon itself of the stuff of the universe, and that transcendent focus we call Omega, the principle which at one and the same time makes this involution irreversible and moves and gathers it in. (p. 307)

While de Chardin's posthumously published works gained a great amount of interest, the concept of the Omega Point was a concept born of de Chardin's desire to reconcile evolution with his religious beliefs, but while no one takes the Omega Point seriously today it does represent an attempt to join science and theology in a tradition that is still presently going on. In considering such attempts to be an intellectual peacemaker, though, we must ask whether science is being bent to the will of theology or whether theology is being considered in the light of the ongoing scientific process, and this distinction is not always clear.

Presently much of the debate over whether evolution has a direction or purpose is contained within the creation/evolution controversy, but despite the ongoing back-and-forth between scientists and creationists the controversies that are more interesting (if not more instructive) are those between evolutionary biologists themselves. The late Stephen Jay Gould was perhaps best known for his often vitriolic exchanges with the likes of Richard Dawkins, Daniel Dennett, and Simon Conway Morris, although at times it appears that the cranky columns each side took out in various media outlets had more to do with the release of hot air than scientific debate. Nevertheless, Gould's argument with Conway Morris is among the most important for us to consider here as it addresses the idea of the direction of evolution head-on. In 1998, each scientist had at each other in the pages of Natural History magazine in a special feature called "Showdown on the Burgess Shale," Conway Morris arguing for an almost teleological trend to evolution (summarized in his book The Crucible of Creation) while Gould, carrying over from his statements in the book Wonderful Life, argued for the role of contingency. After providing a brief review of some of the "Cambrian oddballs," Conway Morris confronts what he finds egregious about the role of contingency in evolution;

Contingency or no, I believe that a creature with intelligence and self-awareness on a level with our own would surely have evolved--although perhaps not from a tailless, upright ape. Almost any planet with life, in my view, will produce living creatures we would recognize as parallel in form and function to our own biota. But first, life must arise, and we have no idea how rare an event that might be. If we are honest, despite our exciting fancies about extraterrestrials, we must admit the real possibility that life arose but once, and that we are alone and unique in the cosmos--with an awesome and, to many, unanticipated role as stewards of all other living things. But were we to let evolution take another route than it did, why not grant (as, Gould will not) that another kind of being would have evolved to fill our special place in nature?

This view seems pretty mild when compared to some of Conway Morris' more recent claims (see this blog post about a talk Conway Morris delivered earlier this year), however, especially in his recent book Life's Solution: Inevitable Humans in a Lonely Universe. Using evolutionary convergence as a starting point, Conway Morris argues that there are recurrent themes present in evolution that would inevitably lead to the existence of our species, or at least one like ours. While there are many animals whose intelligence we are only just beginning to understand, from squid to spotted hyena to baboons and so on, in the opinion of Conway Morris we are not as rare a creature as Gould and others might suggest. Still, it is strange that our own species is singled out; why not talk about the inevitability of elephants or ammonites or oak trees? Conway Morris' preoccupation of recognizing the great Artificer who is involved in what he calls Creation comes into play here, and often his arguments about the "inevitability" of humans comes off as special pleading in an attempt to eat his cake and have it too when it comes to theology & science. Other planets may be something out, but a recent interview presented in the documentary "My Pet Dinosaur" reveals that Conway Morris believes that Homo sapiens (and not just a cephalopod equivalent) is an inevitable product of evolution. Here Conway Morris speaks of the "plausibility" of the "Dinosauroid," a thought-experiment in speculative biology devised by Dale Russell and R. Saguin decades ago;

Conway Morris' comments are a bit odd concerning our species; what makes our "design" better than that of any other creature? Such a call is entirely subjective, and I think a far better case could be made that the cockroach (among an innumerable amount of other invertebrates) is just as successful if not more so. Giving our stamp of approval to one or the other is largely a matter of opinion and there's no way to definitely determine what is better or worse, but if we were to play the game then it would seem that the australopithecine body plan was a much more successful body plan than our own. Looking at the hominid evolutionary bush pictured below, it's clear that we are but a single surviving twig of a group that once had a much greater diversity, australopithecines (including the "robust" forms in Paranthropus) seeming to be a much more successful type of hominid even if they are presently extinct. Given that our own species has only been in its modern form for little more than 200,000 years and we are daily poisoning our own well in terms of the global environment, I don't know if we can rightly say that we are any better or worse than any of the related forms that came before. The extinction of so many different hominids begs the question of why they are no longer around to join us if their forms were so good. If so many hominids so close to us could go extinct, doesn't that reflect that evolution is more contingent than directed towards a certain number of forms? Likewise, while cephalopods like the octopus are quite intelligent they have not evolved the same degree of intelligence that we possess in their own habitat, being out of the reach of our destructive power for most of our evolutionary history. If the evolution of a degree of intelligence comparable to our own being is meant to be, why didn't creatures that Conway Morris would argue started on such a path keep traveling down it? Why are we such a rare creature?

Still, what I find most interesting beyond Conway Morris' somewhat superficial anthropocentric views is that in the interview he envisions an alternative evolutionary history where not only do the dinosauroids evolve, but that humans still evolve as well alongside the hypothetical reptilian creatures. There is no conceivable reason why we should expect this to be so. Would not the survival of non-avian dinosaurs have drastically affected our own evolutionary history, even if we are to provisionally agree with Conway Morris about the inevitability of humans (which I do not) and human evolution is postponed until a little later in history? Fortunately for my own argument, there is no need to conjure up an imaginary creature that reflects more about the love of our own species than evolutionary history; dinosaurs are still alive today and some have developed a high level of intelligence. Birds like parrots and corvids, for example, show high degrees of intelligence and the abilities of Alex the late african grey parrot are well known. Perhaps they are not on par with us, but it does show that highly-intelligent animals do not need to look anything like primates. As I mentioned above, however, the intelligence of a crow is not the same as the intelligence of you or me, so there must be something fundamentally different in our evolutionary paths that has resulted in the present state of things rather than a drive towards a mind that could invent spirituality and other more abstract notions.

The discussion of convergence among modern (or geologically recent) forms is distracting us from the main argument, however. Yes, it is interesting that there was a marsupial saber-toothed predator (Thylacosmilus) in addition to the nimravids, true saber-cats, gorgonopsids, dinocerata, baboons, and other animals that have exceptionally long canines (although I will return to this topic shortly), but what Gould brings up in Wonderful Life is the question of what would have happened if the earliest chordates went extinct, leaving no descendants. Unfortunately we cannot actually "replay the tape" from the Cambrian, but if we are to find life on other planets should we expect such a planet to be home to anything like us? Gould doesn't think so;

I am puzzled that Conway Morris apparently, doesn't grasp the equally strong (and inevitable) personal preferences embedded in his own view of life--especially when he ends his commentary with the highly idiosyncratic argument that life might be unique to Earth in the cosmos, but that intelligence at a human level will predictably follow if life has arisen anywhere else. Most people, including me, would make the opposite argument based on usual interpretations of probability: The origin life seems reasonably predictable on planets of earthlike composition, while any particular pathway, including consciousness at our level, seems highly contingent and chancy.

I don't know how else to interpret the cardinal fact that life did originate on earth almost as soon as environmental conditions permitted such an event--an indication, although surely not a proof, of reasonable expectation and predictability; whereas consciousness has evolved only once, and in a marginal lineage among so many million that have graced our planet's history--an indication, although again not a proof, that such a phenomenon is not inevitably meant to be.

Indeed, perhaps our own hubris makes the idea of a "dinosauroid" or large-brained bipedal aliens seem more plausible than it actually is. While not dealing with this topic specifically, Carl Sagan recognized how we have envisioned intelligent life in the universe in our own image in The Demon-Haunted World;

The typical modern extraterrestrial reported in America in the '80s and early 90's is small, with disproportionately large head and eyes, undeveloped facial features, no visible eyebrows or genitals, and smooth gray skin. It looks to me eerily like a fetus in roughly the twelfth week of pregnancy, or a starving child. Why so many of us might be obsessing on fetuses and malnourished children, and imagining them attacking or sexually manipulating us, is an interesting question.

...the UFO abduction syndrome portrays, it seems to me, a banal Universe. The form of the supposed aliens is marked by a failure of the imagination and a preoccupation with human concerns. Not a single being presented in all these accounts is as astonishing as a cockatoo would be if you had never before beheld a bird. Any protozoology or bacteriology or mycology textbook is filled with wonders that far outshine the most exotic descriptions of the alien abductionists. The believers take the common elements in their stories as tokens of verisimilitude, rather than as evidence that they have contrived their stories out of shared culture and biology.

The known fossil record if full of creatures, both familiar and obscure, that are far more bizarre and wonderful than anything any science fiction writer has ever dreamed up (a cursory look through illustrated guides to space aliens in fiction reveals an overwhelming number of humanoids). Some of these creatures have had counterparts evolve later on in the history of life, but many were unique or remain strange to us, representing forms that do not provide a good case for the convergence of a set pattern of body plans. The world has not seen anything like Anomalocaris and Opabinia since the Cambrian, and while they may be closely related to arthropods they are strikingly different from any marine arthropod yet known. Even among organisms that are often overlooked we find bizarre creatures that take forms that were not repeated during evolution, the reef-building bivalves called rudists (Order Hippuritoida) containing some members where one half of the shell formed a flat lid that fit over the other half which was a cone-shape. The Cambrian fossil Wiwaxia is also difficult to place, its affinities often changing and subject to controversy. Even among living forms there are plenty of creatures that remain enigmatic, the phylum Chaetognatha (commonly known as Arrow Worms), predatory planktonic worms that have a ring of hook-like barbs around their heads, representing a body plan that has similarities with several groups but altogether is quite strange. Likewise, the phylum Cycliophora consists of only one genus (Symbion), discovered on the gills of a lobster in 1995, the creature obviously being strange enough to deserve a taxonomic ranking separate from other invertebrate taxa. I'm sure other examples could be mentioned and expounded upon, but the point is that life does not seem to hone in on merely a handful of "optimum" body plans but is forever changing a large multitude of body plans over the course of history, some dying out and never returning.

Even among animals that are convergent, though, there is still contingency and multiple ways of solving the same problem, and we should not confuse convergence of form with convergence of function as the two are not always the same. If we take a look at marine vertebrate groups with terrestrial ancestors, some have somewhat convergent body plans that have been molded by life in an aquatic habitat, the most famous example being the comparison of a shark, an icthyosaur, and a dolphin to show how evolution has favored a torpedo-like body shape for these aquatic predators. Yet a Leatherback Turtle does not look like an icthyosaur which does not look like a pleisiosaur which does not look like a pliosaur which does not look like a baleen whale which does not look like a penguin which does not look an otter (and so on and so on). Some forms are closer in overall form than others (a plesiosaur being closer to a sea turtle and an icthyosaurs being close to various forms of sharks over their evolutionary history), but the sheer diversity of aquatic vertebrate forms clearly shows that there is not one optimum form for life in the sea; the form that creatures will take is partially determined by their ancestry and partially by the adaptive zone they are moving into, differing combinations favoring different forms that will continue to change through time.

Taking up the example of saber-toothed creatures, an example Conway Morris cites in support of his view, we can detect important differences and contingencies even within groups that exhibit convergence on the same trait. Not only did predators like nimravids, sabercats (see above), and gorgonopsids develop very long canines, but so too have primates like baboons (see above), gibbons, and gorillas, living herbivores like musk deer and muntjacs, extinct herbivores like Eobasileus (see below). The mechanics of the bite of saber-toothed predators has been vexing for some time, but it should be noted that even among predators there is a greater diversity of canine lengths and forms. Even as far as "true" sabercats (members of the Subfamily Machairodontinae) are concerned, there appear to be three ecomorphs or kinds of sabercat with different saber morphology that corresponds to different body types (for more on this see "Three Ways to Be a Saber-Toothed Cat" by Martin et al.), so even among creatures that share a similar feature there are plenty of differences that are lost if our view is too superficial. Like I have mentioned, however, many primates also bear large canines, at least in the males. Indeed, in primates, musk deer, muntjacs, and some extinct members of the dinocerata, large canines are evidence of sexual dimorphism rather than predatory habits, the weapons being more likely a result of sexual selection. Thus there are convergent forms in various groups but the functions differ, and even in the midst of convergence contingency and diversity are still present. I don't think anyone would argue that saber-teeth were inevitable, but extraordinarily long and sharp canines have apparently provided a solution to differing problems across a number of groups over time.

At this point it seems apparent that we should not expect any creature to be inevitable based upon factors within the environment. The earth is not simply cycling through different settings that will allow the successive creation of life forms that are convergent with each other. Instead the earth has changed through time just as the life on it has, and while certain ecologies may appear continuously over time what happens tomorrow is contingent upon what happens today. Similar environments may appear again and again and lead to the evolution of similar forms, but we must be careful in picking our examples as life on earth has gone through many explosive radiations (especially after mass extinctions), the somewhat more charismatic terrestrial vertebrates only representing a relatively small sliver of the entire history of life on this planet. Who knows what environments might favor in the future? Our view from the top is a bit impoverished in this respect, and looking at the fossil record I don't see any reason to think that there is a number of all-purpose optimum body plans that evolution internally favors despite ever-changing environmental conditions. Perhaps we can somewhat safely put the question of contingency vs. determinism to rest in this respect, but some older arguments concerning vitalism and finalism should probably be considered as well.

I've been a bit hard on Conway Morris in this essay, but it should be recognized that his arguments (at least in the most general sense) are hardly anything new. G.G. Simpson devoted an entire chapter to directed evolution or orthogenesis in his popular work The Meaning of Evolution called "Oriented Evolution: Orthogenesis and Trends." Recognizing that horses tended to become larger and exhibit a reduction in toes during the course of their evolution (while also noting that horse evolution is much more "bushy" than often supposed), Simpson asks whether the "direction" of evolution is imposed by physical factors in the environment or whether there is some internal vitalist or finalist mechanism driving creatures towards certain ends over the course of time. What Simpson notes, however, is that most proposed instances of orthogenesis are cherry-picked examples with a large number of exceptions or contradictions, making the notion hardly tenable to begin with;

It is easy enough, in the first place, to observe that evolution along straight lines, or even the rigid orientation of evolution in a less simple way, is far from universal. It is only a tendency with so many exceptions as hardly to constitute a rule. (p. 132)

Even beyond this realization, by the time Simpson wrote this book the term "orthogenesis" had been "kicked around so much that hardly any two students mean exactly the same thing when they use it." In Simpson's case it appears that he is using the term to identify a trend that seems to proceed in one direction, not a sort of "perfecting principle" that can culminate in a given final form or end point. Still, in terms of orthogenesis representing a straight-line progression of one form to another over large periods of time and involving large-scale changes, Simpson rightly notes that evolution in one direction is likely much rarer and less important than others had made it out to be, probably telling us more about the ideas of the researcher than about the tempo and mode of evolution;

The extreme view that evolution is basically or over all an orthogenetic process is evidence that some scientists' minds tend to move in straight lines, not that evolution does.

Some of the straight-line models are even taken to absurd ends. As a child I remember hearing that saber-toothed cats went extinct because their teeth were getting so large that they could no longer close their mouths, but Simpson proved this to be wrong 30 years before I was born. The size of sabercat canines varied widely and there was no trend towards teeth so large that the cats could no longer feed, and if such rare creatures existed then they probably did not make it to reproductive age and were selected out of the population. Adaptation, Simpson concludes, is the only concrete mechanism with evidence, showing us that natural selection working on the genetics of populations allows creatures to evolve and best deal with changing environments. If there was an irreversible trend to larger size, for instance, than creatures would likely go extinct at a much higher rate as they would not be able to escape their orthogenic fate, a hypothesis not borne out by observations of evolution.

Given all this background, we can now move on to the question that spurred this essay in the first place; can we rightly say that evolution is undirected? I absolutely think we can, although the reason why this is so requires a bit of explanation. Like others I often tire of the rhetorical device of calling one feature of life or another "bad design" in arguments with creationists; if we start calling this feature or that a "bad design" we're already arguing the merits of design, our only weapon being our incredulity that a Creator would form the vertebrate eye in a manner that we might deem worse than the way in which an invertebrate eye was fashioned. On the surface saying that life is undirected might appear to be the same sort of argument, effectively saying "Well, I wouldn't have done it that way." Yet this is not what I am arguing at all. There is an absence of any evidence that evolution is being directed from within by some sort of orthogenic force or without by a supernatural force such as a deity, so I see no reason to leave the question as to whether evolution is directed or not up in the air. Saying that life might be directed but we can't tell smacks a bit of Gosse's Omphalos hypothesis that everything was created with the illusion of great age, a position that cannot be falsified but yet has no evidence, there being no reason for us to seriously take it into consideration as reality. Further, if we are to say that evolution is directed we should be able to detect something about the direction and what is driven it towards a given end point. Where is evolution going? Who or what is doing the directing? We might have our own favorite choices based upon personal preference or belief, one person saying the Judeo-Christian God, another aliens, another a vitalistic urge inherent in all organisms, but if such direction is apparent then shouldn't we be able to tell something about the direction other than "Evolution has direction?" If we were to imply direction to evolution but said nothing about it thereafter then we'd be doing the very same thing that intelligent design advocates do when addressing the public, stating that there is design but proferring none of the details of what should be apparent in nature to some degree.

To boil it down to the essentials a bit, I do not reject a "plan" for evolution because evolution doesn't proceed in the way I would do things were I in charge. I reject a direction or plan for evolution because there is no evidence of any such thing. Until such time as there is evidence to the contrary I see no reason to offer my assent to the view that evolution is being directed by a vital force, a deity, or any other factor that science has not yet detected. If I end up being wrong and evidence shows up that evolution has a direction or has been fiddled with by some supernatural force or other, I'll consider that when such evidence arrives. To date it has not, so and for my own part I am not comfortable with saying "We can't say whether evolution is directed or not." As Charles Darwin once wrote "How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service!" and I come down on the side of undirected evolution until some solid evidence to the contrary is presented and holds fast under the intense scrutiny such evidence would be sure to endure.