Convergence

When an octopus and a human being gaze at one another through the aquarium glass, they both do so through a camera-like eye. A human is a vertebrate and the occupant of the tank is a cephalopod mollusc. Their common ancestor lived more than one-half billion years ago, and since it did not have a camera-like eye the fact that humans can exchange gazes with octopuses can only mean that such an eye evolved independently. This is a classic example of convergent evolution—i.e., the emergence of a similar biological feature, not by descent from a common ancestor but from organisms that are effectively unrelated. Yet biologists also know that this eye-type has evolved independently at least four other times. For eyes to work the lens must be transparent. This property is conferred by employing particular proteins called crystallins. Their small molecular size enables a close packing in the watery medium of the lens, thus providing the necessary transparency. Yet the origins of human (and mammalian) and cephalopod crystallins are different. So here is an example of biochemical convergence. In both cases the crystallin is recruited from a protein originally involved with stress control; in mammals it came from a small heat-shock protein, but in cephalopods it derives from a detoxification protein. Both octopus and human end up seeing in much the same way, even though their respective ancestors could not.

Problems with the theory of convergent evolution

In the literature, such examples of convergence often provoke exclamations of "remarkable," "astonishing," and even "uncanny." It is almost as if there was a latent fear of the teleological principle being smuggled back into evolutionary biology. But are the eyes of humans and octopuses really convergent? After all, both employ the protein rhodopsin, which allows a chemical process whereby light is converted into an electrical signal that humans understand as vision. But this does not undermine the principle of convergence; it merely demonstrates that pre-existing structures will be recruited when necessary in a way analogous to the lens crystallins.

There is, however, a more serious obstacle in accepting convergence. This is in the form of the developmental gene known as Paired-Box (or Pax ) 6. This gene now has an almost iconic status: Pax-6 "makes" eyes in most, and perhaps all, groups of animals. Does this not undermine the principle of convergence? Hardly. In the developing embryo, the activity of Pax-6 is much more widespread. Originally it probably evolved in connection with the emerging needs for sensory systems in general: not only vision but also olfaction. Pax-6 is necessary but not sufficient; it is little more than a genetic switch. In human and octopus, it will ensure camera-eyes, but in flies and lobsters it "makes" compound eyes.

As already noted, camera-like eyes have evolved separately at least six times, while compound eyes—most familiar in insects—have evolved independently at least three times. These examples, involving vision, are surely more significant than the other familiar instances of convergence, such as the streamlining of aquatic vertebrates or warm-bloodedness in birds and mammals. This is because such sensory assimilation implies nervous activity and a brain, with the further link to cognition and sentience.

There are also striking instances of convergence in both hearing and olfaction. Even when a nose stops being used for olfaction, as in the star-nosed mole, its tactile sense is actually strongly convergent on the neurology of vision. Even senses that are decidedly alien to humans, such as echolocation (in bats and dolphins) and the generation of electric fields in fish, show splendid examples of convergence.

Scientific and theological implications

Few textbooks on evolutionary biology neglect to mention convergence, but curiously its wider implications are seldom addressed. These concern (1) its ubiquity, which implies (2) the reality of natural selection and thereby adaptation, and (3) the inevitability of evolutionary trends. Moreover, if the natural world is seen as part of God's great order, then convergence may also have theological implications. In brief, how different can this world—or any world—really be? Put another way, if intelligent life exists elsewhere in the universe, will it be humanoid or, in Robert Bieri's phrase, the equivalent of a thinking pancake?

Convergence is, therefore, central to understanding organic evolution. First, it confirms its reality. The eyes of octopus and human are similar, but they are not identical. The structure of the lens and the position of the retina, for example, are different. Convergence does not guarantee the identical, only the emergence of particular biological properties. Second, the ubiquity of convergence implies the prevalence of selective pressure: how else could biological systems come so closely to resemble each other? So too with adaptation; it is a biological reality and not some incidental by-product of effectively random processes. Third, the reality of convergence has the implicit assumption that starting points will be disparate, but there will be defined and repeatable evolutionary trajectories in evolution. Trends are real, and if the end-point is not perfect, is it emphatically better than what came before.

Yet all this is strongly at odds with a widespread perception that contingent happenstance is the determining reality in evolution. Thus, to paraphrase American paleontologist Stephen Jay Gould's metaphor of the tape of life, if the history of the world were to be re-run, the end result would utterly different. Historians might meditate on the untimely demise of Hitler or the death at an advanced age of Alexander the Great, but the consensus amongst biologists is that even a nudge in one direction half a billion years ago would preclude entirely the emergence of humans. As individuals this must be true, given that all humans were conceived by their parents against the odds. Yet biologically this view is deeply credulous. It is no accident that those who suppose the emergence of humans to be the product of individual and contingent history, also believe that humans are not only free (as indeed they are) but may mold their morality to a scheme of their choosing.

The realities of biological evolution and the inevitability of convergence suggest, however, a new view of life. Creation presupposes a history and an end-point, but this does not constrain choice and acceptance (or the opposite). The universe is so arranged that sooner or later, somewhere or other, certain properties, biological and ultimately spiritual, will emerge. The quip by British geneticist and physiologist J.B.S. Haldane remarking upon the creator's inordinate fondness for beetles is thereby turned on its head. Creation is indeed rich, but the modalities of convergence suggest that ultimately it is otiose to speak of accidents. Seeded in the act of creation was the inevitability of sentience endowed with free will.

See also Adaptation; Chance; Design; Evolution; Evolution, Biological; Selection, Levels of

Bibliography

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catania, kenneth c. "a nose that looks like a hand and acts like an eye: the unusual mechanosensory system of the star-nosed mole." journal of comparative physiology a 185 (1999): 367–372

conway morris, simon. the crucible of creation: the burgess shale and the rise of animals. oxford: oxford university press, 1998

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mueller, ulrich g.; rehner, stephen a.; and schultz, ted r. "the evolution of agriculture in ants." science 281 (1998): 2034–2038

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simon conway morris