In 1989, the book Wonderful Life by Stephen Jay Gould made its triumphant appearance - of all the many writings by that prolific author, it was to become perhaps the most famous of all*. In this exploration of (then-)recent advances in our understanding of the animals making up the Cambrian explosion, the relatively rapid appearance of the distant ancestors of most living major animal groups, Gould argued extensively for the role of contingency and chance in evolution. If we were somehow able to turn time back to the Cambrian then let it run all over again, claimed Gould, then we would not see a repeat of the same evolutionary history. Major groups of organisms in our modern environment might fail to appear, while other groups that are currently minor and marginal might diversify to take their place. In particular, humans or intelligent life in general might never come to be.
*Something I find quite surprising - I personally find Wonderful Life rather drekky and overblown, and its arguments ultimately rather weak. To quote Moe Szyslak in the angel episode of The Simpsons (in which Gould actually made a guest appearance, "How about you stop telling us what it ain't, and start telling us what it am?"
Central to this argument of Gould's was his interpretation of the studies on animals from the Canadian Burgess Shale by Simon Conway Morris. Gould argued that very few of the animals present in the Burgess Shale could be definitely associated with taxa that survived the Cambrian, while most of the Burgess animals represented isolated lines that would eventually go extinct. Were one to look at the Burgess fauna in the absence of knowledge about future events, there would be no way of distinguishing which taxa were to survive and which would not. As it turned out, Gould's interpretation of Conway Morris' work was to rankle quite significantly with Conway Morris himself, who has since written two books that essentially counter Gould's book. In the 1998 The Crucible of Creation, Conway Morris attacked Gould's characterisation of the Burgess taxa as phylogenetically isolated oddballs, arguing instead that most were identifiable as stem-taxa showing connections to modern animals. While far more scientifically rigorous than Gould's Wonderful Life, The Crucible of Creation does suffer significantly from the constant shadow of Conway Morris' evident ire at Gould's 'misappropriation' of Conway Morris' work - Richard Fortey was later to comment in his 2000 book Trilobite! that he had "never encountered such spleen in a book by a professional".
I haven't yet read Conway Morris' second book of reply, the 2003 Life's Solution: Inevitable Humans in a Lonely Universe, but I would like to comment on its basic premise. In Life's Solution, Conway Morris addressed Gould's contention that the course of evolution was contingent on past history and essentially unpredictable. Instead, Conway Morris uses the prevalence of convergent evolution, the independent evolution of similar characters in unrelated organisms occupying similar habitats, to argue that selection pressures result in a relatively small number of potentially viable forms, and that even if Earth's history were rerun then a roughly similar assemblage of organisms would result. There might not be humans in exactly the form we now them, but intelligent, self-aware organisms of some kind would eventually appear.
Of course, both Gould's and Conway Morris' propositions are not directly empirically testable - there is no way of actually rerunning the course of evolution. However, convergent evolution is a widely prevalent phenomenon that we can examine and stimate its effect on the form of organisms. Convergent evolution seems to be the result of strong selective pressures - when a certain habitat or lifestyle strongly favours a certain morphology.
Moles, for instance, are definitely inevitable, at least among mammals. At least three different groups of living mammals have independently adopted a burrowing lifestyle - the Holarctic true moles, the African golden moles and the Australian marsupial moles. All three groups have developed a very similar morphology - reduced or lost eyes, dense fur, compact body with short limbs and spade-like forelimbs. Among fossil mammals, the marsupial (or at least metatherian) Necrolestes and the erinaceid Proterix have also been interpreted as burrowers with a very mole-like morphology. Even at least one group of burrowing insects, the mole crickets, went through a similar development of shortened limbs and spade-like forelimbs.
Cacti are fairly inevitable. A number of groups of plants inhabiting arid habitats - most notably various members of the Euphorbiaceae as well as the cacti proper - have evolved thickened water-storing stems with the loss or reduction of functional leaves to reduce water loss. Interestingly, the alteration of leaves to spines has occurred on numerous occassions, and it has been suggested that as well as the obvious benefit of protection, spines may help promote the condensation of moisture onto the plant as dew.
Lice, I'm sorry to say, seem to be inescapable. As well as the lice proper, a number of other insect groups have become ectoparasites living among the fur or feathers of other animals, such as a number of families of flies (including the sheep ked and bat flies) and the earwig Hemimerus. All such groups show a loss of wings, reduction of sensory organs such as eyes and antennae, and development of a flattened morphology that is probably less able to be squashed or scratched off by the host.
On the other hand, antelope are not inevitable. While the primary cursorial grazers of Eurasia and Africa are slender-legged quadrupeds, in Australia their niche is occupied by the bipedal kangaroos. I am not sure why kangaroos became jumping bipeds rather than running quadrupeds, but my suspicion is that their arboreal ancestry (phylogenetic analyses suggest that kangaroos descend from a possum-like ancestor) meant that the ancestral kangaroos started out with a difference in length between hind- and forelimbs. The point is that in this case two very different morphologies have developed to cope with very similar niches.
And humans? Well, it is my personal opinion that we have no reason to regard humans as inevitable. Invoking convergent evolution to claim the inevitability of humans runs up against the major stumbling block that we have no other examples of convergence on the human form. Questioning whether intelligence and self-awareness were destined to arrive as a result of selective pressure demands that we answer the insanely difficult yet crucial questions of how we define "intelligence" and "self-awareness", and how we would recognise them once defined. So difficult are these questions that yours truly is going to be a complete weasel and avoid them (I've spent enough time on this post already), but I will rather weakly point out that most behaviours cited as evidence for self-awareness in humans, such as figurative language and the production of art, are as yet unknown in other organisms when not encouraged by direct human intervention (but refer back to the recognition problem above). How is one to claim convergent evolution to support the existence of something for which no convergences are known?
A coda. In Kurt Vonnegut's 1985 novel Galapagos, a small boatload of people are shipwrecked on the Galapagos islands at about the same time as a plague wipes out humanity in the rest of the world. Over the course of the following million years, the descendants of this small group of shipwrecked survivors lose many of the features that have generally been regarded as the keys to what make us human but which are no longer selectively advantageous in their new environment, such as manual dexterity and large brains. Instead, humans become seal-like animals, covered in dense water-repellent fur with flippers for swimming and catching fish. It doesn't really matter what you're talking about - whether or not it's a good thing really depends on circumstance.
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