Evolutionary explanations: historical trajectory versus convergence

May 9, 2009 | By | 4 Replies More

In the April 16, 2009 edition of Nature (available online only to subscribers), Johan J. Bolhuis and Clive L. Wynne asked “Can Evolution Explain How Minds Work? Their answer is that we need to be careful. Traits don’t always smoothly work their way up from ancestors to contemporary species. Sometimes, traits appear as a result of “convergence,” namely, these traits arise in species that are not closely related because these species were independently subjected to similar selection pressures. A good example of convergence would be the wings of birds and bats, which are not closely related species. Both species, however, were subjected to similar selection pressures,  accounting for the existence of their wings.

Image by bistrosavage at Flickr (creative commons)

Image by bistrosavage at Flickr (creative commons)

The authors argue that many studies suggesting that humanlike traits existed in lower primates lacked sufficient controls. Maybe those capuchin monkeys were not reacting on the basis of assessing “fairness” when they shared or rejected slices of cucumbers. Maybe they were rejecting an inferior reward simply because better rewards were potentially available.

The authors suggest that their reanalysis of these experiments suggests that it is a mistake to assume the continuous development of mind up through the lower primates, culminating in human beings. In fact, many traits that once seemed at one point to be distinctly human can be found more compellingly in birds than in other primates. For instance, rooks rub their bills with one another after confrontations. Magpies seem to recognize themselves when they are put in front of mirrors after a mark has been placed on their bodies. Crows excel at the use of tools. Consider, also, that the “language” employed by bonobos isn’t a true language, because bonobos don’t show the ability to imitate sounds. Compare this substandard primate language to parrots and many other songbirds, who are excellent mimics.

The appearance of similar abilities in distantly related species, but not necessarily in closely related ones, illustrates that cognitive traits cannot be neatly arranged on an evolutionary scale of relatedness.

The authors suggest another major problem: it is impossible to “identify the factors that originally drove the emergence of contemporary animal and human traits.”  They find that culture is one of these selection pressures that is not well accounted for. The authors take evolutionary psychologists to task, in particular. Some traits exist in a species but don’t exist at all in a closely related species. For instance, the marsh tit stores its seeds and is able to retrieve them several days later. Its close relative, the great tit, however, don’t store food at all.

The authors are not criticizing the application of natural selection, but reminding us that we need to be careful in its application.  Sometimes, an aspect of cognition can be historically shown to exist in ancestors such that one can probably conclude that it was passed on through descent. In other cases, including many aspects of human cognition, we might be jumping to false conclusions to the extent that we announce that particular cognitive traits arose as a result of descent.

When reconstructing the evolutionary history of cognitive traits, there is no a priori reason to assume that convergence will be more important than common descent or vice versa. In addition, evolutionary theory may suggest hypotheses about the mechanisms of cognition, but it cannot be used to actually study these mechanisms as long as researchers focus on identifying human-like behavior in other animals, the job of classifying the cognition of different species will be forever tied up in thickets of arbitrary nomenclature that will not advance our understanding of the mechanisms of cognition.


Category: Evolution, nature

About the Author ()

Erich Vieth is an attorney focusing on consumer law litigation and appellate practice. He is also a working musician and a writer, having founded Dangerous Intersection in 2006. Erich lives in the Shaw Neighborhood of St. Louis, Missouri, where he lives half-time with his two extraordinary daughters.

Comments (4)

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  1. grumpypilgrim says:

    Although I would certainly agree with the assertion that traits could result from either convergence or historical trajectory, I would suggest that the source of many traits could be deduced a priori. For example, that humans have five digits on hands and feet, as opposed to some other number, would clearly appear to be historical trajectory, since no obvious environmental pressure would seem to prefer one particular small number of digits over any other. Conversely, having a single bone in the upper arms and legs, and two bones in the lower arms and legs — an arrangement that greatly enhances the utility of these limbs, and which humans share with many other species — could more easily result from convergence than from mere historical accident. Still farther into the convergence end of the spectrum would be a trait like eyes: one that varies considerably in structure among different species, even though the various structures yield similar competitive benefits.

    In sum, if a given structure is seen in different species, and if alternative structures would seem to offer similar functional benefits yet are not seen, then the trait would seem almost certainly the result of historical trajectory; but where vastly different structures are seen in different species to provide a similar functional benefit, then convergence would be the most likely explanation. With this in mind, I would disagree with the assertion that 'it is impossible to “identify the factors that originally drove the emergence of contemporary animal and human traits.”' By viewing "traits" as having both a structural and a functional component, and by considering those components separately, one probably can make reasonable estimates of whether the traits arose from convergence or mere historical trajectory.

    • Erich Vieth says:

      The target of the authors was the cognitive architecture of humans. They further take aim at the sometimes simplistic assertions by evolutionary psychologists that human cognitive functions evolved in an historically traceable way, without producing the evidence to actually demonstrate this. It might be that various human cognitive functions evolved "within" our own species as a result of much more recent ecological pressures than many previously thought, and didn't appear at all in our recent human ancestors. Therefore, much of the primate and ape work might not be directly relevant to the existence of many human cognitive functions.

  2. Tony Coyle says:

    One challenge with 'either historical trajectory OR convergence' is that it is too simplistic.

    All traits are 'a priori'* founded upon a historical trajectory. That is, after all, the basis of evolutionary theory.

    The question, more properly, is whether cognition is a specific derivation of our particular evolutionary tree, or if it is a more general evolutionary property – perhaps an emergent trait in response to environmental complexity (such as sharp claws, or a focusing 'retinal plane' eye).

    We have certainly observed 'apparent' cognition in species as distinct as Macaws, Ravens, Cetacea, Monkeys, and Apes.

    Based on the observations – I'd hang my hat on cognition being a convergent developmental trait, since it appears to provide evolutionary benefit to those creatures who have it.

    Regarding human specific cognition – I'd consider that to be a composite of ape-based historical cognitive traits, alongside evolution of more specific human traits (due in no small part to our larger brain).

    So – I really have no basis on which to make any claim — I just think the evidence points to cognition as being a general emergent trait for creatures (regardless of brain size), but specific cognitive behaviors and capabilities are likely for each branch and variant. Just as octopi or raptors have much 'better' vision in some regards than humans due to their 'better' eye 'design', so too will their cognitive abilities differ.

    * Just to annoy Matt Taibbi! It's not just professors!

  3. Stacy Kennedy says:

    Bolhuis and Wynne seem to be overlooking the fact that our closest relatives are in fact smarter than most other species, (and monkeys are bright, but not as bright as apes), and are quite sophisticated in their use of tools. That being the case, I'd say it's more than reasonable to hypothesize that many aspects of human cognition are the result of evolutionary descent. That of course doesn't rule out the possibility that convergence had a role as well.

    I'm with Tony Coyle on this one. And, as he points out, historical trajectory and convergence aren't mutually exclusive. Sure, we must be careful not to project too much onto our cousins, but there are an awful lot of similarities between ourselves and the other great apes.

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