Homology in vertebrate limbs

One line of evidence for macro-evolution used by school textbooks is comparative biology. If two groups of organisms have a similar feature, textbooks use this as evidence that they are both descended from a common ancestor which had that feature. 

The most common example of this is the limb bones of vertebrates. For example, Longman GCSE Biology (Bradfield and Potter, 2002) pictures the limb bones of humans, bats and birds and claims “they show that all three groups of animals have evolved from a common ancestor with the basic pentadactyl limb structure.” Biology 2, an A-level text published by Cambridge University Press (Jones and Gregory, 2001) states that In all birds and mammals, for example, these limb bones take the same general pattern with a single bone in the upper limb and two bones in the lower limb. You can always pick out this pattern, despite great variations in the way the pattern has been modified in the course of evolution. Such features supply strong evidence that all organisms possessing them had a common ancestor. 

Does the observation of one set of  similar features provide good evidence for common ancestry? The reality of the situation is rather more complicated.

When just one feature is compared between different groups of organisms, it is easy to suggest what their ancestry could have been. But when more than one feature is examined, the evidence can become much harder to follow. The problem is that different features can give conflicting evidence about which groups are more closely related. When this is the case, it is not possible for all of the features to be indicating common ancestry. 

Many features of organisms appear, at first glance, to be shared. For example,  the eye of the mouse is structurally similar to the eye of the octopus. Do Darwinist scientists think that the mouse and the octopus – a mammal and a mollusc – had a common ancestor with well-developed eyes? No, they believe that their common ancestor was a worm-like creature which could do little more than discern light from darkness (Dawkins, 2004). The complex structure of the eyes of mice and octopi therefore had independent origins. When the eyes of mice and octopi were studied closely, they were found to have differences. They show reversed microscopic orientation of some of the light receptors.This was taken as evidence that they were not derived from a common ancestor with well-developed eyes.

But what happens when we look in fine detail at the shared features of vertebrate limbs? It turns out that the limb bones of different vertebrates develop in slightly different ways. The pattern for the bones of vertebrate limbs is laid down in cartilage condensations, which later ‘ossify’ into bones. The sequence of cartilage condensation is an important developmental pathway but differs between vertebrates (Hinchliffe, 1990). For the Darwinist, this should place a question-mark over whether or not the vertebrate limb structures come from a common ancestor. 

Other apparently similar structures start to look less than convincing when examined closely. Yet even if the fine details of a structure and its pathway of development was exactly the same in different organisms, a Darwinist couldn’t be sure if it was due to common ancestry, because this might be contradicted by other features. 

In practice, Darwinists often work out which features they believe to be due to common ancestry by assuming that evolution occurs and would involve the fewest possible number of changes. They construct an evolutionary family tree, and then infer which features can have a common ancestry in keeping with the tree. These similar features with inferred common ancestry are then described as homologous. An example of homology inferred in this way should not be used as evidence for common ancestry, as common ancestry has been assumed in order to identify it.

Similar features that evolutionists do not consider to be homologous are called homoplasies, or convergent structures. Such structures are common in the living world, even though it is arguably hard to see how such high levels of homoplasy could be an intuitive prediction of the neo-Darwinian model.

To give an example from the plant kingdom, the genus Gnetum is made up of 30-35 species of tropical evergreen trees, shrubs and lianas. No other plants are closely similar to them, and they occupy a taxonomic order of their very own. They are usually classified as gymnosperms, but display some traits similar to angiosperms. Like dicot angiosperms, they have net-veined leaves and vessels (chains of empty, open-ended cells) in the wood.  However, these vessel-forming cells bear a closer resembalce to the cells of the  wood of some gymnosperms, none of which contain vessels. They have reproductive structures in cone-like aggregations like some  gymnosperms, but the pollen-bearing parts are more like angiosperm stamens. They use “double fertilization” slightly different from the  “double fertilization” used by angiosperms, but much more different from fertilisation used by gymnosperms.

Patterns of similarity and difference between living organisms are highly complex. Tracing patterns of homology is not as simple, nor as conclusive, as it sounds in school textbooks. 
Bibliography

Bradfield, P. and S. Potter. (2002). Longman GCSE Biology

Dawkins, R. (2004). The Ancestor’s Tale. London: Weidenfield & Nicolson.

Hinchliffe, R. (1990). Towards a homology of process: evolutionary implications of experimental studies on the generation of skeletal patterns in avian limb development In: J. Maynard Smith and G. Vida, eds. Organisational Constraints on the Dynamics of Evolution. Manchester: Manchester University Press.

Jones, M. and J. Gregory. (2001). Biology 2. Cambridge: Cambridge University Press.

Wells, J. (2000). Icons of Evolution Science or Myth? Why much of what we teach about evolution is wrong. Washington DC: Regnery. 

Campbell, J. A. and Meyer, S. C. (2003). Darwinism, Design and Public Education. East Lansing: Michigan State University Press.

Junker, R. Unpublished paper on Homology.

Revised 2 January 2007