Homology is commonly cited as demonstrating, or even proving, common descent and evolution. But there is substantial evidence showing that supposedly homologous structures in fact are not; and in doing so this provides compelling evidence against both common descent and macroevolution. Let me explain:
Homology - what is it?
Historically, homology is simply similarities of biological structure. Probably the most widely-used example to illustrate this is the skeleton of vertebrates, especially the forelimb of tetrapods (vertebrates with four limbs): despite the substantial differences in overall appearance of e.g. a horse’s foreleg, human arm, bird’s wing and whale’s flipper - the underlying bone arrangement is remarkably similar.
The theory of evolution seems to provide an elegant explanation for these similarities. It proposes that the tetrapod leg, from its first appearance with the early amphibians, as their descendants diversified, was itself progressively modified to adapt to differing uses. Similarly, the supposed evolution of tetrapods from fish is supported by the similarities of skeleton of all vertebrates. This evolutionary explanation has become so widely accepted that it now defines homology as referring to those organs which have been derived from the same structure in a common ancestor.
That is, over the course of evolution, modifications of the embryological developmental processes have resulted in divergence from the common embryological source to give the range of modern day organs. And with this evolutionary account of homology, embryology acquired an important role in identifying and interpreting homologies. The point being that, even if adult structures look rather different (wing, flipper, arm), if they are homologous then they will be derived from equivalent embryological sources.
Conversely, even if structures from different species look similar, if they have developed from different embryological tissues then they would not be regarded as homologous, but due to convergent evolution. A good example of this is the vertebrate eye and that of the cuttlefish (a mollusc related to squid). In overall structure they closely resemble each other, notably in having a lens and iris, are equally specialised and with comparable performance. But they are not considered to be homologous, as there is no doubt they have arisen quite independently in separate phyla (chordates and molluscs) which have completely different body plans. And there was no common primitive eye from which they have both evolved.
Homology - the inconvenient truth
Although most evolutionary texts convey a consistent and hence persuasive picture of homology, there are in fact many substantial anomalies. In particular, as we discover more of how tissues are formed embryologically, increasing doubt is being cast on much of the homology that has been perceived for so long at the morphological level.
Notably, in view of the importance attached to the apparent homology of the vertebrate skeleton, and the weight given to embryology for identifying homology, it is especially relevant that vertebrae – a major component of the vertebrate skeleton – form embryologically in significantly different ways for different classes of vertebrate (such as mammals, birds, amphibians and fish), and even from different groups of early embryonic cells. (For example, see Vertebrates: Comparative anatomy, function, evolution by K. Kardong.) This clearly shows that the vertebrae of these different vertebrate classes are not, in fact, homologous - and hence that these different groups of vertebrate do not in fact share a common vertebrate ancestor, despite their superficial similar appearance and contrary to the commonly held view.
Taking this further, in the course of embryological development the members of different classes of vertebrate pass through a similar-looking stage (called the phylotypic stage), which is seen as clear evidence of their common ancestry. However, what is confounding (from an evolutionary perspective) is that even though the phylotypic stage looks similar, and we would have expected it to be formed from a fertilised egg in substantially the same way, there is, in fact, remarkable diversity, including some fundamental anomalies. The fact that the phylotypic stages are formed in different ways prima facie at least undermines, if not completely negates, the notion that they are derived from a common ancestor. And there are similar anomalies in other phyla, not only the vertebrates.
This is not just putting an anti-evolutionary spin on the facts. Here’s what evolutionary biologist Rudolf Raff had to say:
The process of early development from the egg to the phylotypic stage should be at least as conserved as the pattern of the phylotypic stage. One might reasonably expect mechanisms of early development to be especially resistant to modification because all subsequent development derives from early processes.
Homology - why the evidence is important - and hence ignored!
The evidence from homology - or, rather, the lack of it - is very important. Although some people try to dismiss the ‘intelligent design’ case against evolution as a ‘gap’ argument, this excuse is not available over homology. The fact that apparently homologous organs such as the vertebrae in fact have proved not to be, not only removes circumstantial evidence in support of evolution - it constitutes clear counter-evidence against the organisms concerned having evolved from e.g. a common vertebrate ancestor. This evidence unequivocally refutes the theory of evolution.
And this no doubt is why so few, including professional biologists, know about these anomalies - they are kept safely out of the limelight so as not to upset the applecart. As mentioned above, textbooks on evolution only cite the evidence that supports evolution. How many who are reading this article knew previously about the anomalies of so-called homology?