Homology – evidence for or against evolution?

Most evolutionary biologists and textbooks cite homology as clear evidence in support of common descent and evolution. However, contrary to such popular belief, many supposedly homologous structures in fact are not. This is compelling evidence against both common descent and macroevolution. Unfortunately this evidence that clearly challenges evolution is usually ignored, even suppressed, which is why it is not widely known.

What is homology?

Homology refers to similarity or equivalence of bodily structures or organs between different groups of organisms. Although the word homology was not used in its now familiar biological sense until the 19th century, notably by Richard Owen, the concept has a long history. Even in antiquity Aristotle (384-322 BC) recognised that in quite different types of animals there may be many corresponding structures – of soft internal organs as well as bones; and a few others after him such as Galen (130-210), Leonardo da Vinci (1452-1519) and Pierre Belon (1517–1564) compared the human skeleton with that of a monkey, horse and bird respectively. And with the scientific revolution, homologies were the basis for grouping organisms in hierarchical classification schemes.

Homology is illustrated by the vertebrate skeleton: across all groups of vertebrates – fish, amphibians, reptiles, mammals and birds – there are striking similarities in their overall skeleton, notably they all have a spine with ribs, and a skull. And the most widely-used example of homology is the forelimb of tetrapods (vertebrates with limbs, mostly land animals): despite the substantial differences in overall appearance of e.g. a human arm, dog foreleg, bird wing and whale flipper – the underlying bone arrangements are remarkably similar (figure 1).

Homology and evolution

Darwin saw homology as evidence for his theory of evolution, arguing that homologies in related groups of organisms arose because they are derived – through descent with modification and diversification – from a common structure in a common ancestor.

For example, it is proposed 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. In the same sort of way, the homologies of diverse vertebrate skeletons are seen as arising by evolution from common fish ancestors.

This explanation for homologies fits in well with one of the key concepts of evolution. Evolution has no foresight – it cannot anticipate what would be the best form of a wing or flipper; but, it is thought, appropriate structures can arise by progressive modification (through variation and selection, and ignoring the genetic / biochemical implications) of what is already available. Put another way, in evolutionary terms it is much easier to adapt an existing structure to meet new needs than to acquire a completely new structure from scratch. Then, because an ancestral structure, in different lineages, has been modified in different ways, this has produced the diverse structures but with some retained common features – i.e. homologues – of today’s species. Indeed, from an evolutionary perspective, the essence of homology is that the same structure has, through the evolutionary process, been modified in different ways, adapted to different ends.

The theory of evolution, therefore, seems to provide a cogent explanation for these similarities. Indeed, this evolutionary explanation for homology has become so widely accepted that it now defines homology as referring to those features which (are believed to) have been derived from the same structure in a common ancestor.

Homology. Originally, correspondence in fundamental structure of an organ, part, etc. Now chiefly, correspondence in evolutionary origin (of organs, parts, etc.).[2]

Homology and embryology

Structures and organs in a mature organism arise and develop of course through the operation of formative processes as the embryo grows and develops. These embryonic developmental processes operate consistently, such that it is possible to identify which cells in a very early embryo will develop into specific structures of the mature organism.

Embryonic origin

With this in mind (and adopting an evolutionary perspective) it is proposed that from a common ancestor (such as an early amphibian with limbs), over the course of evolution, modifications of the embryonic developmental processes (leading to formation of the limbs) have resulted in divergence from the common embryonic source (e.g. immature limb) to give the range of modern day adult organs (such as the specialised forelimbs mentioned above). 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 (arm, wing, flipper), if they are homologous then they will be derived from equivalent embryonic sources.

Conversely, even if structures from different species look similar, if they have developed from different embryonic 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 clearly resemble each other, notably in having a lens and iris, are equally specialised, and have comparable performance.

However, despite the similarities, they are not considered to be homologous, as there is no doubt they have arisen independently. They are in radically different animal groups (called phyla: chordates and molluscs respectively) which have completely different body plans, so there is no possibility of the eyes being derived from comparable or equivalent embryonic tissues.

Developmental processes

It is also recognised that for homologous organs or tissues the developmental processes should be comparable. It has long been accepted, including by evolutionists, that the early stages of embryonic development will not be susceptible to change: because so much later development depends on it, changes to early development will lead to detrimental deformities rather than something useful.

A necessary component of homology is the sharing of a common developmental pathway. Homologues must, to some extent, follow similar processes of differentiation which, one infers, depends on the same batteries of (regulatory and structural) genes [...] homology is based on the sharing of pathways of development which are controlled by genealogically related genes.[3]

And this view is reinforced the more we learn about the complex and coordinated genetic and biochemical mechanisms involved in embryonic processes – it is evident that random changes to these mechanisms will merely produce deformities. To produce useful developmental changes would require so many coordinated genetic changes that the chance of these occurring is realistically nil.

Criteria for homologies

In other words, if evolution is to explain homology and, reciprocally, for homology to be evidence or support for evolution, it is necessary to show that there is a viable route by which today’s homologues could have arisen from a common predecessor. And – a point I emphasise elsewhere – it's not enough to show how homologues might have arisen via a series of morphological intermediates; it is essential to demonstrate that the underlying genetic changes could have occurred through steps that had a reasonable chance of happening (typically by random mutation). Most obviously this implies a common embryonic source, and secondly the use of comparable developmental processes.

So we have these criteria for homologous organs:

• they must be derived from equivalent embryonic tissues;
• they should be formed by similar embryonic processes, especially in the early stages.

Homology – the inconvenient truth

Although most evolutionary texts convey a consistent and hence persuasive picture of homology, in fact there are many substantial anomalies. In particular, as we discover more of how tissues are formed embryonically, increasing doubt is cast on much of the homology that has been perceived for so long at the morphological level. There are many exceptions: where organs that are generally considered to be homologous –

• are not formed by equivalent developmental processes,
• or even arise from different embryonic tissues.

The diversity of the early embryonic development of vertebrates

As indicated above, probably the most cited examples of homology in support of evolution come from the vertebrates. As well as the similarities of the forelimbs of tetrapods, it is well known that at an early stage of development the embryos of very different classes of vertebrates look quite similar – what is often referred to as the phylotypic stage; and this, too, has reinforced the view that vertebrates have evolved from a common vertebrate ancestor.

However:

• the 'phylotypic stage' arises in very diverse ways, and
• even the vertebrae – the primary defining characteristic of vertebrates – form in at least three different ways,

which clearly challenges their supposed common ancestry.

Why the evidence about homology is important – and hence ignored!

Superficially it had seemed that evolution provided a cogent explanation for homology; but it is clear that there are many substantial examples of apparently homologous structures in fact not being homologous. This is extremely significant: it doesn’t just remove evidence in support of evolution – it constitutes clear counter-evidence against the organisms concerned having evolved from e.g. a common vertebrate ancestor.

Needless to say there is profound reluctance within the scientific community to accept this conclusion; and the incongruence of embryonic development and presumed phylogeny has provoked earnest discussion among biologists working in this area to try to come to terms with the facts within an evolutionary framework. Unfortunately I do not have the space here to describe the responses as fully as I’d wish [4]; in summary:

1. One approach is simply to ignore the significance of the contrary evidence. It is typified by Darwin. He saw embryology as one of the lines of evidence supporting common descent; but even he knew there were anomalies. And what was his response?

‘community of embryonic structures reveals community of descent; but dissimilarity in embryonic development does not prove discommunity of descent’ [5]

And more recently we have:

Although common development processes may aid in the identification of homologous structures (ontogeny as a criterion, not as a mechanism), lack of common development, be it developmental origin, process, or constraint, tells us nothing about lack of homology. [6]

In other words, some scientists will happily use those instances where embryonic development and adult similarities are consistent as evidence of common descent (i.e. evolution), and set aside those instances where they are inconsistent. But it is hardly valid to employ only the evidence that suits your hypothesis, brush the rest under the carpet, and claim the evidence supports your theory.

2. Many assume that the different developmental routes to ‘homologous’ organs demonstrate that developmental processes can evolve while retaining the adult structure. But there are several objections to this:
• It flies in the face of the evidence (and common sense) that tinkering with early development has seriously detrimental (usually fatal) consequences, rather than producing anything useful.
• As said above, the case against this happening is strengthened the more we learn about the genetic and molecular mechanisms of embryonic development. Given the large number of simultaneous coordinated changes that would be required, it beggars belief that such complementary mutations could happen opportunistically.
• Indeed, what rationale is there to account for such a change? What advantage (that natural selection could work on) could drive radical changes in embryonic development to comparable adult forms, given that natural selection acts primarily on the adult?
• Taking the preceding two points together: there must be a viable evolutionary route – by random yet advantageous small steps.
• It is no good simply saying that despite the theoretical objections the evidence shows that it must happen, because that’s a circular argument: rather than accept that different embryonic development shows that the organisms did not have a common ancestry, it is assumed that common ancestry is true, so embryonic processes must have changed, no matter how unlikely that is.
3. Convergent evolution. Some assume that the very similar organs have evolved independently, i.e. they are not homologous but what is called homoplastic. Quite apart from the extreme improbability of this, even if it were true, the non-homologous development still shows that the organisms in question did not share a common ancestor.
4. Revert to a pre-Darwinian view of homology. That is, some argue that homology merely describes features that are similar in various ways, but not necessarily with comparable embryonic development. Again, those adopting this position must recognise that it not only removes homology as support for evolution, but it also does not get rid of the evidence that non-homologous development of so-called ‘homologous’ structures is not consistent with common ancestry.

Teach the controversy? — at least teach the facts!

5. Perhaps the response that is of most concern is that of suppressing the evidence.

The inconsistent embryonic development of so-called ‘homologous’ organs has been known for a long time (e.g. Darwin (1866) [5], Spemann (1915) [7], and de Beer (1971) [8]).

Yet what do we find in evolutionary textbooks? Almost exclusively they present the examples of homology that are perceived as consistent with evolution (notably the tetrapod skeleton) and little if any mention of the inconsistencies.

For example, Berkeley University has part of its website devoted to evolution (evolution.berkeley.edu) which includes an ‘in-depth course on the science of evolution’. This includes homology as one of the ‘evidences for evolution’ and gives tetrapod limbs as an example. There is no mention whatever that any ‘homologous’ organs are formed by non-homologous embryonic processes, not even in the section ‘The big issues’ which ostensibly identifies the outstanding problems with the theory of evolution. Indeed this section is introduced by the grossly misleading sentence that ‘All available evidence supports the central conclusions of evolutionary theory, that life on Earth has evolved and that species share common ancestors.’ [9]

This is typical of what our biology students (and the wider public) are being taught. And no doubt is why so few, including professional biologists, know about the anomalies – they are kept safely out of sight so as not to upset the applecart.

It really is time the scientific community were at least honest about all the scientific facts relevant to evolution.

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Page created 2017, revised October 2020.