Writing for Evolution News, Casey Luskin has already highlighted my current paper in the journal BIO-Complexity, “The Diverse Early Embryonic Development of Vertebrates and Implications Regarding Their Ancestry.” When I wrote the paper, my primary objective was to make this diversity, referenced in the title, better known. So I thank biologist P. Z. Myers for commenting on the paper, because in doing so he has helped to achieve that. See his post here, “Creationist thinks evo-devo ‘refutes’ evolution.”

But His Response Is Extraordinary

Can there be a better example of trying to argue that whatever the evidence, evolution is the answer? Where the evidence is consistent with common descent, then of course that is the explanation — it’s simple. And where the evidence is not consistent, common descent is still the explanation — but it’s complicated! Myers simply uses “complexity” as a smoke screen to avoid facing up to the possible falsification of evolution.

Does the early embryonic development of vertebrates support their common ancestry? On one hand, he says that of course the similarities of the vertebrate phylotypic stages are evidence of common ancestry. This used to make sense, because everyone agrees that where organisms have evolved from a common ancestor, we expect their early embryonic development to be similar. This is obvious, as many biologists have observed. In the paper I quote Rudolf Raff:

One might reason­ably expect mechanisms of early development to be especially resistant to modification because all subsequent development derives from early processes.¹

And Louise Roth wrote:

A necessary component of homology is the sharing of a common developmental pathway.²

Hence, as I acknowledge in the paper, when biologists learned of the similarities of various vertebrate phylotypic stages, it was reasonable to infer that these supported the possibility that they had a common vertebrate ancestor.

A Closer Look

However, subsequent research has shown that these similar stages arise in radically different ways in different classes of vertebrates (e.g., fish, reptiles, mammals). That is, the earlier embryonic development of various classes of vertebrates is not similar but very different. Of particular note is that substantially different parts of the early developmental stage known as the blastula become the subsequent embryo (rather than extraembryonic tissues). This is important because the cardinal criterion for homologues is that they be the same tissue, albeit modified in different ways. 

So the different sources of the embryonic tissues in different vertebrate classes is clear evidence that the resulting embryos — including their phylotypic stages, despite their similar appearance — are not homologous (at least not in the usual or evolutionary sense). And this conclusion is reinforced by the very different processes through which the phylotypic stages arise, such as the different ways in which gastrulation occurs.

The Usual Evolutionary Response

Now what is the usual evolutionary response to all these substantial embryonic differences? In view of their established and well-founded view that early embryonic development should be resistant to change, one might reasonably have thought that unbiased biologists would recognize that this diversity clearly shows that, despite their phylotypic similarities, the vertebrates have not in fact evolved from a common ancestor. But what is their actual response? Even though it flies in the face of common sense, they suppose that there must have been major changes in the early developmental processes. This includes Raff who comments (quoted by Myers):

Early development is highly evolvable, even among closely related species.³

What is the basis for this claim? It’s not because the earlier reasoning that early embryonic development should be resistant to change was mistaken, but solely because it’s the only way belief in common descent can be maintained despite the evidence against it. And Myers tries to make a virtue out of necessity with this topsy-turvy assertion:

Observing that something is complex and diverse is evidence that an evolutionary process created it, rather than an engineer.

A Serious Question Requiring an Honest Answer

Clearly, the question that evolutionists must give a credible answer to is: How might the early processes occurring in a common vertebrate ancestor have diverged so extensively, so fundamentally, such that in the various subsequent vertebrate classes the organism forms from different parts of the early embryo?

Further, should reminder be necessary, because evolutionary processes are blind, i.e., they have no knowledge of embryonic mechanisms and no foresight, all of the supposed changes to embryonic development would need to have arisen opportunistically. That is, solely through undirected mutations to genes and control sequences, all of which must have had a reasonable chance of occurring and of offering some advantage that could be favored by natural selection. Yet, of course, all of the mutations we know of that affect early embryonic development in a significant way, far from being advantageous, are detrimental if not fatal.

Not Alone in His Bias

Unfortunately, Myers is not alone in his biased attitude towards embryonic evidence. For example, Charles Darwin himself wrote:

[C]ommunity of embryonic structures reveals community of descent; but dissimilarity in embryonic development does not prove discommunity of descent.⁴

And more recently, from Brian Hall:

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.⁵

In other words, evolutionists embrace the evidence that is consistent with common descent (and claim it proves evolution is true), and ignore the clear implications of evidence that contradicts it. Will evolutionists ever take falsification seriously?

For more, see Casey Luskin’s earlier post here, “Caught in Contradictions, P. Z. Myers Claims ‘Evolutionary Theory Predicts Differences as Well as Similarities’ (and Therefore Predicts Nothing).”

Notes

1. Rudolf Raff (1999), Larval homologies and radical evolutionary changes in early development, in B Hall (Ed.) Homology: The Hierarchical Basis of Comparative Biology, pp. 110-122, Academic Press.
2. Louise Roth (1984); On Homology, Biolog. J. Linnean Soc., 22:13-29.
3. Rudolf Raff (1999), Larval homologies and radical evolutionary changes in early development, in B Hall (Ed.) Homology: The Hierarchical Basis of Comparative Biology, pp. 110-122, Academic Press.
4. Charles Darwin (1866), Origin of Species, 4th ed., Chapter 13.
5. Brian Hall (1992); Evolutionary Developmental Biology, Chapter 10.