When Can I Trust Scientists About Evolution?

In the first article in this series, asking “When Can I Trust What Scientists Say?,” we reviewed six criteria that can be used to assess the level of confidence we should place in a scientific claim. Now it’s time to apply these criteria to claims made about evolution, specifically universal common ancestry or macroevolution.

Fossil Record and Homology

The most commonly cited areas of evidence in favor of common ancestry include the fossil record and homology (which refers to similarities in genotype or phenotype between different lifeforms, allowing the construction of phylogenetic trees). Richard Dawkins made the bold claim that evidence of homology is the best available evidence for common ancestry:

Just as the vertebrate skeleton is invariant across all vertebrates while the individual bones differ, and just as the crustacean exoskeleton is invariant across all crustaceans while the individual “tubes” vary, so the DNA code is invariant across all living creatures, while the individual genes themselves vary. This is a truly astounding fact, which shows more clearly than anything else that all living creatures are descended from a single ancestor.¹

Bill Nye agrees:

This business of homology is one of the absolutely most compelling indicators of the process of evolution.²

Similarities Between Lifeforms

No one doubts that there are great similarities between lifeforms. The fundamental question is how these similarities came to be. One possible explanation is common ancestry. Another possible explanation is intelligent design, because designers commonly reuse components, producing a new product by modifying a prior product. We would like to apply science to answer the question: “Does common ancestry of all life explain the similarities between lifeforms?” Applying the six criteria for high-confidence evidence, we find:³

1. Is the evidence repeatable? The process that produced existing lifeforms cannot be repeated. Descent with modification can be repeatedly demonstrated and speciation can be demonstrated on rare occasions, but this is merely inferred to operate at higher taxonomic levels; it cannot be repeatedly demonstrated.
2. Can the evidence be directly measured or observed? The passing of genes from parent to child can be directly measured, but the proposed passing of genes from a common ancestor to all existing life certainly cannot be directly measured.
3. Was the evidence obtained through prospective study? Large-scale evolution cannot be studied prospectively.
4. Was bias minimized? Extreme bias is required to extrapolate from the observable descent with modification to accept a single tree of life from a common ancestor. Bias is also displayed when highlighting similarity across lifeforms while dismissing important differences like orphan genes, phylogenetic incongruencies, and differences in the genetic code. This bias is exemplified by Richard Dawkins in the claim I cited above.
5. Were assumptions minimized and openly disclosed? The concept of a common ancestor for all life is based upon the assumption that small observable modifications can be extrapolated to explain all the diversity of life. Thus, the strongest argument for common ancestry is an assumption. A further assumption to refute the alternative hypothesis of intelligent design is that a designer would not reuse parts for different lifeforms.
6. Did they make reasonable claims? The claim by Richard Dawkins exudes confidence that, as he says, “all living creatures are descended from a single common ancestor.” No hedging language is employed to appropriately convey a low level of confidence.

Failure Across the Board

Therefore, the use of homology as evidence for common ancestry fails all six criteria for high-confidence evidence. But, according to Richard Dawkins and Bill Nye, homology is the most compelling evidence for common ancestry. A similar analysis leads to the conclusion that evidence from the fossil record to support common ancestry also fails all six criteria for high-confidence evidence.³

Surely, though, there must be evidence regarding common ancestry that affords a higher level of confidence than what we have seen so far. In my next article, we will explore evidence about evolution that provides high confidence. This evidence must be prioritized over the evidence discussed in this article.

Notes

1. Dawkins, R. The Greatest Show on Earth. New York: Free Press; 2009, p. 315.
2. Nye, B. Undeniable: Evolution and the Science of Creation. New York: Saint Martin’s Press; 2014, p. 149.
3. For more, see Stadler, Rob. The Scientific Approach to Evolution: What They Didn’t Teach You in Biology © 2016. Or watch any of these videos: https://youtu.be/smTbYKJcnj8?si=6m6VjGF0X9ORkTRT; https://youtu.be/kcCV0igIA0U?si=oWsQYxfaW1zwUHMx; https://youtu.be/OhLP-hqOnGw?si=fqC7z5jK8pk7M2-Q.