April 20, 2002 | David F. Coppedge

Can Molecular Clock Relativity Explain the Cambrian Explosion?

Evolutionists seem to believe in a general theory of biological relativity: molecular clocks run at different rates depending on the conditions.  Six Dartmouth College researchers set out to estimate the time when the first bilaterally symmetric animals emerged – the ancestor of humans, vertebrates, worms and everything with two halves.  This event must have occurred, they believe, just prior to the Cambrian explosion, a period in the fossil record that “continues to defy explanation” (see 04/14/2004 entry).  Their solution, published in PNAS,1 depended on running the molecular clock at different rates on different branches of Darwin’s tree of life.  (The “molecular clock” is a dating method that estimates the passage of time by how many genetic changes are observed between two related species, assuming they both diverged from a common ancestor.)
    Their paper begins with the importance of the question: “Accurately dating when the first bilaterally symmetrical animals arose is crucial to our understanding of early animal evolution,” they say.  Yet till now there has been a disconnect between two data sources: “The earliest unequivocally bilaterian fossils are ~555 million years old.  In contrast, molecular-clock analyses calibrated by using the fossil record of vertebrates estimate that vertebrates split from dipterans (Drosophila) [insects with two wings] ~900 million years ago (Ma).”  What happened to 345 million years?  Part of the answer, they claim, is that the molecular clocks ran at different speeds: “comparative genomic analyses suggest that a significant rate difference exists between vertebrates and dipterans, because the percentage difference between the genomes of mosquito and fly is greater than between fish and mouse, even though the vertebrate divergence is almost twice that of the dipteran.”  This is surprising; most would assume a mosquito and fly, both flying insects, would have similar genes, but protein-coding genes between fish and mouse show fewer differences in twice the estimated time.
  The authors suggest two possibilities to explain this conundrum.  Either insects accelerated their rate of molecular evolution, or vertebrates decelerated it.  In this paper, the authors prefer the latter, but they appreciate the magnitude of the difficulties presented by the Cambrian explosion:

Although the Cambrian explosion is of singular importance to our understanding of the history of life, it continues to defy explanation.  This defiance stems, in part, from our inability to distinguish between two competing hypotheses: whether the Cambrian explosion reflects the rapid appearance of fossils with animals having a deep but cryptic precambrian history, or whether it reflects the true sudden appearance and diversification of animals in the Cambrian.  Because each hypothesis makes a specific prediction of when animals arose in time, one way to distinguish between these two hypotheses is to date animal diversifications by using a molecular clock.  A number of previous clock studies (reviewed in refs. 3 and 4) have suggested that the last common ancestor of bilaterians (LCB) lived well over one billion years ago (5, 6), whereas others suggest that LCB arose ~900 million years ago (Ma) (e.g., refs. 7-10), and still others are more consistent with an origination closer to the Cambrian (11-13).  These deep estimates for the origin of LCB raise the question of how hundreds of millions of years of bilaterian evolution can escape detection, given that LCB and its near relatives should have had the capability of leaving both body and trace fossils.

That is why these authors reject the presumption that the LCB existed for over 500 million years without leaving a trace of a fossil, when many precambrian strata appear ideally suited for preservation.
    Their preferred late date, however, contradicts the evidence from the molecular clock, which would put the LCB in “deep time” (i.e., over a billion years ago, long before the Cambrian explosion).  But that is where relativity can help:

Because molecular clocks have several inherent problems, including how the clock is calibrated, how molecular substitution rates are estimated, and how heterogeneity in these rates is detected and corrected, as well as an inherent statistical bias for overestimating dates, a much more recent date for LCB may not yet be refuted.  Of crucial importance for clock accuracy is the calibration of the clock itself, which requires not only accurate paleontological estimates but also rate homogeneity between the calibrated and uncalibrated taxa.  When estimating the origination date for LCB, virtually all analyses use the vertebrate fossil record to calibrate the clock and ask when vertebrates diverged from dipterans.  However, genome-wide sequence comparisons have shown that the average sequence identity of nuclear protein-coding genes between dipterans is lower than that of bony fish, even though the dipteran divergence time, estimated at 235 Ma (19), is only about half as long as the divergence of bony fish at 450 Ma (20).

The answer must be, they claim, that instead of “rate homogeneity” (a constant clock) there was “rate heterogeneity” (relative clocks) on the different branches of the tree.
    Using various mathematical models for building evolutionary trees and estimating the time between the branches, they test their hypothesis that the vertebrate clock ticked slower.  Various adjustments are made to synchronize the molecular estimates with the fossil record; they admit that “the use of molecular clocks to infer divergence times is fraught with difficulties,” and they must apply many assumptions, none of which question the core assumption that a common ancestor existed.  But even within a Darwinian paradigm, their solution leads to another, more challenging problem: the Cambrian explosion was rapid.  Consequently, all the diverse body plans of all animal phyla had to arise quickly from the alleged, unobserved common ancestor.  Even though they express some confidence that their adjustments to the molecular clock produced congruence with the generally-accepted dates from the fossil record, “Because of this congruence, the Cambrian explosion must reflect, at least in part, the diversification of bilaterian phyla.”  Somehow, without leaving a trace, precambrian ancestors gave rise to a rich diversity of animals in a relatively short time.  What genetic mechanisms could produce such rapid invention of body plans and complex organs, they do not say.  But maybe it was triggered by a “snowball Earth,” melting glaciers, an exposed continental shelf or some other environmental change, though “highly speculative at the moment,” that “may have provided the environmental stimuli necessary for the rapid evolution of disparate bilaterian body plans and ultimately the Cambrian explosion itself.


1Peterson et al., “Estimating metazoan divergence times with a molecular clock,” Proceedings of the National Academy of Sciences 10.1073/pnas.0401670101, published online before print 04/14/2004.

Oh what a tangled web we weave, when at first we practice being deceived.  It would appear that an unbiased observer might pronounce Darwinism falsified at this revelation.  If it takes multiple tweaks by orders of magnitude to get a model to work, is there not something fundamentally wrong with the presuppositions?  Tweaks of this magnitude resemble the desperate attempts to keep the Ptolemaic model of planetary orbits from crumbling under the barrage of improved observations.
    Evolutionists have known for years that the molecular clock is broken (see 05/31/2002, 03/26/2002 and 10/01/2001 entries, for instance).  Why even give it the time of day?  It amounts to no more than Skinner’s constant: that quantity which when added to, subtracted from, multiplied or divided by the answer you got, gives you the answer you should have gotten.  It’s time to take away this useless widget from the Darwin Party’s playpen and get them to face the data squarely: every animal phylum (and some extinct ones) appears in the Cambrian, without ancestors, even though many precambrian strata have ideal conditions for fossilization.  Think on these things.
    Getting Darwinism to work with the Cambrian explosion is like getting both ends of a seesaw to be up at the same time.  If you give the last common ancestor more time to evolve, you have to explain why no fossils were recorded for hundreds of millions of years.  If you shorten the time for the last common ancestor to evolve, you have to explain how multiple distinct body plans arose almost simultaneously.  The phylogenetic chart in Peterson’s paper is typical; the actual fossils of the phyla are contemporaneous; the relationships back in deep time are inferred.  If one looks at the evidence without the funky kaleidoscopic Darwin Party glasses on, it does not look like evolution.  It looks like creation.

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Categories: Fossils

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