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New Paper Confirms the Trilobite Explosion

A new paper in PNAS confirms that the Cambrian explosion cannot be denied. Paterson, Edgecombe, and Lee report:

The abrupt first appearance of a multitude of animal fossils in early Cambrian rocks (Terreneuvian to Series 2; ca. 541–509 Ma) epitomizes one of the most significant evolutionary events in Earth’s history. This sudden burst of diversity and abundance across most eumetazoan (especially bilaterian) phyla over a relatively short geologic time span, and lack of obvious Precambrian precursors, poses a conundrum when attempting to reconcile the fossil record with the true tempo of early animal evolution. [Emphasis added.]

It’s important to note that the “true tempo of early animal evolution” exists in evolutionists’ imaginations, not in the rocks. They write:

Fast evolutionary rates during the early Cambrian have been used to explain the rapid emergence of animals, providing support for a more literal reading of the fossil record.   

Translated into active voice verbs, what they are saying is: “We Darwinians explain the rapid emergence of animals by speeding up the rate of evolution.” Their proposal might make Darwin the gradualist uncomfortable, but it allows them to be “fossil literalists.” But how far can the pace of Darwinian evolution be adjusted? 

The solution of speeding up the rate of evolution raises another question: Why did evolution slow down to “Phanerozoic norms” ever since? They decided to look into this by studying trilobites, determining how quickly they evolved during and after the explosion.

100 Percent Trilobite

Paterson et al. studied the most complete dataset of trilobites, “the largest and most comprehensive for trilobites compiled to date, comprising 107 species — representing most Cambrian families,” and compared 115 traits among them. The earliest trilobite in their chart is 100 percent trilobite. The motile, sensory-equipped swimmers were already complex and operational at the earliest stages of the explosion. They did not evolve from Ediacaran ancestors.

Trilobites are often considered exemplary for understanding the Cambrian explosion of animal life, due to their unsurpassed diversity and abundance. These biomineralized arthropods appear abruptly in the fossil record with an established diversity, phylogenetic disparity, and provincialism at the beginning of Cambrian Series 2 (∼521 Ma), suggesting a protracted but cryptic earlier history that possibly extends into the Precambrian. However, recent analyses indicate elevated rates of phenotypic and genomic evolution for arthropods during the early Cambrian, thereby shortening the phylogenetic fuse. Furthermore, comparatively little research has been devoted to understanding the duration of the Cambrian explosion, after which normal Phanerozoic evolutionary rates were established. We test these hypotheses by applying Bayesian tip-dating methods to a comprehensive dataset of Cambrian trilobites. We show that trilobites have a Cambrian origin, as supported by the trace fossil record and molecular clocks. Surprisingly, they exhibit constant evolutionary rates across the entire Cambrian, for all aspects of the preserved phenotype: discrete, meristic, and continuous morphological traits. Our data therefore provide robust, quantitative evidence that by the time the typical Cambrian fossil record begins (521 Ma), the Cambrian explosion had already largely concluded. This suggests that a modern-style marine biosphere had rapidly emerged during the latest Ediacaran and earliest Cambrian (20 million years), followed by broad-scale evolutionary stasis throughout the remainder of the Cambrian.

No “Cryptic History”

What they’re hinting at is that there is no “cryptic history” extending into the Precambrian. Even though they stretch the explosion out to 20 million years, that still represents a remarkably fast rate for a Darwinian to have to swallow. Trying to rescue the father of evolutionary theory from “Darwin’s Dilemma” (their term after quoting his frustration), they used a method showing that the rate was consistent during the later diversification (after the explosion, once the trilobite body plan emerged). But that’s the problem! It’s not whether the subsequent rate varied or not. It’s how a complex body plan, complete with multiple systems and organs, living in a complex ecosystem, originated in the first place. 

The First Appearance Datum (FAD) for trilobites, which they put at 519 Ma, is a complete trilobite. Even though they infer it must have arisen 521 Ma, all the phylogenetic lines below it are empty! 

For much of the paper, they change the focus. They avoid the abrupt appearance problem, and focus on the subsequent diversification. “All analyses reveal that rates of morphological evolution were homogeneous throughout the Cambrian,” they say. So what? They cannot deny the explosion:

Despite ongoing debate over the true origins of animal phyla, our data, as well as the Ediacaran–Cambrian geochemical, body, and trace fossil records, indicate that a modern-style marine biosphere was fully established by Series 2, followed by broad-scale evolutionary stasis throughout the remainder of the Cambrian. Given the apparent paucity of unequivocal eumetazoan representatives in the Ediacaran, it seems that many stem- and crown-group members of most bilaterian phyla had definitively appeared and diversified in 20 My (Fig. 4). Among these novel body plans is rampant convergence in various forms of biomineralization and other anatomical innovations that allowed animals increased mobility and ways of sensing their environment. Notwithstanding the patchy Terreneuvian fossil record, it is clear that a new style of ecological network — including greatly expanded food webs and associated nutrient cycling, plus complex tiering above and below the substrate that helped reengineer the marine ecosystem — rapidly emerged during this interval.

Where Did This Come From?

Animals emerged. Animals appeared. Animals originated. Animals innovated. It was “a truly brief evolutionary burst,” the authors admit. There’s no getting around it. The earliest trilobite fossil just shows up suddenly without ancestors.

Here’s another problem. Trilobite fossils are found around the world, on different paleocontinents. How did that come about? They consider two options. Neither one is helpful to Darwinian belief. Pardon the lengthy citation here, but it’s important to watch the authors struggle with the implications.

The absence of Terreneuvian trilobite body fossils can be explained under two potential scenarios. The first scenario is that the fossil record is a reasonably accurate representation of early trilobite evolution, implying that rates of morphological evolution before 519 Ma were substantially faster than subsequently (SI Appendix, Table S1). This hypothesis forces rapid dispersal between widely separated paleocontinents and is difficult to reconcile with the provincialism observed in the earliest trilobites. However, if correct, this may also explain the perceived diachronism [across time] of the first trilobite fossils on different paleocontinents, with the group potentially originating and rapidly radiating out from Siberia, West Laurentia, or West Gondwana. The second scenario—more consistent with our results, plus trace fossil, molecular clock, and biogeographic data—is that the earliest trilobites (pre-521 Ma) have not been preserved or yet discovered in Terreneuvian rocks. The diversity of other skeletonized animals from a range of environments throughout the Terreneuvian indicates an adequate shelly fossil record. Thus, the absence of trilobites and indeed other euarthropod body fossils in Terreneuvian rocks could be explained by their nonbiomineralized exoskeletons and the unusual dearth of soft-tissue preservation (especially BST deposits) for this time interval (Fig. 4).

Either evolution ran at “substantially faster” (non-gradual) rates, or we haven’t found the transitional fossils yet. Those are the options. So now, 160 years after Darwin complained about the missing ancestors, evolutionists are using the same excuse. Next, though, they show that the second scenario (the one consistent with their belief that ancestors must have existed) doesn’t really help. Endure one more long quote:

The existence of nonbiomineralized trilobites in the Terreneuvian would have required multiple lineages to simultaneously converge upon a calcite exoskeleton at around 521 Ma, unless initial evolutionary rates were much faster, thus bringing their origin and fewer lineages closer to the lower boundary of Series 2 (in support of the first scenario discussed above). Synchronous biomineralization across two or more trilobite lineages is consistent with the observation that other disparate bilaterians, such as echinoderms and rhynchonelliform brachiopods, also acquired calcitic skeletons around this time (Fig. 4). Notably, this time coincides with a change in ocean chemistry, particularly the onset of a calcite sea; ambient seawater chemistry influences the type of biomineral secreted at the time skeletons evolved de novo in a clade. These repeated patterns suggest that compelling environmental [e.g., changing Mg/Ca ratios and oxygen levels] and/or biological factors [e.g., predation were influencing this major episode of biomineralization and diversification during the final stages of the Cambrian explosion. The shelly fossil record of animals thus dramatically improves only around 521 Ma, but by that stage, the Cambrian explosion was largely over.

They’re back to the old grab-bag of explanatory tricks: Oxygen did it. Seawater calcite did it. Predation did it. 

Ask yourself: Are any of these factors necessary or sufficient to invent eyes, swimming gear, legs, sexual reproduction, digestion, body plans, and brains to run all the new “innovations” in trilobites? Does it help to point to other complex body plans that “appeared” around the same time? It’s like arguing that Mars orbiters must have emerged by chance, because Russian and European orbiters did within a few years. There must be something about Mars that causes complex spacecraft to appear. Moreover, they seem to converge on the same materials! The similarity to the old scientific consensus about spontaneous generation is unavoidable: maggots or mice emerge out of the environment.

Despite the Evidence

Intelligent design proponents can take heart at this paper. Six years after the publication of Darwin’s Doubt, nothing new has come forth from believers in Darwinian evolution to answer the Cambrian explosion. No new fossils have shown up as transitional forms. No new theory explains the origin of genetic information to build complex body plans. No vera causa has been put forth to account for the abrupt appearance of hierarchical design by unguided processes. In fact, this new paper, published by the National Academy of Sciences, underscores the nature of the explosion as sudden, unique, and real. What is a synonym for “cryptic history”? How about non-existent.

Meyer proposed a cause that is necessary and sufficient to account for the origin of biological information: intelligence. What’s sad is to see an intransigent scientific community continuing to seek a Darwinian explanation in spite of the evidence.

Photo credit: Mike Peel [CC BY-SA 2.0 uk], via Wikimedia Commons.