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Darwin Wept: Cambrian Brains and Other Challenges for Evolution

Photo source: Current Biology (Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)).

Whenever you find papers and articles dealing with the Cambrian fossil record in the usual science journals, there are commonalities: more evidence of exceptional preservation without fossil ancestors, and complete silence on the doubts this naturally prompts. Here are three that follow that template.

Insignificant Bioturbation

Many fossils become disturbed over time by burrowing organisms, a phenomenon called bioturbation. Gingras and Konhauser, writing for Nature Geoscience News & Views, analyze a study that sought to determine the extent of bioturbation in Cambrian strata. The answer is brief:

The Cambrian evolution of burrowing species is thought to have facilitated sediment mixing. However, sediment fabrics suggest that bioturbation remained insignificant until the appearance of more efficient sediment mixers in the Silurian.

This should be surprising, given that worms and other phyla capable of burrowing appear in the explosion (notice they rebadge it as the “Cambrian evolution”), and the sea floor is generally a soft environment. “A protracted development of marine bioturbation challenges long-held views of the Ediacaran-Cambrian transition,” they say. It also implies that worm burrows did not exist as trace fossils in the late Ediacaran as an attempt to locate complex animals long before the explosion (Meyer, Darwin’s Doubt, p. 85).

The authors of the referenced paper in Nature Geoscience leave it as a mystery why significant “bulldozing” by burrowing animals did not occur until 120 million years later. What it means practically is that the Cambrian fossils were left largely undisturbed for our viewing today, as the next article shows.

Exceptional Preservation

Another paper about Cambrian fossils appeared in Current Biology (see our earlier coverage here and here). This one is about “Preservational Pathways of Corresponding Brains of a Cambrian Euarthropod,” suggesting that it’s going to show brains of a really complex animal, a “true” or “good” arthropod — you know, those complex animals with jointed appendages, brains, and a digestive tract (insects, spiders, crabs, etc.)? Sure enough, the color pictures (see the photo at the top of this article) jump out at you: brown stains at the heads of Chinese arthropod fossils are the actual remains of this animal’s central nervous system (CNS), revealing “tripartite brain organization, cephalic nerves, and optic neuropils.” Earlier finds showed some of these things, but the new fossils remove all doubt.

The record of arthropod body fossils is traceable back to the “Cambrian explosion,” marked by the appearance of most major animal phylaExceptional preservation provides crucial evidence for panarthropod early radiation. However, due to limited representation in the fossil record of internal anatomy, particularly the CNS, studies usually rely on exoskeletal and appendicular morphology. Recent studies show that despite extreme morphological disparities, euarthropod CNS evolution appears to have been remarkably conservative.

Let’s parse this opener and translate the euphemisms. First, notice the quote marks around “Cambrian explosion,” a subtle hint that the term is controversial. It’s not. They state clearly that it is “marked by the appearance of most major animal phyla.” Panarthropoda is a taxon that combines arthropods with tardigrades and onycophorans. The sentence means that yes, lots of different arthropods appear throughout the fossil record, revealing “extreme morphological disparities,” i.e. outward differences. 

Yet these Chinese specimens show that the brains are conservative — not that they vote Republican, but that CNS structures throughout the panarthropod collection are similar, not showing extensive evolution. They’re not just conservative; they are “remarkably conservative.” In terms of general body plan, it’s a picture of sudden appearance and then stasis for the rest of time — not exactly what Darwin hoped the fossils would demonstrate.

You can read this open-access paper and appreciate the delicate features preserved in these fossils. The authors present a theory of taphonomy (the study of how things fossilize) to explain what they see.

Foremost among objections is the lack of taphonomic explanation for exceptional preservation of a tissue that some see as too prone to decay to be fossilized. Here we describe newly discovered specimens of the Chengjiang euarthropod Fuxianhuia protensa with fossilized brains revealing matching profiles, allowing rigorous testing of the reproducibility of cerebral structures. Their geochemical analyses provide crucial insights of taphonomic pathways for brain preservation, ranging from uniform carbon compressions to complete pyritization, revealing that neural tissue was initially preserved as carbonaceous film and subsequently pyritized. This mode of preservation is consistent with the taphonomic pathways of gross anatomy, indicating that no special mode is required for fossilization of labile neural tissue.

These creatures not only appeared suddenly; they were buried so quickly and completely, they left brown stains in the rock preserving details of the brain and CNS. Since there wasn’t significant bioturbation, we have the imprint of its brain — including the original carbon film, overlaid with pyrite, leaving impressions clear enough for the scientists to draw conclusions about the “conservative” evolution of arthropod brains. Is it any wonder there is almost no discussion of evolution in the rest of the paper?

Amazingly, some of the brains retain their original carbon without any pyrite. The authors believe the carbon has been degraded into carbon chains, but accounting for any organic material after 517 million years requires postulating highly unusual circumstances. In addition, some external tissues, such as cuticle from the exoskeleton, have been preserved with their yellow color. Examples of Chengjiang fossils are shown in the color plates of Meyer’s book Darwin’s Doubt. They really are breathtakingly beautiful fossils.

Most importantly, these fossils show that “a tripartite brain comprising three pre-stomodeal neuromeres had evolved by the early Cambrian” — better, it appeared in the early Cambrian without any evidence of evolution before or after. They restate this important fact: “These features demonstrate that by Cambrian Stage 3 (circa 517 million years ago) arthropods had already acquired CNSs generally corresponding to those of extant taxa.” Darwin wept.

Molecular Biology

The third recent article, this one from the University of Leicester, reinforces what we’ve previously said about molecular evidence. Scientists may claim that the common ancestor of the Cambrian animals resides way back in the Precambrian, but without fossils, it’s at best indirect inference. It’s also a circular argument, claiming that the differences are caused by evolution, and then claiming they demonstrate evolution. 

The spinning begins in the first sentences, but watch the word “conserved”:

Researchers at Universities of Leicester and Warwick discover early conserved DNA sequences from almost 700 million years ago.

700 million year-old DNA sequences from ancient animals have been unearthed by researchers at the Universities of Leicester and Warwick, shedding new light on our earliest animal ancestors and how they influenced modern species — including the sponge.

Unearthed? Did they go on a fossil dig? No, they sat in their labs digging through genes. Most interesting are the genes that didn’t evolve for hundreds of millions of years. 

The ancient sequences were found to be present in a wide variety of modern animals including insects, mammals, reptiles, jellyfish, and even our most distant animal ancestor, the sponge…..

Dr Tauber of the Department of Genetics at the University of Leicester said: “We are extremely excited about identifying these ancient DNA elements. Their conservation indicates that they serve an important function, and there is already some experimental evidence to suggest that this is indeed the case.

This non-evolving genetic evidence does nothing to explain its sudden appearance, whether in the Cambrian or 200 million years prior to the Cambrian explosion. Without fossils, furthermore, they have no way to calibrate the molecular clock. The record of the rocks should take priority.

No First Aid for Explosion Victims

So, we find complex animals with stains of their brains preserved in the rocks — brains that look just like modern arthropod brains. The fossils were not disturbed by worms that existed alongside the arthropods, but apparently did not exist earlier in the Ediacaran. And then we see conserved genes that they say haven’t evolved for 700 million years.

Darwin still has good reason to doubt, and his readers have good reason to doubt Darwin.

This article was originally published in 2015.