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What’s New in Cambrian Dodgeball?

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Cambrian

Here are some recent findings about Cambrian and Precambrian strata, and how evolutionists keep dodging the implications of rapid emergence of complex designs. 

The big question before evolutionary paleontologists should be, “What caused the rapid appearance of metazoans?” Keep your eye on that ball. One favorite dodging technique is to speak in passive sentences. Don’t say, “Complex animals appeared because” of some mechanism that is necessary and sufficient. Just talk about “The appearance of” or “the emergence of” complex animals as if it’s a settled fact that they evolved.

Ediacaran Food and Oxygen

If you put out food in a world of microbes, will multicellular animals “emerge” to eat it? That seems to be the thinking of researchers at the University of California, Riverside about the Ediacaran animals that preceded the Cambrian explosion, according to Science Daily:

Love said the Ediacara Biota lived in nutrient-poor regions of the sea on the continental shelf, an extension of land under the ocean that results in relatively shallow water. Despite this oligotrophic environment, the researchers found there were sufficient nutrients and organic debris for feeding sustained by bacterial primary production and dissolved organic matter. [Emphasis added.]

While learning about the cuisine of these enigmatic creatures is helpful, the paper in Nature Communications offers no suggestions about how the eaters arrived. The authors know that “Oxygen has often been proposed as an enabler, if not the driver, of the rise and evolution of multicellular biota and metazoans,” but they also know some disagree with the Oxygen Theory:

The emergence of metazoan fauna at that time has thus often been ascribed to increasing oxygenation of the oceans. This interpretation has been challenged on the basis that anatomically simple metazoans, particularly sponges, could survive under very low dissolved oxygen levels, which may have been sustained in shallow waters prior to the Ediacaran.

They point to Andrew Knoll, author of the slime theory, as one challenger. The UCR team has nothing to offer as an alternative cause for the “emergence” of complex animals. In their last sentence, they conclude that, well, they must have just evolved, because food and oxygen was present. The table setting played a “potentially crucial role in the evolution of” animals.

Whether metabolic requirements or environmental selective pressure restricted Ediacaran soft-bodied multicellular biota to these settings remains uncertain; however, our study highlights that Ediacaran oligotrophic [few-food] settings played a potentially crucial role in the evolution of macroscopic multicellular organisms and marine community ecology.

The dodge is in the word “potentially.” The food settings “potentially” could have been irrelevant, too. 

Causes Inside and Out

Another paper admits up front that the Cambrian explosion remains “one of the major challenges of modern evolutionary biology.” In Nature Communications, Jordi Paps and Peter Holland title their paper, “Reconstruction of the ancestral metazoan genome reveals an increase in genomic novelty.” So what else is new? Stephen Meyer, author of Darwin’s Doubt, already knew that. That is the problem. What caused the novelty? The authors keep their options open by saying the Cambrian explosion might have been due to internal factors as well as to external factors. This allows them to dodge the ball by jumping to either position instead of taking a firm stand on one.

Understanding the emergence of the Animal Kingdom is one of the major challenges of modern evolutionary biology. Many genomic changes took place along the evolutionary lineage that gave rise to the Metazoa. Recent research has revealed the role that co-option of old genes played during this transition, but the contribution of genomic novelty has not been fully assessed. Here, using extensive genome comparisons between metazoans and multiple outgroups, we infer the minimal protein-coding genome of the first animal, in addition to other eukaryotic ancestors, and estimate the proportion of novelties in these ancient genomes. Contrary to the prevailing view, this uncovers an unprecedented increase in the extent of genomic novelty during the origin of metazoans, and identifies 25 groups of metazoan-specific genes that are essential across the Animal Kingdom. We argue that internal genomic changes were as important as external factors in the emergence of animals.

Notice once again the phrasing: “the emergence of animals” and “the proportion of novelties” as if they passively appeared somehow. If you were explaining a painting in an art gallery, would it be acceptable to say, “novelty had a major impact in the emergence of the painting”? How else can we read their hypothesis, as stated in the paper?

However, the role of genome novelty in animal origins has not been fully evaluated. We hypothesize that genomic novelty had a major impact in this transition, particularly involving biological functions which are hallmarks of animal multicellularity (gene regulation, signalling, cell adhesion, and cell cycle).

Are they saying that things as complex as these appeared by mutation and selection? Well, maybe there were other factors, too. They cover all the bases:

We stress the present study focusses on protein-coding genes, and it is possible that the evolution of non-coding genes, regulatory regions, and epigenetic mechanisms also played major roles in this transition.

On their explanatory menu, you get two choices: evolution stew or evolution popcorn. Take your pick:

There are two alternative scenarios that could explain these patterns depending on the length of the branch leading to the metazoan LCA [last common ancestor]. The first assumes that the birth rate of new genes was constant over time, thus the branch leading to the first metazoan was longer than other opisthokont internodes. This would imply an extended ‘stew’ in which the molecular components of animal biology were assembled. However, we note that molecular phylogenetic analyses do not generally show longer branches in the stem lineage of animals, contrary to this scenario. The second possibility involves many new genes emerging during a short ‘popcorn’ stage, caused either by a higher gene birth rate (perhaps produced by environmental factors elevating mutation rates, or due to whole-genome duplications), and/or a lower gene death rate (due to high integration of new genes into regulatory networks). In this scenario, the acquisition of multicellularity would quickly stabilise new molecular systems for cell adhesion, cell communication and the control of differential gene expression, as shown by the increase in proportion of Novel Core HG seen in the metazoan ancestor. These include genes previously hypothesized to be pivotal in the emergence of Metazoa, with additional genes singled out here for the first time as agents involved in the transition. This scenario is also consistent with enhanced rates of gene novelty in the ancestors of Planulozoa and Bilateria when embryonic patterning systems were being elaborated. Further data and analyses are needed to discriminate between the two scenarios.

Since evolution is a fact, those are your choices: stew or popcorn. Eat hearty. We have to hand it to the authors for their clever euphemism for sheer dumb luck: “the birth rate of new genes.” And there’s that passive phrasing again, also seen in, “molecular components of animal biology were assembled,” “the acquisition of multicellularity” and “the control of differential gene expression” – each of which shield the dodger from the ball. As for “high integration of new genes into regulatory networks,” wasn’t that the big issue of the debate between Stephen Meyer and Charles Marshall? You won’t find any mention of it here, because Meyer’s “scenario” of intelligent cooks in the kitchen doesn’t qualify for the menu. You can mix your popcorn into the stew, but you can’t order anything else.

Rock Concert Upset

One more paper deals with geology, not biology, but has wide implications for scientific certainty about fossil-bearing strata. In the Journal of the Geological Society, Vandyk et al. confess that they were fooled by certain Precambrian rocks in Death Valley. The geologist community had all classified the rocks as sills (intrusive magma between layers), but they are not. This team looked at them again, and decided they are olistoliths instead. The Oxford English Dictionary defines olistolith: “Each of the discrete bodies contained within the matrix of an olistostrome.” Gee, thanks. Now what’s an olistostrome, pray tell? “A sedimentary deposit composed of a heterogeneous mixture of materials and formed by the sliding or slumping of semi-fluid sediment.” Whatever it means, it’s very different from a sill.

Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed…. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills.

The geologists didn’t find microfossils in these Precambrian rocks, but their cautionary exemplar remains: scientists can radically change their interpretations of the ages and formation mechanisms of rocks that have long been assumed. Maybe critics of Darwinian evolution regarding the Cambrian explosion will need to take this warning into account. 

That’s it for this episode of Cambrian explosion update. We think we’ll send the Popcorn Theory over to The Gong Show.

Photo credit: Presidio of Monterrey, via Flickr.