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A Beautiful, Wonderful Solution to the Cambrian Puzzle?


Another contender in the race to rescue Darwin from the Cambrian explosion is getting hurrahs from the media. Jordi Paps, a champion in Darwin dodgeball, is giddy with euphoria over the new proposal. In his article, “How animals went from single cells to over 30 different body types,” in The Conversation, he poses as spokesman for the world:

The origin and evolution of animals is one of the most fascinating questions in modern biology. We know that the entire wonderful variety of animals alive today arose from single-celled ancestors. And we know that this transition was likely related to the planet’s environment and how organisms interact with it, as well as changes in their genetic material (genome).

But we don’t know if the diversity of animal shapes, those “endless forms most beautiful and most wonderful” that Darwin described, emerged quickly after the first animal lifeforms or whether it came much later in their evolution. A team of evolutionary biologists from the UK and the US have tried to tackle this question in a most beautiful and most wonderful paper published in the scientific journal PNAS. [Emphasis added.]

Pats on the Back

The authors of said paper pat themselves on the back for their achievement in a statement from the University of Bristol, echoed by cheers from NASA’s Astrobiology Institute, who affirm that the new study “has revealed the origins and evolution of animal body plans.” Finally!

First, they make sure that readers know the required answer before asking the question. They don’t want any Heretic to be seen or heard in the chambers during the debate.

Animals evolved from unicellular ancestors, diversifying into thirty or forty distinct anatomical designs. When and how these designs emerged has been the focus of debate, both on the speed of evolutionary change, and the mechanisms by which fundamental evolutionary change occurs.

It’s a one-party debate, but a debate nonetheless. Even one-party governments can have lively discussions among themselves. Here, it’s not whether Darwinian evolution is a fact, but how it works in the face of evidential challenges from those pesky Cambrian fossils.

Did animal body plans emerge over eons of gradual evolutionary change, as Darwin suggested, or did these designs emerge in an explosive diversification episode during the Cambrian Period, about half a billion years ago?

Valid Empirical Work

Before critiquing their solution, let’s give them credit for some valid empirical work. First, wisely, they included fossils as well as living forms. Additionally, Jordi Paps says:

They collected anatomical data for nearly 2,000 anatomical features for 210 animal groups, sampling many groups within each phylum. Then they analysed their anatomical similarity with cutting-edge statistics. These produce a map of sorts, in which each group is a dot and the distance among groups is proportional to their anatomical similarity. 

The resulting map is the centerpiece of their work. By making both axes the same (evolutionary distance), their data points spread into clusters, allowing easy visualization of just how disparate the animal groups are. The grid forms a morphological “design space” (where “design” to them means a successful bodyplan as opposed to an “impossible” one). The caption explains:

This image is based on the presence and absence of anatomical features, like jointed legs and compound eyes, neurons and boney skulls. Considering all of these features, animals that are similar group together, far away from animals that are dissimilar. Most of this ‘design space’ is unoccupied, in part because of extinction of ancient ancestors that are unrepresented, in part because animals have only been around for half a billion years and that is not enough time to explore all possible designs, but most of the design space is unoccupied because those designs are impossible.

Did you hear that right? Ancestors that are “unrepresented”? What are those? Apparently, they are potential bodyplans that were never actualized because of limited time for evolution to “explore” those parts of “design space.” The paper appears to say that the authors simply imagined animals in the spaces where no remains are known:

Thus, by comparing only living taxa, it could be argued that we have captured only net historical disparity. Therefore, we coded a phylogenetically diverse and representative sample of Cambrian taxa, principally the earliest representatives of ordinal level clades. This entailed coding 70 fossil taxa for the existing character set and adding 111 mostly autapomorphic characters. Coding these fossil taxa was potentially problematic in that most of the characters (54.1%) are not preserved, and therefore unknown.

Visualizing Intermediates

For Darwinians, visualizing intermediates in those empty spaces is not a problem, because imagination is a key tool in their toolkit. It’s legit, because they already “know” that “animals evolved from unicellular ancestors, diversifying into thirty or forty distinct anatomical designs.” The “unrepresented” forms, therefore, must have appeared, and then disappeared without a trace. (Compare this tactic with the use of “ghost lineages” as gap fillers.)

The map shows that a few phyla, especially the arthropods and chordates, continued to diversify extensively after the Cambrian. Consider that both fish and giraffes are vertebrates! Co-author Bradley Deline quips:

“Many of the animals we are familiar with today are objectively bizarre compared with the Cambrian weird wonders. Frankly, butterflies and birds are stranger than anything swimming in the ancient sea.

They take this extreme diversity within phyla as justification to Darwinize the extreme disparity between phyla in the Cambrian. If evolution was powerful enough to generate birds from Metaspriggina, it surely was powerful enough to generate trilobites from microbes. That makes sense, doesn’t it? We mustn’t underestimate the creative power of mutation and natural selection!

The Bristol evolutionists do take Darwin to task about his gradualism, though. Co-author Philip Donoghue shows that it’s OK to adjust the emperor’s clothes as long as you don’t oust him from the parade:

“Our results show that fundamental evolutionary change was not limited to an early burst of evolutionary experimentation. Animal designs have continued to evolve to the present day – not gradually as Darwin predicted – but in fits and starts, episodically through their evolutionary history.”

In other words: “Sure, there was an early burst of evolutionary experimentation, but what’s the surprise? That’s been the pattern throughout evolutionary history.”

Taking Stock

Let’s take stock of the story so far. They admit that the appearance of animals was explosive, but assert that is the normal pattern in evolutionary history. They imagine transitional forms that are “unrepresented” in design space, but went extinct, leaving the appearance of gaps. What’s lacking so far is a mechanism to generate the initial body plans. “Distilling the phenomenon of animal disparity is one thing; establishing its causality is another,” they admit. The causes they consider (only unguided causes, of course) are either intrinsic (e.g., genome expansion, protein fold expansion, gene regulation) or extrinsic (e.g., environmental challenges, such as the expansion to land). Or, evolution might just be doing a “random walk” through morphospace.

Their favored conclusion is that the evolution of gene regulation is the primary cause, but not the only one. Co-author Jenny Greenwood cuts through the paper’s jargon, stating succinctly, “it is the evolution of genetic regulation of embryology that precipitated the evolution of animal biodiversity.” Colleague Kevin Peterson agrees, saying “Our study confirms the view that continued gene regulatory construction was a key to animal evolution.” (Note: They reference Davidson and Erwin on this point, but not Charles Marshall.)

In short: animals evolved, because they evolved. Evolution is fast, except when it operates in fits and starts. It’s the nature of living things to explore possibilities. In their random walks, gene regulatory networks hit on some bodyplans that worked. When you see gaps between the bodyplans, just imagine some intermediate forms that were exploring “shape space,” but went extinct, leaving gaps. Cambrian explosion solved? Almost. With thanks to Neil Shubin for refereeing their work, they end their “most beautiful and most wonderful paper” with promissory notes:

Our results also suggest that debate on whether early animal evolution has been underpinned by uniformitarian or nonuniformitarian processes has been misplaced. Animal evolutionary history does not appear to have been characterized by a uniform rate and scale of change but rather by a high frequency of small changes and low frequency of changes of large magnitude within the context of intrinsic genetic and developmental variation and extrinsic environmental change. Such patterns are readily open to modeling in the same manner as nucleotide and amino acid substitution frequencies. Future research in this direction will inform understanding of the nature of phenotypic evolution, its relation to molecular evolution, underpinning the development of phylogenetic methods. However, it will also provide for a more precise characterization of the tempo of metazoan diversification and the processes that underpinned the establishment of animal bodyplans.

Shallow thinking about major problems in evolution continues because its defenders have shielded themselves from real debate. Once again, this paper and its cheerleading articles completely ignore the issues raised by Stephen Meyer in Darwin’s Doubt.

Photo: Trilobite fossil, by Jakob Vinther, University of Bristol, via EurekAlert!