Last month Science Alert reported on “How 2 Teens Accidentally Solved Charles Darwin’s Most Vexing Problem.” Did they really? The “vexing problem” in question is a reference to the Cambrian explosion. The article claims that the Ediacaran-aged Precambrian fossil Charnia was a “a Precambrian fossil of a plant, which probably once lived on the seafloor” and that somehow this fossil provides an “answer to Darwin’s dilemma” — i.e., how biological diversity appeared so quickly in the Cambrian period. Here’s what the article says:
It looked like a fern. But as a budding geologist, [UK teenager Tina] Negus knew these 600 million year old rocks were too old to host such a plant. According to the established fossil record, the first complex species of flora wouldn’t appear for at least another sixty million years, following the event known as the Cambrian explosion.
Sometimes called the biological big bang, the ‘explosion’s’ abrupt burst in biodiversity was the most vexing dilemma facing Charles Darwin’s theory of evolution. In his famous book, On the Origin of Species, Darwin could not find a satisfactory answer to two nagging questions: Where did all this diversity of life suddenly come from? And how did it evolve so quickly?
An Eruption of Animals
Even if this fossil were a “plant” it would not solve the “dilemma” of the Cambrian explosion because the Cambrian explosion describes the abrupt origin of animals, not plants. Paleozoo.com explains why Charnia could not have been a plant:
Charnia was a sessile organism that existed in the deep ocean away from sunlight — which means that it couldn’t have been a form of photosynthesizing plant or alga. As it also seems to lack any animal characteristics like a gut or mouth its taxonomic affinity remains a mystery. If not flora or fauna then what?
This sort of reasoning is found in the technical literature. One paper on Charnia says, “the presence of some Ediacaran frond taxa in life position in deep-water, presumably aphotic environments rules out light-gathering as the function for at least these genera and implies that nutrient absorption is most likely.” Another paper states: “sedimentological studies implying a deep-water axial/slope environment well below storm wave base for the Mistaken Point assemblage and a community organization similar to that of Phanerozoic and modern filter-feeding animals rules out any affinities for Charnia that rely on photosynthesis for feeding (such as plants, algae, or lichen)…”
If Not a Plant, Then What?
So if Charnia was not a plant, does that mean it was an animal that could have been ancestral to the Cambrian animals? The latter paper cited above argues that the impossibility of Charnia being a plant “implies that an animal origin is most likely.” But many leading experts argue that the Ediacaran fossils were neither animals nor plants — they were enigmatic fossils that are not thought to have been ancestral to organisms that appeared later in the Cambrian explosion. That means they don’t solve “Darwin’s dilemma,” as Stephen Meyer explains in Darwin’s Doubt:
Paleontologists James Valentine, Douglas Erwin, and David Jablonski distill the confusing welter of conflicting views about the Ediacaran animals: “Although the soft-bodied fossils that appear about 565 million years ago are animal-like, their classifications are hotly debated. In just the past few years these fossils have been viewed as protozoans; as lichens; as close relatives of the cnidarians; as a sister group to cnidarians plus all other animals; as representatives of more advanced, extinct phyla; and as representatives of a new kingdom entirely separate from the animals.” What’s more, Valentine, Erwin, and Jablonski note that those paleontologists who do regard the Ediacaran fauna as animals rarely classify them the same way, underscoring their lack of clear affinities to any known animal groups. As they note, “Still other specialists have parceled the fauna out among living phyla, with some assigned to the Cnidaria and others to the flatworms, annelids, arthropods and echinoderms.” The uncertain standing of these fossilized forms is partly due to their early extinction, but it also stems from an absence of defining characteristics shared with known groups. They conclude: “This confusing state of affairs arose because these body fossils do not tend to share definitive anatomical details with modern groups, and thus the assignments must be based on vague similarities of overall shape and form, a method that has frequently proved misleading in other cases.”
Other leading paleontologists also doubt that the Cambrian animals descended from these Ediacaran forms. In a phylogenetic diagram showing the evolutionary relationship of Precambrian and Cambrian fossils, Oxford biologists Alan Cooper and Richard Fortey depict the Ediacaran fauna as lying on a line of descent separate from the Cambrian animals rather than being ancestral to them. In another paper, Fortey asserts that the beginning of the Cambrian “saw the sudden appearance in the fossil record of almost all the main types of animals (phyla) that still dominate the biota today.” He concedes that there are a variety of fossils in older strata, but insist that “they are either very small (such as bacteria and algae) or their relationships to the living fauna are highly contentious, as is the case with the famous soft-bodied fossils from the late Precambrian Pound Quartzite, Ediacara, South Australia.”
Similarly, paleontologist Andrew Knoll and biologist Sean B. Carroll have argued: “It is genuinely difficult to map the characters of Ediacaran fossils onto the body plans of living invertebrates.” Although many paleontologists initially showed interest in the possibility that the Cambrian animal forms might have evolved from the Ediacaran organisms, paleontologist Peter Ward explains that “later study cast doubt on the affinity between these ancient remains preserved in sandstones [the Australian Ediacaran] and living creatures of today” (that is, animals representing phyla that first arose in the Cambrian). As Nature recently noted, if the Ediacaran fauna “were animals, they bore little or no resemblance to any other creatures, either fossil or extant.”Darwin’s Doubt, pp. 83-85
The Cambrian Mystery Remains
The Science Alert article ends with a statement that is striking in that it has one sentence that is totally correct and another that is highly dubious:
It is now clear that complex plants once flourished before the Cambrian explosion. And yet it’s still unknown why diversity suddenly exploded more than 500 million years ago.
The first statement is wrong because it’s very unlikely that Charnia was a plant. The second statement, however, is exactly right: We don’t know why animal diversity exploded during the Cambrian period. Charnia certainly does not solve this mystery, nor does it resolves Darwin’s dilemma of the evolutionary origin of the animals.