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Worming Evolution into the Cambrian Explosion

Cambrian explosion

It’s dated at 540 million years old. A worm-like bilaterian animal left a track in mud, and this time its body was preserved along with the trail. Is this the proof Darwin needed to show precursors before the Cambrian explosion? Does it show that diversification into bilaterian animals was underway at the end of the Ediacaran? Is Darwin’s doubt becoming less dubious?

“Ancient worm fossil rolls back origins of animal life,” according to Nature. “Half-a-billion-year-old creature challenges theory that animals burst onto the scene in an abrupt event known as the Cambrian explosion.” [Emphasis added.]

“Death march of a segmented and trilobate bilaterian elucidates early animal evolution.” This is the primary paper in Nature. 

Here we describe the fossil of a bilaterian of the terminal Ediacaran period (dating to 551–539 million years ago), which we name Yilingia spiciformis (gen. et sp. nov). This body fossil is preserved along with the trail that the animal produced during a death march. Yilingia is an elongate and segmented bilaterian with repetitive and trilobate body units, each of which consists of a central lobe and two posteriorly pointing lateral lobes, indicating body and segment polarity. Yilingia is possibly related to panarthropods or annelids, and sheds light on the origin of segmentation in bilaterians. As one of the few Ediacaran animals demonstrated to have produced long and continuous trails, Yilingia provides insights into the identity of the animals that were responsible for Ediacaran trace fossils.

Join the Chorus

The popular media add to the chorus. “Spiky Worm’s Ancient ‘Death March’ Might Be Earliest Known Animal Journey on Earth,” says Live Science. The 550-million-year-old worm “may represent the earliest known evidence of an animal walking on Earth’s surface, incidentally proving that animals have been mobile since at least the Ediacaran period (635 million to 539 million years ago).”

Virginia Tech quotes Rachel Wood about this “remarkable finding of a highly significant fossil.” She says, “We now have evidence that segmented animals were present and had gained an ability to move across the sea floor before the Cambrian, and more notably we can tie the actual trace-maker to the trace.”

According to the BBC News, Wood also said, “They show that a type of complex behaviour had evolved before the Cambrian (when multi-cellular life exploded into a wide variety of forms) the ability to move over the sea floor.”

New Scientist claims that this “540-million-year-old worm was [the] first segmented animal that could move.” The article speculates that Yilingia might have been an arthropod or annelid, or an ancestor of one or the other — or both.

Solving Darwin’s Doubt

So while most of these articles avoid calling it a “missing link” or “transitional form” outright, they surely imply that it’s part of a solution to Darwin’s doubt — his dilemma that he called the gravest objection that could be lodged against his theory. Here is a bilaterian mover in the Ediacaran! Colin Barras is the least inhibited in his Nature commentary:

The fossil, which formed some time between 551 million and 539 million years ago, in the Ediacaran period, joins a growing body of evidence that challenges the idea that animal life on Earth burst onto the scene in an event known as the Cambrian explosion, which began about 539 million years ago.

“It is just pushing things further and further back into the Ediacaran,” says Rachel Wood, a geoscientist at the University of Edinburgh, UK. The Cambrian explosion no longer appears to be such an abrupt event in the history of life on Earth, she says.

Now for the Data

If this were an early arthropod with articulating limbs, that might be something. How clear is that evidence? Barras is not embarrassed to admit:

Exactly which animal lineage Y. spiciformis belonged to is unclear. The researchers suggest it might be a relative of insects and crustaceans such as shrimp and lobsters, because it seems to have leg-like structures. If further analysis shows that those structures are actually an artefact of the fossilization process, the animal might instead be some sort of primitive segmented worm.

Some caution is clearly advised. Many Ediacaran creatures were segmented. Some also showed bilateral symmetry. Controlled movement, however, would imply muscles, nerves, some kind of “skeleton” (not necessarily bone) able to hold its shape. The legs would have to be coordinated. How clear is the data on movement? It doesn’t sound too convincing in the Virginia Tech piece:

Remarkably, the find also marks what may be the first sign of decision making among animals — the trails suggest an effort to move toward or away from something, perhaps under the direction of a sophisticated central nerve system, Xiao said.

Maybe, suggest, perhaps. It would be necessary to rule out all other sources of movement, say, from waves and currents, which could cause a long worm-like thing to shift along the sea floor and even change direction. In a video, lead author Shuhai Xiao relies on animation to show his vision of a bilaterian animal, with head and tail and left and right sides and top and bottom, crawling with legs through the mud. The actual fossil, though, seems less clear. The BBC quotes Wood, “Though it’s not well-preserved, it has the hint that it has a front and a back… so this animal has already got some sense of unidirectional movement.” 

New Scientist takes “the hint” and runs with it. “An extinct creature that looked like a cross between a millipede and an earthworm was one of the first animals that could move under its own power.” Yet later the article says that other Ediacarans moved passively with the water current, while perhaps Kimberella could “slither across the sea floor.” None of those organisms had a central nervous system, a gut, or a head with mouth parts. Did Yilingia? It’s not clear.

Room for Doubt

The actual paper in Nature leaves plenty of room to doubt the hype. Motility is inferred, because there is no clear evidence of actual legs or any other organs; no mention of a head, either. How is a motile creature supposed to move without eating? The authors impose wishful thinking on a paucity of data from one spot in China. If this were such a monumental discovery, clear evidence of complex organs at this stage would have been found by now, and would be widespread all over the earth. Watch how the authors’ first sentence — an admission of ignorance — launches a Darwin party cruise.

Thus, although its internal anatomy and developmental genetics are unknown, Yilingia adds substantially to the current discussion on the evolutionary transition from serial pattern formation, through metamerism, to segmentation in early ecdysozoans, lophotrochozoans or even bilaterians. In addition, Yilingia — insofar as it is one of the few Ediacaran animals known to have been capable of producing long and continuous trace fossils such as trails and trackways — provides key insights into the identity of the trace makers that were responsible for the abundant trace fossils of the terminal Ediacaran period. Considering the correlation between segmentation and motility among bilaterian animals, the evolution of segments may have led to a transformative innovation that eventually precipitated the substrate and agronomic revolutions in the subsequent Cambrian period.

Notice “terminal Ediacaran period.” This creature sits at the end of the end of the Ediacaran fossil record, essentially on the borderline of the Cambrian explosion. Did the authors consider the possibility of bioturbation from a Cambrian animal descending into the layers beneath? Apparently not; the paper and articles are silent about that. Ediacaran.org, describing this layer, notes that “A [sic] unusual aspect of the Shibantan Member is an abundance of bilobed trace fossils, which may include evidence for vertical burrowing (Chen et al. 2013; Meyer et al. 2014), alongside the macrobiota.”

Significance or Insignificance?

Even giving evolutionists the most generous benefit of the doubt, this is not really big news. Stephen Meyer and others in the ID community have recognized the possibility of movement before the main Cambrian explosion. They readily acknowledge the “small shelly fossils” and worm burrows found below the Cambrian. On page 81 and pages 85-86 of Darwin’s Doubt, Meyer grants evolutionists the most favorable interpretation that four animal phyla existed in the Ediacaran, including sponges, mollusks, and cnidarians, though the latter two are questionable (see the chart on page 32). These still fall far short of the burst of complexity displayed in the other 20 animal body plans, which include unmistakable guts, heads, eyes, and articulating limbs.

If an evolutionary sequence were so clear from the fossil record, it would not have taken 160 years to see it. Caution requires not trusting the artwork showing Yilingia as an active, decision-making centipede on the move. Given the poor clarity of the actual fossil, the suggestive nature of the interpretations of this organism, and the paucity of evidence to connect it to anything as complex as a trilobite or chordate, it’s enough to congratulate the discoverers on something that is interesting, of course, but irrelevant to the big question: what is the source of the genetic information for 20 new complex body plans that appear suddenly and simultaneously in a geological instant in the Cambrian explosion? That is the question.

Image credit: Nanjing Institute of Geology and Paleontology (press release).