Pushing the Limits of Cambrian Ancestry - Evolution News & Views

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Pushing the Limits of Cambrian Ancestry

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As Stephen Meyer documents in Darwin's Doubt, the Ediacaran fauna (enigmatic colonial organisms in the Precambrian) are mostly simple organisms that evolutionists do not believe bear any relationship to the complex body plans that appear suddenly in the Cambrian explosion. Mostly shaped like fronds or air mattresses, the Ediacaran organisms lack the bilateral symmetry of most of the Cambrian body plans, to say nothing of complex organ systems.

Scientists at the University of California at Riverside found tracks of a tubular organism in Ediacaran rocks from Australia. It appears they are trying to get a lot of mileage out of their discovery. The photographs of the fossils show little more than bumps in the rock looking like a string of sausages, but they allege that the organism responsible was possibly one of the first bilaterian animals. If so, it must have been our ancestor! "Likely related to our ancestors, 'Plexus ricei' was much like a tapeworm or modern flatworm, say UC Riverside researchers." (Emphasis added.) Their find was published in the Journal of Paleontology. More:

Named Plexus ricei and resembling a curving tube, the organism resided on the Ediacaran seafloor. Plexus ricei individuals ranged in size from 5 to 80 centimeters long and 5 to 20 millimeters wide. Along with the rest of the Ediacara Biota, it evolved around 575 million years ago and disappeared from the fossil record around 540 million years ago, just around the time the Cambrian Explosion of evolutionary history was getting under way.

"Plexus was unlike any other fossil that we know from the Precambrian," said Mary L. Droser, a professor of paleontology, whose lab led the research. "It was bilaterally symmetrical at a time when bilaterians -- all animals other than corals and sponges -- were just appearing on this planet. It appears to have been very long and flat, much like a tapeworm or modern flatworm."

These claims echoed around the news media. Plexus is "a distant ancestor of other bilateral organisms -- including humans," says Live Science. It's "likely related to our ancestors," NASA's Astrobiology Magazine teases.

How is this misleading? Let us count the ways.

  1. Like the other Ediacarans, this organism (whatever it was) died out before the Cambrian explosion.
  2. The scientists admit there is no clear path of descent from Ediacaran to Cambrian. "Ediacaran fossils are extremely perplexing," the first author of the paper states; "they don't look like any animal that is alive today, and their interrelationships are very poorly understood."
  3. The discoverers make a big issue of whether it is a "trace fossil," because if that's all it is, it's not nearly as exciting. "Plexus is not a trace fossil," they assert, but then hedge a little: "What our research shows is that the structure we see looks very much like a trace fossil, but is in fact a new Ediacaran tubular organism, Plexus ricei."
  4. Giving something a scientific name does not mean you understand it. Ernst Haeckel famously dredged up mud from the sea and gave it a name Bathybus haecklii ("Haeckel's mud"), claiming it was turning into living matter.
  5. The drawing seems to use ample artistic license compared to the photographs of the rock impressions.
  6. If there is any bilateral symmetry at all, it is extremely minimal. Are the scientists sure the tubes were not squashed by subsequent geological processes?
  7. Tapeworms and flatworms are much more complex organisms than this. Belonging to phylum Platyhelminthes, flatworms are true bilaterians, have three cell layers, and specialized "brains," digestive and excretory systems. Nothing like that is discernible in Plexus, so making a big deal of the resemblance is misleading.
  8. After all the hype, the discoverers are not even sure Plexus is a bilaterian! "At this time, we don't know for sure that Plexus ricei was a bilateral," the team leader admits, but then adds, boisterously, "but it is likely that it was related to our ancestors."

There is a distinct impression of desperation here. Under the best of circumstances for Darwinian evolution (assuming this is an early bilaterian), this organism still leaves a huge gap to overcome when the Cambrian explosion hits.

Most likely, the circumstances are not the best. An unbiased observer would not likely be convinced by the little bumps in the rock that he is beholding his ancestor. If this is the best evidence that evolutionary paleontologists can put forward, Darwin's Doubt remains unanswered.

Image source: Droser Lab, UC Riverside.


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