More complexity in the earliest multicellular animals has only intensified what Stephen Meyer calls “Darwin’s Doubt.” Some examples follow.
A paper in Current Biology comes closest to demonstrating “the deep homology between exoskeletal features in an evolutionary continuum of taxa with distinct types of body organization.” Author Javier Ortega-Hernández takes on “the euarthropod head problem” by finding similarities between two specimens from the Burgess Shale (Middle Cambrian). His analysis, though, only compares positions of complex tissues, not how they originated. Amid various controversies, he focuses on a relatively simple structure, the anterior sclerite (a front plate of cuticle) within one phylum, the true arthropods.
Despite its ubiquitous nature, the significance of the anterior sclerite remains controversial, as there is little agreement on the correspondence of this structure among stem- and crown-group euarthropods. [Emphasis added.]
His attempt at finding phylogenetic relationships, given such a trifling structure in a narrow range of animals, is less than convincing. Worse, he ignores the weightier matters of the explosive origin of the complex body plans of these animals.
Science media were excited to report on a “spiky monster worm” from China, named Collinsium ciliosum (original paper is in PNAS). If this is supposedly an ancestor of modern velvet worms, as researchers at the University of Cambridge claim, it was already complex, with spikes, a mouth with teeth, antennae, and filter feeding appendages. Its 72 spikes in rows down its back are particularly noteworthy. If anything, it looks more complex than “Today’s 180 or so species of velvet worms [that] all look and act pretty much the same” (Science Magazine). “This isn’t the first time that an ancestral group has displayed more diversity than its modern-day relatives,” Live Science comments. If this is evolution, it’s going backwards.
The critter is one of the first known animals on Earth to develop protective armor and to sport specialized limbs that likely helped it catch food, the researchers said. This newfound species lived during the Cambrian explosion, a time of rapid evolutionary development, they said.
Abrupt appearance? Rapid development? How this helps the evolutionary story is not clear. The Cambridge news item offers word salad as a distraction from the issues raised by Dr. Meyer, which they simply ignore.
“Animals during the Cambrian were incredibly diverse, with lots of interesting behaviours and modes of living,” said Ortega-Hernández. “The Chinese Collins’ Monster was one of these evolutionary ‘experiments’ — one which ultimately failed as they have no living direct ancestors — but it’s amazing to see how specialised many animals were hundreds of millions of years ago. At its core, the study of the fossil record seeks answers about the evolution of life on Earth that can only be found in deep time. All the major biological events responsible for shaping the world we inhabit, such as the origin of life, the early diversification of animals, or the establishment of the modern biosphere, are intimately linked to the complex geological history of our planet.”
The apex predator Anomalocaris was mentioned in connection with a more recently discovered member of its family. This one, a giant named Aegirocassis benmoulae, was found in Morocco, indicating the global extent of the anomalocaridids. Unlike its more famous relative, “this anomalocaridid from the Ordovician exposes a second set of body flaps and reopens the question of how the two branches of arthropod legs evolved,” Gregory Edgecombe notes in Current Biology. The authors of the paper in Nature are not much help to Darwin, having to invoke “convergent evolution” again:
Among arthropods, the size of A. benmoulae (over 2 m in length) is paralleled only by some pterygotid eurypterids and terrestrial arthropleurids. The evolution of gigantic filter-feeders within clades of nektic macrophagous predators is well documented in Mesozoic pachycormid fish and Cenozoic sharks and whales. The huge size of A. benmoulaerepresents a much earlier example of a filter-feeding lifestyle correlating to gigantism. The abundance of gigantic anomalocaridid filter-feeders in the high palaeolatitude Fezouata Biota points to a complex planktic ecosystem. Early Cambrian anomalocaridid filter-feeders also fed on zooplankton, but they remained relatively small. Although the Cambrian Explosion saw the establishment of the first complex planktic ecosystems, the convergent (Supplementary Text) rise of giant filter-feeding anomalocaridids during the Ordovician followed an increase in the abundance and diversity of phytoplankton and a consequent zooplankton radiation as part of the Great Ordovician Biodiversification Event.
Once again: abrupt appearance of complex body plans, complex ecosystems, and convergent evolution. None of this helps the evolutionary story or answers the key issue: where did the genetic information come from to build complex body plans with hierarchical structures and functional organs composed of new tissues and cell types?
When Hallucigenia (pictured above) was first found in the Burgess Shale a century ago, paleontologists couldn’t tell top from bottom or front from back. The bizarre creature with paired spines pointing away from its paired legs was missing an important part: its head. Since then, the head has been found. It’s complex, with a pair of eyes and rows of teeth. This requires explaining more cell types and tissue types than before, exacerbating the problem Stephen Meyer identifies.
Martin R. Smith from Cambridge, with Jean-Bernard Caron from the Toronto Museum of Natural History, announced the discovery in Nature. Other sites, like Phys.prg and New Scientist, picked up the story and showcased the new artwork. For BBC News, Smith described his initial reaction:
By delicately chipping away at the rock, scientists found a spoon-shaped head with some surprising features.
“When we put it into the electron microscope, we were delighted to see not just a tiny pair of eyes looking back at us, but also beneath them a really cheeky semi-circular smile.
“It was as if the fossil was grinning at us at the secrets it had been hiding,” explained Dr Smith.
Inside the creature’s mouth, the researchers found a ring of teeth and then another set of teeth running from its throat down towards its stomach.
Most of the chatter is preoccupied with where to put this creature in a Darwinian phylogenetic tree. For a long time, animals were lumped together by their type of body cavity (coelom). That’s changed; in 1997, Aguinaldo invented the category “ecdysozoa” (“molting animals”) based on ribosomal RNA comparisons. This lumped together everything from butterflies to roundworms, from tardigrades (“water bears”) to centipedes, from velvet worms to spiders. But is such a clade meaningful? “These disparate phyla are united by their means of molting, but otherwise share few morphological characters — none of which has a meaningful fossilization potential.” Smith and Caron note. “As such, the early evolutionary history of the group as a whole is largely uncharted.”
The purpose of the grouping was to try to unite all the creatures that supposedly had a common ancestor. A more meaningful designation would account for the complexity and unique features of each animal, without forcing it into preconceived notions of common ancestry. Hallucigenia is a prime example. This creature had eyes, a mouth, teeth, a throat, a foregut, a stomach, and an anus. It had appendages that could reach its mouth. It had seven pairs of spines, each emerging from “a buttress of soft tissue,” arranged with curvatures from front to back, protecting the entire animal. It had claws on the ends of the legs.
It’s not just the cell types that need to be explained, but their arrangement into functional structures. These structures, moreover, need to be integrated into a functional animal in its ecosystem. And, they need software in some central nervous system that allowed the animal to use all of it. This is hierarchical organization, none of which is seen in the Precambrian layers beneath.
Live Science has a nice gallery of these and other Cambrian critters. Interesting animals, but nothing new here. More of the same complexity. More of the same just-so storytelling that assumes undirected evolution. More distraction from the main question: what is the source of complex specified information to build a complex animal? How could it emerge from a blind, unguided process?
This article was originally published in 2015.