Evolution News keeps a lookout for new excuses that pop up to explain away the very un-Darwinian sudden emergence of 17 or more animal body plans in the Cambrian explosion. Ever since Stephen Meyer published Darwin’s Doubt in 2014, we have seen a gamut of excuses pass by. One of the strangest was that it was caused by (or correlated with) one or more “carbon isotope excursions” or CIE (see here, here, and here). Presumably, the major CIE was also associated with more oxygenated ocean water, giving microbes the magic elixir of oxygen to drive them to become animals (see here and here). The dates of those carbon excursions, however, have been uncertain and hard to correlate at different sites around the world.
A new study published in PNAS1 has tried to nail down the dates of the so-called “Shuram CIE” to see if it correlates with the Ediacaran or Cambrian fossils.
The rise of animals occurred during an interval of Earth history that witnessed dynamic marine redox conditions, potentially rapid plate motions, and uniquely large perturbations to global biogeochemical cycles. The largest of these perturbations, the Shuram carbon isotope excursion, has been invoked as a driving mechanism for Ediacaran environmental change, possibly linked with evolutionary innovation or extinction. [Emphasis added.]
The authors measured isotopes in northwest Canada, Newfoundland, and Oman — three geographically distinct sites on “multiple paleocontinents and paleolatitudes” — to bracket the onset, duration, and ending of the Shuram CIE. Unfortunately, the earliest Ediacaran fossils appeared well before this excursion. This undercuts the notion that the CIE somehow facilitated the explosive appearance of multicellular life.
Our understanding of the interactions between animal evolution, biogeochemical cycling, and global tectonics during the Ediacaran Period (635 to 541 Ma) is severely hampered by lack of a robust temporal framework. The appearance and extinction of the earliest fossil animals are hypothesized to correlate with upheavals in biogeochemical cycles — foremost the Shuram carbon isotope excursion, possibly the largest known disturbance to the global carbon cycle. However, without age constraints on the excursion’s timing and duration, its driving mechanisms, global synchroneity, and role in Ediacaran geobiological evolution cannot be evaluated. We provide radioisotopic ages for the onset and termination of the Shuram, evaluate its global synchroneity, and show that it is divorced from the rise of the earliest preserved animal ecosystems.
An Absurd Idea
How any of the alleged “driving forces” could create new hierarchical body plans with irreducibly complex systems and functions is never explained. What is it about redox conditions, carbon cycles, or plate motions that possesses such creative power? The idea seems absurd, yet paleontologists have little else to reach for in their desperation to explain the Cambrian explosion, and the Ediacaran explosion as well.
The paper reasonably assumes that if the Shuram CIE was global, the beginning and ending dates must reflect the overlapping time ranges measured between the three locales. Taking that overlap, they date the onset to 574 Ma and the ending to 562 Ma. The first Ediacaran creatures, however, date to about 598 Ma — almost 25 million years before the Shuram CIE. The CIE cannot account for even a correlation with the Ediacaran explosion, therefore, let alone a causation. Fossil evidence suggests that the “these organisms were globally distributed and likely appeared before the Shuram CIE,” including the characteristic frondose filter-feeders that waved passively like leaves from their holdfasts in the ocean floor. Those were not animals, however, and were not bilaterian animals like the later Cambrian phyla. The Ediacarans appeared suddenly, and disappeared mysteriously, having little or nothing to do with the Cambrian explosion.
More Bad News for Evolutionists
There’s more bad news in the paper: some had theorized that the CIE was caused by the Gaskiers deglaciation, but that event, estimated at ~580 Ma, “suggests at least 5.2 ± 4.8 My between Gaskiers deglaciation and the onset of the CIE.” Those two events, therefore, are also “stratigraphically and geochronologically distinct” from each other, as are several other hoped-for correlations between geochemistry and biology.
Calibration of the Shuram CIE also unexpectedly divorces the stratigraphic and/or evolutionary appearance and disappearance of the Ediacara biota from previously hypothesized environmental drivers.
In addition, the Shuram CIE was over before the onset of the Cambrian explosion. Try as they might, they cannot connect animal appearances with geochemistry, as if that would have helped in any event.
There are still important nuances to consider in connecting the Shuram CIE with records of animal evolution, including preservational controls on the appearance of fossils and long-enduring questions of the permissible temporal lag between environmental change and biotic response. However, our temporal framework for the terminal Proterozoic suggests that evolutionary dynamics in the Ediacaran Period were, at face value, potentially decoupled from dramatic environmental changes. This finding highlights the importance of a robust geochronological framework for linking Proterozoic environmental change with the fossil record.
Good news never comes. They have only removed an excuse for the sudden appearance of animals. This Shuram CIE, “potentially one of the largest global carbon cycle perturbations in Earth’s history,” does not correlate to the evolution of animals, either Ediacaran or Cambrian. Their disappointing conclusion is notably understated:
This emerging chronology provides essential context for evaluating the mechanisms capable of driving an extraordinarily negative and extended CIE and assessing its impact on the habitats and evolution of early animals.
The words “explosion” and “burst” appear in another paper about arthropods. In the biology preprint server bioRxiv, three scientists attempted a new method of arranging insects and arthropods into a Darwinian tree.2 This time, instead of genomes, they assembled proteomes to see if the major insect orders reflected the genomic phylogeny.
Results: An alignment-free whole-proteome tree of the arthropods shows that (a) the demographic grouping-pattern is similar to those in the gene trees, but there are notable differences in the branching orders of the groups and the sisterhood relationships between pairs of the groups; and (b) almost all the founders of the groups have emerged in an explosive burst near the root of the tree.
These authors use the word “burst” a good 13 times, in phrases like “arthropodal burst” and “sudden burst” for all the major orders of insects. Sound familiar?
“Arthropodal burst” near the root of ToAr [tree of arthropods]: Figures 3 shows the whole-proteome ToAr with CGD values. It reveals that the “founders” (for definition, see Supplemental information, Fig. S3) of all major groups (at Subphyla and Order levels) emerged in a burst within a short evolutionary progression span between CGD [cumulative genomic divergence] of 1.6 and 5.8 (marked by a small red arc), near the root of the tree in Figs. 3, 4 and 5. This burst is reminiscent of the “deep burst” of the founders of all kingdoms of Life in the whole proteome tree of Life.
Something about life likes to burst onto the scene without ancestors. One thing is for sure; that is a very un-Darwinian picture. And building complex organisms — whether Ediacarans, Cambrian body plans, or arthropods — takes more than excursions of carbon and oxygen.
- Rooney et al., “Calibrating the coevolution of Ediacaran life and environment,” Proceedings of the National Academy of Sciences, 6 July 2020. https://doi.org/10.1073/pnas.2002918117.
- Choi, Kim, and Kim, “Whole-Proteome Tree of Arthropods: An ‘alignment-free’ phylogeny of proteome ‘books’,” bioRxiv, 14 July 2020. https://doi.org/10.1101/2020.07.11.198689.