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Cambrian Explosion Story Hour: Oxygen as a Creative Force, Again


Gather round, children. It’s the Cambrian Explosion Story Hour and Douglas Erwin is back. The theme of the hour: “Debunk Stephen Meyer and Darwin’s Doubt without naming them.” Yes, this is familiar ground. Since we examined his proposals a year ago, Erwin seems to have joined the Oxygen Party (see our reviews from 2013, 2014, and 2016).

In his latest paper, co-authored by scientists from Yale, Georgia Tech, and UC Riverside, the renowned paleontologist from the National Museum of Natural History does an interesting double take. Erwin roundly criticizes the notion that a rise in oxygen in the Precambrian oceans could have fueled the emergence of complex body plans. But in the conclusion, he ends up embracing oxygen anyway as a “creative evolutionary force” — a mystical retreat from scientific explanation.

The paper in the Proceedings of the National Academy of Sciences, “Earth’s oxygen cycle and the evolution of animal life,” demonstrates with geophysical models that for most of the Proterozoic — just before the Cambrian explosion — oxygen levels were likely insufficient to fuel the complex metabolic pathways required for active, motile organisms. Would this part of the Abstract sound encouraging to evolutionists desperately needing an answer to Meyer?

Herein, we quantitatively explore one aspect of the evolutionary coupling between animal life and Earth’s oxygen cycle — the influence of spatial and temporal variability in surface ocean O2 levels on the ecology of early metazoan organisms. Through the application of a series of quantitative biogeochemical models, we find that large spatiotemporal variations in surface ocean O2 levels and pervasive benthic anoxia are expected in a world with much lower atmospheric pO2 than at present, resulting in severe ecological constraints and a challenging evolutionary landscape for early metazoan life. We argue that these effects, when considered in the light of synergistic interactions with other environmental parameters and variable O2 demand throughout an organism’s life history, would have resulted in long-term evolutionary and ecological inhibition of animal life on Earth for much of Middle Proterozoic time (∼1.8-0.8 billion years ago). [Emphasis added.]

The paper counters oft-stated assumptions that multicellular organisms began to innovate all kinds of new tricks as oxygen levels steadily rose.

Although it should be stressed that many key biological innovations must have preceded the Late Proterozoic emergence of basal metazoan clades, we suggest that early multicellular organisms would have had to contend with a “patchy” and evolutionarily restrictive oxygen landscape for much of Proterozoic time, despite background local O2 levels that may, in some cases, have been sufficient to fuel their metabolic needs. Although our discussion here focuses on metazoans, if periodic anoxia was common throughout shallow oceans during the Neoproterozoic, all eukaryotic life may have been oxygen-stressed on evolutionary timescales. This relationship may, in part, explain the early rise but protracted diversification of eukaryotes.

Erwin and his co-authors have pulled the rug out from under his colleagues who trusted in oxygen. But now comes the double take. It’s not a bug; it’s a feature! Suddenly, oxygen hit a threshold in the Late Proterozoic. Like magic, it created complex animals.

Finally, we hypothesize that the interplay between long-term changes in base-level pO2 and some degree of spatiotemporal oxygen variability may promote rather than inhibit biological and ecological innovation. For example, theory predicts that environmental fluctuation can often reinforce evolutionary transitions in individuality, such as multicellularity. A critical combination of baseline pO2 and spatiotemporal variability may thus have ratcheted the evolution of more advanced forms of multicellularity in some eukaryotic lineages. In addition, variability in the marine oxygen landscape against the backdrop of increasing baseline O2 during the Late Proterozoic may have acted as a creative evolutionary force, ultimately favoring rapid phylogenetic diversification and ecological expansion among early animal lineages. Future work pinpointing when and how this transition occurred will be essential for fully understanding how changes in surface O2 levels have affected the tempo and mode of early metazoan evolution.

Indeed. When and how. Fully understanding. Keep up the good work.

Nature, too, retells the Tale of the Creative Oxygen. Referring to a study of salt crystals that contained “11% oxygen, more than expected” for 815 million years ago, they say: “The authors suggest that high oxygen levels drove animal evolution, rather than the other way around.” How about an empirical test for that hypothesis? Add more creative oxygen to Lenski’s E. coli test tubes, perhaps?

A Snipe Hunt for Eukaryote Origins

Another double-take occurs in an open-access paper in Trends in Cell Biology, “On the Archaeal Origins of Eukaryotes and the Challenges of Inferring Phenotype from Genotype.” Gautam Dey, Mukund Thattai, and Buzz Baum undercut a favorite evolutionary notion, only to embrace it in the end.

You can see the “bad news” part of the story in this summary from Cell Press on EurekAlert. How long have you heard that the first eukaryote was born when a bacterium engulfed an archaeal cell, creating a wonderful symbiosis that led to today’s eukaryotic cells replete with chromosomes in a nucleus, mitochondria, and all the other organelles? Think again. The textbooks are wrong.

The first eukaryote is thought to have arisen when simpler archaea and bacteria joined forces. But in an Opinion paper published June 16 in Trends in Cell Biology, researchers propose that new genomic evidence derived from a deep-sea vent on the ocean floor suggests that the molecular machinery essential to eukaryotic life was probably borrowed, little by little over time, from those simpler ancestors.

“We are beginning to think of eukaryotic origins as a slow process of growing intimacy — the result of a long, slow dance between kingdoms, and not a quick tryst, which is the way it is portrayed in textbooks,” says Mukund Thattai of the National Centre for Biological Sciences in India.

But lest evolutionary colleagues worry, the authors have no intention of pandering to Darwin critics. They’re only rearranging the timeline, offering “Slow Eukaryogenesis” instead of Fast Eukaryogenesis. They found some genes (but not the organism) near a deep-sea vent that suggest that a mysterious ancestor named Loki is lurking around. Loki (short for a proposed Lokiarchaeota) might be a snipe ancestor (snipe are birds, and they are eukaryotes, right?). The hunt is on for this cell that they think has precursors to true eukaryotes — that is, machines doing jobs for archaea that were later co-opted for eukaryotes of the future:

Loki is so far unique among archaea in having a large number of highly conserved actin and actin-like proteins, proteins with homology to gelsolin, representatives of all three ESCRT complexes, and a putative BAR domain protein. However, these ESPs [Eukaryotic Signature Proteins] most likely function within a typical TACK family archaeal cellular milieu. It follows that we should look to other TACK archaea as a guide to potential function in addition to eukaryotes….Given its larger complement of actin-like and potential gelsolin-like regulators, Loki may then be capable of assuming different forms. However, given the recent discovery of actin’s involvement in nuclear functions, actin homologs could function to regulate gene expression in Loki. In a similar vein, since all cells have to divide, it should be no surprise that all bacteria and archaea encode machinery that enables their membranes to undergo regulated or unregulated membrane scission…. Thus it is likely that, in Loki, ESCRTIII does this job: inducing a change in membrane topology that is as old as cellular life itself.

More magic, whether performed in a long, slow fashion or at a quicker pace. Evolutionists need to stop the “bad news, good news” jokes. Enough magic acts and storytelling. The goal of science is to provide a vera causa for observed effects. Except in some highly imaginative fiction, oxygen is not a creative evolutionary force. A gas cannot devise body plans. A living archaeal cell at a deep sea vent is not the ancestor to all eukaryotes. Complex specified information is present. Molecular machinery is present. What necessary and sufficient cause other than intelligence can explain these observed realities?

Photo: Story hour at the library, by United States Army, Cpl. Hwang Joon-hyun, Yongsan Public Affairs [Public domain], via Wikimedia Commons.

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