Evolution News and Views (ENV) provides original reporting and analysis about the debate over intelligent design and evolution, including breaking news about scientific research.

Evolution News and Views
Evolution NEWS

Geologists Kick Out Props for Evolutionary Theories of the Cambrian Explosion

Ediacaran_GSSP_-_closeup (1).jpg

Reading the Darwinist explanations for the Cambrian explosion is like watching a game of "Who's Got the Button?" The biologists are sure the paleontologists have the reasons for this unprecedented appearance of animals in the fossil record. The paleontologists, in turn, point to the geologists for it. And the geologists rely on the opinion that the biologists have handfuls of explanatory buttons. When you inspect each one's hands, though, they come up empty.

It was the geologists' turn to show their empty hands. In the Geological Society of America Bulletin, two papers undermine popular assumptions used by the biologists and paleontologists to account for the explosion. Ask yourself if their work sounds like "settled science."

No Oxygen Rise Here, Mate

In the first paper by Sperling et al., geologists from Harvard and Queens College undermine the notion (commonly asserted by the biologists) that it was a rise in oxygen levels that triggered the explosion. Not that we ever found that explanation credible (#1, #2, #3, #4), but at least it gave the biologists a story with which to entertain the uninformed. No longer; these geologists found no evidence to support a global oxygen rise in the Ediacaran period that preceded the Cambrian.

The causes behind the appearance of abundant macroscopic body and trace fossils at the end of the Neoproterozoic Era remain debated. Iron geochemical data from fossiliferous Ediacaran successions in Newfoundland suggested that the first appearances correlated with an oxygenation event. A similar relationship was claimed to exist in the Mackenzie Mountains, Canada, although later stratigraphic studies indicated that the sections analyzed for geochemistry were incorrectly correlated with those hosting the fossils. To directly connect fossil occurrences with geochemistry in the Mackenzie Mountains, we conducted a multiproxy iron, carbon, sulfur, and trace-element geochemical analysis of stratigraphic sections hosting both the Cryogenian "Twitya discs" at Bluefish Creek as well as Ediacaran fossils and simple bilaterian traces at Sekwi Brook. There is no clear oxygenation event correlated with the appearance of macroscopic body fossils or simple bilaterian burrows; however, some change in environment -- a potential partial oxygenation -- is correlated with increasing burrow width higher in the Blueflower Formation. Data from Sekwi Brook suggest that these organisms were periodically colonizing a predominantly anoxic and ferruginous basin. This seemingly incongruent observation is accommodated through accounting for differing time scales between the characteristic response time of sedimentary redox proxies versus that for ecological change. Thus, hypotheses directly connecting ocean oxygenation with the appearance of macrofossils need not apply to all areas of a heterogeneous Ediacaran ocean, and stably oxygenated conditions on geological time scales were not required for the appearance of these Avalon-assemblage Ediacaran organisms. At least in the Mackenzie Mountains, the appropriate facies for fossil preservation appears to be the strongest control on the stratigraphic distribution of macrofossils. [Emphasis added.]

For non-specialists, this basically means that the evidence connecting oxygenation (a geochemical condition) with Ediacaran and Cambrian fossils is equivocal at best. The geologists appear to be trying hard not to rule it out ("a potential partial oxygenation"), but throughout the paper, they reveal contradictions with the oxygen hypothesis. Some of the best fossils appear in non-oxygenated rocks, and other well-oxygenated rocks have no fossils. Moreover, earlier studies that seemed to find a correlation were flawed.

The first step in distinguishing coincidence from correlation is determining whether temporal linkages represent a global pattern, regional events, or simply the unrelated appearance of organisms during a time interval characterized by broadly increasing oxygen levels. Neoproterozoic oxygenation, if present, is increasingly being recognized as regionally heterogeneous (Kah and Bartley, 2011). This is reflected in iron speciation data from the southern Canadian Cordillera showing an increased prevalence of anoxic conditions during the mid- Ediacaran (Canfield et al., 2008), in contrast to the Newfoundland data, and data from the Wernecke Mountains of northwestern Canada which show no change at all (Johnston et al., 2013). Other regions such as Namibia also show heterogeneous but generally anoxic and ferruginous conditions through the late Ediacaran (Wood et al., 2015). Analyzed collectively and statistically, a global database of Proterozoic and Paleozoic iron speciation data shows no overall change to the oxygenation state of marine environments between the Ediacaran and Cambrian (Sperling et al., 2015). These database analyses do not rule out an increase in oxygen through this time period, but they do limit the magnitude of such a change to much less than is normally depicted (e.g., Holland, 2006).

Their research in Canada also finds interesting geological evidence that contradicts the picture of slow, gradual deposition. For instance, some of the fossils are found in turbidites, which represent underwater landslides. Other fossils appear encased in strata that appear to have been formed in offshore storm surges. There are unconformities, slumps, and gaps.

In sum, the idea that biologists and paleontologists can expect to find a gradual increase in oxygen below the Cambrian boundary is mistaken. Figure 7 of the paper correlates outcrops from China, Africa, the U.S., and Canada. There's no pattern. "Redox change [i.e., change in reducing vs oxidizing conditions] may correspond to the appearance of megascopic fossils in some sections, but stable oxygenation on geologic time scales is not required." So if it's not oxygen, what is it? "The most obvious explanation is simply the distribution of beds capable of preserving fossils." Some places were suitable for preserving fossils, and some weren't. That's all.

Importantly, these data suggest that oxygenation of a basin is not required for the appearance of many Ediacaran taxa. To a first-order approximation, more so than oxygenation, the appearance of fossils of large eukaryotes throughout the entire Cryogenian and Ediacaran succession in NW Canada is dictated almost entirely by the appearance of event beds suitable for their preservation and presentation.

More on the second paper tomorrow.

Image credit: Golden spike indicating the base of the Edicaran period, by Bahudhara (Own work) [CC BY-SA 3.0], via Wikimedia Commons.