Evolution Icon Evolution

Low Oxygen Suffocates Darwinian Explanations for the Cambrian Explosion

oxygen bubbles.jpg

When the American Association for the Advancement of Science publishes a theory seeking to explain the Cambrian explosion, you can be confident you’re getting some of the best thinking out there. In Science Magazine’s Halloween issue, nine researchers from eight prestigious universities, including Yale, University of Illinois, and Caltech advanced their latest offering on the emergence of nearly twenty new animal body plans in the geological blink of an eye.

The basic idea: animals wanted to emerge, but oxygen held them back. That’s evident in the title: "Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals."

The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans. [i.e., the Cambrian explosion, emphasis added.]

Surely there must be something more substantive in the body of the paper than the mere suggestion that explosive diversification is a function of oxygen levels. Can you find it?

Right off the bat one notes that the bulk of the short paper focuses on constraining values of chromium levels in various rocks and attempting to date them. Here are the only connections with the Cambrian explosion. The opening says:

It remains unclear whether the appearance and diversification of animals are linked to a change in environmental oxygen (O2) levels or if this dramatic shift in the structure and complexity of the biosphere simply reflects the timing of genetic and/or developmental innovation independent of any environmental control….

…. Such a comparison is essential for delineating the potential role of Earth’s oxygen cycle in the early evolution of animal life.

Then the final paragraph draws the conclusions:

Under previous estimates of atmospheric pO2 during the mid-Proterozoic (1% PAL < pO2 < 40% PAL), there was potentially sufficient atmospheric oxygen for the earliest sessile and mobile animals to thrive well in advance of their ostensible emergence…. In any case, our results suggest a temporal overlap between the appearance of stable environments favorable for animal life and the divergence of basal metazoan clades — which, according to recent estimates [e.g., (2)], occurred between ~0.8 and 0.7 Ga (Fig. 3). Though the emergence and eventual ecological dominance of animal life must, at its core, be tied to genetic and developmental innovations, our results implicate Earth’s oxygen cycle as a crucial factor shaping the evolutionary landscape from which animal life emerged and help explain the delayed appearance of animals in the late Proterozoic.

DebatingDDsmall.jpegPardon the long quote with its dense scientese, but we should give the authors their full say, to allow you to hunt for their explanation, too. Where is it? They admit up front that it’s "unclear" if the Cambrian explosion was due to oxygen (it might have nothing to do with it). Oxygen only has a "potential role" in the "ostensible emergence" of new body plans. In other words: We don’t know if oxygen plays a role at all, but just in case it does, oxygen might have been too low when animals were trying to evolve, if our proxy measurements are meaningful.

Satisfied? Remember, this was published just days ago in the AAAS journal. It must represent the best thinking to date that Darwinian evolutionists can muster. It was received for publication in July this year, well after Darwin’s Doubt was published in 2013. With the publicity of Meyer’s best-selling book fresh in mind, it would seem Darwinists would rally to counter his ID message with a solid, coherent, reasoned response.

Here’s the response in a nutshell: It’s the environment’s fault that animals didn’t "emerge" sooner. Their ancestors were swimming around gasping for breath. When the environment finally turned up the air, "the evolutionary landscape" was set for animal life to emerge. All that pent-up energy exploded in a profusion of new complex body plans.

This should be testable. Just put some anaerobic microbes in test tubes with varying levels of oxygen. Watch for ones that evolve into new phyla the fastest. Lenski should try this on some of his E. coli samples.

In the same issue of Science, Carolyn Gramling tries to sell this theory:

Between the appearance of complex cells about 2 billion years ago and the explosive diversification of multicellular animals some 800 million years ago was the so-called boring billion — a long span of time during which evolution seems to have mired. Now, a new study looking at chromium isotopes in ancient ironstones suggests that the oxygen content of the atmosphere during that time was only 0.1% of present oxygen levels — at least an order of magnitude smaller than previous lower limits for oxygen content at the time. Oxygen levels rose again about 800 million years ago, coinciding with animal diversification — and suggesting that such environmental conditions can play a powerful role in controlling biological evolution.

Correlation is not causation. A rise in oxygen about the time of "the explosive diversification" of multicellular animals does not mean oxygen caused the diversification. The only new idea in the paper is the suggestion that oxygen was lower than previously thought during the "boring billion" before evolution started to move.

It’s not clear why they are stretching back the explosion to 800 million years ago, unless they are tossing in the first eukaryotes. The Ediacaran multicellular organisms exploded onto the scene 635 million years ago in the evolutionary time sequence; the onset of the Cambrian explosion itself (unrelated to the Ediacaran, most scholars believe) began about 540 million years ago. Gramling skirts over this gap: "It wasn’t until about 800 million years ago that oxygen again began to rise, causing reactions that triggered global cooling and then — about 542 million years ago — a rapid diversification of animals called the Cambrian Explosion."

The dates, though, are not the issue. As Meyer makes clear, it’s the sudden appearance of new genetic information necessary for new cell types, tissues, organs and systems that make up a body plan, like an arthropod with a gut, circulatory system, eyes, and articulating limbs.

Preachers know to pound the pulpit harder when the point is weak. In that spirit, we’re told the new study is not only "exciting" but "very exciting":

The finding by a team led by Noah Planavsky of Yale University is "very exciting," says geochemist Sean Crowe of the University of British Columbia, Vancouver, in Canada. "There’s been a tendency to shy away from changes in atmospheric or oceanic oxygenation as important for driving the evolution of animals. It’s great to see that we now have a little more information pointing toward oxygen as an important driver."

A statement from Yale also tries to get the audience excited:

"If we are right, our results will really change how people view the origins of animals and other complex life, and their relationships to the co-evolving environment," said co-author Tim Lyons of the University of California-Riverside. "This could be a game changer."

Their case is extremely weak. No one is certain if chromium makes a good proxy for oxygen levels. Even if it does, not everyone is convinced that oxygen had any role in evolutionary progress. At the very, very most, all the new paper does is bring back an old, discredited idea to the table for further brainstorming. Gramling writes:

Reinhard says the study reemphasizes the role of oxygen in biological evolution. Recent studies have stressed genetic innovation over environmental factors, he says, but "we bring that back to the table."

A slight variation on the low-oxygen hypothesis was offered up by the University of Texas at Austin. Science Daily begins with a tantalizing promise for Darwin’s dilemma:

A new analysis of geologic history may help solve the riddle of the "Cambrian explosion," the rapid diversification of animal life in the fossil record 530 million years ago that has puzzled scientists since the time of Charles Darwin….

The Cambrian explosion is one of the most significant events in Earth’s 4.5-billion-year history. The surge of evolution led to the sudden appearance of almost all modern animal groups. Fossils from the Cambrian explosion document the rapid evolution of life on Earth, but its cause has been a mystery.

The sudden burst of new life is also called "Darwin’s dilemma" because it appears to contradict Charles Darwin’s hypothesis of gradual evolution by natural selection.

Again, though, we find them claiming that whatever tectonic events moved some continents around resulted in a rise of oxygen, as if that "allows" evolution to proceed:

Beyond the sea level rise itself, the ancient geologic and geographic changes probably led to a buildup of oxygen in the atmosphere and a change in ocean chemistry, allowing more complex life-forms to evolve, he [Ian Dalziel] said….

"I’m not claiming this is the ultimate explanation of the Cambrian explosion," Dalziel said. "But it may help to explain what was happening at that time."

The strategy seems to be: (1) ignore Meyer, (2) offer tantalizing suggestions, (3) sound excited, and maybe some people will think evolutionists are still on top of this "nagging mystery" as Yale calls it.

There is little new in the evolutionists’ toolkit of explanations. In February, David Klinghoffer presented other published evidence showing that low oxygen is not a constraint on evolution. Later in February, we critiqued the whole idea of oxygen levels having anything to do with the timing of the Cambrian explosion. Now here we are at the beginning of November and they are still pushing the same discredited hypothesis. It all can be considered a backhanded support for Meyer’s case.

Image: Oxygen bubbles, Ben Adams/Flickr.

Evolution News

Evolution News & Science Today (EN) provides original reporting and analysis about evolution, neuroscience, bioethics, intelligent design and other science-related issues, including breaking news about scientific research. It also covers the impact of science on culture and conflicts over free speech and academic freedom in science. Finally, it fact-checks and critiques media coverage of scientific issues.



Cambrian NewsColleagues' ResponsesDarwin's DoubtNews and EventsResponse to Criticism