Next on the Cambrian Explosion Gong Show: The Slime Theory
Some culture analysts have complained that many millennials seem to feel entitled to honors just for participating. They don’t have to win anything; just showing up is enough to get a trophy. Enablers of this behavior, whether teachers or parents, may smother them with hugs and praises for their attempts at self-expression, no matter how pathetic.
Does this sound familiar, from a different, namely scientific context? It should. A similar situation obtains in the research journals when it comes to addressing the Cambrian explosion. Anything goes. It’s like the old Gong Show with Chuck Barris, but without the gong, where every act gets a standing ovation.
Consider a new paper in Nature by Brocks et al., “The rise of algae in Cryogenian oceans and the emergence of animals.” It basically says that slime caused animals. Here’s what they say in the Editor’s Summary:
The sudden appearance of complex animals in the Cambrian period puzzled Darwin. He regarded it as one of the most important problems to beset his theory of evolution by natural selection. Here, Jochen Brocks and colleagues show that the Cambrian ‘explosion’ was preceded by a ‘rise of algae’ during an interval in which the world may have been largely frozen over. Various steroids preserved in sediments are distinctive markers of eukaryotes, but steroids typical of algae only abound for a short interval in the Cryogenian period between the Sturtian (720–660 Ma) and Marinoan (650–635 Ma) glaciations. In this relatively short, warm interval, phosphorus released by Sturtian weathering allowed eukaryotes to flourish. This broke the stranglehold on Earth’s ecology by cyanobacteria, which can get by in lower phosphorus concentrations. This ‘rise of algae’ created shorter, more efficient food webs, driving an escalatory race towards larger and increasingly complex organisms and the rise of animals. [Emphasis added.]
The paper itself doesn’t actually offer much more than a plate of slime for animals to eat. It says nothing about animal body plans. There’s nothing about natural selection (except in the Editor’s summary quoted above). The authors seem to think that if you just set the menu for animals, they will come.
The engulfment of a cyanobacterium by a heterotrophic eukaryote approximately 1,900 to 900 Myr ago marks the origin of algae. Yet, the corrected record of fossil steroids demonstrates that algae only broke the incumbency of phototrophic bacteria as the principal marine primary producers 659 to 645 Myr ago. This new timeframe offers a network of explanations for a Neoproterozoic/Paleozoic rise in atmospheric oxygen levels, establishment of more modern nutrient and carbon cycles, and the evolution of an increasingly complex biota. We propose that rising phosphate levels and the radiation of algae in late Cryogenian and early Ediacaran marine ecosystems were the bottom-up drivers for the evolution of eumetazoan animals.
Look, all you animal phyla! We set the table for you. Look at all this delicious phosphate and algae. Come forth and eat! The authors say:
We propose that the incumbency of cyanobacteria was broken by a surge of nutrients supplied by the Sturtian deglaciation. The ‘Rise of Algae’ created food webs with more efficient nutrient and energy transfers, driving ecosystems towards larger and increasingly complex organisms.
As usual, the paper ignores the reality that Stephen Meyer makes plain in Darwin’s Doubt, that the geologically sudden emergence of some twenty animal body plans requires enormous amounts of complex specified information indicative of intelligent causes, and inaccessible to Darwinian processes. It doesn’t matter; the authors get a trophy for participating.
They didn’t even offer an explanation for the Cambrian explosion. They offered (interesting term) a “network of explanations” for “an increasingly complex biota.” Perhaps said network is like a brier patch where the rabbit of explanation can hide from growling critics.
We can take some heart from Figure 1 in their paper, which shows the “rise of algae” a good 100 million years before the Cambrian explosion, raising doubt about its causal adequacy. If slime brings forth animals, why did the animals wait so long before flooding into the dining hall? The figure also confirms an Ediacaran explosion, a break at the Cambrian, and a very brief Cambrian explosion (the only mention of it in the paper). Nothing new here.
In the same issue of Nature, Andrew H. Knoll comments on this slime-breakfast proposal. Back in 2012, Knoll had opined that “It is genuinely difficult to map the characters of Ediacaran fossils onto the body plans of living invertebrates” and so their connection to Cambrian phyla “remains equivocal.” He thus acknowledges the suddenness of the Cambrian explosion. What does he say now? Basically, he dances around:
Ever since microorganisms first emerged, Earth and the life upon it have engaged in something of a pas de deux. Seldom during this long history have the dance’s tempo and the dramatic heights of the jeté leaps exceeded those that occurred near the end of the Proterozoic eon, which spanned the time from 2.5 billion to 541 million years ago. A supercontinent broke apart and then began to reassemble; ice ages twice whitened the planet from pole to Equator; and redox conditions underwent a fundamental shift. Amid all of this, animals large enough to be visible to the eye arrived on the scene, among them the bilaterians, which are symmetrical along their front-to-back axis, have complex organs and overwhelmingly represent present-day animal diversity.
They arrived on the scene. Animals. Complex organs. The judges stand and applaud. The only contentious points Knoll acknowledges is the “timing of key events and the mechanisms of interaction” between earth and life during the “dramatic leaps” of their dance, and the need for “biologists to articulate mechanisms that illuminate the relationship between life and the surrounding physical environment.”
He feels that Brocks et al. “make a substantial contribution to both, reorienting discussions of early animal evolution.” Reorienting; is that like rearranging proverbial deck chairs?
Knoll continues the dance act, leaping upon the notion that nutrients and oxygen drive complex animal body plans. He calls this the “ecological model.” It should be testable; fill a large tank with algae, phosphate, and oxygen in the conditions described in the paper. Watch trilobites emerge. (We would score them points, too, if Ediacarans emerged.)
Key questions about the history of oxygen levels remain. The view that Proterozoic ecosystems had more oxygen than their earlier Archaean counterparts (of more than 2.5 billion years ago) but less than those of modern times is reasonable, but a more quantitative understanding of this history remains elusive. Better models than those currently available are needed to describe phosphate and nitrogen cycling in the Proterozoic biosphere, and an improved understanding of the physiological characteristics of early animals is also required. Brocks and colleagues’ contribution will change the conversation. Continuing discussions of early animal evolution can now be embedded in a more-complete ecosystem framework that includes primary producers.
Key questions remain. An improved understanding of early animals is required. It’s time to change the conversation.
Photo: Chuck Barris, The Gong Show, 1976, by NBC Television Network [Public domain], via Wikimedia Commons.