New Solution to Darwin's Doubt: Earth Wandered into Cambrian Explosion
Since the debut of Darwin's Doubt, we have welcomed all comers to give Darwinian explanations for the Cambrian explosion. We've seen scientists propose a rise in oxygen, the emergence of hard parts, a rise in fecal material, and other creative ideas, including sheer dumb luck. If nothing else, it's entertaining, like watching Britain's Got Talent but, it sometimes seems, a bit light on the talent.
Now there comes along a new solution to the Cambrian enigma, proposed by a team from the University of St. Andrews. The announcement arrives under the promising title, "Unlocking one of the great secrets of Earth's evolution."
They start by making huge promises:
An international team including scientists at the University of St Andrews has unlocked the secret of one of the great events of Earth's evolution -- the Cambrian explosion. [Emphasis added.]
Around 520 million years ago, a wide variety of animals burst onto the evolutionary scene in an event known as the Cambrian explosion. In perhaps as few as 10 million years, marine animals evolved most of the basic body forms that we observe in modern groups.
The event has sparked fierce debate all the way back to Darwin. Some paleontologists see the Cambrian explosion as a real, astonishing episode of unprecedented, fast evolution. Others suggest it is a false artifact of an unreliable fossil record.
The conflict has been established: the realists vs. the artifactualists. It's good to see that this contestant doesn't sidestep the brevity of the record. The team acknowledges that the "unprecedented, fast evolution" occurred in "as few as 10 million years." The alternative, the suggestion of a "false artifact of an unreliable fossil record," leaves much to be desired, since that idea perished quickly after Darwin. The team seems in a bind. Can they pull off a miracle?
Yes! They're going to get the whole planet involved in unlocking the great secret. Here is the main theme:
Now work published in the American Journal of Science shows that these competing theories can be unified by the geography of Cambrian Earth, as it underwent a wholesale lurch that clustered most of Earth's continents around the equator.
Co-author Dr Timothy Raub of the Department of Earth and Environmental Sciences at the University of St Andrews said: "In a nutshell both camps were right. The particular locations of Cambrian continents relative to each other was special in a way that supercharged animal speciation while preserving an unusually good record of those early fossils."
Is this the "lurch" theory of Cambrian animals? OK, you get the continents together; so what? How exactly does that "supercharge speciation"? Fossils can be preserved; great. Fossils of what?
They argue that the Earth lurched about in its orbit so that the continents cluster near the equator. This is dubbed "true polar wander." By chance, the continents created a hothouse ready to nurture the new complex animals.
Earth's continental and oceanic plates are in constant motion relative to one another, but in rare episodes of true polar wander, the entire solid Earth slips about its liquid outer core over the course of five to ten millions years, causing the geographic locations of Earth's plates to shift altogether in the same sense.
The paper suggests that about 520 million years ago a lurch of more than 60 degrees moved most continents from polar to tropical latitudes. For reasons that are still debated, biological diversity reaches a global peak around the equator and tapers off closer to the poles. This early Cambrian rotation therefore would have dramatically increased shallow coastal area in Earth's biodiversity hotspot.
Then, they say, a great bulge of water swamped continental coastlines, burying organisms under floods of sediment. The setting "would have opened up new habitats for rapid diversification, in particular vast continental seaways rife with previously unexplored ecological niches" and then preserving whatever, well -- whatever took advantage of the habitats.
That rather awkward ending deserves to be greeted with courtesy applause. One imagines Simon Cowell responding, "Thank you for that, but I do have a question; I'm a little confused where the animal body plans come from. You've set the table for them, surely; you've provided them with a nice hothouse and some unexplored habitats, but isn't the challenge how to explain the animals themselves?"
They never really grapple with the problem of new body plans, settling for vague language:
Cambrian true polar wander happened at a time when Earth was already seeded with many of the traits that subsequently radiated throughout the Tree of Life.
If one of us were a judge on this imaginary competition, the dialogue might go something like this: "I see, but...seeds? Are you saying that simple Ediacaran animals planted trilobite seeds and worm seeds and jellyfish seeds? I'm not sure I follow this line."
Dr Raub said: "A bunch of wonderful ideas have been published emphasising one or another aspect of the Cambrian biosphere as the crucial link in the explosion of animal life. An appealing aspect of our study is that a geographic contingency -- the shape and arrangement of the Cambrian continents and the direction of the remarkable true polar wander shift -- can support almost all those ideas simultaneously. At the same time, it turns out that preservation of Cambrian fossils really was enhanced over that of other ages....
Another judge chimes in: "Geographic contingency...are you telling us that it happened by chance? We were hoping to hear you explain the origin of nearly twenty unique animal body plans through a plausible biological mechanism in a very short period of time."
Another judge: "You can't fossilize what doesn't exist. Even if the continents lined up near the equator like you say, there should have only been Ediacaran organisms at the time. Where did the information come from to build all those body plans?"
They really haven't addressed the main issue. What could this presentation possibly do to, as they claim, "unlock the secret" of the Cambrian explosion?
"This new geographic framework answers a debate going back over a hundred years. It should encourage scientists to review all sorts of old and new hypotheses, which no longer must fit into the evolution or preservation camp exclusively."
The uncomfortable exchange continues: "You mean, like the intelligent design camp perhaps?" With a shocked expression, he responds, Well, no; of course not. Cowell continues, "Doesn't that qualify for inclusion in "all sorts of old and new hypotheses?" The respondent gulps. I thought we were talking scientific hypotheses. "So contingency is a scientific hypothesis?" Well, no, but... "Where did the information come from to build twenty new animal body plans?" Our model wasn't concerned with that, but rather with how the continents became rearranged to make whatever happened possible.
Thank you for your time. Next.
Image source: University of St. Andrews.