The Surprising Chemical Story behind the Cambrian Explosion
How did the mysterious creatures of the Cambrian explosion come to be? Chemistry may hold a clue.
INTRO BY KATHRYN: We’re going on a fossil hunt in today’s essay by chemist Dr. Ben McFarland. Not for just any fossils, but 540 million-year-old ones from the Cambrian Explosion. How did those mysterious creatures come to be? Ben thinks chemistry may point to an answer.
Once I took my children on a hike to see an explosion. Since I’m a chemistry professor, you might conclude that this was some sort of chemical explosion, like igniting a hydrogen balloon (or, better yet, a hydrogen-and-oxygen balloon). Maybe you’d think of a different kind of chemical explosion, like what happens when ice freezes inside a cast-iron ball. But it wasn’t that kind of explosion either—this was a biological explosion.
The biological event in question is called the Cambrian explosion. In several places around the world, in rocks dated 540 million years old, complex fossil forms are found that don’t exist in rocks dated 550 million years old. It’s like they suddenly appear on the scene. Many of these are tiny forms the size of dimes and quarters, yet they bristle with spines and bulge with armor plates. My collaborator, the artist Gala Bent, drew some of them (image above).
Last year we were vacationing in the Canadian Rockies, near where these fossils were first discovered. A web page said that a trail in Kootenay National Park led to a newly-discovered cache of these fossils. I wanted to go, but had several calculations to make. I had to consider the rest of my family, especially my youngest son, who was four years old at the time. I’d probably have to carry him for half the hike. Also, the hike started with a steep, steady climb until it reached the valley with the fossils. Would this path take us on a wild goose (or fossil) chase? My wife and I read and reread the description and talked it over. We guessed that we could just barely make it if we could pass that initial climb.
We started the hike after lunch on a warm, clear day in early September. At first, it was worse than I expected. I heard my first “I’m tired” while we were still within sight of our car. I pretended not to hear it. The next “I’m tired” prompted a quick water break. It came too soon.
The kids needed motivation, so I told them stories of the wonderful fossils they would find. They walked a little faster, energized by the idea. But even as I talked it up, a voice in my head questioned everything. Could I really trust those internet reports? Were there really enough fossils up there that we would be able to find some? I imagined us trudging back to the car, disappointed and exhausted. My stories of what we’d find faded a bit as I felt the gnawing sensation that I didn’t know this path firsthand.
After a few rounds of rest and encouragement (directed to myself as much as my kids), we reached a log bridge that marked the halfway point of the hike and the end of the big climb. At this point, everything changed. As the hiking reports predicted, the path leveled and the great U shape of the hanging valley opened up ahead. Rock walls on either side loomed as large as skyscrapers, tan and slate with a black, almost oily sheen. A cold blast of air blew into my face. The air flowed down from the Stanley Glacier, which had carved this valley long ago.
The directions told us to look for a waterfall on the right side of the valley. After half a mile, it appeared, a silver line against dark cliffs. At its base were thousands of plates of green-gray shale that it had pulled down and scattered for us to search. We clambered over and started flipping rocks.
I heard the echoing shout of my eleven-year-old. He held a rock crisscrossed with tiny, irregular tracks. Were those worm trails? Then my wife showed me a tear-shaped indentation, something like a kidney bean with feathers. My six-year-old set up a “fossil shop” displaying his finds. Even my four-year-old found a worm-like shape. I let out a breath. The reports were right—at the end of the path was an abundance of fossils.
After an hour the sun dropped and the valley darkened. We left our fossils for future explorers to find, and our walk back to the car was much faster. I didn’t have to carry the four-year-old nearly as much. It was a downhill trip, physically and emotionally.
Upon explaining what we found to my kids, they wanted to know why there were so many fossils there. I realized that, in a sense, a second kind of path led to that same destination. This was the path that life took through time. Could we know why the fossils suddenly appeared?
This is a question that has been asked many times before, with different answers. One possible path is presented in Stephen Meyer’s 2013 book, Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design. The “doubt” referred to in the title concerns whether natural processes can explain the explosion of life. Meyer claims that the Cambrian explosion is “one set of facts that troubled Darwin—something he conceded his theory couldn’t adequately explain, at least at present” (p. 6).
Meyer thinks the path life took to the Cambrian explosion is as discontinuous, and as unlikely, as a path scaling a sheer hundred-foot cliff. To him, the gap between pre-Cambrian life and Cambrian fossils seems too great to bridge with any natural cause—like asking my four-year-old to climb up the waterfall.
But Meyer’s description does not mesh with the opinion of the mainstream scientific community. One 2013 issue of Science magazine (Vol. 341, No. 6152) included both a book review of Darwin’s Doubt (p. 1344) and a review article about the Cambrian explosion (p. 1355). The article lists more than a dozen possible natural causes for the Cambrian explosion in a figure with a dense thicket of arrows and causes running back and forth across the page, looking more like a scribble than an actual explanation.
After reviewing the evidence, I agree with the scientific community, with one twist. For one thing, the calculated rate of evolution during the Cambrian explosion was only about five times faster than the normal rate. That seems within the reach of natural processes. Even my four-year-old can be prodded to walk up a slope five times greater than normal (and he did, for a while). These calculations were published in Current Biology as Lee, et al. “Rates of Phenotypic and Genomic Evolution during the Cambrian Explosion.”
However, I object to the way the 2013 Science review article presents the data. It seems deliberately oversaturated. At the end of the day, a dozen causes don’t tell the story of the Cambrian explosion that much better than Meyer’s single unknowable cause. I think the path that life took is clear enough to be explained with a coherent story, backed up by experiment.
I have more details in Chapter 9 of my own book, A World from Dust: How the Periodic Table Shaped Life. In brief, the dozen arrows from that Science article can be sorted into four categories. Each of those four categories might have have been caused by the increase of a single element in the air—oxygen.
Such an explanation requires evidence, which has recently come to light. A geological event called the Great Unconformity provides evidence for a drastic, worldwide chemical shift about 550 million years ago. At this time, old rock was scrubbed and eroded away in an “unconformity” before new rock was added 540 million years ago—a date which coincides with the Cambrian explosion.
Geological events like the Great Unconformity release oxygen into the air, and the oxygen eats away at the rocks in a feedback loop, releasing calcium, phosphate, and other rocky elements into the ocean. Life could have used those chemicals to grow new forms. Oxygen and phosphate would have given life new energy for building complex structures. Oxygen had bonding ability to knit these structures together.
Calcium would have given life new structures, too. Just like we use calcium for teeth, Cambrian organisms could have used it to build proto-teeth and jaws for chewing and burrowing through the environment. One study, titled “A vanished history of skeletonization in Cambrian comb jellies” and published in Science Advances (DOI: 10.1126/sciadv.1500092), showed that these newly complex organisms built intricate exoskeletons out of calcium, looking like so many Christmas ornaments wrapped in calcified spikes.
The 2012 Nature article describing how this might have worked is Peters and Gaines, “Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion” (DOI: 10.1038/nature10969). I think it makes a lot of sense and fits with the current data.
We’re not sure yet about many pieces of evidence for this story, especially the all-important oxygen levels. Future data will do much to clarify how much oxygen rose and when, especially whether it was before or after the Cambrian explosion. Right now, it’s very close.
I came to this story after weighing all options, including Meyer’s Intelligent Design explanation. But Meyer’s explanation didn’t really explain anything to me. I believe that God made a lawful world and made our minds to understand it. If so, God probably wants us to understand how he created life, and that means God could have worked through the laws of chemistry that still work today. He created the valley that held that path through secondary causes like glaciers, erosion, and chemical weathering. If the same kind of events made the Great Unconformity and the Cambrian explosion, then a story connects these two events. If such a story exists, then it is a story God ultimately caused and wants us to know.
The chemical story to explain the Cambrian Explosion is a more elegant mechanism and, in my option, the better story. It may seem strange to make a scientific argument based on elegance, but physicists do it all the time. The physicist Brian Greene has even written a book titled The Elegant Universe in which he says that good science tells a good story:
We all love a good story. We all love a tantalizing mystery. We all love the underdog pressing onward against seemingly insurmountable odds. We all, in one form or another, are trying to make sense of the world around us. And all of these elements lie at the core of modern physics. – Brian Greene, 2003, p.ix, The Elegant Universe
This chemical story also fits with a theology where God is what’s called the “primary” cause, the cause of all other causes, and chemistry (especially oxygen) is the “secondary” cause, accomplishing God’s purposes like a messenger or faithful subordinate. Rising oxygen is one component of a logical chemical sequence that unfolds from the chemical rules in the periodic table. That’s why the subtitle of A World from Dust is How the Periodic Table Shaped Life.
In this story, God isn’t so much a distant watchmaker as a patient shepherd that guides his flock. (In this case, his flock is all of living creation, guided toward complexity by the multiple powers of oxygen.) In the same way, God worked consistently through the natural environment to guide life along the right path for his name’s sake.
Right now, I’m not certain that this particular chemical story is correct, any more than I was certain that my family could find fossils at the end of that path to the Stanley Glacier. But, starting from the assumption that we live in a world that we can understand, I see clues that the chemical story may be on the right path, like the clues that told my family we were on the right path. This adds an element of suspense to the story, because I’m not sure how it will turn out in the end. But even that uncertainty and suspense improves the story, and makes this path worth walking…
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