For years evolutionary biologists have grappled with the Cambrian explosion, an event around 520 to 530 million years ago where many of the major animal phyla appear abruptly in the fossil record without any apparent direct evolutionary precursors. Stephen Meyer wrote about the Cambrian explosion in his best-selling 2013 book Darwin’s Doubt, and I served as his research assistant during the writing of that book. I remember that just before his book came out, another book by two leading Cambrian paleontologists, Douglas Erwin and James Valentine, was released, aptly titled The Cambrian Explosion. As I recounted here, that book affirmed Meyer’s basic take that the Cambrian explosion was a real event in the history of life and represented the rapid and abrupt appearance of many major animal groups. But what was most interesting was that this book also affirmed that key challenges posed by the Cambrian explosion remain “unresolved” — at least if you’re approaching them from an evolutionary perspective:
The patterns of disparity observed during the Cambrian pose two unresolved questions. First, what evolutionary process produced the gaps between the morphologies of major clades? Second, why have the morphological boundaries of these body plans remained relatively stable over the past half a billion years?
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Taking the Easy Way Out
I always appreciated Erwin and Valentine’s forthrightness about major unexplained aspects of the Cambrian explosion. But many evolution-defenders seek quick and easy answers. They claim that some “trigger” somehow rapidly caused all these diverse animal groups to burst on to the scene. For many evolutionary biologists, that trigger is a sudden rise in atmospheric oxygen just before the Cambrian — and the claimed sudden rise of oxygen is enough to satisfy them that the Cambrian explosion is explained (or at least explained away). As I noted here:
Many claim the Cambrian explosion was triggered by a sudden global increase in oxygen levels. We’ve discussed this many times, observing over and over that oxygen doesn’t generate new genetic information. But such information had to be the proximal cause of the Cambrian explosion.
David Coppedge has done excellent work critiquing what he calls the “Oxygen Theory” over the years, and once put it this way: “Translation: Oxygen didn’t cause the Cambrian explosion. It just gave animals their ‘get up and go.’ What did cause the Cambrian explosion, then? No answer.”
Locking the Trigger
The oxygen “trigger” theory for the Cambrian explosion has now taken another hit. A new paper in Proceedings of the National Academy of Sciences, “Breathing life into the boring billion: Direct constraints from 1.4 Ga fluid inclusions reveal a fair climate and oxygenated atmosphere,” suggests that oxygen might have been quite high long before the Cambrian period, casting doubt on the idea that it played a role as a trigger. The basic story is fairly simple — Gizmodo explains it nicely:
Researchers have retrieved samples of 1.4 billion-year-old air from ancient crystals and found something surprising about a supposedly “boring” time period.
The team studied the gases and fluids locked in halite crystals (rock salt) from Canada, shedding light on the composition of the atmosphere hundreds of millions of years before dinosaurs walked the Earth. It turns out the planet was sporting more oxygen than expected — at least, in that exact moment in time — as they explained in a study published last month in PNAS.
Of course, 1.4 billion years ago is over 800 million years before the Cambrian explosion, which is generally thought to have gotten going in earnest around 530 million years ago. This raises a question: If oxygen is supposedly this powerful trigger that caused animals to evolve, then why did complex animals not evolve when the atmosphere was highly oxygenated at 1.4 Ga? Gizmodo sees the problem:
The atmosphere also had 3.7% of today’s oxygen levels. While this might not seem like a lot, it’s still an unexpectedly high quantity. At the time, life was ruled by bacteria, and red algae was a newcomer, but these oxygen levels could have sustained complex animal life, such as animals and plants, that would not emerge until 800 million years later.
Why so slow, animals?
So why was animal life so slow to appear? “It may reflect a brief, transient oxygenation event in this long era that geologists jokingly call the ‘boring billion,’” Park explained. In other words, the readings reflect a brief moment in time. The “boring billion” is an epoch during which there were low oxygen levels, stable atmospheric and geologic conditions, and little evolutionary shifts.
A news release from Rensselaer Polytechnic Institute makes a similar argument:
One question that naturally arises: if there was enough oxygen to support animal life, why did it take so long to finally evolve?
Park emphasizes that the sample captures just a snapshot of geologic time. “It may reflect a brief, transient oxygenation event in this long era that geologists jokingly call the ‘boring billion,’” he said. It was an epoch of Earth’s history marked by low oxygen levels, widespread atmospheric and geologic stability, and scant evolutionary change.
“Despite its name, having direct observational data from this period is incredibly important because it helps us better understand how complex life arose on the planet, and how our atmosphere came to be what it is today,” Park said.
Previous indirect estimates of carbon dioxide during the period pointed to lower levels incompatible with other observations showing that there were no significant glaciers during the Mesoproterozoic era. The team’s direct measurements of high carbon dioxide levels, combined with temperature estimates from the salt itself, suggest that the Mesoproterozoic climate was milder than previously thought — comparable to today’s.
Schaller notes that red algae arose right around this point in the Earth’s history, and that they remain a significant contributor of global oxygen production today. The relatively high oxygen levels could be a direct consequence of the increasing abundance and complexity of algal life.
“It’s possible that what we captured is actually a very exciting moment smack in the middle of the boring billion,” he said.
Likewise, the technical paper notes:
Our results show this was a period of equable climate and that atmospheric oxygen concentrations, at least transiently, surpassed the metabolic requirements of early animals long before their emergence. … Our results point to an Earth system state characterized by a moderate climate with sufficient O2 for early animal respiration.
Transient Oxygen?
So, the authors argue that maybe this was just a brief “transient oxygenation event” that was not sufficient to give rise to oxygen. I have another theory: Maybe the rise of oxygen is not what caused the Cambrian explosion! Higher oxygen levels may be a necessary, but far from sufficient condition for complex animal life to exist. If this is the case, then oxygen is not the real “trigger” that caused the Cambrian explosion. Oxygen can do whatever it wants throughout earth’s history and animals won’t come into existence until the actual trigger is pulled. Given that (a) thousands of new genes must have arisen to generate the new animal forms that arise in the Cambrian explosion, (b) those new genes represent a huge amount of new genetic information, and (c) information comes from a mind, I think the real trigger behind the Cambrian explosion is something a lot more powerful than merely a higher pO2 (partial pressure of oxygen) level in the atmosphere.
Challenging the Trigger
The technical paper acknowledges that this high level of oxygenation, if sustained, would indeed “challenge the view” that oxygen was a trigger for animal evolution:
Our results confirm atmospheric pO2, at least transiently, exceeded the thresholds necessary for early animal respiration. A short pulse of oxygenation may help explain the lag between the origin and ecological rise of complex eukaryotes. However, if these O2 levels were sustained for hundreds of My, it would challenge the view that rising atmospheric oxygen concentrations drove animal evolution.
But the authors don’t want to challenge the oxygen trigger model, so they opt for the “transiently” high oxygen hypothesis. Perhaps that is indeed correct, but whatever the case, this data point contradicts the model that oxygen was always too low for animal life prior to the Cambrian. That’s bad for the oxygen-trigger model. And most importantly, it doesn’t come anywhere near explaining how oxygen could generate the new genetic information needed for the Cambrian animals to arise.