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Sorry, Faster Mutation Rate Will Not Explain the Cambrian Explosion

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Strange ideas are percolating at the University of North Carolina at Chapel Hill. Certain biophysicists think that hot vents and boiling pools accelerated the emergence and complexity of life. Why? The hotter the water, the more the mutations. When Earth boiled soon after its birth, there were more opportunities for mistakes in the genetic code to come up with new things. As the temperature cooled over 4 billion years to modern levels in the 70-degree range, there was a “precipitous decline in spontaneous mutation” during Earth’s history, leading to fewer opportunities for evolution to advance. Read all about it in the Proceedings of the National Academy of Sciences.

This is a new one on us. Intrigued by news from UNC, we wondered if the idea predicts a linear relationship between heat and evolution.

Early life forms on Earth are likely to have mutated and evolved at much higher rates than they do today, suggests a new analysis from researchers at the University of North Carolina.

In a study published this week in the Proceedings of the National Academy of Sciences, Richard Wolfenden, PhD, and his colleagues found that the rate of a certain chemical change in DNA – a key driver of organisms’ spontaneous mutation rates and thus of evolution’s pace — increases extremely rapidly with temperature. Combining that finding with recent evidence that life arose when our planet was much warmer than it is now, the scientists concluded that the rate of spontaneous mutation was at least 4,000 times higher than it is today.

Think of all the things Darwin could do with four thousand times more mistakes! Is that the gist of this idea? Natural selection must have been gaming the casinos at a record pace.

“At the higher temperatures that seem to have prevailed during the early phase of life, evolution was shaking the dice frantically,” said Wolfenden, Alumni Distinguished Professor of Biochemistry and Biophysics at the UNC School of Medicine.

A much faster pace of evolution means that species could have proliferated much more rapidly than they do now, affording the flora and fauna of Earth ample time to acquire their enormous diversity and complexity.

Although they do not mention any geological eras, it appears that they are offering a new explanation for the Cambrian explosion: turn up the heat! Not only did it help the Cambrian animals emerge, it helped at the origin of life. Life appeared as early as 4.1 billion years ago, Wolfenden asserts, “almost in the blink of an eye after the appearance of liquid oceans.” That can only mean one thing. Heat is the engine of evolution.

Where did this idea come from? If you read the news item with a skeptic’s eye, you sense a hint of frustration with the status quo in Darwin circles. Those ID advocates are turning up some heat of their own.

That issue — whether life could have evolved to its present level of complexity within the time available — has lingered ever since Darwin published his theory more than a century and a half ago. Throughout that debate, both skeptics and proponents of evolutionary theory have often assumed that evolution’s pace has stayed more or less constant over the eons.

The pressure is on to speed up evolution. UNC is simply trying to help Darwin. Well, we can make a few predictions based on their hypothesis that heat drives evolution.

  1. Life will never emerge at Titan, Mars, or Enceladus, because it’s too cold.

  2. Thermophiles should be the fastest-evolving organisms today, while arctic species should be frozen in place.

  3. Evolutionary innovations should trend downward over the age of the Earth.

Let’s see if the paper in PNAS addresses any of these predictions.

First, we can’t find anything about exoplanets or life on other worlds. There’s just this statement: “There is widespread, if not universal, agreement that life originated when the earth was warmer — perhaps much warmer — than it is today.” Evidence please? Just that life seems to go back 4.1 billion years when consensus theory demands the Earth was boiling hot. Many geophysicists, however, acknowledge that the sun was much cooler then, and quite a few consider that life originated during a “snowball Earth” phase.

Second, there’s nothing in the paper about thermophiles evolving faster or cold species evolving slower. It would seem that if heat-induced mutations speed up evolution, you could see that trend in operation today.

Third, do they really think that heat increases biological progress? You can’t find anything in the paper about innovation, complexity, or information resulting from hot temperatures. In fact, the authors readily acknowledge that without DNA repair mechanisms, life would be sweating bullets.

To appreciate the burden borne by systems for nucleic acid repair in thermophiles, and the problems that may have been faced by primordial organisms before the advent of sophisticated systems for DNA repair, it would be desirable to have accurate information about the rate at which cytosine deamination and related reactions proceed at elevated temperatures.

Cytosine deamination is the one main empirical detail in the paper. They show that heat increases the rate at which cytosine, the “C” in the DNA code, loses its ammonia-like amine group of atoms. The authors rely on the fact that “occurs from time to time in all cells and may be the single most frequent cause of spontaneous DNA mutations.” So if mutations are the raw material for evolution, more must be good, right? And if heat speeds up chemical reactions, the hotter the early Earth, all the better! Here’s the concluding paragraph:

Recent work has uncovered the startling extent to which slow reactions are accelerated by elevated temperatures. This acceleration would have collapsed, by many orders of magnitude, the time that would have been required for chemical events on a warm earth before the advent of enzymes. It has also been shown that a primitive enzyme, if it enhanced the rate of a reaction by lowering its enthalpy of activation, would have produced a rate enhancement that increased — automatically — as the environment cooled. Simple catalysts have been shown to exhibit that behavior in aqueous solution, as do most present-day enzymes. That scenario for enzyme evolution does not take into account improvements arising by natural selection. The present findings confirm and extend earlier evidence that the chemical events that lead to spontaneous mutation are highly sensitive to temperature, furnishing a mechanism for accelerating evolution during Earth’s early history. Later — at lower temperatures — DNA would have become a much more stable repository for genetic information.

We would like to charitably pre-empt further vain speculation on this hypothesis by offering the authors a copy of Darwin’s Doubt. It provides prima facie evidence that the Cambrian explosion was a relatively cool event in which radically new body plans appeared — not from heat or the rise of oxygen — but from new genetic information. It’s unlikely that heat, cytosine deamination, or any other mutagen was responsible for eyes, guts, or articulated limbs controlled by central nervous systems. If they want to insist that mistakes create information, they are certainly welcome to boil living cells and see if they grow new body plans, or to turn off the air conditioning in their computer rooms to see if new integrated systems emerge.

Such a desperate attempt would never catch the respect of the National Academy of Sciences unless Darwin is really in trouble. Keep the heat on.

Photo: Iceland geyser via Pixabay.

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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.



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