Today’s New Reason To Believe

Posted by Fazale ‘Fuz’ Rana, Ph.D.

Metabolic Cycles on Early Earth Deemed Implausible by Famous Origin-of-Life Researcher

When someone speaks to you from beyond the grave, you better pay attention. A paper, submitted posthumously on behalf of the late origin-of-life researcher Leslie Orgel, has just been published in the journal PLoS Biology. And Orgel has a message for origin-of-life researchers: metabolic cycles, and consequently, metabolism-first scenarios are unrealistic. This wake-up call causes a grave concern for evolutionary explanations of life’s beginnings.

Evolutionary origin-of-life models require pathways that ultimately generate two of life’s defining biochemical features: self-replication and metabolism. As such, there are two fundamental approaches to explain life’s beginning from an evolutionary standpoint: (1) replicator-first, and (2) metabolism-first scenarios.

From a molecular standpoint, self-replication describes the capacity of a complex molecule to guide its own reproduction, typically by serving as a template that directs the assembly of chemical constituents into molecules identical to it self.

DNA is a self-replicating molecule. DNA not only orchestrates its own reproduction, but also houses the information needed to carry out the cell’s operation. Prior to cell division, the cell’s biochemical machinery generates two identical DNA molecules from the “parent” DNA. These two molecules become partitioned into the daughter cells during the cell division process. In this way, the information needed to operate the cell is passed on to the next generation.

Metabolism defines the entire set of chemical pathways in the cell. Foremost are the ones that chemically transform relatively small molecules. Metabolic pathways: (1) generate chemical energy through the controlled breakdown of fuel molecules like sugars and fats; and (2) produce, in a stepwise fashion, the building blocks needed to assemble proteins, DNA, RNA, and cell membrane and cell wall components. Life’s metabolic pathways often share many molecules. The shared molecules serve as connection points, causing the cell’s metabolic routes to interconnect into complex, reticulated webs of chemical pathways.

Replicator-first scenarios face significant, perhaps insurmountable hurdles. For a detailed discussion of problems with replicator-first explanations for the origin of life see the book I wrote with Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off. Also see an earlier article I wrote for the Today’s New Reason To Believe feature.

In the face of the seemingly intractable difficulties with replicator-first scenarios, some origin-of-life researchers postulate that once prebiotic materials formed, these relatively small molecules self-organized to form chemical cycles and networks of chemical reactions that over time gave rise to life’s metabolic systems. Once encapsulated or sequestered within a membrane, these complex, reticulated systems of reactions became the first prebionts.

According to this view, molecular self-replicators emerged later along with the enzymes that catalyzed each step in the chemical cycles and networks. Some proponents of metabolism-first scenarios maintain that these cycles and networks closely resembled the metabolic pathways found in the cell today. In other words, “metabolism recapitulates biogenesis.” Metabolism-first adherents suggest that either: (1) individual chemical species that were part of these cycles and networks catalyzed these same reactions—a type of autocatalysis; or (2) that mineral surfaces catalyzed the protometabolic pathways.

(I also critiqued metabolism-first scenarios in a past TNRTB article).

Leslie Orgel was critical of metabolism-first ideas and produced a manuscript before his death detailing some of his concerns.

Acknowledging the importance of metabolism-first ideas for the origin-of-life question, Orgel notes,

“If complex cycles analogous to metabolic cycles could have operated on primitive Earth, before the appearance of enzymes or other informational polymers, many of the obstacles to the construction of a plausible scenario for the origin of life would disappear.”

Still, metabolism-first models can only be viewed as truly valid if they are chemically plausible. According to Orgel, chemical plausibility must be assessed based on the efficiencies and specificities of the proto-metabolic cycles in the context of the conditions of the early Earth. Orgel illustrates the importance of these criteria for assessing the likelihood of metabolism-first scenarios by applying them to the reverse citric acid cycle.

Certain bacteria utilize the reverse citric acid cycle to fix carbon, converting carbon dioxide and water into organic compounds. Some origin-of-life researchers have proposed that the reverse citric acid cycle was one of the first metabolic pathways to emerge and that its genesis pre-dated the origin of information-based molecules like RNA (and proteins and DNA).

Orgel concludes that the conditions of the early Earth permit the reverse citric acid cycle to operate sufficiently provided that it is stable for long periods of time and that disruptive side reactions don’t reduce the overall efficiency of the cycle below fifty percent. On the other hand, the reverse citric acid cycle (in fact, all proto-metabolic cycles) appears to be implausible on early Earth because the catalysts needed to drive the cycle lack the necessary specificity.

The reverse citric acid cycle consists of eleven steps, each one requiring a specific mineral catalyst on the early Earth. The cycle also depends on six fundamentally distinct chemical transformations. Inside cells, this metabolic process employs complex enzyme catalysts possessing high specificities and capacities for molecular-level discrimination among the components of the cycle. Orgel rightly argues that it’s not likely that the right types of minerals needed to carry out these reactions would coexist at a particular locale on the early Earth in such a way to support the reverse citric acid cycle.

The specificity problem becomes exacerbated if the reverse citric acid cycle is to evolve toward greater complexity, a requirement if life is to originate from a proto-metabolic cycle. Presumably, evolution of complexity results when additional reaction sequences are appended onto the core reactions of the cycle. According to Orgel,

“Given the difficulty of finding an ensemble of catalysts that are sufficiently specific to enable the original cycle, it is hard to see how one could hope to find an ensemble capable of enabling two or more.”

Discrimination, or more appropriately, the lack of discrimination is also a problem. Many of the compounds in the reverse citric acid cycle share structural similarities. Enzymes inside of cells can readily distinguish these similar compounds, but mineral catalysts can’t. This lack of specificity will cause key components of the cycle to be siphoned off into unwanted disruptive side reactions. Over time, this disruption will more than likely drive the efficiency of the cycle below fifty percent, thus quenching it.

The problems identified for the reverse citric acid cycle apply to any conceivable proto-metabolic cycle on the early Earth.

Consequently, even though metabolism-first scenarios are becoming more prominent within the evolutionary paradigm for the origin-of-life, they ultimately provide little hope to explain the emergence of life from inanimate systems. Orgel’s last words to the origin-of-life community admonish both metabolism-first and replicator-first adherents,

“solutions offered by supporters of geneticist or metabolist scenarios that are dependent on ‘if pigs could fly’ hypothetical chemistry are unlikely to help.”

by Jeff Zweerink

Last Christmas, my family flew back to the midwest in a Boeing 737. Getting an airplane to stay up in the air requires a tremendous amount of design, but most of the flight delivers all the excitement of a long bus ride. The action usually occurs during the takeoff (and to some extent, the landing).

New geophysical discoveries show similar action for the tectonic history of Earth. Plate tectonics requires three essential processes to operate. First, rigid continental and oceanic plates must float and move around on the more fluid upper mantle below. Second, hot magma from the mantle must well-up through cracks that form as the plates move apart. Third, where the plates collide with one another, one of the plates must move below the other, back into the mantle, in a process called subduction. Scientists largely understand the mechanisms of the first two steps as well as how subduction functions. However, the initiation of subduction remains more enigmatic.

As described in Science, recent research demonstrates that Earth’s tectonic activity fluctuated significantly in the past because the subduction zones shut down (as a result of continental plates coming into contact). In particular, the time period when the Rodinia “supercontinent” formed corresponds to a dramatic decrease in subduction. (On a side note, when subduction reinitiated leading to the break-up of Rodinia, Earth experienced a glaciation which covered almost the entire globe.) Most of the current subduction zones reside in the Pacific Ocean. Consequently, the closing of the Pacific basin in roughly 350 million years will bring another period of tectonic inactivity.

As continental plates collide, subduction inevitably terminates. However, such a process does not necessarily lead to subduction initiating somewhere else. For example, the Indian and African continents collided with Eurasia 35-50 million years ago, shutting down a subduction zone (this collision caused the formation and continued growth of the Himalayas—home to Mount Everest). Yet since the collision, no new subduction zones have formed in the region nor does it appear that any will.

Most scientists assumed that plate tectonics operated slowly and continuously over the bulk of Earth’s history. But new scientific results argue for an “on-again/off-again” tectonic past. Because of the importance of subduction initiation for plate tectonics activity, RTB’s creation model predicts that scientists will find substantial fine-tuning as they better understand this critical process.

(As an added bonus, a more sporadic nature of Earth’s plate tectonic activity also solves another long-standing geological problem. Scientists measure the tectonic activity and the rate at which heat escapes from Earth today. Extrapolating those two quantities just two billion years in the past leads to an unacceptably high interior temperature for Earth. However, if tectonic activity experienced periods of stagnation, heat would escape more slowly in the past compared to extrapolations from the measured values assuming continuous plate tectonics. Extrapolations incorporating intermittent tectonic activity give reasonable temperatures all the way back to Earth’s formation.)

Going back to the plane analogy, sustaining flight necessitates a high degree of fine-tuning on the part of the pilots and the engineers of the aircraft. Air travel requires an even greater amount of fine-tuning because planes must take off and land—without a successful takeoff, no one can go home for the holidays. Similarly, without subduction initiation the life-essential process of plate tectonics would never take off on Earth.

Kenneth Richard Samples

In the first installment of this series I wrote about the phenomenon known as the “new atheism.” In 2007, four books advocating atheism (the view that no God or gods exist) made the bestseller list. These four horsemen of atheism include Richard Dawkins (The God Delusion), Sam Harris (Letter to a Christian Nation), Christopher Hitchens (God Is Not Great), and Daniel Dennett (Breaking the Spell).

In part one I listed two of five reasons why these new “in-your-face” atheists are so zealous for their worldview. I mentioned that while traditional atheists have always viewed religion as being illusory, the rise of radical Islam and the perceived mixing of religion and politics, among other things, have influenced new atheists to now view religion as dangerous.

In this article I will explore a third factor that has contributed to the emergence of this more strident type of atheism.

3. Reaction to the Intelligent Design Movement:

In the last decade or so a new movement has appeared in the marketplace of ideas that has sought to challenge the atheistic, naturalistic, Darwinian paradigm concerning the origin and development of life on planet Earth. This new movement is known as Intelligent Design (ID). It advocates the “scientific” view that the complexity of life on this planet is best explained in terms of an intelligent causal agent. Some of the leaders of this crusade include Phillip Johnson, Michael Behe, and William Dembski. The efforts of these leaders and others to make ID a part of public school science curriculums has been hotly contested in courts throughout the country.

The new atheists, by and large, view ID as religion, not science. They also equate ID with young-earth creationism (YEC), the scientifically inaccurate view that God created the world by divine fiat some six to ten thousand years ago. Put off by YEC, new atheists feel that the whole ID campaign has invaded their turf by advocating religion under the guise of science. Worse still, new atheists believe that the advocates of ID resort to politics to win a battle that can’t be won through objective scientific analysis.

Unfortunately, amidst the media firestorms and controversies, the various differences amongst Christian opinions on the origin of the universe (such as old earth creationism and theistic evolution) often get overlooked. Alternative models that allow and invite scientific testing to determine the credibility of their explanations (like RTB’s creation model) end up neglected or ignored.

Additionally, new atheists contribute to the struggle between Darwinism and ID by dogmatically guarding evolution as irrefutable fact. (Frankly, physicists seem far less self-conscious about criticizing Einstein’s theories than biologists in their treatment of Darwin’s theory.) At its purest level, the scientific enterprise ought to be a dispassionate and objective pursuit of truth about the natural world, submitting all theories and models to continuous and exhaustive testing.

Yet it seems that some within the secular scientific community have decided to permit only naturalistic explanations and anyone who challenges this position will not be granted a place at the table of scientific inquiry. Many atheists appear to hold on to the theory of evolution with a seemingly religious fervor, as illustrated by comments made by Dawkins:

“It is absolutely safe to say that, if you meet somebody who claims not to believe in evolution, that person is ignorant, stupid or insane (or wicked, but I’d rather not consider that).”

If the atheists are right about religion being irrational and illusory, then it would seem that creationism would be dealt a greater defeat if secular scientists would allow its responsible advocates a place at the table and then dispassionately falsify its claims.

Also, the Christian worldview would likely be viewed with more legitimacy if science was perceived as being of great value to believers. Christians trained in the natural sciences can and should work hard to earn a rightful place at the table of scientific inquiry.

In part three of this series I will briefly explore two other issues that I think are helpful in understanding the emergence of the so-called new atheism.

For more on the Christian worldview and how it relates to the atheistic worldview of naturalism, see my new book A World of Difference: Putting Christian Truth-Claims to the Worldview Test.