Today’s New Reason To Believe

No Good Options for the Origin of Life

The hall-of-fame catcher Yogi Berra is reputed to have said, “When you come to a fork in the road, take it.” Origin-of-life researchers have been following his advice for years as they search for an evolutionary explanation for life’s origin.

There are two fundamental approaches to explain life’s beginning from an evolutionary standpoint: (1) replicator-first; and (2) metabolism-first scenarios.

Chemist Robert Shapiro argues in the cover article of the June 2007 issue of Scientific American that the replicator-first approach to the origin-of-life is a failed paradigm. From his vantage point, metabolism-first scenarios offer the best hope to explain the origin of life.

This week I would like to explain how Shapiro reaches this conclusion. Next week I will describe Shapiro’s proposal for life’s origin and point out some of the chemical difficulties with metabolism-first models.

Evolutionary origin-of-life models require pathways that ultimately generate two of life’s defining biochemical features: self-replication and metabolism. 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.

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, cell membrane, and cell wall components. Life’s metabolic pathways often share many molecules. This sharing causes the cell’s metabolic routes to interconnect to form complex, reticulated webs of chemical pathways.

Replicator-First Scenarios

Most origin-of-life researchers maintain that the first step toward a living entity took place when a self-replicating molecule emerged. Only later did this naked self-replicator become encapsulated within a primitive membrane. According to this view, after encapsulation, metabolism emerged as a means to support the production of the self-replicator by providing the necessary building block molecules to sustain its activity.

Origin-of-life researchers have proposed a number of possible candidates for the original self-replicator. But Shapiro has pointed out that the identity of the first self-replicator doesn’t matter. Why? All replicator-first scenarios suffer from a fatal flaw known as the homopolymer problem. Let's observe.

Candidates for the first self-replicating molecule possess common chemical features. All potential self-replicators are relatively complex molecules made up of smaller chemical subunits that link up to form chain-like molecules. The side groups that extend from the self-replicator’s backbone must be chemically and physically varied to provide the physicochemical information necessary to carry out the self-replication process. However, the self-replicator’s backbone must mundanely consist of a repetitious structure.

To function as a self-replicator, a molecule must serve as a template to direct the assembly of subunit molecules into an identical copy of itself. Self-templating, and hence, self-replication is possible only if the backbone’s structure repeats with little, if any, interruption. This means that the subunit molecules that form the self-replicator must consist of the same chemical class.

Chemists call chain-like molecules with structurally repetitive backbones homopolymers. (Homo = same; poly = many; mer = units). DNA, RNA, proteins, and the proposed pre-RNA world self-replicators, such as peptide-nucleic acids, are all homopolymers and satisfy the chemical requirements necessary to function as self-replicators.

Shapiro has pointed out that while undirected chemical processes can produce homopolymers under carefully controlled, pristine laboratory conditions, they cannot generate these types of molecules under early Earth’s conditions. The chemical compounds found in the complex chemical mixture that origin-of-life researchers think existed on early Earth would interfere with homopolymer formation. Instead, polymers with highly heterogeneous backbone structures would be produced. And these molecular entities could not function as self-replicators. The likely chemical components of any prebiotic soup would not only interrupt the structural regularity of the self-replicator’s backbone, but they would also prematurely terminate its formation or introduce branch sites.

The homopolymer problem is fundamentally intractable, devastating all replicator-first models. The only remaining option is to explain life’s origin via a metabolism-first scenario. And this approach has troubles of its own as I will discuss next week.

For a detailed discussion of problems with evolutionary models for the origin of life, see the book I wrote with Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off.

Last month’s multiverse musings delineated some philosophical objections to the existence of actual infinities. However, an extremely large but spatially finite universe could still negate the significance of the fine-tuning arguments used in Christian apologetics. Today, I want to address an email question particularly pertinent to this issue. The question is:

How do scientists measure the total density of the universe, and also the split between conventional matter, dark matter, and dark energy?

The importance of this question relates to the fact that the only hard experimental evidence currently weighing in on the size of the universe is the geometry of the universe. The WMAP image of the cosmic microwave background (CMB) radiation provides the most potent tool for measuring the geometry although the large-scale structure also contributes significantly. In a nutshell, a closed universe cannot be spatially infinite.

The total density of the universe, Ω_total, defines the universe’s geometry. For an open universe, Ω_total < 1 whereas a closed universe has Ω_total > 1. Ω_total = 1 means the universe is flat. Additionally, a measured value of Ω_total > 1 also constrains the size of the universe (not just our observable universe). The best measurements of Ω_total indicate a closed universe but are consistent with a flat or open universe within the error bars.

To get from the all-sky WMAP temperature measurements to a measurement of the cosmological parameters involves three steps:

1. Transform the CMB sky map into a set of numbers that characterize the fluctuations.
2. Run simulations to see how changing the various cosmological parameters affects the numbers characterizing the fluctuations.
3. Find the parameters that best fit the data.

The first step utilizes the common mathematical tool of spherical harmonics. Basically, you convert this into a sum of these , then you determine the amount of power at each harmonic and make a graph like this.

The next step is where all the physics enters. The CMB photons were all emitted at a common time (when the universe cooled enough for nuclei and electrons to combine and form atoms) roughly 380,000 years after the big bang. The temperature fluctuations measured by WMAP record a snapshot of the physical scale and gravitational distribution of the universe’s density. Multiple factors affect this distribution, such as the total density, the number of baryons, the relative amounts of the components making up the total density, and the spectrum of the initial density fluctuations. Using simulations, cosmologists can form pictures of the CMB fluctuations assuming different cosmological parameters. For a more detailed description of how these processes interact, see Wayne Hu’s tutorial.

The final step involves comparing the simulated CMB distribution of harmonics with the distribution calculated from the WMAP data. Max Tegmark’s CMB movies provide nice illustrations of how changing the various cosmological parameters would affect the measured distribution of harmonics. Once the best fit to the data is found, you have your measurement of Ω_total (and a number of other parameters). The current best values for some relevant parameters are:

• Ω_total = 1.02 +/- 0.02
• Normal matter = 4%, Exotic dark matter = 22%, and Dark energy = 74%.
• Spectral index n_s = 0.95 +/- 0.17 (inflation predicts a value less than one).

So now you know how to convert the WMAP data into cosmological parameters.

A student in the logic class I was teaching said that the Trinity doctrine was contradictory. He argued the following: Since the Father is God, the Son is God, and the Holy Spirit is God; and since the Father is not the Son, the Father is not the Holy Spirit, and the Son is not the Holy Spirit; then the result is that each person is simultaneously God and not God. This is, he reasoned, a violation of the law of noncontradiction (A cannot equal A and equal non-A).

This evaluation of the Trinitarian formulation is also a [straw-man](http://en.wikipedia.org/wiki/Straw_man argument) (misrepresentation), for again it fails to recognize the essence/subsistence distinction. The members of the Trinity all share equally the one divine nature and are thus the one God. However, the relational (personal) distinctions in the Godhead do not in any way subtract from each individual person’s possession of the divine nature. Thus the three persons are distinct from each other, but they nevertheless remain fully and equally God.

How one Being can simultaneously be three persons is an unfathomable mystery, but it is not a formal contradiction. It is not contradictory to predicate deity to all three members of the Trinity, while simultaneously asserting that they possess distinct personal identities: Father, Son, Holy Spirit.

The Christian view of God is different from other forms of monotheism in asserting that God is superpersonal (more than merely personal).

For more on the historic Christian doctrine of the Trinity, see “How Can God Be Three and One?” in Kenneth Samples’ book Without a Doubt: Answering the 20 Toughest Faith Questions.