LECA Front-loaded Metazoa?

In the previous posting, we saw that the last common ancestor of all eukaryotic organisms was quite modern-like in terms of its complexity.  Doubt me?  Well, here is Figure 3 – Major transitions in evolution of the endomembrane system – from Evolution of the eukaryotic membrane-trafficking system: origin, tempo and mode).  Have a look:

Ancestral Eukaryote with modern-like complexity

Recall that the eukaryotic cell plan is needlessly complex.

Recall the evidence suggests this needless complexity was essential for the emergence of metazoan-type existence.

And it looks like the key feature that facilitated the emergence of metazoan-type complexity is the nucleus (see here and here).

Of course, the nucleus, even without the chromosomes within, is a very complex and sophisticated structure.  Yet just how old is this complexity?

Another front-loading prediction

In the previous posting, the researchers noted that “NRs appear to be a metazoan innovation, because they are absent from the genomes of choanoflagellates, fungi, plants, and prokaryotes.”

Yet the hypothesis of front-loading would predict these nuclear receptors that play key roles in animal development and homeostasis would exist in some single-celled organisms.  So I would argue that this observation of the NRs as a metazoan innovation is premature.  That is, from the perspective of front-loading, I predict that NRs will be found to predate the appearance of metazoa and be found in some unicellular organisms.

Front-loading just became even more plausible

A recent research article nicely illustrates how front-loading is an increasingly plausible perspective on evolution.  The article is Protein Evolution by Molecular Tinkering: Diversification of the Nuclear Receptor Superfamily from a Ligand-Dependent Ancestorby Bridgham et al.

The researchers analyzed the superfamily of nuclear receptor transcription factors.  As they note, these proteins represent “a diverse superfamily of transcriptional regulators that play key roles in animal development, physiology, and reproduction.”  I briefly described one such receptor earlier – the retinoic acid receptor – as it appears the Alu elements are front-loaded to generate DNA binding sites for these proteins.

This class of receptors share common features:

a modular domain structure, including a highly conserved DNA-binding domain (DBD) and a moderately conserved ligand-binding domain (LBD)—which in most receptors contains a ligand-regulated transcriptional activation function—along with extremely variable hinge and N-terminal domains.

To recognize why this study is relevant to front-loading, pay close attention to this paragraph.

Myc-n-max

First, consider this abstract:

Annu Rev Cell Dev Biol. 2000;16:653-99. The Myc/Max/Mad network and the transcriptional control of cell behavior. Grandori C, Cowley SM, James LP, Eisenman RN.

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.

Keeping in mind that front-loading allows us to predict that major evolutionary transitions would be indebted to preadaptations, consider this recent finding:

Designing Evolution

A version of social engineering known as “nudging” has many parallels with front-loading evolution. Key to the concept of nudging is the “choice architecture”:

Amazon.com: What is “choice architecture” and how does it affect the average person’s daily life?

Thaler and Sunstein: Choice architecture is the context in which you make your choice. Suppose you go into a cafeteria. What do you see first, the salad bar or the burger and fries stand? Where’s the chocolate cake? Where’s the fruit? These features influence what you will choose to eat, so the person who decides how to display the food is the choice architect of the cafeteria. All of our choices are similarly influenced by choice architects.

Let’s rephrase this. A choice architecture is a context that has been set up to favor certain choices or outcomes. The choice architecture is the design and it is this design that influences the choices or outcomes later in time.

At this point, we simply ask whether choice architecture/context must necessarily be environmental?

Consider human choice. Do you accept the idea that a person’s genetics can predispose him to make certain choices? There is plenty of evidence that says it does. So the context that influences choice does not necessarily need to be environmental. The choice can be influenced by both environmental and genetic contexts. In principle then, the choice architecture behind the nudge can be internal/genetic.

So if we posit that the original life forms were designed, instead of viewing the composition and architecture of life as something that natural forces cannot possibly account for, consider the more tantalizing possibility that the composition and architecture of the first cells as representing a choice architecture designed to influence/nudge the “choices” made during subsequent evolution. The manner in which the various pieces and parts of life were hooked up would represent the architecture of life and this, in turn, would amount to a logic that would help guide and facilitate subsequent evolution. The actual pieces and parts of life would represent the composition of life and this, in turn, would amount to various preadaptations that would favor certain evolutionary trajectories over others.

And if you think about it, nudging from an internal state has a huge advantage over nudging from an environmental state. The internal state of every organism is faithfully replicated and thus travels through time. Reproduction is not merely a brute given that lies at the bottom of biological reality, but is a mechanism for design, and nudging, to persist over time and influence the future from the past.

When is a Flaw not a Flaw?

As you know, I have often pointed to the proteins as an incredible design material, raising the question as to just how much the blind watchmaker is dependent and indebted to protein biochemistry.  A recent study highlights yet another way in which protein biochemistry may be an essential prerequisite for the evolution of complex life.

First, you can read the summary of this research by Ariel Fernández and Michael Lynch here.  Then, come back and I will help you detect the teleological echo of this work.