Evolutionary Thought Experiment

Let me provide you with a little evolutionary thought experiment.  Stephen J. Gould once noted that “evolution is a bush, not a ladder.”  Quite true.  Consider the following representation:

 

Notice that the evolution of mammals does not entail a straight shot from some ancestral chordate to mammal.  On the contrary, the evolution of mammals also involves the evolution of sharks, bony fish, frogs, snakes, and birds along the way.  What’s more, the bush shows a nesting pattern, where the tetrapods, for example, nest together to the exclusion of all other chordates.  This is because the tetrapods derive from an ancestral tetrapod state that was not shared by the other chordates.  In fact, the following arrangement makes this more clear.

First questions about LECA

We have seen that science has discovered the last eukaryotic common ancestor was essentially as complex as a modern day eukaryotic cell (see here and here and here).  Furthermore, I have argued that this complex cell plan that has defined eukarya since the time of LECA has worked to facilitate the eventual emergence of metaozoan-type complexity.

So perhaps it is time to begin contemplating the origin of this complexity.

Electric Faces

From here:

For the first time, Tufts University biologists have reported that bioelectrical signals are necessary for normal head and facial formation in an organism and have captured that process in a time-lapse video that reveals never-before-seen patterns of visible bioelectrical signals outlining where eyes, nose, mouth, and other features will appear in an embryonic tadpole.

Genetic Switch for Limbs and Digits Found in Primitive Fish

From here:

Genetic instructions for developing limbs and digits were present in primitive fish millions of years before their descendants first crawled on to land, researchers have discovered.

Genetic switches control the timing and location of gene activity. When a particular switch taken from fish DNA is placed into mouse embryos, the segment can activate genes in the developing limb region of embryos, University of Chicago researchers report in Proceedings of the National Academy of Sciences. The successful swap suggests that the recipe for limb development is conserved in species separated by 400 million years of evolution.

“The genetic switches that drive the expression of genes in the digits of mice are not only present in fish, but the fish sequence can actually activate the expression in mice,” said Igor Schneider, PhD, postdoctoral researcher in the Department of Organismal Biology and Anatomy at the University of Chicago and lead author on the paper.

Google Mystery

What does it mean?

Scientific discovery, not tautology

I previously showed that the scientific discovery of a complex LECA was not a tautology.  DrREC replied:

Mike Gene, do you recognize the difference between FIRST and LAST?

L as in LUCA or LECA is LAST-The Last Eukaryotic Common Ancestor, the most RECENT (not oldest) organism from which all organisms/all Eukaryotic organisms (respectively) living on Earth descend.

Not the First! Do you understand LECA isn’t the first Eukaryote?

This is a very strange line of questioning given that nowhere did I argue that the last eukaryotic common ancestor was the first Eukaryote.  Nor do I think so.  Thus, DrREC’s first objection amounts to shadow boxing with the straw man he invented.

But it gets worse.

Complex LECA is no tautology

Someone with the moniker DrREC replied to my posting about the complexity of the last eukaryotic ancestor as follows:

This is almost a tautology. The last Eukaryotic common ancestor had the defining features of a Eukaryote….which happen to be more complex than prokaryotic life.

There is no tautology at work here.  Not even close.  We can appreciate this by simply recognizing that scientists could very well have discovered that LECA was remarkably simple.  For example, it could have been a cell with a nucleus, but lacking protein-coding introns, mitochondria, golgi bodies, ubiquitin, and flagella.  And its nuclear pore complex, cytoskeleton, and endomembranous system could have been rather simple.  But as it turned out, LECA had a level of complexity that rivals modern day cells.

Of course, we don’t need to be hypothetical about this.  Back in the 1980s, biologists expected LECA to have been rather simple.  Consider the simplest of eukaryotic cells – microsporidia.

Koonin and LECA

Below the fold you will find some excerpts from Eugene Koonin’s article, The origin and early evolution of eukaryotes in the light of phylogenomics.

 

Oh my. LECA was really complex.

Earlier I showed you that the last eukaryotic common ancestor (LECA) was quite modern-like in terms of its nuclear pore complex, mechanisms of transport through this complex, and the entire endomembranous system.  Yet the modern-like features do not stop there.