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.

DrREC argues:

So “it could have been a cell with a nucleus, but lacking protein-coding introns, mitochondria, golgi bodies, ubiquitin, and flagella” is an argument that might apply to “FECA” the first common Eukaryote–which we can’t reasonably infer the details of–but not LECA!

LECA could very well have turned out to be simple.  This is why scientists had to engage in experimental reconstruction to determine the complexity status of LECA.  It was an open question.  The discovery of a complex LECA was just that – a scientific discovery, not some tautological deduction from the principles of common descent.

But here is where DrREC seriously goes astray:

Arguing LECA was simple and lacking all those things leads to the poor hypothesis that introns, spliceosomes, mitochondria, golgi, ubiquitin and flagella evolved multiple independent times in Eukarya-in a homologous manner, having the same features and detectable sequence homology in each lineage!

Wrong.  He must have skimmed over the observations of Keeling and Fast that I originally provided:

However, in 1983, attention was drawn to the possible evolutionary significance of microsporidia in a new way. Cavalier-Smith proposed that the origin of eukaryotes might have preceded the endosymbiotic origin of the mitochondrion by some considerable span of time, implying that there may be protists that evolved before the mitochondrial origin. In other words, there may be primitively amitochondriate eukaryotes, and focusing attention on these protists could unlock some of the secrets surrounding the origin of eukaryotes.

If it turned out that microsporidia were primitive protozoans, it would not mean thatmitochondria…..evolved multiple independent times in Eukarya-in a homologous manner, having the same features and detectable sequence homology in each lineage!”  It would simply mean that microspordia represent a lineage that split off the branch that later acquired mitochondria.  That was precisely the hypothesis behind the Archezoa classification.

And guess what?  This same logic would apply to flagella, golgi bodies, etc.  This is why I am entirely correct in noting that LECA could have turned out to be 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.

In fact, consider how Patrick Keeling described the situation back in 1998:

The taxon Archezoa was proposed to unite a group of very odd eukaryotes that lack many of the characteristics classically associated with nucleated cells, in particular the mitochondrion. The hypothesis was that these cells diverged from other eukaryotes before these characters ever evolved, and therefore they represent ancient and primitive eukaryotic lineages. The kingdom comprised four groups: Metamonada, Microsporidia, Parabasalia, and Archamoebae. [1]

See?  Metamonada, Microsporidia, Parabasalia, and Archamoebae would have still shared a common ancestor with plants and animals.  They would simply have split off the lineage that led to plants and animals earlier on and thus “represent ancient and primitive eukaryotic lineages” closer to LECA.

Or consider how Poole and Penny explain it:

In the mid-1990s, a somewhat pedestrian view of eukaryotic origins, the ‘archezoa hypothesis’, held sway. This maintained that a protoeukaryote (with nucleus) engulfed the mitochondrial ancestor. Supporting the theory were ‘archezoa’, anaerobic eukaryotes with no mitochondria. Archezoa apparently populated the oldest branches of the eukaryote tree, suggesting that eukaryotes began diversifying before mitochondria entered the picture.

The archezoa hypothesis is thus composed of two independent hypotheses: (a) that a protoeukaryote host (PEH) engulfed the mitochondrial ancestor, and (b) that modern archezoa are ‘missing links’ that never possessed mitochondria. Hypothesis (b) is now unanimously rejected: every archezoan examined bears vestigial mitochondria, or genes inherited from mitochondria. Thus, all modern eukaryotes evolved from a mitochondrionbearing ancestor

If LECA was roughly as complex as extant eukaryotes, there would be no basis for thinking of the ‘archezoa’ as ‘missing links.’  They were considered ‘missing links’ because they were supposed to link a simple LECA to a complex extant eukaryotic state. They were supposed to reflect cellular states that existed prior to the acquisition of much of that eukaryotic complexity.

So in conclusion, the complex state of LECA is a genuine scientific discovery and cannot be dismissed as some trivial tautology.  Science could have discovered a simple LECA, but instead uncovered

that the characteristic eukaryotic complexity arose almost ‘ready made’, without any intermediate grades seen between the prokaryotic and eukaryotic levels of organization


The results of all these reconstructions consistently point to a complex LECA, in terms of both the sheer number of ancestral genes and, perhaps even more importantly, the ancestral presence of the signature functional systems of the eukaryotic cell  [2]

1. Keeling, PJ. 1998. A kingdom’s progress: Archezoa and the origin of eukaryotes.  Bioessays 20:87-95.


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