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:
Click to enlarge
Go ahead and compare the early eukaryotic cell to the extant eukaryote. See any major differences? Nope. The extant eukaryote is tweaked to add some more endocytotic pathways, but that’s about it.
Now, I think it is fair to argue that the early eukaryote could represent an example of Original Mature Design. It is an original design in the sense that it is built around a set of innovations – folded substrate export, vesicular transport, endocytosis, phagocytosis, and lysosomal degradation. These processes are tied to structural innovations – nucleus, endoplasmic reticulum, vesicles, golgi complex, lysosomes.
It is a mature design because it has not been significantly changed since it appeared prior to a billion years ago. In fact, the changes simply tweak the cell – multiple modes of endocytosis, multiple modes of exocytosis, tissue-specific pathways, lineage-specific pathways, and secondary losses.
All of this is quite consistent with foresight. That is, once this cell plan appeared, it was sufficient to support the emergence of complex mammalian organisms. There was no need for redesign of the cell to meet the needs of a complex, mammalian organism. What appeared long before any metazoan-type creature would turn out to work splendidly. In other words, whatever put LECA together got it right from the very start. That original design did not need to be retooled or significantly expanded. It was already mature from the start.
So as we can see, if front-loading involves a cellular architecture and composition that can guide and nudge subsequent evolution, the eukaryotic cell plan from the last common ancestor of eukaryotes would constitute such a context that housed all subsequent eukaryotic evolution.
Or put it this way. If you designed LECA as shown in Figure 3, would you truly be surprised to find the extant cell design later in time? And wouldn’t it mean that the extant cell plan evolved within the context you had designed?