A few months back, I used the hypothesis of front-loading evolution to outline a subtle, but very important, shift in our perspective of evolution. Instead of viewing evolution as all about selection picking from a pool of variants, consider the possibility that some variants are more special than others. In this case, let’s pick a variation that is very special – the complete redesign of the cell plan, as it is this change that was necessary for setting the stage for the emergence of metazoan life. I explained this as follows:
This is about whether the cell design – the composition and architecture of the prokaryotic cell – is capable of generating something as structurally complex as a mouse (for a mouse, like all animals, is an assembly of cells). Seen from this angle, the endosymbiotic hypothesis supports my position. That is, in order for prokaryotes to ultimately spawn eukaryotes, they first had to go through a radical re-design of cell structure.
So here is what we have. Prokaryotic cells can be viewed as the highest expression of mutation and selection, for there is no better cellular candidate for a “self-replicator.” Yet after billions of years, the prokaryotic cell plan has failed to achieve anything near the level of structural complexity as exhibited by the eukaryotic cell plan. To reach such structural complexity, the cell design had to be radically retooled, partly through endosymbiotic union, a one-time event given the widely accepted monophyly of eukaryotes. Once the eukaryotic cell design was established, prior to the radiation of all extant eukaryotes, the basic cell design was now capable of supporting the emergence of complex, metazoan life. The evolution of metazoa did not require further extensive retooling of the eukaryotic cell plan, given that metazoan cells are so similar to protozoan cells; it was more like the natural outflow of the potential inherent in the eukaryotic cell plan.
It turns out there is now more scientific evidence to support the contention that the emergence of the eukaryotic cell plan was a necessary prerequisite for the emergence of something as complex as a mouse:
For 70 years scientists have reasoned that evolution of nucleus was the key to complex life. Now, in work published in Nature, Lane and Martin reveal that in fact mitochondria were fundamental to the development of complex innovations like the nucleus because of their function as power stations in the cell.
“This overturns the traditional view that the jump to complex ‘eukaryotic’ cells simply required the right kinds of mutations. It actually required a kind of industrial revolution in terms of energy production,” explained Dr Lane.
Scientists now know that this common ancestor, ‘the first eukaryote’, was a lot more sophisticated than any known bacterium. It had thousands more genes and proteins than any bacterium, despite sharing other features, like the genetic code. But what enabled eukaryotes to accumulate all these extra genes and proteins? And why don’t bacteria bother?
By focusing on the energy available per gene, Lane and Martin showed that an average eukaryotic cell can support an astonishing 200,000 times more genes than bacteria.
“This gives eukaryotes the genetic raw material that enables them to accumulate new genes, big gene families and regulatory systems on a scale that is totally unaffordable to bacteria,” said Dr Lane. “It’s the basis of complexity, even if it’s not always used.”
“Bacteria are at the bottom of a deep chasm in the energy landscape, and they never found a way out,” explained Dr Martin. “Mitochondria give eukaryotes four or five orders of magnitude more energy per gene, and that enabled them to tunnel straight through the walls of the chasm.”
“If evolution works like a tinkerer, evolution with mitochondria works like a corps of engineers,” said Dr Martin.
“The underlying principles are universal. Energy is vital, even in the realm of evolutionary inventions,” said Dr Lane, UCL Department of Genetics, Evolution and Environment. “Even aliens will need mitochondria.”
So we can now point to two aspects of the eukaryotic cell design that explain why there are no prokaryotic mice: introns and mitochondria. And they appear to be linked!
Keep in mind how nicely this all fits three others themes I have explore on this blog:
- The endosymbiotic union was an event that was itself front-loaded
- The crucial importance of oxygen, a byproduct of a biotic process used to terraform the planet
- The conceptual ties between the complex processes of photosynthesis and aerobic respiration
There is a logic and structure that lies behind our evolution. But those who view evolution as nothing more than the environment selecting from among a vast pool of variants can not be expected to see this logic. To them, there is no logic and structure to evolution other than what selection happened to pick out. It is the design perspective, opposed to the anti-evolution and anti-design perspectives, that gives us the edge. It is the design perspective that helps us to view evolution as a biological process instead of a series of events that just happened.