In Search of Cell History

I’ve slowly worked my way through In Search of Cell History by Franklin Harold, professor emeritus at Colorado State University. The subtitle of the book is The Evolution of Life’s Building Blocks. The focus of the book is on the cell as the building block or “atomic unit” of life, and tracing cell evolution through the three main branches of the phylogenetic tree: Bacteria, Archaea and Eukarya. Based on the similarities and differences among the three branches, Harold speculates on the nature of the cell underlying the tree’s root, the Last Universal Common Ancestor (LUCA).

Harold does several important things in this book. He re-emphasizes the importance of cell theory: “Cells are the atoms of life, and life is what cells do.” He also builds upon Virchow’s dictum: “Every cell from a previous cell.” Modern molecular biology's focus on genes and the genome as the most important players in the deciphering of life is counter-balanced by Harold emphasizing the crucial role played by the cell and its internal organization that distinguishes life from non-life. He also carefully presents the evidence comparing and contrasting Bacteria, Archaea and Eukarya. Although Bacteria and Archaea are prokaryotes and share many similar structural and organizational features (as viewed through a microscope), their molecular constituents show large differences. Harold also discusses the role and evidence of lateral gene transfer at the microbial level and the nature of viruses, along with how these contribute to the complicated lineages near the roots of the “tree of life”.

Eukaryotes are an enigma. On some level, they share characteristics of both Archaea and Bacteria. Harold writes that while Bacteria and Archaea can be separated distinctly by the differences in their ribosomal RNA, “the essential unity of eukaryotes is indisputable; the devil is in their diversity”. For example, the genetic core of eukaryotes shares many similarities to Archaea, but the plasma membranes are akin to those of Bacteria. The seemingly more complex eukaryotes use the very inefficient mechanism of glycolysis for their energy needs. Prokaryotes, on the other hand, are highly diversified and make use of respiration and photosynthesis, much more efficient metabolic systems. Somewhere in deep history, the precursor to the eukaryotes “swallowed” prokaryotes leading to what seems like a mutually beneficial relationship. Mitochondria in our cells that generate ATP (our energy currency) resemble relics of bacteria. Chloroplasts in plant cells that run photosynthesis look remarkably like cyanobacteria.

In Chapter 10, Harold tackles the origin of life. He first sets up practical constraints to the scientific problem, i.e., “[assuming] that life originated here on earth by a natural and probable outgrowth of the chemical and physical circumstances prevailing some four billion years ago.” Given this starting point, “the origin of life can therefore be construed as a problem in geochemistry; but it is also the black hole at the root of biological organization, and there’s the rub.” That is the rub indeed! Harold writes that “a survey of the literature devoted to the beginnings of life leaves one in no doubt that all the critical questions remain open.” I agree with this statement because on my last sabbatical I decided to learn something new and read as much as I could about the chemical origins of life. As a scientist, I also read Harold's statement in a positive light, i.e., there is plenty of room for plenty of interesting scientific work. (I wrote about this topic briefly in an earlier post about the nature of research.) I still try to keep up with the latest research but I admit that I’m falling behind thanks to my administrative duties.

I will leave more detailed discussion about the chemical origins of life to later posts. Instead I will speculate on a point that Harold makes about the enigma of eukaryotes and the rise of multi-cellularity – the evolution of more complex organisms made up of large collections of differentiated cells rather than a single-celled organism. Harold (citing Lane and Martin) suggests that a consequence of eukaryotes incorporating much more efficient energy transducers by “eating” prokaryotes (thereby obtaining chloroplasts and/or mitochondria) could be what leads to multi-celled organisms. Now that the energy needs are taken care of, the organism can evolve into something more complex with specialized parts that contribute not just to survival but rather to growth and propagation.

One could speculate that Harold’s suggestion is the second time this “great leap forward” has taken place. The first time was the “evolution” of LUCA, by incorporating simple energy transducers within a primitive genetic and metabolic framework. The organism that is able to incorporate a more efficient energy transducer will outcompete all others and at this point we can invoke the mechanisms of biological evolution that give rise to the diversity of life.

The speculated third leap forward comes from an intriguing suggestion by Richard Wrangham in his book CatchingFire: How Cooking Made Us Human. (If you do a search, don’t just use “Catching Fire” because you’ll be inundated with the second book in the Hunger Games trilogy.) Wrangham speculates that cooking foods allowed for much more efficient energy transduction, thereby transforming the human brain and society leading in a sense to the next step of evolution beyond mere survival. Before cooking, subsistence through food requiring lots of chewing left little energy to do anything else.

Perhaps the fourth leap forward was the industrial revolution coupled with the taming of electricity. Our plethora of time-and-energy-saving electrical devices free us of having to use large amounts of our own energy and allow us to lead more complex lives. (We could have chosen more leisure instead – I wrote about this briefly when discussing the book Free Time.) And now with computers, and electronic devices of all sorts, we have started to become cyborgian. As we store more of our information on dedicated “cloud” services, it seems we are recapitulating the eukaryotic solution – sequestering the information we need into a specialized nucleus. Our complex lives now extend far beyond our (flesh and blood) cells and are stored/accessed in our digital toys; some of us might be so strongly dependent on our devices that we cannot imagine living without them. Is this the fifth leap forward?

Humans are quite the enigma among living beings. We have even come up with the word “enigma” to allude to what we ponder but do not understand.