If the recA gene is an evolution gene, we would predict that its removal would somehow negatively impact the ability to evolve. So let’s see what happens when RecA function is removed by mutation.

To start off, it does not appear RecA function is essential to unicellular life or reproduction. For example, a commonly used lab strain of E. coli is known as DH5alpha. This strain has a mutation in its recA gene such that it cannot carry out recombination. Researchers exploit this inability to make their genetic transformations more efficient. Yet these bacteria clearly survive and reproduce. The same theme holds true for yeast, a single-celled eukaryote. While yeast without Rad51 function (the eukaryotic version of recA) are quite sensitive to agents that cause DNA damage, they remain viable.

Yet if we focus on eubacteria as a group, we’d find that the distribution of recA is nearly universal. And when we compare the encoded amino acid sequence of various recA genes from distantly related species, it becomes clear this is a highly conserved gene. For example, RecA sequence from E. coli (a gm negative, enteric bacteria) shows 65% identity and 83% similarity with sequence from B. subtilis (gm positive, soil bacteria). If we couple the way RecA is dispensable for life and reproduction in single-celled organisms to the widespread distribution of the gene and strong conservation of sequence, this suggests RecA’s functions are more fully realized across generations, something we would expect from an evolution gene.

But then something changes in the multi-cellular context. Mice that have both copies of their rad51 gene removed show the embryonic lethal phenotype. In other words, such embryos die and are unable to properly develop. Thus, Rad51 function is essential in the complex process of development, at least in mammals.

Could it be that RecA’s role in evolution is to facilitate the generation of complexity? I just noted the distribution of recA is nearly universal among eubacteria. But nearly universal is not universal. So you might be asking yourself about these bacteria that lack recA. It turns out there is a theme that is shared by these few examples of recA-less bacteria – they are endosymbionts (for example, Buchnera sp.). And what characterizes endosymbiotic bacteria is extreme genome reduction and degeneration. In fact, one comparative study indicates that recA (and other DNA repair genes) are lost early in the symbiotic relationship. In other words, it is the removal of recA that might serve as the trigger for genome reduction and thus the glue that establishes the symbiotic existence.

And it is thus from this angle that the hypothesis of front-loading evolution returns, as our evolution gene could be front-loading the appearance and maintenance of multi-cellular life. But that’s the next part of the story.