Deep Homology and Front-loading
Deep Homology and Front-loading
I argue that the FLH predicts that proteins of major importance in eukaryotes and advanced multi-cellular life forms (e.g., animals, plants) will share deep homology with proteins in prokaryotes. I have discussed this prediction with various critics of the FLH, and the most common objection seems to be that non-teleological evolution also makes this prediction. I disagree, so let me explain.
Life seems to require a minimum of about 250 genes (Koonin, Eugene V. How Many Genes Can Make a Cell: The Minimal-Gene-Set Concept, 2002. Annual Reviews Collection, NCBI) – a proto-cell would not require that many genes. Thus, it would be perfectly acceptable, under the non-teleological model, that the last common ancestor of all life forms had approximately 250 genes, add or take a few. From this small genome, gene duplication events would have occurred, subsequently followed with mutations in the new genes, leading to the origin of novel proteins. Over time, then, and through gene and genome duplication/random mutation, this small genome would evolve into larger genomes. This model is perfectly acceptable with the non-teleological hypothesis, and the non-teleological hypothesis does not predict otherwise. However, this model – where a minimal genome gradually evolves into the biological complexity we see today, through gene duplication, genome duplication, natural selection, and random mutation – is not compatible with the front-loading hypothesis. This is because front-loading requires that the first genomes have genes that would be used by later, more complex life forms. Of the 250 or so genes required by life, none of them could encode proteins that would be used later in multicellular life forms (excluding the proteins that are necessary to all life forms). A front-loading designer couldn’t possibly hope to “stack the deck” in favor of the appearance of plants and animals, for example, by starting out with a minimal genome.
Look at it this way. With a minimal genome of 250 genes that are involved in metabolism, transcription, translation, replication, etc., evolution could tinker with that genome in any way imaginable, so that you couldn’t really front-load anything at all with a minimal genome. You couldn’t anticipate the rise of animals and plants. Such a genome would not shape subsequent evolution. If the last common ancestor of all life forms had a minimal genome, and if you ran the tape of life back, and then played it again, a totally different course of evolution would result. But if you loaded LUCA with genes that could be used by animals and plants, you could predict that something analogous to animals and plants would arise. If you loaded this genome with hemoglobin, rhodopsin, tubulin, actin, epidermal growth factors, etc. – or analogs of these proteins – something analogous to animal life forms would probably result over deep-time.
Given that you couldn’t really front-load anything with a minimal genome consisting of about 250 genes, under the front-loading hypothesis, it is necessary that the LUCA contain unnecessary (but beneficial) genes that would later be exploited by more complex life forms. Non-teleological evolution does not require this. It has no goal, unlike front-loading. It tinkers with what is there – and if a minimal genome was all that was there, it would tinker around, eventually producing “endless forms most beautiful” as Darwin so famously put it. On the other hand, front-loading is goal-oriented: a minimal genome does not allow one to plan the origin of specific biological objectives.
Thus, under the front-loading hypothesis, we would predict that important proteins in eukaryotes, animals, and plants will share deep homology with unnecessary but functional proteins in prokaryotes.
Non-teleological evolution does not predict this. Non-teleological evolution could explain that observation, but it does not predict this. And this is the important point to understand. There is nothing in non-teleological evolution that requires multi-cellular proteins to share deep homology with unnecessary prokaryotic proteins – but front-loading demands this. There is nothing in non-teleological evolution that requires that the LUCA have a genome larger than the minimum genome size – but for front-loading to occur, this must be the case. I conclude, then, that this prediction is made by the front-loading hypothesis, but it is not made by non-teleological evolution, and so front-loading is certainly testable.