As stated in the last blog post, the evidence is in: We appear to be a genetic landfill for whatever kinds of organisms can invade our cells and dump some of their genes into us. This includes viruses, bacteria, fungi, and even plants! Scientists call this kind of genetic modification, horizontal gene transfer as opposed to vertical gene transfer (from parents to progeny). The first question is: How long has this been happening? Well, the best assumption would be for as long as viruses, bacteria, fungi, and plants have been around! That has been a very long time, indeed.
- viruses: (at least as old as bacteria)
- bacteria: 3.6 billion years
- complex cells (eukaryotes) 2 billion years
- eukaryotes which use sex: 1.2 billion years
- fungi and animals cells separate from plant cells (all of which are eukaryotes): 1.1 billion years
- multicellular eukaryotes: 1 billion years
- Complex animals: 600 million years (Cambrian explosion)
- Complex land plants: 475 million years
The next obvious question to ask is how genes from such divergent groups of cells manage to not only integrate into each other’s genomes, but actually survive and persist as part of a “hybrid” organism? Well, that is pretty much what this blog has been all about. Let me try to explain it in a very anthropomorphic way using business models: Let us start out with three companies: a hardware store, a gas station, and a grocery store. Each one is autonomous from the others and represents their own “species”. Since they each occupy different economic niches with negligible competition from the other two, they can survive in the business ecosystem very well together. One of these companies grows much bigger than the others because of local resources that are available mainly to that kind of company, say the grocery store, for example. It decides to undergo a merger with one of the other smaller companies to gain the benefits and resources of the niche the smaller company enjoys. Over time, redundant positions like cashiers, accounts, clerks, etc are eliminated to save both space and resources. Soon, the company grows large enough to merge with the third company, and the cycle continues on and on.
This is what may have occurred with cells. Acquisition (engulfing), merging (integration of genomes), and downsizing or purging of redundant genes. Such a complex system may have lead to cellular differentiation wherein the different attributes of each “company cell” are expressed in particular tissues of the host cell (nerves, muscle, etc) leading to a complex multicellular organism, be it plant, animal or fungus.
The mechanics of how this may have occurred are discussed and displayed with models throughout this blog. Take some time to review it and get back to me with any questions or comments at: firstname.lastname@example.org.