And "abandonment of a hunter-gatherer lifestyle in favor of an agrarian lifestyle" occurred at least 4,000 years before writing appeared (leaving aside the few subcultures which still practice hunting and gathering).
Moving right along ....
True, but the ancient Israelites, from whom the oldest bits of the Bible spring, came a bit late to the table. They were, for example, still in the Bronze Age when all the surrounding civilizations were Iron Age. (It's a testament to their ferocious appetite for guerrilla warfare and the quality of their tacticians that they were able to kick the collective asses of a number of societies far more technologically advanced than they were.)
Interesting timing on this conversation; I just went to a lecture by an anthropologist who talked about the story of the Fall as a kind of lament of the loss of the hunter/gatherer lifestyle.
Are there actually single-celled organisms (other than cancers) that would live forever if it was not for external effects like predation, accident? I assume disease wouldn't have an effect on something that generates telomerase.
Yep. Almost all single-celled organisms, and some simple multicellular organisms (including jellyfish) are biologically immortal. There's a brief introduction to a few of them at the
Wikipedia entry on biological immortality.
The most famous line of immortal cancer cells are
HeLa cells, cultivated from the ovarian cancer tumors that killed Henrietta Lacks and now used to test cancer treatment drugs (among other things). HeLa cells are a great example of how cancer cells can differ so much from the healthy cells in the host that they are, for all intents and purposes, a completely different species. Human beings have 23 chromosomes; the cancer cells that killed Mrs. Lacks have 82. Some of the extra chromosomes are duplicate copies of normal human chromosomes; some are made up of mutated human chromosomes; and some contain HPV DNA. (HPV, or human papilloma virus, is the virus responsible for almost all cases of cervical cancer. It copies bits of the viral DNA into the host DNA, in a process called "lateral transfer.")
There are some cells in human beings that produce telomerase, such as the cells in bone marrow that create blood cells. Since every human cell has a complete copy of all the human genes, it's possible for any human cell to start producing telomerase. Normally, in most cells, the gene for telomerase is switched off, but if it is accidentally switched on, by cellular damage or exposure to environmental toxins or through an error in cellular reproduction, the cell may start dividing out of control. This uncontained rapid dividing can introduce other errors in gene transcription or replication, and the result may be a cancer cell that differs genetically from the original cell.
Genes can also be switched on accidentally if a cell has shortened telomeres but fails to become senescent. If that happens, when the cell divides, the telomeres may break, exposing the raw ends of the chromosomes. We have repair mechanisms in our cells designed to repair damaged DNA, but it doesn't always deal with that situation well. If the telomeres are completely removed from a set of genes, the repair mechanisms can sometimes end up fusing those genes together end-to-end, which can have all sorts of consequences for the cell. The genes may not be read correctly, resulting in transcription of bits that should be switched off, or failure to transcribe bits that should be switched on.
The result, again, is a cancer cell that differs genetically from its host.
This is a quick and dirty overview, of course, but it gets the idea across.
We have a number of cellular defense mechanisms designed to destroy damaged cells. One of those is apoptosis, or "programmed cell death." If a cell begins to malfunction badly, the mitochondria in the cell release a protein that triggers cellular self-destruction...at least in theory.
In cancer cells, this mechanism doesn't work. There is currently a line of research aimed at figuring out why it doesn't work, and causing apoptosis in cancer cells. In lab cultures, it works really well against some cancer cells but not at all against others, and in a few types of cancer it can actually make the tumors grow more rapidly. The strains of HPV that can cause cancer carry a gene for a protein that binds to and inactivates the protein that triggers apoptosis. Since the cancer treatments that work by triggering apoptosis generally do it by prodding the mitochondria into greater activity, HPV-related cancers, which are essentially immune to apoptosis, can actually flourish in the presence of these experimental drugs.