The truth is stranger than fiction sometimes.

I think I've mentioned chimeras before. They are the opposite of identical twins, where two embryos merge creating a composite human with two genetic codes, instead of one embryo splitting to form two humans with (nearly) identical genetic codes. I think I remember reading it happens in maybe one in one million births. I've seen it mentioned a couple of times outside the scientific field: once when there was a case where a woman had a paternity test done on her children so she could receive public assistance benefits - the fellow she claimed as the father turned out to be the father, but it turned out she was not the mother - she almost lost her kids because the court thought she had kidnapped them; a second on a detective CSI type show where a fellow who knew he was a chimera thought he could get away with a crime because of the mismatched genetic types of his blood and skin.

In the scientific field, they've made chimeras out of different species. The experiments I remember from the early 80s involved fusing goat and sheep embryoes to create Geeps. In one particular case, I recall they fused two different types of bird, and got a bird that had the appearance of one species, but sang the bird song of another. They've also fused rabbit and human, and human and sheep embryos (although these were not allowed to develop). Pretty amazing, eh? Don't believe me? Check the article on wikipedia.

Cancer is interesting from an evolutionary standpoint. Multicellular organisms can be thought of as very large colonies of single celled creatures: each adhering to a strict program of differentiation and planned senescence to ensure ultimate long term survival. Evolution and competition can still exist though, even within the context of a multi-cellular body. Every time a human cell divides, it has on average 100 mutations. While this may seem large, consider that the human genome is 3,000,000,000 base pairs, for a mutation rate of .00000003333%. Sometimes, either mutations of this sort or mechanisms related to cell differentiation result in cells that stop following the ordinary instructions for cell types and just start reproducing uncontrollably.

Runaway reproduction is usually a bad thing in the long term, because the cells destroy the body that is supplying them nutrients. In two very strange cases though, cancers have acquired a sort of immortality.

In once case, a woman named Henrietta Lacks had cervical cancer which technically killed her in 1951. Her cancer cells, the HeLa strain, lived on, and live on today in research laboratories all over the world. It's estimated that the total mass of the HeLa cells in the world today exceeds the mass of Henrietta Lacks by several orders of magnitude.

Another more recent find is that certain types of cancers in dogs are infectious. This is not the typical viral infection that causes cancer, like the ones that cause cervical cancer that scientists developed a vaccine for. No, in fact the infectious agent is an actual dog cell line. My daughter's dog had these lumps on him as he got older that were cancerous. Not immediately fatal mind you, just weird lumps. Well, those lumps were the cancerous cells from another dog that now survive as an infectious parasitic growth. (Incidently a similar problem is causing a massive die off in tasmanian devils) Presumably, we could make a clone from those cells and figure out what the original dog actually looked like. Maybe we could do the same with Henrietta Lacks. We could have cancer woman and cancer dog going for a walk.