SET = serial endosymbiosis theory

	Serial = in a series: one step follows the next
	endosymbiosis = concept in biology in which one or more organisms lives inside another organism
	Theory = a scientific idea that has a fairly high level of confidence, based on observation and experimentation

In a nutshell, SET is the notion that complex living cells result from evolutionary processes in which earlier cells merge in a series of successive combinations to eventually form larger, more complex cells

Although some of the ideas of SET surfaced in the early 20th century, Lynn Margulis was the first to articulate a well developed theory in a 1967 journal article and then more completely in 1981 in her book Symbiosis in Cell Evolution.

See the quote from Margulis below

Although not accepted by most scientists at first, mounting experimental evidence has led to an almost universal acceptance of many elements of SET

This theory is still a "working model" and continues to be refined by Margulis and others There are alternative hypotheses that have been proposed as well Lynn Margulis (originator of SET) and Kevin Patton (right) ponder the meaning of life click for larger image

To [over]simplify, SET suggests that each human cell can be thought of as a merging of several earlier cell types and that some organelles were in fact at one time separate bacterial cells

Of particular interest in our course is the mitochondrion According to SET, the mitochondria now in our cells were once bacteria that were capable of energy conversions (cellular respiration) different from, but complementary to, the energy pathways of the original host cells The mitochondria have a protected environment with plenty of surplus glucose from the [host] cell available to "recharge" ATP The [host] cell can use the surplus ATP from the mitochondrion to run a larger, more complex cellular system

Other organelles such as cilia and flagella may have once been independent organisms, as well

If mitochondria were once independent bacteria, one would expect bacterial structures and functions

	They do.  For example: Highly folded internal membranes A bacterial genome (in the form of a single DNA strand looped into a ring shape) Self-replicating (at least within the cell) in a manner similar to bacteria Metabolic processes similar to some bacteria
	The "mitochondrial genome"
	DNA strand in the molecule is called mitochondrial DNA (mDNA or mtDNA) A ring, as in bacteria, not a folded string as in the nucleus All mitochondria of a cell seem to be genetically identical, as are all bacteria of the same colony Mitochondria are "descended from" the mitochondria in the mother's egg cell (sperm mitochondria, if any enter the egg during fertilization, must all "die off") This implies that all the mtDNA in your cells is identical to that in your mother and any other relative related "maternally" to you (including all your children, if you are female) but not identical to the mtDNA of your father or "paternal relatives" (or your children, if you are male) Since there is far less variation in mtDNA than in nuclear DNA from generation to generation, scientists have used mtDNA to discover relationships between different populations of humans with greater accuracy

Mitochondrial function, in light of SET, can be viewed as "subcontracting" in some ways

	Mitochondria "perform a service" for the cell (transfers energy from glucose [acetyl] to ATP)
	The cell "pays" the mitochondria with glucose for its own use ("room and board")
	When the subcontractor (mitochondria) have malfunctioning enzymes, either through direct damage or genetic mutation of mtDNA, then the whole cell may suffer because the subcontractor isn't doing its contracted services properly For example, certain mtDNA mutations can be involved in Parkinson Disease, Alzheimer Disease, certain forms of blindness, and other inherited conditions Why would these diseases of mitochondrial origin be passed by the mother but not the father?  Might male children of affected women also be affected --or just the female children?

In both her writings and in person, Lynn Margulis has entertained the notion that SET plays into the concept that we are all [organisms] "part of each other" and that perhaps the very idea of an "independent organism" is false because we are all made up of smaller organisms and interact with with other organisms to form an even larger "organism."

	What does this statement mean?  How does it extend or apply SET?
	How does this fit in with the Gaia hypothesis (mentioned when we studied "levels of organization" in the course introduction A&P 1)

In the passage above, what situation caused Margulis to stay home and work out details of her SET concepts?

Was Margulis's book about SET accepted for publication eagerly?

When does Margulis say that the ideas that led to SET first appear?

Why should Margulis fret over SET's uncritical acceptance today?